972 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009

Problems of Manufacturing NanocrystallineYttrium Silicate Materials

Lili Nadaraia, Nikoloz Jalabadze, Roin Chedia, Tengiz Kukava, Archil Mikeladze, and Levan Khundadze

Abstract—Technology of producing single crystal scintillatorsis rather a hard and complex technological task connected with avery low output issue (15–50%) of the material and the restrictedarea of its application. Recent efforts of scientists are directed to-ward searching for alternative ways for producing these materials.It was established that high-performance ceramic scintillationmaterials for producing the scintillators could be prepared byusing powder metallurgy techniques. However despite of certainachievements in this area, transparency of ceramic scintillatorsis still under the question. In our opinion a key to the solutionof the problem lies in preparing scintillators in nanocrystallinestructural state. A technology developed on the basis of the modi-fied Sol-Gel method shows possibilities of manufacturing numberof standard (as well as new hard to obtain as single crystals)scintillation materials. Present work is dedicated to the problemsof manufacturing yttrium orthosilicate ��������—as of one ofthe perspective scintillation materials.

Index Terms—Nanocrystalline materials, nanotechnology, scin-tillators, sintering, sol-gel process, SPS, synthesis.

I. INTRODUCTION

Y TTRIUM silicate scintillation materials are distinguishedfor their high properties if compared to those of other in-

organic scintillators. These materials are under intensive devel-opment in order to meet contemporary needs of medicine, sci-ence and industry. Suitably doped host materials are expectedto reveal favorable scintillation performance and therefore theyare attributed to most frequently studied systems. These mate-rials are hard to obtain as single crystals, the procedure is verycomplex and expensive, and many scientists search for the waysof developing alternative technologies for achieving favorablescintillation performance at lower costs.

Transparent thin films of andwere prepared by

using a metal-organic decomposition sol-gel process. A roomtemperature strong red emission with predominant wavelengthat 614 nm, corresponding to the intra- electrictransition of , was observed as a function of the annealingtemperature; optical properties were in good correlation withthe film structures. The detected changes in luminescence andCIE coordinates of -doped at 1050 were

Manuscript received July 21, 2008; revised November 07, 2008. Current ver-sion published June 17, 2009. The research described in this publication wasmade possible in part by Award No NSS #-14/07 of the U.S. Civilian Research& Development Foundation (CRDF), the Georgia National Science Foundation(GNSF), and the Georgian Research and Development Foundation (GRDF).

The authors are with Republic Center for Structure Researches, TechnicalUniversity of Georgia, Tbilisi 0175, Georgia (e-mail: nadaraia@gtu.ge, jal-abadze@gtu.ge, chediageo@yahoo.com, t.kukava@posta.ge, archilm@mail.ru,khundadze@gtu.ge).

Digital Object Identifier 10.1109/TNS.2009.2016961

attributed to phase transformation of nanocrystalline films.Exposure to the UV source revealed bright red-phosphorescentcolor with excellent color saturation [1]. There are two types of

crystal: monoclinic -type (crystallographic group) and monoclinic -type (crystallographic group B2/b).

The high-temperature synthesis leads to the -type andlow-temperature synthesis to the -type [2].

Sol-gel process and mixed-powder methods were used forthe synthesis of different polymorphs of . Ranges oftheir stability were discussed as a function of heat-treatment andsynthesis methods. Crystallization of amorphous sol-gel sam-ples was conducted following the route: amorphous

(even in the cases of isothermal heat-treatment regard-less of temperature). Preferential crystallization of (in the ini-tial amorphous charge) was attributed to the silicon structuralunit types similar to phase. High-resolution NMR spectra(linewidth 21 Hz) were reported for the , , , and poly-morphs which were consistent with the accepted structural datafrom the point of view of the number of sites, the populations,and the relative chemical shifts [3].

Sol-gel chemistry was used to produce a phos-phor having good chromaticity and brightness. Although, thechromaticity of was similar to that of the com-mercial phosphor, it was 20–25% brighter. Yttriumsilicate:cerium phosphors containing a range of gadoliniumconcentrations were also prepared by the sol-gel route, and theirluminescence properties under cathodoluminescent conditionswere measured. Over the range of gadolinium concentrations,there was no significant change in the chromaticity of thephosphors, but adding of gadolinium resulted in a 20% lossin brightness compared to synthesized by thesol-gel route [4].

Crystallization of three precursor powders hy-drothermally synthesized at temperatures ranging between 991and 1038 was studied via using transmission electron mi-croscopy. Powders prepared in acidic or near-neutral conditionswere found to be highly inhomogeneous both chemically andstructurally, with a wide range of crystalline phases formed.In contrast to the abovementioned, the powder prepared inalkaline conditions was found to be quite homogeneous. Smallcrystalline nuclei formed on heating to 1006 , grew rapidly at1038 to form large single crystal particles. The majority ofthe particles were attributed to the phase howeversome were of the phase. It was supposed that heattreatment at 1200 would promote transformation.Weaknesses in the current crystal structure data for yttriumdisilicate phases were identified and suggestions were made forrectifying them [5].

An doped - nanocomposite was obtained bythe sol-gel technique; the sample was characterized by usingthe methods of powder X-ray diffraction, transmission electron

0018-9499/$25.00 © 2009 IEEE

NADARAIA et al.: PROBLEMS OF MANUFACTURING NANOCRYSTALLINE YTTRIUM SILICATE MATERIALS 973

microscopy, NMR and laser-excited luminescence spec-troscopy. It was found that interfaces of small (2–3 nm)nanoparticles (whose sizes were not supposed to be changed atheat treatment at 500–900 ) interacted with the matrix.Luminescence spectroscopy showed a probable preferential dis-position of inside of the highly disordered nanopar-ticles. These luminescent nanocomposites form a class of thematerials with high potential of application in the field of phos-phors [6].

Principles of metal oxide phase formation from multicom-ponent molecular gels have been reviewed. The critical phaseseparation mechanisms operating at each stage of the gelprocess, viz. gel synthesis, gel thermolysis and oxide crys-tallization, were described with examples from the synthesisliterature on aluminosilicates, cuprates and lead-based per-ovskite. It was demonstrated that direct crystallization of anequilibrium metal oxide was possible through the synthesis of acation-homogeneous gel, with avoiding phase separation duringthermolysis, and providing a low energy barrier for nucleationof the equilibrium phase. Influences of synthesis parametersand heating conditions on chemical phase separation wereexplained and guidelines for regulating the direct formation ofmetal oxides were outlined [7].

A new and simple method for the synthesis ofnanoparticles has been reported. The method provided a reac-tion of and TEOS in an adequate molar ratio to forma precursor complex gel composed of two cations (Si and Y, inthe 1:1 ratio). The resulted crystalline nanopowder ( 40 nm) of

was obtained at 1060 [8].There are several known transparent ceramics (Garnet family)

in particular those with cubic structure. To achieve transparencyin bulk ceramics it is necessary for all the comprised crystalsto transmit a light beam. It is possible in the case if structureof crystals is cubic or nanosized. Light waves are not diffractedon nanosized crystals that being a precondition for transparencyof ceramics. It is less probable to obtain transparency in ce-ramic particles of a noncubic structure having the same orien-tation (i.e., if texturing throughout the volume occurs). In ouropinion transparency in ceramic with non-cubic structure willbe obtained in the cases when nanocrystallinity is preserved inbulk samples.

In recent years a number of methods have been proposedfor compacting powders in fully dense bulk nanocrystallinesamples [9]–[14]. Preparation of nanocrystalline bulk pieces iscommonly connected with the procedures of compacting andsintering of nanocrystalline powders. Major problems ariseduring the processes of sintering as due to an excessive thermalenergy release: it is very difficult to maintain nanocrystallinestructure of powders in the bulk. Standard methods for man-ufacturing bulk samples are: cold compaction and sintering,hot pressing, sintering under high pressure, electric dischargesynthesis, shock-wave sintering and gasostat sintering. Typicalfor all these processes is extensive grain growth resulting information of an ordinary macro-structural material instead ofthe desired nanosized one. Admixtures of inhibitors might bepreventive for the grain growing processes however contam-inations brought by that way into the powders may changethe material composition and hence may reduce the improvedcharacteristics to be acquired through nanocrystallinity. An-other way for preventing the grain growing processes is rapid

sintering. This principle is realized in an installation based onusing SPS method for in situ preparation and synthesis of nano-sized composites. There are designed some SPS method-basedindustrial installations however a physical essence of the pro-vided processes is still unclear.

II. EXPERIMENT

The methods used for preparing powders of scintillationmaterials are: sol-gel method, co-precipitation method anda combination of sol-gel method with combustion reactions.As starting materials were used: yttrium oxide (purity99.9%), tetraethoxysilane (purity 99%), ceriumnitrate (purity 99%). Purity of other reagents was ofan analytical range.

Metallic content in the reagents was determined by themethod of atomic-absorption (Perkin-Elmer atomic-absorp-tion spectrometer AAnalist 600). Organic compounds wereanalyzed by using the Agilent Technologies GC/MS methods(6890N/5975).

For composing the gels were used the methods of sol-gelprocesses. A procedure for the synthesis of was thefollowing: 2.258 g (0.01 mol) was dissolved in 10 ml56% nitric acid; the solution was evaporated to 3–4 ml (up tothe formation of syrup-like mass); then, at an intensive stirringof the mixture to the mass were added: 10 ml , 50 ml

(96%), 1.5 ml formamid , 1.4 ml ceriumnitrate (0.00306 mol ) and 2.258 ml (0.01 mol)of tetraethoxysilane. Adding of a small amount of HNO( 1.4 ml) to the area of reaction mixture has turned it to thetype with . At stirring, the hydrolysis of TEOS re-sulted in the formation of polysilicon acid (ySiO xH O) anda gradual transformation of sol to gel started. Via the gradualevaporation of the solvents from the reaction area (being understirring (60–70 ) during 60 min) there was created a plastictransparent yellowish gel. By heating of the gel at 100–120there was formed a yellowish amorphous powder. After the1 h air calcination of the powder in an inert atmosphere at800–1100 the resulting product was obtained in the form ofa white-colored voluminous powder.

A fact of the development of a technology for fabricating yt-trium silicate based scintillators from different silica-containingmaterials (Fig. 1) may become of a great significance due to aneconomic efficiency of using cheap product for serial produc-tion of scintillators. The applied nanotechnology will promotethe decrease of mass of detrimental impurities and envisage ho-mogenous distribution of components in the obtained materialthose being a precondition for improving performance of theend product.

A procedure of preparing a bulk of the sample from the scin-tillation nanopowder was the following: a sample of the cerium-doped nanopowder was brought into a graphite en-vironment with boron nitride as a protective layer. Then thesample was hot pressed at 1300 for 2 hours under 8000 psi.The resulting bulk sample was a nontransparent ceramic of alight grey color with a porous structure. The sample was pre-pared at the Boston University.

YSO ceramic scintillator was fabricated as well in the instal-lation designed and constructed at the Technical University ofGeorgia. The design was based on the method of Spark PlasmaSyntheses (SPS). The applied graphite pressing mould had inner

974 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009

Fig. 1. Technological scheme for the synthesis of nanophase powders of yttrium silicate materials.

diameter of 12 mm; synthesizing pressure was 20–50 MPa; tem-perature—1200–1400 . The temperature measurements wereperformed by a thermocouple.

Particle sizes of the sample was were measuredon the scanning electron microscope (FESEM, Zeiss, SupraVP40). XRD measurements were carried out on the XZG-4Aapparatus. Emission spectra and lifetime measurements ofcerium embedded in the nano-sized host lattice wereperformed at room temperatures after excitation into the 5D4f absorption band at 410 nm.

III. RESULTS AND DISCUSSION

Different modifications of yttrium silicates are known as:with triclinic, monoclinic, orthorhombic structures;

the silicate with yet unidentified structure; —withmonoclinic structure as well as with an unidentified struc-ture; with hexagonal structure andwith an unidentified structure. We have synthesized differenttypes of yttrium silicates (Fig. 2): (a), yttriumpyrosilicate with monoclinic structure (b), yttriumoxyorthosilicate with monoclinic structure (c). Our ex-perience has shown that any deviations from the certain modesas well as from the Y:Si ratios were resulting in the formationof together with the mixture of and

silicates [Fig. 2(d)].Structure conditions of the powders obtained after ther-

mochemical synthesis were studied at different temperatures(Fig. 3). XRD investigations of the samples show that the sili-cate obtained as a result of synthesis at 800 is amorphous forX-rays [Fig. 3(a)]. Synthesis at 900 results in the formationof monoclinic type nanocrystalline structure [Fig. 3(b)]. Thesame structure is formed at 1000 (PDF-2 # 00-052-1810)[Fig. 3(c)]. Such a monoclinic structure is stable up to

Fig. 2. XRD patterns of nanopowders of different yttrium silicates:a-� ���� � �; b-yttrium pyrosilicate � �� � c- � type yttrium oxy-orthosilicate � ��� : d- with the mixed structure of � � , � ���� � �and � ���

1050 ; above this temperature it transforms into monoclinicstructure of another type (PDF-2 # 00-036-1476) [Fig. 3(d)].

Fig. 4 shows the results of SEM investigations of the as-pre-pared ceramic synthesized at different temper-atures. As seen from the images, increase of the synthesis tem-perature promotes substantial increase of graininess of powders.E.g., graininess of the synthesized at 900 powder is of an

NADARAIA et al.: PROBLEMS OF MANUFACTURING NANOCRYSTALLINE YTTRIUM SILICATE MATERIALS 975

Fig. 3. XRD patterns of yttrium oxyorthosilicate � ��� synthesized ata-800 �, b-900 �, c-1000 � and d-1100 �.

Fig. 4. SEM images of the YSO synthesized at different temperatures:a-powder (900 �), b-powder (1100 �), c-powder (1300 �); d- hot-pressed1300 �, 2 h.

order of 15–20 nm [Fig. 4(a)], at 1100 it increases to 50–70nm [Fig. 4(b)]. During the procedure of synthesis at 1300grains of the created yttrium silicate start fusing and their sizeachieves 200–300 nm [Fig. 4(c)]. Therefore 1300 was se-lected as an optimal temperature for sintering of bulk samples.Fig. 4(d) shows SEM images of YSO hot pressed at 1300 .However pressing at such temperature for 2 h makes impossibleto preserve nanocrystallinity in the bulk. Fig. 5 shows SEM im-ages of the YSO synthesized at 900 . From the images one cansee that the YSO powder synthesized at 900 is comprised of50–100 nm conglomerates [Fig. 5(a)] composed of 15–20 nmsize grains [Fig. 5(b)].

The emission spectra for under excitationwavelength at and excitation spectra at

are shown in Fig. 6. Despite of such a low concentrationof Ce (0.005 at%) intensity spectra of the powder is very high.

Our experience showed that using of SPS method [Fig. 7(a)]was not effective for nonconductive materials since in such ma-terials arc between particles could not be created. lack of con-

Fig. 5. SEM images at the different magnifications of the YSO synthesized at900 �.

Fig. 6. Photoluminescence excitation and emission spectra of YSOnanopowder.

ductivity in some of the dielectric materials can be compensatedby high temperatures. DC passes through a graphite mould andthe released heat promotes heating of the powder in the mould,making it conductive. Then through the realization of plasmaand appropriately sintering processes, nanocrystalline state ofthe material is preserved [Fig. 7(b)]. It is evident that if the ap-plied powder is not conductive at high temperatures, then thearc-plasma processes will not be conducted; these processescould have been provided at increasing of the mass temperaturehowever in such situations nanocrystalline state of the materialwould not be preserved. There is also another problem arisingdue to agglomeration of nanopowder particles. In such agglom-erated powders, arc-plasma processes go between aggregatedmasses (not between nanoparticles) and appropriately sinteringprocesses proceed between the aggregated particles. Therefore,pores existing within the aggregated particles remain unchangedand the sintered product (which despite of being nanosized, hasa number of pores) can not have high performance. Non-aggre-gated powders could be produced through using inhibitors, how-ever as stated above, this measure is also connected with certainproblems.

Our approach to the solution of the above mentioned prob-lems is connected with creation of a new device equipped withthe measures for simultaneous using of the methods of SPS,condenser discharge, pulsed pressure and ultrasonic excitation(Fig. 7(c)). Condenser discharge creates a spark between non-conductive powder particles thus giving start to plasma pro-cesses. then the procedure follows the processes realized in stan-dard SPS device: compaction processes envisage preservationof nanostructure in the bulk material; using of a pulsed pressure

976 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009

Fig. 7. Scheme of the SPS devices: old (a)—PDC—pulsed DC, GD—graphitedie, S—powder sample, P—pressure loading and new (c)—LVPG—Lowvoltage pulsed generator, HVPG—High voltage pulsed generator,MIXER—Special unit for mixing pulsed current of low and high voltages,Ps—Statically loading, � —Pulsed dynamic loading, USE—Ultrasonicexcitation unit and scheme of the process of sintering (b) EC—electric current,s—spark, sp—spark plasma and p—powder particles.

and ultrasonic method reduces pores of the bulk material to theminimum.

IV. CONCLUSION

Nanocrystalline powders of yttrium silicatedoped with cerium (0.005 at%) were successfully

synthesized by using the modified sol-gel method. Exactmaintenance of the technology modes and the Y:Si ratio pro-vides formation of monophase systems of yttrium silicate.The developed technology for preparing powders makes pos-sible to obtain highly efficient yttrium silicates from varioussilica-containing starting materials. Economy of the technologydeveloped for production of scintillation materials (if comparedto the technology of single crystal growth) is provided by timesaving (a couple of hours instead of a couple of days), powersaving (some tens of KWT/h instead of some hundreds ofKWT/h), and high yield of the product (90–100% instead of15–50%). Economic efficiency of using the obtained cheapmaterial for serial production of scintillators premises to berather high. The developed technological processes envisagemaximal homogeneous distribution of doping elements in thematerial: it is a precondition for achieving high scintillationperformance in the compacted material even at low percentage

of the doping agent. Nanopowders of YSO were compacted byhot pressing. The experiments showed that nanocrystallinitycould not be preserved in bulk materials at high- temperaturepressing: at low-temperature pressing. Nanocrystallinity couldbe preserved, however porosity of the compacted sampleswould be rather high making obstacles for preparing fullydense transparent scintillation ceramic. As yttrium silicate wasa dielectric material, we could not apply a conventional SPSmethod for achieving our main goal that being preservationof nanocrystallinity in the bulk material. For the solution ofthe stated problems there has been developed a technologyand an appropriate device was designed on the basis of themodified SPS method. Further works will be directed towardthe solution of the above mentioned problems with the help ofthe developed device.

ACKNOWLEDGMENT

The authors gratefully acknowledge Prof. Vinod Sarin(Boston University) and Prof. Charles Melcher (University ofTennessee).

REFERENCES

[1] Q. Y. Zhang, K. Pita, W. Ye, W. X. Que, and C. H. Kam, “Effectsof composition and structure on spectral properties of �� -dopedyttrium silicate transparent nanocrystalline films by metallorganicdecomposition method,” Chem. Phys. Lett., vol. 356, no. 1–2, pp.161–167, 2002.

[2] X. Qin, Y. Ju, S. Bernhard, and N. Yao, “Europium-doped yttrium sil-icate nanophosphors preparedby flame synthesis,” Mater. Res. Bull.,vol. 42, pp. 1440–1449, 2007.

[3] J. Parmentier, P. R. Bodart, L. Audoin, and G. Massouras, “Phase trans-formations in gel-derived and mixed-powder-derived yttrium disilicate,� �� � , by Y-ray diffraction and �� MAS NMR,” J. Solid StateChem., vol. 149, no. 1, pp. 16–20, 2000.

[4] P. J. Marsh, J. Silver, A. Vecht, and A. Newport, “Cathodolumines-cence studies of yttrium silicate: Cerium phosphors synthesised by asol-gel process,” J. Lumin., vol. 97, no. 3–4, pp. 229–236, 2002.

[5] I. Maclaren, P. A. Trusty, and C. B. Ponton, “A transmission elec-tron microscope study of hydrothermally synthesized yttrium disilicatepowders,” Acta Mater., vol. 47, no. 3, pp. 779–791, 1999.

[6] C. Cannas, M. Casu, A. Musinu, G. Piccaluga, A. Speghini, and M.Bettinelli, “Synthesis, characterization and optical spectroscopy of aY2O3-SiO2 nanocomposite doped with Eu3+,” J. Non-Cryst. Solids,vol. 306, no. 2, pp. 193–199, 2002.

[7] Y. Narendar and G. L. Messing, “Mechanisms of phase separation ingel-based synthesis of multicomponent metal oxides,” Catal. Today,vol. 35, no. 3, pp. 247–268, 1997.

[8] J. S. Moya, M. Diaz, C. J. Serna, and S. Mello-Castanho, “Formationof nanocrystalline yttrium disilicate powder by an oxalate gel method,”J. Eur. Ceram. Soc., vol. 18, no. 9, pp. 1381–1384, 1998.

[9] Y. V. Bykov, K. I. Rybakov, and V. E. Semenov, “High-temperaturemicrowave processing of materials,” J. Phys. D: Appl. Phys., vol. 34,pp. R55–R75, 2001.

[10] V. Mamedov, “Spark plasma sintering as advanced PM sinteringmethods,” Powder Metall., vol. 45, no. 4, pp. 323–328, 2002.

[11] J. R. Groza, “Nanocrystalline powder consolidation methods,” inNanostruct. Mater., C. C. Koch, Ed. New York: Noyes Publications,Williams Andrew Publishers, 2002, pp. 115–178.

[12] K. C. Cho, R. H. Woodman, B. R. Klotz, and R. J. Dowding, “Plasmapressure compaction of tungsten powders,” Mater. Manuf. Process.,vol. 19, no. 4, pp. 619–630, 2004.

[13] D. Jia, K. T. Ramesh, and E. Ma, “Effects of nanocrystalline and ultra-fine grain sizes on constitutive behavior and shears bands in iron,” ActaMater., vol. 51, pp. 3495–3509, 2003.

[14] E. Lassner and W. Schubert, Tungsten: Properties, Chemistry, Tech-nology of Element, Alloys, and Chemical Compounds. New York:Kluwer Academic/Plenum, 1998.