synthesis of inorganic solids using microwaves

Reviews

Synthesis of Inorganic Solids Using Microwaves

K. J. Rao,* B. Vaidhyanathan, M. Ganguli, and P. A. Ramakrishnan

Solid State and Structural Chemistry Unit, Indian Institute of Science,Bangalore 560 012, India

Received May 29, 1998. Revised Manuscript Received March 3, 1999

Use of microwaves in the synthesis of materials is gaining importance. Microwave-assistedsynthesis is generally much faster, cleaner, and more economical than the conventionalmethods. A variety of materials such as carbides, nitrides, complex oxides, silicides, zeolites,apatite, etc. have been synthesized using microwaves. Many of these are of industrial andtechnological importance. An understanding of the microwave interaction with materialshas been based on concepts of dielectric heating and of the resonance absorption due torotational excitation. This review presents a summary of recent reports of microwavesynthesis of inorganic materials. Various observations regarding microwave interaction withmaterials are also briefly discussed.

Contents

Introduction 882Interaction of Materials withMicrowaves and Dielectric Heating

883

Case Studies 885Epilogue 894References 894

Introduction

Development of new routes for the synthesis of solidsis an integral aspect of materials chemistry.1 Some ofthe important reasons for this are the continuing needfor fast and energy-efficient techniques, necessity toavoid competing reactions in known processes, and thechallenge implied in the synthesis of metastable phasesbypassing thermodynamically reversible routes. In re-cent times, development of techniques such as sol-geland other soft-chemical methods have led to the prepa-ration of engineered materials.2-5 The microwave-assisted route is yet another novel method of synthe-sis6-10 and is a very rapidly developing area of research.Clark and Sutton have reviewed various aspects micro-wave applications reported in the literature.11 In recenttimes, several reports have appeared where conven-tional preparative techniques have been substituted bymicrowave methods. Microwaves have been in use foraccelerating organic reactions for a quite awhile.7,9,12,13

Microwave synthesis is generally quite faster, simpler,and very energy efficient. The exact nature of microwaveinteraction with reactants during the synthesis ofmaterials is somewhat unclear and speculative. How-

ever, energy transfer from microwaves to the materialis believed to occur either through resonance or relax-ation, which results in rapid heating. This knowledgeis widely used as the basis in the discussion of reactionmechanisms.

Microwaves are electromagnetic radiation, whosewavelengths lie in the range of 1 mm to 1 m (frequencyrange of 0.3 to 300 GHz). A large part of the microwavespectrum is used for communication purposes and onlynarrow frequency windows centered at 900 MHz and2.45 GHz are allowed for microwave heating purposes.Very few microwave applications involving heatinghave been reported where frequencies of 28, 30, 60, and83 GHz have been used.14-17 As is well-known, micro-waves are produced by magnetrons, which are princi-pally thermionic diodes with heated cathodes acting assources of electrons. From the magnetrons the micro-waves are generally directed toward a target (placed inso-called microwave cavities) with the use of micro-waveguides. These guides are usually made of sheetmetal,andtheintensitydistributionwithinthewaveguidesis homogenized by the use of mode stirrers.16

Many microwave preparations reported in the litera-ture have been made on the laboratory scale of only afew grams. These preparations have all been made withthe use of domestic microwave ovens operating at 2.45GHz and with a maximum output power of 1 kW.However, use of higher power levels for specializedapplications have also been reported.15,17-21

Recent reports of the syntheses of several inorganicsolids have been reviewed in this article, and the newinsights gained in some of them have been highlightedand discussed. Many of the illustrative reactions havebeen chosen from the work of the authors. Processingof ceramics using microwaves22-35 and (theoretical)modeling of microwave interaction with ceramic

* Corresponding author; FAX: +91-80-334 1683; Tel: +91-80-3443897/309 2583; E-mail: [email protected].

882 Chem. Mater. 1999, 11, 882-895

10.1021/cm9803859 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 00/00/0000

materials36-38 have also been reported extensively in theliterature; however, these aspects are not discussed inthis review. The present understanding of the natureof microwave interaction with materials is summarizedin the following section, which provides the necessarybackground for the various case studies discussed in thesubsequent section. The final section is a brief critiqueof the state of microwave synthesis.

Interaction of Materials with Microwaves andDielectric Heating

In general, materials fall into three categories, withrespect to their interaction with microwaves; (i) micro-wave reflectors, typified by bulk metals and alloys, suchas brass, which are therefore used in making micro-waveguides; (ii) microwave transmitters which aretransparent to microwaves, typified by fused quartz,zircon, several glasses, and ceramics (not containing anytransition element), Teflon etc.; they are thereforeemployed for making cookware and containers forcarrying out chemical reactions in microwaves; and (iii)microwave absorbers which constitute the most impor-tant class of materials for microwave synthesis; theytake up the energy from the microwave field and getheated up very rapidly.

Many inorganic materials are known to couple stronglyto microwaves at ordinary temperatures. Table 1 listsa number of such minerals and inorganic compounds.39,40

The temperatures attained by these materials and thecorresponding exposure times when irradiated by mi-crowaves in ordinary domestic microwave ovens (DMO)are also listed in the table. Several chalcogenides, notincluded in Table 1, such as cinnabar (HgS), molybden-ite (MoS2), orpiment (As2S3), sphalerite (ZnS), etc. alsointeract with microwaves but are not heated so rapidlyas those listed in the table. It may be noted from Table1 that a number of transition metal containing oxides,halides, etc. also get heated up very fast in microwaves.Most forms of carbon interact with microwaves in theirpowder form. Amorphous carbon powder, in particular,absorbs microwaves (2.45 GHz frequency) most rapidlywith its temperature rising to 1550 K in just 1 min ina DMO operating at 1 kW (Table 1).

It is well-known that the interaction of dielectricmaterials with microwaves leads to what is generallydescribed as dielectric heating.41 Electric dipoles presentin such materials respond to the applied electric field

of microwaves. The reorientation dynamics of the di-poles in the applied alternating field is significant formicrowave heating. When the dipolar reorientation isunable to respond to the frequency of the alternatingelectric field of the microwaves, there results a phaselag in the reorientation and this gives rise to a polariza-tion current which is in phase with the applied field. Ifthe phase lag, field strength, and current are δ, E ,andI respectively, then the component of the in phasecurrent is I sin δ, (since E and I are 90° out of phase inan ideal dielectric). As a consequence, resistive heatingoccurs in the medium. A convenient measure of theheating effect which occurs in an applied field istherefore, sin δ ≈ tan δ ) ε′′/ε′, which is the energydissipation factor or loss tangent. In materials wherethe dipoles rotate freely, such as in liquids, the rota-tional frequency of the dipole determines the dissipationof energy from the applied field. In general, the dipolarspecies in any medium possesses a characteristic relax-ation time, τ, and the dielectric constant is, therefore,frequency-dependent. This is expressed by representingthe dielectric constant as a complex quantity, ε* ) ε′ +iε′′, where ε′ and ε′′ are the real and imaginary parts ofthe dielectric constant. If the dipolar relaxation ischaracterized by a single relaxation time τ and ω is thefrequency of the applied field (such as of the micro-waves), then the relation between dielectric quantitiesand the frequency are represented well by the Debyeequations:

εs and ε∞ are the zero frequency and infinite frequencydielectric constants, respectively. Thus, ε′′ varies withfrequency giving rise to characteristic peak at ωτ ) 1or ω ) 1/τ. For water at 20 °C, the value of therelaxation peak frequency is about 18 GHz and ε′′ isquite significant at 2.45 GHz, which is why in com-mercial ovens, operating at 2.45 GHz, there is rapiddissipation of energy and hence heating of water.

In ionic materials, where ions can drift in the appliedfield, joule heating can occur because of the ionic currentitself. Thus in such situations, the dipolar reorientation

Table 1. Microwave-Active Elements, Natural Minerals, and Compounds

element/mineral/compoundtime (min) of

microwave exposure T, K element/mineral/compoundtime (min) of

microwave exposure T, K

Al 6 850 NiO 6.25 1578C (amorphous, <1 µm) 1 1556 V2O5 11 987C (graphite, 200 mesh) 6 1053 WO3 6 1543C (graphite, < 1 µm) 1.75 1346 Ag2S 5.25 925Co 3 970 Cu2S (chalcocite) 7 1019Fe 7 1041 CuFeS2 (chalcopyrite) 1 1193Mo 4 933 Fe1-xS (phyrrhotite) 1.75 1159V 1 830 FeS2 (pyrite) 6.75 1292W 6.25 963 MoS2 7 1379Zn 3 854 PbS 1.25 1297TiB2 7 1116 PbS (galena) 7 956Co2O3 3 1563 CuBr 11 995CuO 6.25 1285 CuCl 13 892Fe3O4 (magnetite) 2.75 1531 ZnBr2 7 847MnO2 6 1560 ZnCl2 7 882

ε′ ) ε∞ +εs - ε∞

1 + ω2τ2

ε′′ )(εs - ε∞)

1 + ω2τ2

Reviews Chem. Mater., Vol. 11, No. 4, 1999 883

current σd and ion drift current, σi can be combined sothat effectively ε′′ ) (σd + σi)/ωε0, where ε0 is dielectricconstant of free space. In semiconducting and semi-metallic materials, the conduction loss is dominated bythe electronic transport. Hence σelectronic, which increaseswith both increasing temperature and decreasing bandgap, leads to high degree of microwave absorption.

The microwave power dissipation per unit volume ina material, P, is dependent upon the total current σandthe square of the electric field E in the sample:

or, in terms of ε′:

In conjunction with the Debye relations given above

When the heat lost by the system during microwaveinteraction is negligible, one can estimate the rate ofheating as approximately equal to ∆T/t ) σ|E|2/FC,where F is the density and C is the specific heat capacityof the material.

The microwave heating process has been treated moreelaborately by using known material properties andMaxwell’s equations. For the case of a microwaveapplicator with single-mode resonant cavity, whichsimplifies the method of calculation, the heating ratedT/dt is formulated as42

where F is the mass density of the sample, Vc is thecavity volume, and P0 is the microwave power insidethe cavity, S is the Stefan-Boltzmann constant, and êis the surface emissivity of the sample. The experimen-tal temperature versus power plot obtained for the caseof a few ceramics under single mode irradiation condi-tions is shown in Figure 1, along with the modelprediction using the above expression.

It is therefore evident that coupling to microwaves of2.45 GHz frequency in microwave reactors such as aDMO is possible only if the material has dipolarabsorption in the region of the reactor frequency (2.45GHz). However, dipole relaxation times are affected bytemperature and the microwave absorption of the mate-rial can therefore be optimally increased by matchingthe relaxation frequency of the material with thefrequency of the reactor (DMO). This is achieved byeither varying the temperature or the reactor frequencyitself. Also, in a material in which the dielectric losspeak frequency is higher than the microwave reactorfrequency, the initial microwave heating shifts theabsorption peak further away from the reactor fre-quency as a result of which microwave coupling de-creases and further heating stops. Choice of microwavecontainer materials for safe handling are based on theseconsiderations.

The temperature profile within the material exposedto microwaves depends on several geometrical factors

related to design of the microwave cavity. However,when microwaves are incident perpendicularly on thesurface of the material, its intensity decreases progres-sively inside the material in the direction of incidenceas the microwave energy gets progressively dissipated.This is expressed through the parameter known aspenetration depth, D, which is the distance in thedirection of penetration at which the incident power isreduced to half its initial value:

where λ0 is the wavelength of the microwaves. Theequation suggests that there is a slight advantage inworking at lower frequencies when large samples areinvolved, but it is associated with a payoff in terms ofpower absorbed per unit volume. In bulk metals andalloys which generally reflect the microwaves, themicrowave penetration is rather low and it is commonpractice to describe the microwave penetration in metalsthrough a quantity known as skin depth, δ. This is givenby

where ν is the microwave frequency (ω ) 2πν), µ is thepermeability of free space, and σ is the electricalconductivity.43

Dielectric properties are dependent on both thechemical composition and on the physical state of thematerial. For example, impurities, aliovalent substitu-tion, and the chemical nature of the material (whetherit is a transition metal compound or a chalcogenide, etc.)can give rise to high dielectric constants and losses andresult in strong microwave coupling. Similarly, powderswhich consist of surface defects, surface charge, andpolarization can also enhance the dielectric parameters

P ) σ|E|2 ) (ωε0ε′′)|E|2

P ) (ωε0ε′ tan δ)|E|2

P )ε0(εs - ε∞)ω2τ

1 + ω2τ2|E|2

dTdt

) 4tan δ

1FC

ε′′xε′

1Vc

P0êSFC( area

volume)sample(273 + T )4

Figure 1. Steady-state temperature vs power plots for threedifferent ceramics under single mode irradiation conditions.Solid lines were calculated from equations for heating rate ina single-mode resonant cavity (see text) (data from ref 42).

D )3λ0

8.686π tan δ(ε′)1/2

δ ) 1/xπνµσ

884 Chem. Mater., Vol. 11, No. 4, 1999 Reviews

which are desirable for microwave coupling. Crystalstructures which support permanent polarization cansimilarly be expected to give rise to good microwavesusceptibility. In turn it is to be anticipated that mate-ials containing highly polarizable elements (e.g., PbS)should be good microwave susceptors. Powders of inor-ganic materials which are piezoelectric can give rise toresonance absorption because the thickness vibrationalmodes in micrometer-sized particles which lie in themicrowave regime.43 In highly ionic materials (fast ionconductors), both polarization effects associated withsite to site jumps of ions and the ionic current enablethem to couple well with microwaves.

Microwave interaction of two other types can also leadto dissipation of energy very rapidly. One of them is theexcitation of weak bonds such as secondary bonds inchalcogenides or interlayer bonds in graphite (andsimilar materials). We may note that microwave fre-quencies correspond to rotational excitation energies inmaterials. Thus the incident microwaves may exciterotational modes in a material. They are state-to-stateexcitations and therefore the energy absorption occursby resonance. The absorbed energy may be completelydissipated as heat during deexcitation via internal modecoupling. The excitation of weak bonds may fall into thesame general category of state-to-state excitations.

The behavior of large metal pieces and correspondingmetal powders under microwave irradiation are start-lingly different. As noted earlier, skin depths in bulkmetals are very low and vary as 1/xσ, and hence, thereis little penetration of microwaves. In large metalsamples and also in metal films, large electric fieldgradients occur in the microwave cavity, which givesrise to electric discharges. On the other hand, in metalpowders, no such discharge takes place and due to eddycurrents and localized plasma effects, very rapid heatingtakes place which may be as high as 100 K/s.44 Eddycurrent phenomena arises from the alternating mag-netic field associated with microwaves. Other manifes-tations of the microwave coupling via magnetic fieldeffects include hysteresis loss, dimensional resonancesin magnetic fields, and magnetic resonance from preces-sion of magnetic moments of unpaired electrons, etc.43

Dielectric heating in microwaves has thus severalorigins. In a broad sense, they can occur either viaresonance modes of absorption, or via relaxationalmechanisms as a result of phase lag between alternatingmicrowave field and the motion of the polar species inthe material. The dielectric properties themselves beingtemperature-dependent, both microwave coupling andthe resulting temperature profiles of the sample can bevery different for different materials. In Figure 2, thebehavior of three oxide samples is shown which il-lustrates this point.8 Silica (SiO2), whose dielectricconstant and dielectric loss do not show much disper-sion, does not get heated up much when exposed tomicrowaves even after a long duration. But the tem-peratures of the two oxides, NiO and Cr2O3, escalateafter some initial exposure times. In the case of NiO,the high dielectric loss results in almost a parabolicincrease in temperature, while increased dielectric lossat the high temperature is responsible for the fairlyabrupt rise in temperature in Cr2O3. The regions of highslopes in the temperature-time plots are referred to as

“thermal runaways”. However, it may be noted that thetemperature time profiles are dependent on the micro-wave power levels as well. The temperature correspond-ing to the beginning of thermal runaways is designatedas Tcritical.

The expression for the microwave penetration depthsuggests that for most materials, D is significantly highand, therefore, microwave power dissipation is fairlyuniform throughout the material as long as the samplesizes are not large. This leads to an important featureof microwave heating, namely, volumetric heating,which results in a temperature profile exhibiting adecreasing slope toward the geometrical borders of thesample in a manner opposite to that found in conven-tional heating.

Case Studies

In this section, we consider examples of the synthesisof the materials achieved through the use of micro-waves. These are illustrated under various categories,viz., (i) direct reactions, including those involving theuse of simple compounds; (ii) preparation of solids whichinvolve decompositions and combinations giving rise tosolids of complex composition; (iii) nitridation reactions;(iv) reactions brought about in liquid media; (v) prepa-ration of glasses; (vi) selective deoxidation reactions; and(vii) plasma-assisted reactions.

(i) Direct Reactions. Simplest, very illustrative, andtechnologically important microwave preparation start-ing from the elements is that of â-SiC.45 SiC is a largevolume ceramic used in many industrial applicationssuch as for grinding wheels, etc. It has been preparedvery simply by exposing Si and C (charcoal) in theirpowder form to microwaves. Slight excess of carbonpowder is used in the reaction mixture, which is keptin a silica crucible and irradiated with microwaves for4-10 min in an ordinary domestic microwave oven(DMO) at a maximum power of 1 kW and operatingfrequency of 2.45 GHz. The temperature attained by themixture at various power settings is shown in Figure3a as temperature-time plots. It may be seen that thehighest temperature attained is dependent on the powersetting and the temperature rises to a maximum withinthe first 2 min of exposure. The X-ray diffraction patternof the product obtained is shown in Figure 3b. It wasfound that an inert ambient was essential for purity ofthe product. In air, up to 15% SiO2 was formed in such

Figure 2. Microwave heating of silica, chromium (III) oxide,and nickel (II) oxide. Note that SiO2 with low dielectric lossdoes not heat, whereas NiO and Cr2O3 couple well withmicrowaves and get heated up rapidly. Cr2O3 exhibits “thermalrunaway” due to increased dielectric loss at higher tempera-tures (data from ref 8).


a reaction while only monophasic â-SiC was formedwhen surrounded by a shroud of iodine vapor. Theshroud of iodine vapors was produced in situ in themicrowave oven by a separate but simultaneous reaction(see later). The entire reaction was completed in lessthan 10 min. Temperatures were measured by inter-rupting the microwave irradiation and inserting athermocouple into the reaction mixture.

It was interesting to note that in this microwavepreparation, the Si + C reaction occurred at tempera-tures less than 1250 K, unlike in a conventional heatingprocess, which requires much higher temperature (1673K).46 Therefore, Si does not melt during the reaction.

Also, it was found that Si is not a susceptor to micro-waves at ordinary temperatures, whereas charcoal is.Therefore high temperatures are initially achieved bythe heating of carbon which is attributed to the excita-tion of weak graphitic bonds by the microwaves. Sincethe reaction Si + C f SiC is exothermic (∆G°298 K )-16 kcal/mol), the rate of reaction increases very rapidlyonce it is initiated. It was also found that the productwas free from any hot spot formation and excess carboncould be simply burned out even at 700 °C. It was evenpossible to float out any excess carbon powder. Theproduct was remarkably clean, monophasic â-SiC.

Several important chalcogenides have been preparedusing microwave irradiation. They include PbSe, PbTe,ZnS, ZnSe, Ag2S,47 and ZnTe.48 Stoichiometric quanti-ties of fine powders of the respective metals andchalcogens of high purity were sealed in evacuatedquartz ampules (about 10 g of mixture). The sealedampules were irradiated with microwaves for 5-10 minin a DMO with a power setting of 800-950 W. Reactantswere found to melt, and the reaction was often associ-ated with emission of splashes of light. The ampuleswere then allowed to cool by turning off the microwaveoven and then broken to recover the products of goodphase purity and crystallinity (see XRD in Figure 4).Whittaker and Mingos prepared several chromiumchalcogenides, R-MnS, Fe7S9, TaS2, and SnS2 by irradia-tion of the elemental mixtures for times less than 10min (however, it was found necessary to anneal theproducts for several hours).44

Ternary chalcogenides, CuInS2 and CuInSe2, whichhave high potential for application in solar cells (as theyare semiconductors with direct band gaps of 1.53 and1.04 eV), have been synthesized by microwave irradia-

Figure 3. (a) Time-temperature graph for Si-C reaction.The maximum temperature attained during the reaction isdependent on the microwave power. Even at the maximumpower used, the temperature attained is only about 1200 K,which is less than the temperature required for conventionalpreparation of SiC (data from ref 45). (b) XRD patterns for (i)silicon-carbon initial reaction mixture, (ii) reaction productsformed in air after 10 min, and (iii) reaction product formedin iodine atmosphere after 10 min. When the reaction wascarried out in air, SiO2 impurities are observed, whereas whenthe reaction is carried out in iodine atmosphere, pure â-SiC isobtained (data from ref 45).

Figure 4. XRD patterns of the metal chalcogenides preparedusing microwaves. The reaction was carried out in evacuated,sealed quartz tubes under microwave irradiation for less than10 min (data from ref 47).


tion of elemental mixtures.49 The synthesis was ac-complished by microwave irradiation of elemental mix-tures taken in quartz tubes and sealed under vacuum.Blue flashes were initially observed which has beenattributed to the formation of sulfur plasma. Severalexperiments were performed to identify the reactionmechanism, and it was concluded that Cu first reactswith the chalcogen to form copper chalcogenide and theheat released by the reaction quickly melts the indium.Further reaction takes place in the indium solution ofcopper chalcogenide and the chalcogen. Copper indiumchalcogenide crystallizes from the liquid melt. Scanningelectron microscopic studies have been used to provideevidence for the involvement of molten phases.

It may be noted that the conventional preparation ofthe chalcogenides50 involves heating in rotary furnacesup to 48 h. It appears that the rapidity of the reactionin microwave method is a consequence of simultaneousheating of the metal powders through induction of eddycurrents and of the chalcogens through activation of thesecondary bonds. The release of reaction enthalpiesleads to rapid escalation of the reaction rates.

The direct reaction of elemental reactants has beenused for the microwave preparation of carbides such asTiC and Mo2C.51 The microwave susceptibility of gra-phitic carbon has been exploited since Mo powder itselfis not a very good susceptor. Formation of the carbidesis also assisted by the reaction enthalpy. The metalshave a strong tendency to form oxides when the irradia-tion of elemental mixture is performed in the ambienceof air. However, use of an iodine shroud as in the caseof SiC enables formation of products with high phasepurity. However, Kozuka and Mackenzie achieved prepa-ration of SiC, TiC, NbC, and TaC using microwavemethod where carbidation was preceded by a carbo-thermal reduction of the required oxides. The temper-atures of the reaction were of the order of 1773 K andthe product morphologies were found to be dependenton the nature of both the form of carbon used and thephysical nature of the oxide.52

Several industrially important silicides have beenprepared by direct microwave reactions.53 Notable is thepreparation of MoSi2.54 When a stoichiometric mixtureof only Mo and Si is irradiated, no reaction seems tooccur. When graphitic carbon is mixed with the reac-tants and irradiated, a pronounced tendency of Si tocombine with C is noticed. Mo-Si reaction is neverthe-less initiated when the mixture is heated along withamorphous C powder, but small amounts of SiC areformed in the course of reaction. However, when Mo andSi powders are mixed well (by repeated grinding),pelletized, covered with amorphous carbon, and thenmicrowave irradiated in a DMO, Mo and Si reactcompletely, and the product MoSi2 is found to form asa well-sintered button. The surface of the button wasscraped and either dusted or washed. The reaction, Mo+ 2Si f MoSi2, seems to be initiated only at very hightemperatures (>1200 K). As noted earlier, microwavecoupling of various forms of carbon is very different fromone another. Shown in Figure 5 is the time-tempera-ture behavior of two well-known forms of carbon. It maybe seen that amorphous carbon attains temperature ofapproximately 1500 K in less than 2 min while tem-peratures attained by graphitic carbon is only 1200 K

which does not increase further. Therefore MoSi2 for-mation is initiated only by amorphous carbon and thereaction drives itself to completion due to the liberationof reaction enthalpy. In the case of TiSi2 (and also CoSi2)however, the temperature attained by graphitic carbonis sufficient to initiate the reaction. Formation of WSi2follows the pattern of MoSi2, requiring high tempera-tures for initiation and hence the use of amorphouscarbon. The phase purities of these silicides are foundto be excellent. The microwave coupling of the metalpowders were found to be in the order Ti > Co > W >Mo. It may be noted that the high melting point,oxidation resistance, carbidation resistance, and metal-lic conductivity of MoSi2 makes it an important indus-trial silicide used in making heating elements and hightemperature engine parts. The conventional preparationof these silicides, particularly MoSi2 involves tedious,cumbersome, and expensive procedures such as high-temperature vacuum fusion and laser pulse treatments.

Microwave-induced direct reactions between simplecompounds is exemplified by the formation of bismuthand lead vanadates,47 BaWO4 and CuFe2O4,55 and La2-CuO4 and YBa2Cu3O7-x.56 In all these preparations,stoichiometric mixtures of the corresponding oxideshave been irradiated by microwaves for times up to 30min, sometimes followed by longer durations of anneal-ing. The reaction times are much lower than in theconventional procedures. For example, while prepara-tion of Pb- and Bi-vanadates by conventional proce-dures requires up to 10 h of heating at high tempera-tures, the microwave-induced reactions are completedin under 15 min in a DMO operating at 900 W. Theannealed Bi4V2O11 sample is found to exhibit R f âtransition at ∼725 K, while the unannealed sampleshows only a weak transition. This suggests that unan-nealed product is in metastable state, which is consid-ered to be structurally disordered, the disorder beingrandom orientation of bismuth-oxygen and vanadium-

Figure 5. Time-temperature profiles of the graphitic andamorphous forms of carbon under microwave irradiation (linesare drawn as a guide to the eye). Maximum temperatureattained is dependent on the particle size. Amorphous carbongets heated to a higher temperature than graphite (data fromref 51).

Table 2. Microwave-Assisted Preparation of SomeMixed Oxides

compoundstarting

materialstime of

exposurepower used

(W) ref

BaWO4 BaO, WO3 30 min 50-500 55CuFe2O4 CuO, Fe2O3 30 min 50-500 56La2CuO4 La2O3, CuO ≈10 min,

(30 min anneal)130-500 56


oxygen polyhedra. It is suggested that the randomorientation leads to anomalies in XRD intensities.Relevant reaction details are summarized in Table 2.

Recently, a convenient microwave irradiation methodhas been developed57 for preparing YBa2Cu3O7-x inwhich pellets of the reactant oxide powders are embed-ded in the same mixture of powders and the heat lossfrom the reaction mixture is prevented by surroundingit with glass wool. Reaction was carried out in a DMOat a power level of 200 W and was found to be completedin just 25 min.

Direct reaction in microwaves has been used in anovel way to prepare composites by Clark et al.58 Al2O3-TiC composites have been prepared by microwaveirradiation of titania along with aluminum and carbonpowders. TiC, TiB, and Al2O3-TiB composites have alsobeen prepared similarly by the microwave ignitionmethod. Products of relatively high density were ob-tained in the process. The initial microwave absorptionhas been both by metal powders and graphite andpowerful industrial multimode oven having a maximumpower output of 6.4 kW was used.

Microwave-initiated reactions between metal powdersand gases have been reported by Whittaker and Min-gos.59 A number of transition metal halides, oxyhalidesand nitrides as also indium and tellurium halides havebeen prepared by this method. Fluidized beds were usedto carry out the reactions.

(ii) Complex Solids. Conventional methods of prepa-ration of industrially important niobates and titanates,such as BaTiO3, LiNbO3, and lead zirconate titanate(PZT), require several hours of high-temperature reac-tions. Use of microwave methods appears not feasibleas a first thought since several of the oxides used forthese preparations are not microwave susceptors. It isnot convenient to make these preparations by usingsuitable carbonates or nitrates as starting materialsbecause such carbonates and nitrates are also not goodmicrowave susceptors. Microwave methods can be modi-fied for the preparation of this class of materials byincluding a secondary susceptor in the reaction mixture.The important requirement is that the secondary sus-ceptor only assists the initial heating but does not reactwith other reactants. For the preparation of titanates,niobates, etc., the transition metal oxide and the nitrateor carbonate of the other metal in the compound areground together, pelletized under moderate pressures,embedded in graphite powder in a silica or aluminacontainer, and then irradiated with microwaves in aDMO.60 Due to its high microwave susceptibility, carbon

(graphite) catches fire and heats the pellets therebyinitiating the decomposition of the nitrates or carbon-ates. The resulting hot oxide mixture of the pelletappears to couple much better with microwave at thosetemperatures and an extremely fast reaction ensues,giving rise to the desired products. Several of thesereactions are summarized in Table 3. The products havebeen found to possess both good crystallinity and phasepurity.

In an illuminating experiment directed toward un-derstanding the effect of microwaves, the reaction ratesand activation energies have been evaluated for theBaCO3 + TiO2 f BaTiO3 + CO2 reaction by makingmeasurements at three different temperatures (achievedby adjusting the power level).67 It was found that thereaction was characterized by an activation barrier of58 kJ/mol, compared to much higher values in aconventional diffusion-controlled reaction.68

When one of the reactants is a microwave susceptor,such as in a mixture of K2CO3 and V2O5 (V2O5 issusceptor), decomposition of the carbonate and combi-nation of V2O5 and K2O does not require the presenceof graphite. As noted earlier, the role of graphite in thesepreparations is as a secondary heater and this propertyof graphite can be fully exploited in any other similarsituation provided the oxides are not reduced carboth-ermally by graphite. BaTiO3 has also been preparedby the microwave irradiation of stoichiometric mix-tures of respective metal-organics (Ba(OCOCH3)2 andTi(OC3H7)4).69

Polycrystalline Y3Fe5O1270 has been synthesized by

starting from Y2O3 and Fe3O4 mixtures and using amonomode microwave source and the reaction temper-ature has been found to be below 900 K. Microwaveirradiation enabled preparation of YFe2O4 also in asimilar manner.

A wide range of complex oxides are used as cathodesin lithium batteries.71 LiMn2O4 is a cubic spinel and isconsidered as an important cathode material. LiMn2O4has been prepared using microwave irradiation of amixture of LiI and MnO2 in a DMO for just 6 min.72

The product is found to be high-purity monophasic cubicspinel. Iodine, which is liberated in the reaction, actsas an antioxidation shroud. However when Li2CO3 andMnO2 are used as reactants, only a defect spinel phaseis found to result after microwave irradiation.

Similarly, a stoichiometric mixture of CuO and Bi2O3,irradiated with microwaves for just 5 min, has beenfound to yield monophasic, tetragonal CuBi2O4. Thiscompound is reported to have been tested as cathode in

Table 3. Preparative Conditions for the Microwave Synthesis of Niobates and Titanatesa

conventional methodcompound initial reactants

power and duration ofmicrowave exposure conditions ref

LiNbO3 Li2CO3 + Nb2O5 15 min (600 W for 10 min+ 800 W for 5 min)

needs more than 12 h at 770 K 61

NaNbO3 Na2CO3 + Nb2O5 17 min (600 W for 10 min+ 800 W for 5 min + 980 W for 2 min)

several hours of heatingat 1523 Kwith intermittent grinding

62

KNbO3 K2CO3 + Nb2O5 12 min (600 W for 7 min+ 800 W for 5 min)

needs 30 h of heating at 1273 K 63

BaTiO3 BaCO3 + TiO2 25 min (600 W for 5 min+ 800 W for 10 min + 980 W for 10 min)

several hours of heating of theball-milled powders at 1673 K

64

PbTiO3 PbNO3 + TiO2 9 min (600 W for 9 min) more than 8 h heating at 633 K 65PbZr0.52Ti0.48O3 PbNO3 + m-ZrO2 + TiO2 10 min (600 W for 5 min

+ 800 W for 5 min)more than 6 h of heating at

different temperatures66

a Comparison with conventional route is made.


Li batteries using glassy electrolytes.73 The conventionalpreparation of CuBi2O4 requires different stages of heattreatment and the total heating time of about 18 h withthree intermittent grindings.

Microwave preparation of LiCoO2 using metal-organics has been reported by Yan et al.74 The synthesisinvolves an initial calcination at 673 K, and from theXRD pattern, it appears that the compound is formedeven at this stage and microwave irradiation helps infurther crystallization of the product.

The reaction between LiI and V2O5 has been used toproduce an iodine shroud (referred earlier in prepara-tion of SiC). This simple microwave reaction betweenV2O5 and alkali iodides was found to lead readily to theintercalation of alkali ion into the layered V2O5 resultingin the formation of alkali vanadium bronzes and libera-tion of iodine.75 However, these bronzes appear to becharacterized by significant cation disorder leading tointensity disparities of the XRD peaks compared tothose prepared by conventional methods and needfurther annealing.

Preparation of phosphates and phosphatic materialsgenerally involves long reaction times and high tem-peratures in conventional methods. But phosphates arenot susceptible to microwaves and therefore it is difficultto use microwaves for the preparation of phosphatesexcept when one of the other reactants is a susceptor.However, recently it has been found that crystallineNaH2PO4‚2H2O is a good microwave susceptor.76 WhenNaH2PO4‚2H2O is irradiated by microwaves, heatingoccurs initially due to the presence of water of crystal-lization. This type of microwave susceptibility is com-mon to most crystalline hydrates. But in the case ofNaH2(PO4)‚2H2O the elimination of 2H2O is followed bycontinued microwave heating, resulting in the loss ofthe remaining chemically bound (structural) watermolecule. At this point the product, metaphosphateNaPO3, attains a temperature of ∼700 K, with areasonably high microwave susceptibility of its own.NaPO3 gets heated further to its melting temperature.Finally NaPO3 melt attains a temperature of about 1000K. These decomposition stages while heating have beenidentified by thermal analysis.

Initial microwave absorption in hydrated phosphatesis likely to be due to rotational excitation of the H2Omolecules. Phosphates such as KH2PO4 and (NH4)2-HPO4 in which water of crystallization is absent do notcouple to microwaves at all. The vibrational-rotationalenergy spectrum of water of crystallization appears toinfluence the microwave activity. Thus, for example, inNa2HPO4‚2H2O, where a proton has been substituted

by Na+ ion (in a chemical sense), the microwavesusceptibility of the water of crystallization is reducedenormously and Na2HPO4.2H2O does not dehydrate likeNaH2(PO4)‚2H2O does under microwave irradiation. Thebehavior of some of the related alkali phosphates issummarized in Table 4. Na2HPO4‚2H2O is thereforeunique in its microwave susceptibility and the natureof its heating in microwaves can be likened to a step-ladder in which the first stage of heating with the lossof two molecules of water of crystallization takes it to atemperature of ∼500 K and the next stage of loss ofwater takes it to ∼800 K due to increased tan δ. In thefollowing stage, temperature rise occurs for the samereason (increased tan δ) and takes the phosphate to themolten state around 1000 K.

Phosphates of the general formula AxBy(PO4)3 whereA is an alkali element and B is a transition element orSi or Ge are generally designated as NASICONs, whichis an acronym for sodium superionic conductors and thearchetypes, NaZr2(PO4)3 and NaSiGe(PO4)3, have beengood Na+ ion conductors.77,78 This class of crystallinephosphates possesses unique open structure and prop-erties, like high ionic conductivity, low thermal expan-sivity, etc. They have also been considered as possiblehosts for nuclear waste disposal. The conventionalmethod of preparation of NASICONs involves severalhours of controlled heating. It has now been foundpossible to prepare NaZr2(PO4)3 and Na3Fe2(PO4)3 inless than 8 min of microwave irradiation in a DMO frommixtures containing Na2HPO4‚2H2O and the otheroxides (ZrO2 and Fe2O3) in proper stoichiometry. TheXRD of the microwave-prepared products are shown inFigure 6.

(iii) Nitridation Reactions. There has been arevival of interest in nitrides because of the versatilityof these materials. While Si3N4 and SiAlONs representsuperior structural ceramics,79,80 GaN is a blue lasermaterial81 and AlN is a superior material for sub-strates;82 GaN together with AlN and InN span widesemiconductor band gaps of 1.9-6.2 eV. They formcomplete (wurtsitic) solid solutions and, therefore, pro-vide versatile semiconductors with tunable band gaps.TiN is similarly a well-known diffusion barrier mate-rial83 in the form of coatings and is also used as gold-colored hard coating on a variety of consumer products.VN is used as a catalyst.84 Nitrides in general, however,are relatively unstable compared to oxides at hightemperatures, which accounts for their virtual absencein earth’s crust. The unreactivity of N2 is the mainreason for the great difficulty encountered in thepreparation of nitrides.

Table 4. Microwave Susceptibility of Different Phosphates

compoundmicrowave

exposure time observations and inferencetemperature

reached in 5 min (K)

NaH2PO4‚2H2O 7 min the two coordinated water molecules get completelyremoved after 50 s; he third water fully leaves after125 s and the material is melted within 150 s; theweight loss observed corresponds to three H2O molecules

951

Na2HPO4‚2H2O 9 min no weight loss, no red hotness and no melting observed;not a good microwave susceptor

328

Na3PO4‚12H2O 7 min weight loss observed; found dehydrated, but no furthermicrowave bsorption noticed after 2 min; no melting orred hotness seen

365

KH2PO4 9 min no microwave absorption; no weight loss and no melting 316(NH4)2HPO4 8 min no weight loss and no melting; not a good microwave susceptor 323


The rotational modes of N2 may be expected to beexcited in microwaves and help nitridation. NH3 is alsooften used to introduce N into chemical compoundsparticularly when replacing O by N. NH3 can also beexpected to be microwave-active because of its rotationalmodes. It should therefore be anticipated that nitrida-tion by the use of N2 or NH3 is assisted by microwaves.This approach has been used effectively for the prepara-tion of nitrides of the third group elements and some ofthe industrially important nitrides of transition metals.

AlN has been prepared by a microwave method viadirect nitridation of Al powder.85 Al and charcoal powdermixtures taken in an all-quartz container with anattached inlet and outlet arrangement was microwaveirradiated in a DMO in the ambience 99.999% purityN2. It was found that nitridation proceeded to about 40%in 15 min but no further. The partially nitrided mixturewas taken out, ground well, and again subjected tonitridation in the same manner. It required six repeatedgrinding and nitriding operations for achieving completeconversion of Al into AlN. The X-ray diffractograms ofthe nitrided powders at various stages of nitridationwere recorded. In Figure 7, the percentage of conversionand the actual time for nitridation are shown along withthe X-ray diffractograms before and after the completionof the reaction. The repeated grinding and microwaveirradiation leads to the formation of extremely finepowders, as noted from electron micrographs of thepowders. The nanosized AlN particles appear as ag-gregates in the final product. The temperature of thereaction was not found to exceed 1173 K at any stage.This temperature is significantly lower than tempera-tures employed in conventional processes. The temper-ature of the charge was found to increase rapidlyinitially due to both the susceptibility of carbon andformation of eddy currents in Al particles. The nitrida-tion, which occurred at the surface of each Al particle,resulted in the formation of a N2-impervious coating ofAlN which therefore required grinding to expose unre-acted Al to N2 in subsequent stage of microwave

irradiation. Since there was no evidence of melting andaggregation of Al, particularly since 2Al + N2 f 2AlNis a highly exothermic reaction, it appears that the highthermal conductivity of AlN coating could have quicklydissipated the heat to the surroundings. The nitridationoccurring at metal surface may involve excitation of theNtN bonds but the mechanism of nitridation is still notclearly established.

Yet another approach to the preparation of nitrideswith the use of a microwave is exemplified by thecarbothermal reduction and nitridation using high-purity NH3.86 In the case of GaN, for example, high-purity gallia (Ga2O3) was ground with an excess of high-purity amorphous carbon black and kept in a fusedquartz container that was exposed to microwaves in aDMO, which allowed continuous flow of high purity NH3over the irradiated mixture. Due to the high microwavesusceptibility of amorphous carbon, the mixture at-tained a high temperature within a short time. Galliawas completely converted to GaN (Figure 8). However,when N2 was used in place of NH3, complete nitridationdid not occur even after 35 min of exposure. As notedearlier, amorphous carbon attains much higher tem-peratures on microwave exposure than other forms ofcarbon and is a particularly powerful reducing agent.However, in addition to reduction (removal of oxygen),

Figure 6. XRD patterns of microwave-prepared crystallineNASICONs. Na2HPO4‚2H2O couples well with the microwaveraising the temperature and assists the reaction resulting inthe formation of NASICONs (data from ref 76).

Figure 7. Amount of AlN formation with microwave exposuretime. The reactants were subjected to periodic grinding byinterrupting the reaction every 15 min. Top inset: XRDpattern of the product mixture after 15 min of microwaveexposure (no grinding). Bottom inset: XRD patterns of theproduct mixture after 120 min of microwave exposure (eightgrindings) (0, with intermediate grinding; +, without inter-mediate grinding). Single phase AlN is obtained only withintermittent grinding and reexposure to microwaves (datafrom ref 85).

Figure 8. XRD pattern of the microwave-prepared GaN. Thenitridation was complete only when NH3 was used. Also, thereduction was possible only with amorphous carbon (data fromref 86).


nitridation requires attachment of nitrogen to thegallium ions at the same time as reduction occurs. It issuggested that a plasma is created around the reactionmixture, through the coupling of NH3 with microwaves,which may consist of species like NH2

- and NH2-. Thisincreases electron density on nitrogen and creates amore favorable condition for the attachment of nitrogento Ga3+ ions. Eventually other hydrogen atoms areeliminated and GaN is formed. However, no confirma-tion of the presence of amide (NH2

-) or imide (NH2-)has been made.

(iv) Reactions in Liquid Media. Reactions inaqueous media where microwaves play an importantrole is exemplified by the preparation of hydroxyapatite(HAp).87 HAp is the inorganic material which composesthe skeletal structure in living beings. It is also natu-rally available as an inorganic mineral. Stoichiometricapatite is a hydroxy phosphate of calcium with theformula Ca10(OH)2(PO4)6 and is very useful for pros-thetic applications.88 The conventional preparation ofHAp is an elaborate process in which solutions of Ca-(NO3)2, (NH4)2HPO4, and NH4OH are reacted at highand critical values of pH for long times when HAp isprecipitated.89 The hydrothermal process of preparationof HAp is even more tedious.90 This important biocer-amic material is, however, prepared very easily by themicrowave method. An aqueous suspension of a sto-ichiometric amount of Ca(OH)2 and a solution of (NH4)2-HPO4 are mixed together in a glass container andirradiated in a DMO. The power level is adjusted so asnot to allow the solution to spill over while boiling offwater. In less than 25 min, a fine, dry powder of HApis formed. The X-ray diffraction pattern of the productcompares extremely well with that of standard com-mercial samples (Figure 9); the IR spectrum clearlyreveals the presence of a triplet band in the region of550-650 cm-1, which is very characteristic of HAp(Figure 10). The fast reaction is attributed to themicrowave coupling of water bound to Ca2+ ions in anaquation sheath. The rotational excitation of the watermolecules by microwaves disengages them from Ca2+

ions. This has the effect of denuding the Ca2+ ions andenabling them to interact with the phosphate andhydroxyl ions much more readily than when they (Ca2+

ions) have to get out of the aquation sheaths bythemselves. It almost mimics a soft chemical route inthat the entire process occurs in an aqueous solution

although microwave power has the effect of tearingaway the water molecules from the calcium ions.Precipitation of nanosized hydroxyapatite using micro-wave irradiation from citrate-phosphate solutions (whosechemistry controls the growth kinetics of apatites) hasbeen reported recently.91

An important application of the microwaves in thesynthesis of materials from aqueous media is that oflarge pore zeolites and aluminum phosphates(AlPOs).92-95 Synthesis of zeolites is achieved throughcrystallization of aluminosilicate gels under hydrother-mal conditions over long times. This stage of crystal-lization from the gel is found to be accelerated to a highdegree by exposure of gels to microwaves. Synthesis ofcloverite which is a large pore zeolite with ring openingsof the size of ∼13.2 Å and cavity diameters of up to 30Å has been achieved by exposure of appropriate gels tomicrowaves.92 It has been emphasized in this work thatthe initial step in microwave synthesis of zeolites is notjust a simple replacement of thermal heating by micro-wave heating but it involves gel preparation withadditional optimizations. The time required for crystal-lization of gels has been found to decrease from 1-2days to 20 min with the use of microwaves. It has alsobeen found possible to achieve isomorphous substitution(e.g., by Ti, Co, or Ni) or metal atom attachments insidethe pores. The synthetic scheme used for the prepara-tion of cloverite92 is reproduced in Figure 11.

Microwaves have also been used for the isomorphoussubstitution in AlPOs to introduce Brønsted acid sitesat a very fast rate and with high efficiency. Indeed, largesingle crystals of AlPO-5 and cloverite have beensynthesized by the use of microwaves. It has been shownrecently that with the use of microwaves, Al-rich zeolitescan be prepared within minutes.92 The absorption ofmicrowaves has been attributed to increased dipolarnature in AlPOs, GaPOs (gallium phosphates), and Al-rich silicates, but the energy transfer from microwavesin Si-rich preparations is considered to be mainlythrough interaction with water. Several zeolites havebeen synthesized by combining pressure with microwaveirradiation using Parr autoclaves. Microwave powerlevels were maintained at just 600 W, and less than 3min of microwave exposure was found sufficient. Zeo-lites so prepared include sodium zeolite A, faujasite, andhydroxy sodalite.92

Microwave treatment of gels made from potassiumpermanganate and maleic acid yielded cryptomelane ata lower temperature (593 K) than in conventionalsynthesis and in a much shorter time.18 It was also

Figure 9. XRD pattern of microwave-prepared hydroxyapa-tite. The reaction was carried out with optimum microwavepower and was over in less than 25 min (data from ref 87).

Figure 10. Infrared spectrum of microwave-prepared hy-droxyapatite. The spectrum reveals the triplet band around550-650 cm-1, which is characteristic of HAp (data from ref87).


found that the same gels yield bixbyite at a similarlylower temperature, compared to conventional methods.The rates of formation of cryptomelane and bixbyite isattributed to the presence of Mn4+ (MnO2) which has amuch higher microwave susceptibility and thereforehelps in microwave-assisted crystallization.

The microwave heating method has also been usedsuccessfully for the preparation of layer and openstructure compounds such as synthetic hollandite (K2-Mn8O16‚nH2O) and synthetic todorokite (typicallyMgMn2+

2Mn4+4.5O12‚4.5H2O).96 These are powerful cata-

lysts, used for oligomerization of hydrocarbons such asmethane, and are prepared in ∼16 h of microwaveheating compared to 4-7 days required by conventionalmethods. Microwave-prepared samples are found to begenerally free of impurities such as bixbyite (Mn2O3).

Microwave irradiation in aqueous media has beenused effectively in intercalation of pillared clays.97

Recently, Al-intercalated montmorillonite has beenprepared by irradiating montmorillonite clays sus-pended in water to which aluminum chlorohydrate wasadded and held in sealed tubes. A low microwave powerlevel of 90 W was used for various times up to 15 min.Good quality, Al intercalated product, with greatersurface area than the samples obtained by conventionalmethods, have been obtained.

Mingos and co-workers98 found that R-VOPO4‚2H2Ocan be intercalated with pyridine, and its derivatives

at extremely fast rates in microwaves (2 orders ofmagnitude faster than in the absence of microwaveirradiation). Intercalation reactions were carried out inTeflon containers and the maximum temperature em-ployed was about 473 K. The microwave products werealso far more crystalline than the products obtained byconventional methods. The rapidness of the reaction wasattributed to an autocatalytic effect which operates asa consequence of better microwave coupling of interca-lated products.

Intercalation of Li in transition metal sulfides hasbeen achieved by using microwaves.99 In these reactions,Li is introduced as butyllithium. Suspensions of the hosttransition metal sulfides (MoS2 and TiS2) along withbutyllithium solutions in n-hexane and benzene wereirradiated by microwaves at low power (350 W), and thetemperature was maintained at about 333 K. Theintercalation reaction was found to occur in 5-6 min inthe case of MoS2, compared to more than 24 h inconventional method. Such a 100-fold increase in therate of intercalation under microwave irradiation hasalso been noticed in intercalation of pyridine andsubstituted pyridines into layered R-VOPO4‚2H2O; theintercalation was carried out at 473 K and a highpressure of about 50 atm.98

(v) Preparation of Glasses. Glasses are preparedby quenching the corresponding melts. But the processof melting and homogenizing in conventional methodsinvolves undesirable decomposition, oxidation/ reductionof component oxides, loss of materials by evaporation,etc. Therefore the rapid heating attainable by micro-wave exposure can be expected to assist in glasspreparation. The only requirement is that at least onecomponent of the charge used for making glass ismicrowave-active. Many ionic materials are quite mi-crowave-active because of the ionic current which causecoupling to microwaves. Therefore, rapid heating occurswith the use of microwaves when highly ionic com-pounds are present in any multicomponent glass com-position. Once the components are melted which occursvery rapidly, within about 5 min in a DMO, the meltsappear to couple to microwaves to a lesser extent (thedipolar relaxation times become much too short com-pared to 2.45 GHz frequency) and thus melts exhibit atemperature plateau which is insensitive to the powersetting of the DMO. These temperature plateaus appearto be determined dominantly by the composition. Themelts are then quenched in the usual way. Hencemicrowave heating has been particularly useful inpreparing a variety of glasses. Microwave-preparedglasses reported in the literature include silver iodidecontaining fast ion conducting glassses as well asseveral plumbovanadate glasses, containing ZnO, MoO3,CuO, WO3, and Bi2O3.100-102 Glasses with compositionscorresponding to crystalline NASICONs have also beenprepared by this method.76 The microwave heating routebeing very fast helps avoid undesirable decompositionof the components, particularly when silver and coppersalts are involved. The interesting feature of the mi-crowave preparation is the temperature plateau that theglass-forming melts attain which acts as an automatictemperature control for the melt.

Solid electrolytes based on bismuth vanadate compo-sition have been prepared recently with substitution of

Figure 11. Microwave synthesis scheme of cloverite. Thecrystallization of gels takes approximately 20 min in micro-waves. Also it is possible to achieve isomorphous substitutionof metal atoms such as Ti, Co, or Ni. (data from ref 92).


vanadium sites by several lower valent ions, like Ag+,Mn4+, Ga3+, Y3+, and Ce4+. The resulting materialsexhibit very high oxygen ion conductivities. The prepa-ration involves exposure of the pellets made frommixtures of oxides taken in appropriate proportions tomicrowaves in a DMO for about 15 min.103 The productsreferred to as BIMEVOXs were all found to have beenstabilized in their high-temperature γ phases.

(vi) Selective Deoxidation Reactions. A veryinteresting application of microwaves is in bringingabout highly structurally selective reduction reactions.It is observed that by microwave irradiation of mixturesof graphitic carbon and layered oxide materials theinterlayer oxygens are rapidly and completely removedin very short irradiation times.104 The resulting reducedoxides generally possess nonlayered structures and thereaction stops at the end of this stage of reduction. ThusMoO3 (to MoO2), CrO3 (to Cr2O3), V2O5 (to VO2),R-VOPO4‚2H2O (to VPO4), and Ag6Mo10O33 (to Ag +MoO2) were all found to reduce readily in microwavesin less than 5 min and at temperatures lower than 1000K. Nonlayered oxides such as WO3, etc. are unaffectedunder identical conditions. This reduction is, in fact, aselective deoxidation which can be used as a verygeneral strategy in microwave preparation of materials.The mechanism appears to be that carbon atom layersfirst emerge after the excitation of interlayer weakbonds in graphite and the sheet of carbon atomsthemselves slip into the interlayer regions of the oxides,leading to reaction between the reactive sp2 carbons andthe weakly bound oxygens in the interlayer regions ofthe host as depicted in Figure 12.

(vii) Plasma-Assisted Reactions. Several micro-wave plasma-assisted reactions have been described inthe literature and the subject cannot be adequatelyreviewed here. Only a few plasma-based synthesisreported in the literature are considered here. A numberof nitrides such as TiN, AlN, and GaN have beenprepared recently with the use of a plasma reactionstarting from oxides by exposing them to a plasma ofN2/H2.105

Microwave-assisted synthesis of binary (TiN, AlN,and VN) and ternary (Li3FeN2, Li3TiN2, Li3AlN2) ni-

trides have been reported where a direct reactionbetween metal powder and nitrogen was established byfirst striking a N2 plasma for which a low powermicrowave source was used.106 Ternary nitrides wereproduced by direct reaction of either the componentnitrides or a mixture of Li3N and metal powder againreacted in nitrogen plasma. However, it was also notedthat sustaining a plasma in the domestic microwaveoven is unfeasible. Microwave-generated N2 plasmareactions have been used to demonstrate formation ofSi3N4 and BN as well.

A rapid synthesis of alkali metal fullerides has beenreported by Douthwaite et al.107 by using microwave-induced argon plasma. Reaction times appear to bereduced to only a few seconds while the conventionalpreparation takes a very long time. The microwaveplasma was generated in argon held at a pressure of10-5 mbar in evacuated quartz tubes. Fullerenes andalkali metals are kept physically separated, and thetube was so positioned that the microwave amplitudewas highest at the position where alkali metal was kept.The alkali metal quickly heated and vaporized, and theC60 reacted with condensed K under the Ar plasma.Under these conditions, K intercalated easily. It wasalso confirmed that uncondensed K in the plasma didnot intercalate. It has been shown by alteration ofreaction conditions that there is a thermal action of theplasma which is responsible for the rapid synthesis.

Another interesting microwave plasma enhancedCVD process is the formation of films of carbon nitridecompounds over Si wafers using an intermediate layerof Si3N4. The plasma was first struck in a mixture ofmethane and nitrogen, and a negative dc bias was usedto obtain a covering layer of carbon nitride.108

Microwave applications for dispersing cobalt clustersin sodium zeolites using low-power microwave plasmaof cobalt carbonyl vapors diluted with argon is wellknown.109 Several possible mechanisms were suggestedto be operative in these reactions which together leadto deposition of cobalt atoms in zeolite matrixes. Severalof these mechanisms which operate in microwaveplasma reactions were discussed by Sharp.110

Figure 12. A schematic representation of the mechanism of microwave-assisted deoxidation; (a) Layered MoO3 structure. Twolayer blocks are shown. Oxygen atom between the layers is reacted away by carbon as marked. (b) The coordination of Mo atomsin MoO3. The longest bond has a Mo-O distance of 2.33 Å (data from ref 104).


Epilogue

In the preceding sections, a summary of recent reportson microwave-assisted preparation of inorganic solidshas been presented. The microwave-synthesized materi-als encompass a wide variety of solids and most of themare of industrial and technological importance. Themotivation for use of microwaves obviously has been todesign cleaner, faster, and economically viable methodsof synthesis. While the preparation of intended materi-als has been achieved, an understanding of the reactionmechanism in microwave field is still not satisfactory.Since it is a relatively new field of activity in materialschemistry, there is a growing volume of empiricalknowledge about microwave-material interaction.

One of the most important issues in microwaveapplications for syntheses of materials is whether thereis a genuine nonthermal effect of microwaves whichSutton11 has most appropriately referred to as “micro-wave effect”. It is observed almost universally thatreactions occur faster under microwave irradiation thanin conventional methods. Rate increases have been seeneven in mineral leaching such as leaching of complexcopper sulfide concentrates using FeCl3 under micro-wave irradiation.111 Several references were made ear-lier to the occurrence of microwave reactions at tem-peratures lower than in conventional methods. Therehave been careful studies to measure activation barriersin microwave-assisted reactions such as in sintering ofY3Fe5O12 garnets. It was noted by Lin et al.112 that veryhigh densities (97.3%) were achieved at 1573 K within20 min, while the conventional process required atemperature of 1693 K and a reaction time of 8 h. Theactivation energy in microwave sintering was found tobe almost half (83.3 kcal/mol) of that observed inconventional sintering (163.2 kcal/mol), and this valueis even lower than the barrier for grain boundarymigration (94.8 kcal/mol). As a consequence, not onlywas the sintering process very rapid and very effective,but it also leads to extremely fine grains, a feature notedin ZrO2-CeO2 sintered products.113 Reduction in barrierheights appears to be a genuine microwave effect. Butin liquid media, increased reaction rates under micro-wave irradiation has been carefully analyzed and at-tributed to a purely thermal effect since organic solventssuch as ethanol have been found to be capable of beingsuperheated under microwave irradiation by up to 26°C.114 Although the temperatures of the reactions andthe activation barriers are found to be lowered, excep-tions have also been noted in the literature. Forexample, the transition temperature of BaCO3 (at 811°C) was found to be unaffected by microwave irradia-tion.115

A very thoughtful experiment was performed byFreeman et al.116 who examined the effect of microwaveirradiation on ionic diffusion currents in single crystalNaCl, which were measured using varying external biasvoltages. Diffusion current was found to increase whenirradiated by microwaves. This increase was attributedto a rectified potential arising from the rectifying effectof uneven distribution of surface charge and the associ-ated space charge polarization. The rectified potentialis coupled to the microwave field. This indeed is agenuine microwave effect, since it produces a variationin transport coefficient under microwave irradiation.

This effect can manifest quite generally in a variety ofmicrowave-induced reactions and processes such asmicrowave reaction of BaCO3 + TiO2, leading to forma-tion of BaTiO3 as noted by Zhang et al.67 It is possiblethat when a plurality of pathways and activationbarriers characterize a process, microwaves may abetthat path which has the lowest activation barrier.

While analyzing the effect of microwave irradiation,it is tempting to draw parallels between the action oflasers and microwaves since they are just two regionsof electromagnetic spectrum. In suitably designed mi-crowave processes, it should be possible to exploit fullythe aspect of coherence of microwave beams. Since theregime of rotational-vibrational excitations is centralfor chemical reactions, the role of microwave irradiationin chemical reactions can be readily appreciated. Thevery fact that the energy can be supplied where it isneeded (a microwave coupling material can be intro-duced into the system by design) has opened up averitable treasure trove for synthetic chemists. Theobservation that the temperatures of reaction in micro-waves are almost always lower than in conventionalprocesses, and correspondingly the activation energiesare also significantly lower, are features of significancein developing synthetic strategies.

However, the reported literature suggests that nomaterial has been prepared using microwaves which hasnot been prepared earlier by conventional methods. Thisaspect itself provides new opportunities and challengesin the field of microwave synthesis. Similarly, the useof pressure, coupled with microwave heating has beenreported in only few cases (see earlier sections) since itrequires development of nonmetallic pressure vesselsthat can stand both reasonably high temperatures andpressures. Such developments are bound to occur andenhance the advantages associated with microwavesynthesis.

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