8

Recent Developments

In recent decades, calorimetry has achieved considerable progress and has gainedmuch in importance. The method is now well established, the different existingcalorimeter types presented so far along with the theoretical background of theirfunction principle are well understood, and the respective advantages anddisadvantages are known. The classic methods to measure heat (see Chapter 1) andthe basic construction principles (see Chapter 5) of calorimeters have not changedin the course of time. But during the past few decades, there has been hugeprogress in electronics, computer technology, materials science and processtechnology, which, of course, has had a great influence on the development ofcalorimetric equipment. Besides the increased precision of measurements, thanksto modern electronics, and automated data processing and evaluation, thanks topowerful computers, integrated circuit (microchip) technology has made theconstruction of ultrafast calorimeters for very small sample masses possible.There is no doubt that automatic data processing has found an increasing

application in calorimetry. Microprocessor technology permits automatic controlof calorimeters with a continuous adjustment of such parameters as heatingpower, heating rate, temperature, and so on in accordance with the pertinentdata of the caloric process in the sample. There are already calorimeters inwhich the results of the measurement are given automatically in a corrected,calibrated, desmeared form. Such a computer-controlled calorimeter may becalled a smart calorimeter.The downside of the progress is, however, that modern calorimeters have

become more and more “black boxes,” that is, the basic function principle isoften hidden, and users have to trust in what the electronics and the softwareoffer them concerning the presented results.In this chapter, some of the recent developments are presented to give an

overview of what is going on and to show the trends and the advantages aswell as the disadvantages of the progress in calorimetry.

213

Calorimetry: Fundamentals, Instrumentation and Applications, First Edition.Stefan M. Sarge, G€unther W. H. H€ohne, and Wolfgang Hemminger.� 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

8.1

Microchip Calorimetry

Traditional calorimeters offer high resolution and precision in measuring heat andheat flow rates only when sample masses and heating rates are not too small. Inapplications in which these conditions are not fulfilled, the signal-to-noise ratio canbe very low, particularly if the heat flow rate into the sample is reduced. Reductionof the masses of the measuring system together with an improvement of thethermal properties can help to increase the resolution and signal-to-noise ratio sothat small samples can be measured even at low heating rates. In addition, theresponse time (time constant) of small devices with high thermal conductivity andsmall samples is substantially reduced, allowing very high heating and coolingrates as well as temperature modulation (see Section 7.9.5) at high frequencies.To our knowledge, Denlinger et al. (1994) were the first who successfully

built a microchip based calorimeter using a silicon chip they made themselves.Such small systems were increasingly used during the past decade (Schick andH€ohne, 2003, 2005); they were normally fabricated with silicon microchipsavailable at that time. The purpose of these devices was originally a differentone, but they proved to be suitable for calorimetric purposes. As a result, thechips were more and more optimized for thermal sensing or calorimetricapplications (Herwaarden, 2005).These so-called “nanocalorimeters” consist mainly of such a calorimeter chip

mounted with good thermal contact inside a temperature-controlled furnace orthermostat. The calorimeter chip in its turn consists mainly of a thin siliconmembrane with a thin-film heater wire and a thin-film thermopile positioned onone side of it. The membrane is embedded into a silicon frame that is fixed into aceramic housing. The latter is mounted inside the furnace. The geometricconstruction principles of a typical calorimeter chip are shown in Figure 8.1.Normally, the sample has to be placed in the middle of the membrane, where a

square or rectangular area is just above the heater. Around the heater area, the hotjunctions of the thermopile are positioned. The cold junctions are located on thesilicon chip frame. Some chips provide an additional temperature sensor on the chipframe so that the reference temperature for the cold junctions can be measuredseparately.

1 Ceramic frame

2 Sample

3 Thermopile

4 Heating resistor

5 Membrane

6 Glue

7 Isolation layer

8 Frame

Figure 8.1 Schematic of a typical calorimeter chip, according to Xensor Integration.

214 8 Recent Developments

The “underlying” temperature of the experiment, typically linear with time orconstant, is applied by temperature control of the surrounding furnace to which thecalorimeter chip is connected. The heater, which is part of the chip, is used forfurther temperature programs such as continuous (scanning) or periodical orpulse-like heating.The construction principles of such chip calorimeters are similar to those of

conventional calorimeters: The heater corresponds to the furnace, and the center ofthe membrane corresponds to the calorimeter system, including the samplecontainer. The thin membrane serves as the thermal path between the heater andthe sample with very low thermal resistance and very low effective heat capacity.The thermopile measures the temperature difference between the sample site andthe chip frame (surroundings). Because of the much larger lateral dimension of themembrane of at least two orders of magnitude, the heat exchange between thesample and the frame can be neglected. The chip calorimeter can therefore beconsidered a quasi-adiabatic calorimeter when vacuum is applied.An important point is to have sample geometries that do not differ too much

because this would have a strong influence on the heat transfer behavior to thesample. Crucibles cannot be used because they are too large and have much toohigh a mass. The necessarily very small sample must be placed directly on the chipright in the responsive area, a rather tricky and sensitive task. Normally, it is notpossible to remove the sample after the experiment; the chip has to be exchangedfor every new sample. This is, of course, a huge disadvantage for chip calorimeters.Fortunately, commercial chip technology allows producing large series of properchips, available at a reasonable price, which keeps this disadvantage in anacceptable relation to the advantages of the nanocalorimeters.Although the fundamental function principle of nanocalorimeters is not

changed, the theory of such calorimeters and the mathematics for the decon-volution of the sample properties from the measurements are much morecomplicated than for common calorimeters. The pathway of the heat flow cannotbe approximated by a one-dimensional model, and the heat capacity of the sampleis often in the same order of magnitude as the heat capacity of the calorimeter“system,” which in many chips consists only of a very thin silicon membrane. Inclassical calorimeters, the “calorimeter system” is much larger in mass and heatcapacity than the sample. This is desirable to avoid an influence of the sample onthe sensitivity of the calorimeter and to keep the calibration factor a device propertyand free it from sample properties.At the times when the classical calorimeters were built, no computers existed and

all evaluation was done by hand. Therefore, there was a need for simple formulas tocalculate the quantities of interest from the measured curves. The construction ofthe calorimeters was such to give a signal strictly proportional to the heat flow rateinto the sample itself with a “calibration factor” almost not influenced by the heattransfer to the sample and its heat capacity. The price to be paid for this comfortwas a rather low sensitivity of the calorimeter with a need for large samples andlarge time constants in the range from some seconds up to many minutes in thecase of very sensitive “microcalorimeters” (see Section 7.9.2).

8.1 Microchip Calorimetry 215

Nowadays personal computers are available for everyone with a computing powercomparable to mainframe computers filling a large room in the 1970s. With thishardware and modern computer algebra systems, it has become possible foranyone to solve even complicated differential equation systems and calculate thequantities of interest. It is possible to calculate the heat flow rate into the sample independence of the sample temperature “online” while the measurement proceeds.Such techniques enable scientists to construct nanocalorimeters with low timeconstants in the millisecond region and get reliable results even for very smallsamples and fast processes. This opens the door to caloric properties as well as tothermodynamics of thin films.Several methods exist and have successfully been used to describe the thermal

behavior of the microchip calorimeters (H€ohne and Winter, 2005). With propermethods, it is also possible to calculate the heat capacity of the sample from themeasured signal of a microchip. This is also possible during thermal events releasinglatent heat in the sample and causing a so-called excess heat capacity and thus a time-dependent apparent heat capacity curve. The pulse- or step-response method hasproved to be a powerful tool to determine the model parameters by fitting theexpected function to the measured step (or pulse) response function (see Section 6.3).Modern computer technique together with powerful software allows solving rathercomplex problems without larger difficulties. This enables the modeling of these verysmall and sensitive single calorimeters with time constants in the millisecond regionand the determination of thermal properties even of thin films without the need for adifferential measurement with twin calorimeters as in conventional differentialscanning calorimetry.

8.1.1

First Example: Thin-Film Differential Scanning Calorimeter

Lai et al. (1997) fabricated a chip calorimeter with a 100 nm thick Si–N membranewith two Ni thin-film stripes (30 nm thick and 0.4mm wide) as the differentialheater pair, one serves as the sample heater and the other as the reference heater.With two synchronized electrical current pulses, the two heaters can be heatedup to 300 �C at a rate of about 30 000K s�1. The heater stripes also function astemperature sensors. The apparent heat capacity of the chip is about 6� 10�7 J K�1

at 300K and the lowest detectable heat is given as 0.2 nJ.With this calorimeter, the authors originally investigated the melting behavior of

very small tin samples (1–20�A), which were vapor deposited on the back of the

membrane with the help of a shadow mask. The temperature sensor was calibratedwith a “bulk” tin sample of 190

�A thickness. The authors found that the melting

point decreases from 231 �C for the 190�A sample to about 120 �C for 1

�A samples.

The equilibrium (bulk) melting temperature of tin is 232 �C. The size-dependentmelting point depression found is described in thermodynamics with the well-known Gibbs–Thomson equation (H€ohne, 2003). Considerably more interesting isthe result that the heat of fusion also decreases with decreasing size from 62 J g�1

for the 190�A sample down to 17 J g�1 for the 1

�A sample. The cause for this huge

216 8 Recent Developments

decrease is the surface energy of small grains: the smaller the particle, the moredominating it becomes.The same type of nanocalorimeter was used by Kwan et al. (2001) to investigate

the melting behavior of solution-grown single crystals of polyethylene. The size ofone such crystal was about 20mm� 13mm� 12 nm� 3� 10�18m3 according to amass of about 3 pg per crystal. The authors succeeded in measuring the meltingtemperature and the heat of fusion of down to only 20 single crystals (total massca. 0.06 ng) to 123 �C and 198 J g�1, respectively, at heating rates of 20 000–100 000K s�1. The latter was performed by 2–10ms pulses of 9–25mA currentheating the stripes to about 200 �C at the pulse end. The effective heating rates wereas large as to perform adiabatic operation under high vacuum conditions.

8.1.2

Second Example: Low-Temperature AC Nanocalorimeter

Minakov et al. (2005) constructed an alternating current (AC) calorimeter (seeSection 7.10.1.3) for low temperatures (4.2–300K) and high magnetic fields up to 8T. It is based on a commercially available thin-film chip (TCG 3880, XensorIntegration) with a submicron amorphous SixN membrane of 1� 2mm2 lateral sizewith an active heated area of 50� 100mm2. The sample size should be such as notto extend behind the heating area. To increase the thermal contact of thesubmicrogram sample to the membrane and the heater, special grease is used. Thechip is mounted in a chamber filled with helium exchange gas (0.01–1bar) inside ahelium cryostat. The nonadiabatic conditions allow fast heating as well as fastcooling rates and allow larger frequencies without deformation of the thermal wavefrom the heater through the sample to the exchange gas.With this nanocalorimeter, it is possible to measure heat capacities with a

resolution (noise) of 1 nJ K�1 at temperatures <100K or 3 nJ at 300K using the ACmethod in the frequency range of 10�1–104Hz at temperature amplitudes of1–100mK. The calibration and the deconvolution procedure (to get the sample heatcapacity from the measured signal) was successfully proved with a silver sample(30� 60� 70mm3). With that calorimeter, the magnetic phase transitions ofruthenium-doped CeFe2 in magnetic fields up to 8 T were investigated in thetemperature range of 20–280K.Calorimeter chips suitable for such nanocalorimeters are commercially available;

for details, see Herwaarden (2005).

8.2

Ultrafast Calorimetry

With the microchips now available, it is possible to construct nanocalorimeters thatallow the investigation of submicrogram samples at very high heating and coolingrates because of the very fast response time of such devices. The thermal lag andthe “smearing” of the measured signal on heating and cooling increases with

8.2 Ultrafast Calorimetry 217

increasing the heating rate and the time constant of the calorimeter (see Chapter 6).The time constant depends on the apparent heat capacity C of the sample andthe calorimeter system on the one hand and on the effective thermal resistancebetween the heater and the sample Rth on the other (see Chapter 6 andSection 7.9.2.3):

t ¼ C � Rth ð8:1ÞIn the case of nanocalorimeters (Section 8.1), the heat capacity is so small andthe thermal path so short that the time constant drops to milliseconds only. Asa result, the thermal lag is so small that very high heating rates are possiblewithout too large a “smearing” of the measured signals. The maximumheating rate bmax can be estimated with the following equation:

bmax ¼DTmax

t

With DTmax¼ 1K as the maximum acceptable thermal lag and t¼ 1 s as the timeconstant (response time) of conventional differential scanning calorimeters(DSCs), a maximum heating rate of 1 K s�1 or 60 Kmin�1 can be achieved.Nanocalorimeters with time constants well below 1ms allow heating rates up to105 K s�1 and more (Minakov and Schick, 2007). For rapid cooling, adiabaticconditions are unfavourable, and a quick heat exchange with an exchange gas ofhigh thermal conductivity is needed.With very high heating and cooling rates, it is possible to override time-dependent

processes such as crystal growth, crystal ordering (on heating), and early crystal-lization (on cooling). With nanocalorimeters, it is possible to study fast crystallizationand melting kinetics and the superheating effect occurring in polymers.

8.2.1

First Example: Ultrafast Nanocalorimeter

Minakov, Adamovsky, and Schick (2005) developed a nanocalorimeter toinvestigate the melting and crystallization behavior of polymers at heating andcooling rates up to 105 K s�1 based on the chip TCG 3880 (Xensor Integration).This chip (Figure 8.2) has a submicron amorphous Si3Nx membrane of

Figure 8.2 An ultrafast calorimeter chip (left), the membrane (middle), and its sensitive part

(right), with permission of Ahrenberg et al. (2012).

218 8 Recent Developments

1� 2mm2 lateral size with an active heated area of 50� 100 mm2 with a thin-film heater and a thin-film thermopile arranged around the active area andelectrically insulated with a thin layer of SiO2. The thermopile hot junctionshave a distance of 50 mm to the active center; the measured temperaturedifference is therefore not directly that of the active area (with sample) and thesurroundings (thermostat), but this disadvantage could be compensated byproper calibration (Adamovsky, Minakov, and Schick, 2003).The basic heat balance equation for the calorimeter chip is given as

ðC þ C0Þ � b ¼ W0ðtÞ � ðTðtÞ � T0Þ � Gwhere C is the heat capacity of the sample, C0 is the apparent heat capacity ofthe chip, b is the heating rate, T� T0 is the temperature difference detected bythe thermopile, and W0 is the input heat flow rate from the heating resistance.The parameter G, an effective thermal conductance between the active part andthe surroundings, depends strongly on the exchange gas and its pressure andmust be determined by proper procedures. For air at room temperature, it isfound to be about 2� 10�5WK�1, but for helium it is about five times larger.Most of the heat in the active center is transferred to the exchange gas; theamount transported laterally through the membrane is about 50 times smallerwhen using helium gas. The temperature-dependent heat capacity C0 can bedetermined from the respective measurements with the empty calorimetersystem; it proved to be about 100 nJ K�1 at 100 K, increasing monotonously to200 nJ K�1 at 600 K. With these parameters and a given heating rate, theunknown heat capacity C(T) of the sample can be calculated. For a detaileddescription of the thermal behavior and the temperature distribution in sucha nanocalorimeter system and the theoretical background of the evaluationprocedure, see Minakov et al. (2006, 2007).With this calorimeter, the specific heat capacity of a 480 ng poly(ethylene

terephthalate) sample was determined in air (2� 103 Pa) at a heating rate of3 000 K s�1 after fast cooling with 104 K s�1. Such a chip calorimeter wasused to investigate the crystallization behavior of polyethylene (Adamovsky,Minakov, and Schick, 2003), poly(ethylene terephthalate) (Minakov, Mordvint-sev, and Schick, 2004), and polypropylene (Gradys et al., 2005) at variouscooling rates down to 30 000 K s�1 following melting at different heating rates.The respective nanocalorimeter is commercially available from FunktionaleMaterialien Rostock e.V.The experience gathered from these experiments inspired the manufacturers

to develop special microchips for calorimetric purposes (Minakov et al., 2007).With these special chips, it was possible to increase the heating and coolingrates of nanocalorimeters up to 106 K s�1 (Minakov and Schick, 2007). Othersuitable chips have been developed recently: Guenther et al. (2011) reportabout a nanocalorimeter chip operating in various gas atmospheres at differentpressures and temperatures up to 1100 K. Another nanocalorimeter suitablefor AC and temperature-modulated DSC measurements was developed byTagliati, Krasnov, and Rydh (2012).

8.2 Ultrafast Calorimetry 219

8.2.2

Second Example: Flash Differential Scanning Calorimeter

Mathot et al. (2011) described a power-compensated differential scanning calorimeterfor very fast heating and cooling rates based on a specially developed microchip. Thechip contains two identical membranes consisting of 2mm-thick silicon nitride/oxide.Each membrane has two thin-film heaters; the heaters are insulated with silicondioxide and coated with a thin aluminum layer to achieve a homogeneous temperatureprofile. The heated area is surrounded by eight thermocouples that measure thetemperature difference against the silicon frame. One membrane serves as the sampleside and the other as the reference. The difference between the two thermopiles’signals is a measure for the temperature difference between the sample and thereference. The differential temperature signal is either proportional to the differentialheat flow rate or is used to compensate, via the second heater, temperature differencesoccurring on controlled heating or cooling (power compensationmode). The extra heatis automatically switched to the parallel heater of that side where the temperature isfound to be too low compared with the set value of the program. Because of the above-mentioned reasons, the sample must be placed directly on the chip, which serves asthe sample container and must be changed for a new sample. To avoid the time-consuming calibration of every single chip, they have been produced with narrowtolerances and precalibrated in the factory. As a result, the caloric uncertainty is provedin the factory to be 1–3%, making any further caloric calibration unnecessary. Only thetemperature calibration remains if the user wants a more precise temperature than�5K in the range of�65 to 420 �C.The temperature range of this Flash DSC with a suitable cooler is �95 to 420 �C.

Heating rates of up to 40 000K s�1 and cooling rates up to 4 000K s�1 are possible.From this point of view, the calorimeter fills the gap between conventional DSC andthe ultrafast nanocalorimeters presented earlier. Sample masses should lie between20 ng and 10 mg, depending on the heating and cooling rates, to give reliableresults. For polymer samples, a mass of 200 ng proved to be optimal.Different polymer samples were used to test the performance of the instrument.

Measurements with polyethylene glycol showed that on cooling from the melt, thecrystallization can be totally suppressed with cooling rates of 1000K s�1 and above.This calorimeter is commercially available as Flash DSC 1 from Mettler-Toledo

Intl. The specifications are: temperature range: �95 to 450 �C, temperatureresolution (noise): 0.01 K, chip time constant: 1ms, sample size: 10 ng–1mg,cooling rates: 0.1–4 000K s�1, heating rates: 0.5–40 000K s�1, maximum heat flowrate: 20mW, heat flow rate noise: <0.5mW, and sampling rate: maximum 10 kHz.

8.3

Extreme Ranges of State

The improved technology and computer techniques allow performing calorimetricmeasurements in extreme ranges of state such as high pressure, high temperature,

220 8 Recent Developments

strong magnetic fields, or even in plasma surroundings. Up until now, all of theseexperiments were performed with calorimeters made by the researchers them-selves; the interest in such special calorimeters has not been high enough toinspire manufacturers to develop suitable equipment. Here we want to point outthe peculiarities of calorimetry in extreme ranges of state by means of only someexamples to show what is currently possible.

8.3.1

High Pressure

The term “high” refers to pressures well above 100MPa (1000 bar). For investiga-tions in the region below 100MPa, there are some commercially availablecalorimeters. The problems arising with calorimetry at really high pressures aredifficult to solve. One major limitation is the need for a fluid medium to transferthe pressure to the sample. The density of gases at pressures above 100MPaapproaches that of liquids, and the same is true for thermal conductivity, whichbecomes comparable with the conductivity of solids. As a result, the heat flowsthrough the pressure medium to the surroundings rather than through the thermalpath constructed to measure the heat flow rate in a calorimeter. Consequently, areally large thermal leakage occurs, leading to a much reduced sensitivity of thecalorimeter. The large heat capacity of the unavoidable pressure medium, whichhas to be heated together with the sample, increases the heat flow rate to thesample and the reference and makes maintaining the symmetry more difficult andincreases the thermal noise, which reduces the sensitivity even more.Another problem arises from the fact that high pressures are dangerous and not

easy to handle from the safety point of view. A pressure of 500MPa is nearly doublethe pressure inside a gun being shot. If a part of the high-pressure equipment failsduring an experiment, the effect in a laboratory may be worse than that from abullet. Because of this reason, an autoclave driven with a gas as the pressuremedium must be operated in an impact-resistant room, and the operator must beoutside in a safe place. For this reason, the use of oil as the pressure mediumshould be preferred; because of its much smaller expansion coefficient, the storedmechanical energy is not that high.The problem with the very large heat capacity, thermal conductivity, and huge

thermal leakage toward the surroundings via the pressure medium remainshowever and makes the use of heat-flow DSCs for high-pressure purposesimpossible. One possibility to overcome some of the problems is to constructsample and reference containers that are stable against high pressures andposition them in a Calvet-type DSC (see Section 7.9.2.3). This was successfullytried (Randzio, Grolier, and Quint, 1994; Zhu et al., 2004), but the necessarilymassive container increases the problems with the large heat capacity of thecalorimeter system even more.For these reasons, most designers preferred to make use of the power

compensation DSC for their construction (see H€ohne and KaletunSc, 2009, andreferences given there).

8.3 Extreme Ranges of State 221

8.3.1.1 Example: Power-Compensated High-Pressure DSC

Ledru et al. (2006) describe a power-compensated high-pressure DSC based on theconstruction of Blankenhorn and H€ohne (1991). The construction makes use of acommercial power compensation DSC (PerkinElmer) with the original measuringhead replaced with self-made silver furnaces with the same electrical and sensorproperties but positioned inside an autoclave (SITEC-Sieber Engineering) that canbe pressurized up to 500MPa by means of a spindle pump. Branched silicone oilwas used as the pressure medium because linear silicone oil tends to crystallize athigh pressures. To avoid too large a convectional heat transfer, the viscosity shouldbe as high as possible but low enough to keep the oil flowing at high pressures.Depending on the pressure region of the respective experiment, the use of oils withdifferent viscosities is advisable. To avoid interaction with the oil, the sample mustbe hermetically sealed, although any cavity must be excluded; otherwise, heavydeformations of the crucible and the sample may occur.Between the two silver furnaces and the autoclave, there are ceramic housings

that fill the gap and reduce the convectional heat transfer between the hot furnacesand the autoclave (isoperibol surroundings). Nevertheless, there is a huge heattransfer between the two calorimeter systems, which can reach up to 30Wat higherfurnace temperatures. This poses high demands on the symmetry of the twofurnaces because the heat flow rate to the sample (the differential signal) is morethan three orders of magnitude lower than the heat loss to the surroundings. Everyasymmetry increases the signal of the empty systems significantly.Another peculiarity of high-pressure DSCs is the more time-consuming

calibration because the heat and temperature calibration (see Section 7.9.4) dependnot only on temperature but also on pressure and the respective pressure medium.Because of these special problems, the specifications of high-pressure DSCs are notcomparable to those of a common power compensation DSC. The temperaturerange is limited by the stability of the oil used; with silicone oil, the maximum isabout 300 �C. The thermal noise is given as about �0.1mW, and the baselinestability is poor; depending on the pressure, it changes up to 30mW within thetemperature range of 200K. This is, however, not too much compared with thehuge heat leakage (up to 30W) through the oil. Heating and cooling rates of0.5–20Kmin�1 are possible. The uncertainties of temperature and transition heatafter careful calibration are 2K and 5–10%, depending on pressure. The detectionlimit for transitions (peak area) is given as about 5mJ.With such a high-pressure DSCs, it was possible to measure the phase behavior

of polymers at pressures up to 500MPa (H€ohne, Rastogi, and Wunderlich, 2000;Ingram et al., 2008). To our knowledge, this is the only high-pressure calorimeterthat is able to measure transition heats of solids up to 500MPa.

8.3.2

High Temperature

The term “high” applies to temperatures well above 1000 �C. In thistemperature region, special problems arise. One point is that many of the

222 8 Recent Developments

materials used for the construction of calorimeters and for the encapsulationof the samples do not tolerate high temperatures and have to be exchangedwith, say, ceramic materials with very different thermal properties. Anothermore urgent point is the dramatically increasing radiation heat exchange, whichcannot be neglected at higher temperatures. The radiation heat transferdepends on T4 as well as on the emissivity coefficient of the involved surfaces(see Section 4.3). As a result, the heat calibration factor depends strongly ontemperature in a nonlinear way and on the surface material of the samplecontainer. If the surface changes, the calibration changes too. These problemsmake high-temperature calorimetry a difficult task that needs much experienceto gain reliable and convincing results. Modern material technology and com-puter techniques are, however, very useful to overcome these problems.Possible solutions are the use of levitation techniques by which the samplefloats in a levitation cell by means of an external force such as electromagnetic,electrostatic, aerodynamic, acoustic, or microgravity forces (Herlach et al., 1993;Frohberg, 1999) or the application of very short measuring times in which thesample has no time to react chemically with the surrounding materials(Boivineau and Pottlacher, 2006).

8.3.2.1 Example: Levitation Calorimetry on Nickel, Iron, Vanadium, and Niobium

Schaefers, R€osner-Kuhn, and Frohberg (1995) developed a levitation calorimetrysetup that allowed the investigation of liquid and supercooled samples ofnickel, iron, vanadium, and niobium. The setup consists of an electromagneticlevitation apparatus, a temperature-measuring unit, and a drop calorimeter. Acirculating gas cooling system was used to modify the sample temperature toreach the supercooled state without crystallization. The sample vessel allowedmeasurements in vacuum and under purge gas. A high-frequency generator(65 kW, 450 kHz) was necessary to keep the sample floating. The temperature ofthe sample was measured with a two-pyrometer system working at twofrequencies each. This enabled the simultaneous determination of the black-body temperature of the samples together with their hemispherical spectralemissivities. The drop calorimeter (see Section 7.9.1.3) consisted of a nickel-plated copper block in a water-filled thermostat. The calibration constant of thecalorimeter was determined by Joule heating. The temperature of thecalorimeter block was measured with a quartz thermometer with a resolutionof 100 mK. Two movable gates prevented the exchange of heat between thelevitation apparatus and the calorimeter. They were opened only duringthe drop, which lasted only 0.32 s. Within on average 13min, the heat of thesample was transferred to the calorimeter, where it caused a temperatureincrease of 6 K at the maximum. The enthalpies of the said materialswere determined by this method between 1463 and 2818 K and supercooled by186–272 K. In all cases, the enthalpies of the supercooled states turned out tobe linear extrapolations of the enthalpies in the stable liquid state. The authorsestimated the accuracy of their values to �5% whereby the deviations from theaccepted literature values were always smaller than 2%.

8.3 Extreme Ranges of State 223

8.3.3

Strong Magnetic Fields

At low temperatures and high magnetic fields, many materials and substancesreveal interesting properties, for example, about energetic interactions, magneticcontributions, or defects that can easily be detected by measuring their heatcapacities.

8.3.3.1 Example: Influence of Magnetic Fields on Point Defects

Mei�ner and Strehlow (2004) measured the low-temperature heat capacity ofsilica glasses in magnetic fields to determine the different contributions ofphonons and low-energy excitations, which were either intrinsic or due to pointdefects. For this purpose, the heat capacity was measured by a relaxation-typecalorimeter (see Section 7.9.2) between 0.35 and 15 K and magnetic fields up to8 T. The commercially available calorimeter (Oxford Instruments: MagLabHC

calorimeter, discontinued) consisted of a sapphire disk carrying a thin-filmtungsten heater and a resistive thermometer chip. A special grease of about0.5mg was used to establish the thermal contact between the sample and thecalorimeter. The sample masses ranged between 5 and 35mg. The heat capacityof the sample was calculated from the temperature versus time response aftersupplying a step-like amount of energy to the heater. The uncertainty of themeasured values was estimated to be 2%.

8.3.4

Plasma Surroundings

Space craft are exposed to extreme conditions on reentry into the atmosphere. It isof interest to investigate the thermal effects quantitatively. For this purpose,calorimeters are used to determine the heat flux and enthalpy flows in a plasmaatmosphere that is created by an arcjet. Heat fluxes on the order of 50MWm�2 andenthalpy flows of about 20MJ kg�1 have to be measured at temperatures around10 000K. This can only be accomplished using transient techniques. To do so, thecalorimeter, or the heat flux meter, is traversed through the arc and exposed to theplasma for only a fraction of a second, and the temperature increase of athermocouple is measured. The calorimeter usually consists of a conical-shapedaxisymmetric body of 25–50mm diameter with a central bore in which athermocouple is embedded close to the surface. Typical exposure times are 30ms,resulting in temperature increases of several hundred kelvin. Traditionally, themeasured temperature and the geometric parameters of the sensors are used tocalculate the heat flux under the assumption of linear one-dimensional heatconduction.

8.3.4.1 Example: Calibration Using a Laser Beam

L€ohle et al. (2007) describe a setup for the characterization and calibration of such aheat flux sensor by the use of short laser pulses. A Nd:YAG laser was used to

224 8 Recent Developments

irradiate the tip of the heat flux meter with pulses of different lengths and atvarying intervals, and the response of the thermocouple was recorded. A modelfor the heat flow through the sensor was developed and used to calculate theincident heat flow from the measured temperature recording. From the knownproperties of the laser pulse (pulse duration, shape, and energy), the parameters ofthe model could be quantified and used for the determination of unknown heatfluxes in arcjets. An uncertainty assessment showed (L€ohle et al., 2008) that theheat flux can now be determined with an overall uncertainty of 16% compared withthe 20% uncertainty attributed to the conventional evaluation technique.

8.4

Calorimetry as an Analytical and Diagnostic Tool

Recently, calorimetric methods have proved suitable for applications not commonin classical calorimetry. Again, the microchip technology together with advancedcomputer techniques opened the path to new developments in analytics anddiagnostics. Two applications will be presented here to show future possibilities.

8.4.1

First Example: “Artificial Nose”

The idea of an “artificial nose” to detect dangerous or otherwise interestingcomponents in gases is founded on the concept of an enzyme loaded thermistorinside a flow-through calorimeter. The specific enzyme reacts with the specimen tobe detected and produces some heat, which then is detected in the calorimeter andproduces a signal that provides information about the concentration of the specificcomponent.A somewhat different approach with a pyroelectric detector chip was suggested

by Schreiter et al. (2006). The advantages of such a lead–zirconate–titanate-baseddetector array are the low heat capacity and high thermal insulation of the sensor.The surface of the sensor was coated with specific reagents to detect differentchemicals. As an example, the coating with poly(methylsiloxane) was chosen todetect heptane with a detection limit of 10 ppm. With a bacterial surface layer andsmall Pt clusters, it was possible to study the catalytic oxidation of hydrogen in therange of 0.5–3.5 vol%.

8.4.2

Second Example: Infection Diagnostics

The metabolism of microorganisms is connected with heat production. Thereplication rate in a culture and with it the development of the heat flow rate intime often follows a certain course that is characteristic of different organisms. The“thermogram” obtained from the development of a culture in a calorimeter issimilar to a fingerprint to identify a certain microorganism. In suitable culture

8.4 Calorimetry as an Analytical and Diagnostic Tool 225

media, such a characteristic thermogram can be measured within 24 h. This provedto be a faster method to identify microbial contaminations than the traditionalmethod with culture plates.Trampuz et al. (2007) used a commercial microcalorimeter (TAM III, TA

Instruments) to detect contamination and identify different bacteria in transfusionproducts. They showed that different bacteria develop very different – andtherefore characteristic – heat flow rate–time functions that allow the detectionof contaminations within 10–60h, depending on the specific bacterium and itsamount in the sample (see Section 9.10.3). The experiments show thatmicrocalorimetry could become a helpful diagnostic method in medicine inthe future.

References

Adamovsky, S.A., Minakov, A.A., and Schick,C. (2003) Scanning microcalorimetry at highcooling rate. Thermochim. Acta, 403, 55–63.

Ahrenberg, M., Shoifet, E., Whitaker, K.R.,Huth, H., Ediger, M.D., and Schick, C.(2012) Differential alternating current chipcalorimeter for in situ investigation of vapor-deposited thin films. Rev. Sci. Instrum., 83,033902.

Blankenhorn, K. and H€ohne, G.W.H. (1991)Design, specifications and application of ahigh pressure DSC cell. Thermochim. Acta,187, 219–224.

Boivineau, M. and Pottlacher, G. (2006)Thermophysical properties of metals atvery high temperatures obtained bydynamic heating techniques: recentadvances. Int. J. Mater. Prod. Technol., 26,217–246.

Denlinger, D.W., Abarra, E.N., Allen, K.,Rooney, P.W., Messer, M.T., Watson, S.K.,and Hellman, F. (1994) Thin filmmicrocalorimeter for heat capacitymeasurements from 1.5 to 800 K. Rev. Sci.Instrum., 65, 946–959.

Frohberg, M.G. (1999) Thirty years of levitationmelting calorimetry�a balance. Thermochim.Acta, 337, 7–17.

Gradys, A., Sajkiewicz, P., Minakov, A.A.,Adamovsky, S., Schick, C., Hashimoto, T.,and Saijo, K. (2005) Crystallization ofpolypropylene at various cooling rates.Mater.Sci. Eng. A, 413–414, 442–446.

Guenther, G., Aulbach, E., Hahn, H., andGuillon, O. (2011) High-temperature chip

calorimeter with atmosphere control.Thermochim. Acta, 522, 77–85.

Herlach, D.M., Cochrane, R.F., Egry, I., Fecht,H.J., and Greer, A.L. (1993) Containerlessprocessing in the study of metallic meltsand their solidification. Int. Mater. Rev., 38,273–347.

Herwaarden, A.W. van (2005) Overview ofcalorimeter chips for various applications.Thermochim. Acta, 432, 192–201.

H€ohne, G.W.H. (2003) Calorimetry on smallsystems�a thermodynamic contribution.Thermochim. Acta, 403, 25–36.

H€ohne, G.W.H. and KaletunSc, G. (2009) High-pressure differential scanning calorimetry,in Calorimetry in Food Processing (ed. G.KaletunSc), Wiley-Blackwell, Ames,pp. 51–66.

H€ohne, G.W.H., Rastogi, S., and Wunderlich,B. (2000) High pressure differentialscanning calorimetry of poly(4-methyl-pentene-1). Polymer, 41, 8869–8878.

H€ohne, G.W.H. and Winter, W. (2005) Aboutmodels and methods to describe chip-calorimeters and determine sampleproperties from the measured signal.Thermochim. Acta, 432, 169–176.

Ingram, M.D., Imrie, C.T., Ledru, J., andHutchinson, J.M. (2008) Unified approach toion transport and structural relaxation inamorphous polymers and glasses. J. Phys.Chem. B, 112, 859–866.

Kwan, A.T., Efremov, M. Yu., Olson, E.A.,Schiettekatte, F., Zhang, M., Geil, P.H.,and Allen, L.H. (2001) Nanoscale

226 8 Recent Developments

calorimetry of isolated polyethylene singlecrystals. J. Polym. Sci. B: Polym. Phys., 39,1237–1245.

Lai, S.L., Ramanath, G., Allen, L.H., andInfante, P. (1997) Heat capacitymeasurements of Sn nanostructures using athin-film differential scanning calorimeterwith 0.2 nJ sensitivity. Appl. Phys. Lett., 70,43–45.

Ledru, J., Imrie, C.T., Hutchinson, J.M., andH€ohne, G.W.H. (2006) High pressuredifferential scanning calorimetry: aspectsof calibration. Thermochim. Acta, 446,66–72.

L€ohle, S., Battaglia, J.-L., Batsale, J.-C., Enouf,O., Dubard, J., and Filtz, J.-R. (2007)Characterization of a heat flux sensor usingshort pulse laser calibration. Rev. Sci.Instrum., 78, 053501.

L€ohle, S., Battaglia, J.-L., Jullien, P., vanOotegem, B., Couzi, J., and Lasserre, J.-P.(2008) Improvement of high heat fluxmeasurement using a null-point calorimeter.J. Spacecr. Rockets, 45, 76–81.

Mathot, V., Pyda, M., Pijpers, T., Vanden Poel,G., van de Kerkhof, E., van Herwaarden, S.,van Herwaarden, F., and Leenaers, A. (2011)The Flash DSC 1, a power compensationtwin-type, chip-based fast scanningcalorimeter (FSC): first findings onpolymers. Thermochim. Acta, 522, 36–45.

Mei�ner, M. and Strehlow, P. (2004) Lowtemperature heat capacity of high puritysilica glasses in magnetic field. J. Low Temp.Phys., 137, 355–369.

Minakov, A.A., Adamovsky, S.A., and Schick,C. (2005) Non-adiabatic thin-film (chip)nanocalorimetry. Thermochim. Acta, 432,177–185.

Minakov, A.A., Mordvintsev, D.A., andSchick, C. (2004) Melting andreorganization of poly(ethyleneterephthalate) on fast heating (1000 K/s).Polymer, 45, 3755–3763.

Minakov, A., Morikawa, J., Hashimoto, T.,Huth, H., and Schick, C. (2006) Temperaturedistribution in a thin-film chip utilized foradvanced nanocalorimetry.Meas. Sci.Technol., 17, 199–207.

Minakov, A.A., Roy, S.B., Bugoslavsky,Y.V., and Cohen, L.F. (2005) Thin-filmalternating current nanocalorimeter for

low temperatures and high magneticfields. Rev. Sci. Instrum., 76, 043906.

Minakov, A.A. and Schick, C. (2007) Ultrafastthermal processing and nanocalorimetry atheating and cooling rates up to 1MK/s. Rev.Sci. Instrum., 78, 073902.

Minakov, A.A., van Herwaarden, A.W., Wien,W., Wurm, A., and Schick, C. (2007)Advanced nonadiabatic ultrafastnanocalorimetry and superheatingphenomenon in linear polymers.Thermochim. Acta, 461, 96–106.

Randzio, S.L., Grolier, J.-P.E., and Quint, J.R. (1994) An isothermal scanningcalorimeter controlled by linear pressurevariations from 0.1 to 400MPa.Calibration and comparison with thepiezothermal technique. Rev. Sci. Instrum.,65, 960–965.

Schaefers, K., R€osner-Kuhn, M., and Frohberg,M.G. (1995) Enthalpy measurements ofundercooled melts by levitation calorimetry:the pure metals nickel, iron, vanadium andniobium.Mater. Sci. Eng. A, 197, 83–90.

Schick, C. and H€ohne, G.W.H. (eds) (2003)Thermodynamics and calorimetry of smallsystems. Thermochim. Acta, 403, 1–152.

Schick, C. and H€ohne, G.W.H. (eds) (2005)Thermodynamics and calorimetry of thinfilms. Thermochim. Acta, 432, 125–268.

Schreiter, M., Gabl, R., Lerchner, J., Hohlfeld,C., Delan, A., Wolf, G., Bl€uher, A.,Katzschner, B., Mertig, M., and Pompe, W.(2006) Functionalized pyroelectric sensorsfor gas detection. Sens. Actuators B, 119, 255–261.

Tagliati, S., Krasnov, V.M., and Rydh, A. (2012)Differential membrane-basednanocalorimeter for high-resolutionmeasurements of low-temperature specificheat. Rev. Sci. Instrum., 83, 055107.

Trampuz, A., Salzmann, S., Antheaume, J., andDaniels, A.U. (2007) Microcalorimetry: anovel method for detection of microbialcontamination in platelet products.Transfusion, 47, 1643–1650.

Zhu, S., Bulut, S., Le Bail, A., and Ramaswamy,H.S. (2004) High-pressure differentialscanning calorimetry (DSC): equipment andtechnique validation using water–ice phase-transition data. J. Food Process Eng., 27,359–376.

References 227

Manufacturers

Funktionale Materialien Rostock e.V., Rostock,Germany; www.materials-days.uni-rostock.de/fmr/ (February 11, 2013).

Met t l e r To l e d o I n tl ., Gre i f e n s e e ,Switzerl and; www.mt .co m(February 11, 2013 ).

Oxford Instr um ents , Abingdon, O xf ordshire,U K ; w ww.o x f o r d - i n s t r u me n ts . c o m(February 11, 2013).

PerkinElmer, Waltham, MA, USA; www.perkinelmer.com (February 11, 2013).

SITEC-Sieber Engineering, Maur/Zurich,Switzerland; www.sitec-hp.ch (February 11,2013).

TA Instruments, New Castle, DE, USA;www.t a instrume nts.co m (Februar y 11,2013).

Xensor Integration, Delfgauw, TheNetherlands; www.xensor.nl(February 11, 2013).

228 8 Recent Developments