WO1999031493A1

WO1999031493A1 - Differential thermoanalysis device

Info

Publication number: WO1999031493A1
Application number: PCT/EP1998/007916
Authority: WO
Inventors: Jürgen SCHILZ; Wolf Eckhard MÜLLER; Gerhard BÄHR; Lutz Rauscher
Original assignee: Deutsches Zentrum für Luft- und Raumfahrt e.V.
Priority date: 1997-12-17
Filing date: 1998-12-05
Publication date: 1999-06-24
Also published as: DE19756069C1

Abstract

A differential thermoanalysis device (10) used to examine or determine the parameters of a test sample, especially the phase transition point or specific warmth of the test sample, by means of measurement. The inventive device consists of a heat source (12) and a sensor plate (16) coupled thereto. A test sample coupling zone (18) is arranged on said sensor plate for thermal coupling of a test sample (20), in addition to a reference sample coupling zone (22) for thermal coupling of a reference sample (24) exhibiting a known behaviour with respect to the parameter to be measured or examined. The sensor plate (16) consists of a ceramic or a monocrystalline or polycrystalline thermoelectric semiconductor material, at least inside the test sample and reference sample coupling zones (18, 22). The Seebeck coefficient and temperature development of the semiconductor material can be periodically measured and calculated in order to take the effects of ageing into account.

Description

Difference thermal analysis device

The invention relates to a differential thermal analysis device for the metrological examination or determination of a thermophysical parameter of a test sample, in particular for the investigation of phase transitions or for determining the specific heat of the test sample.

Differential thermal analysis devices (DTA devices) are used for (quantitative) investigation of phase transitions and for measuring the specific heat of a substance. Examples of this are described in PCT-A-94/06000 and US-A-4 126 032. In the case of the quantitative measurement option, one also speaks of the heat flow differential calorimeter (DSC). In the following, no distinction should be made explicitly between these two types, since they are based on the same measuring principle.

In differential thermal analysis, the test sample to be examined and a reference sample are subjected to a predetermined temperature curve at the same time. If there is a phase change in the sample or there is a difference in the specific heat between the sample and the reference, this is noticeable in a temperature difference between the sample and the reference substance. This temperature difference is detected via thermocouples suitably attached to the sample and reference substance and converted into an electrical voltage. This measuring method has been established for decades and is used both at low temperatures down to the milli-Kelvin range and at high temperatures up to over 2000 ° C.

The "quality" or "efficiency" of a system with the measuring principle explained above is determined by the sensitivity of the temperature resolution ξ and the time constant of the arrangement τ

certainly. The sensitivity of the temperature resolution depends in particular on the level of the thermal voltage that forms on the thermocouples. This should advantageously be particularly large.

In order to obtain a large sample throughput, the greatest possible sensitivity, i. H. Large temperature resolution with the shortest possible time constant is desirable.

So far, it was due to the relatively low thermal power of the metallic thermocouple used

Materials only possible, only one of these parameters

^■ at the expense of the other towards greater efficiency of the

Measuring system to increase. A high resolution was e.g. B. achieved in that not a single thermocouple was used for temperature measurement, but one

Thermopile integrated in the sample pan. The resulting increase in sensitivity of such a so-called cup system, however, goes with one

Enlargement of the time constant goes hand in hand, since the effective thermal mass of the measuring head increases due to the design in the form of a cup.

Fast conventional measuring arrangements contain a single sensor plate made of a metallic thermocouple material such as. B. Chromel, constantan or platinum, at which two thermocouple wires are attached. The two crucibles with the measurement sample and the comparison or reference sample are located at these positions. Such a system is also called a disc measuring system. As possible, the plate at the crucible positions and in their immediate vicinity is thin, this area has a very small thermal mass and the arrangement thus achieves a small time constant. However, the system is relatively insensitive due to the low thermal stresses of the metallic thermocouple materials.

Conventional disk measuring systems made of metallic thermoelectric alloys have time constants of around 10 s, which, depending on the measuring task, allow a heating rate of up to 100 K / minute, with a maximum sensitivity of 10 μV / mW being achieved. Cup systems achieve sensitivities of up to 100 μV / mW, although here, due to the poor temporal resolution (time constants greater than 30 s), only heating rates of up to 20 K / minute make sense.

It should also be noted that e.g. B. from US-A-5, 059, 543 radiation detectors are known in which semiconductor material is used as an active sensor element.

The task of a differential thermal analysis system is to measure a temperature difference between the sample and a reference substance in a dynamic temperature curve in such a way that quantitative conclusions about conversion energies or specific heat are possible. The temperature difference can be measured in different ways depending on the type and task. Is it z. B. on the detection of an extremely low conversion heat, so far preferably a cup system with a thermopile has been used. Because of the large time constants in the temperature difference-over-time display, this results in a flat signal which decays only slowly. If several conversions close to each other on the temperature scale have to be resolved, the case can occur, that the time resolution is no longer sufficient due to the large time constants.

In this case, if you switch to the faster conventional disk measuring system, you can separate the different signals from each other due to the shorter time constants; but at the same time loses sensitivity in the resolution of the temperature difference, so that the signals become smaller and may disappear entirely in the noise.

The invention has for its object to provide a differential thermal analysis device which ensures maximum sensitivity of the temperature resolution with low time constants.

To achieve this object, the invention proposes a differential thermal analysis device for the metrological examination or determination of a thermophysical parameter of a thermophysical test sample, in particular for the investigation of phase transitions or for determining the specific heat of the test sample, the differential thermal analysis device being provided with - a heat source, a sensor plate thermally coupled to the heat source, on which a measurement sample coupling zone for the thermal coupling of a measurement sample to be measured and a reference sample coupling zone for the thermal coupling of a reference sample with a behavior known with regard to the parameter to be examined or measured are formed The sensor plate, at least within its measurement sample and reference sample coupling zones, consists of thermoelectric semiconductor material which is in ceramic or monocrystalline or polycrystalline form. two first thermal sensors each designed as thermocouples, which are connected to the coupling zones of the sensor plate and output a voltage difference representing the temperature difference between them, two second temperature sensors for determining the temperatures of the coupling zones of the sensor plate and an evaluation unit which is connected to the thermocouples and the second temperature sensors is, the evaluation unit during a heating caused by the heat source and / or cooling of the sensor plate at time intervals the voltage difference between the two thermocouples and the temperature values of the two second

Queries temperature sensors on the basis of the temperature difference resulting from the temperature measurement values of the second thermal sensors and the voltage difference the Seebeck coefficient of the semiconductor material for the

Temperature of the sensor plate determined at the respective times of inquiry, forms a characteristic curve of the Seebeck coefficient against the temperature of the sensor plate from the calculated values for the Seebeck coefficient and carries out a subsequent thermal analysis taking into account the measured characteristic curve of the Seebeck coefficient.

In the differential thermal analysis device according to the invention, a heat source feeds a sensor plate which is thermally coupled to the heat source. At least one coupling zone for thermal coupling of the measurement sample to be measured and at least one coupling zone for thermal coupling are on the sensor plate of the reference sample. Two temperature sensors are thermally coupled to each coupling zone, one of which is in particular designed as a thermocouple. Each thermocouple acts as a contact point for a supply line (thermo line) with the test sample or Reference sample coupling zone. The two thermal lines each consist of a material that is different from the material of the coupling zones. Together with the sensor plate, the thermal lines connected to form a closed electrical circuit thus form a differential thermocouple. Due to the Seebeck effect, there is an electrical voltage when heated between the two leads of the differential thermocouple to the coupling zones, which is characteristic of the temperature difference between the two contact points of the measuring sample coupling zone and the reference sample coupling zone cable.

A voltage difference thus arises across the two thermal lines connected to the measurement sample and reference sample coupling zones if these two zones are heated to different extents. On the basis of the temperature-dependent course of the Seebeck coefficient of the material of the coupling zones, a statement can be made about the temperature difference.

According to the invention, the sensor plate of the differential thermal analysis device consists, at least within its measurement sample and reference sample coupling zones, of thermoelectric semiconductor material which is in ceramic or monocrystalline or polycrystalline form.

The decisive disadvantage of the known disk systems, namely the relatively low sensitivity, can be achieved according to the invention by using ceramic, single or polycrystalline semiconducting material with thermoplastic electrical properties can be overcome. Such materials show a significantly increased sensitivity with the same or even reduced time constants in a temperature range, even if limited, since they have significantly higher Seebeck coefficients in this temperature range. The limited temperature range results from the fact that, combined with the temperature profile of the conduction behavior typical of semiconductors, the Seebeck coefficient (often also referred to as "thermal force") forms a characteristic maximum in the range of its highest values.

However, the use of semiconducting materials for the sensor plate brings with it the difficulty that such materials have signs of aging which are included quantitatively in the thermal analysis measurements. The device according to the invention is therefore provided with a self-calibration for checking and adapting the characteristic of the Seebeck coefficient of the semiconductor material with regard to its age-related changes. This self-calibration makes it possible to regularly and automatically check long-term aging processes (i.e. measurable changes in the characteristic curve in the period from a few hours to a few weeks or months) and, in particular, to correct them with the aid of a computer. This makes the use of highly sensitive ceramic sensor plates made of single or polycrystalline semiconducting material practically usable over a longer period of time.

Self-calibration is possible in the device according to the invention by measuring the temperature difference between the two coupling zones. This can be done using temperature sensors that measure the local temperatures of the two coupling zones. In particular, these temperature sensors are in turn as Thermocouples formed, it being sufficient to connect an additional supply line, consisting of a different material than that of the thermal lines of the differential thermocouple, which are attached to the coupling zones to generate the temperature difference proportional voltage, with each of the coupling zones. The voltage difference and the temperature difference can be used to calculate the Seebeck coefficient of the semiconductor material of the sensor plate or the coupling zones, using the formula

S = ΔU / ΔT.

The Seebeck coefficients calculated in this way for different temperatures are used as support points for a characteristic curve which is calculated and stored in the evaluation unit. The actual thermal analysis is carried out on the basis of this characteristic.

It is expedient to repeat the calculation of the characteristic curve from time to time, which can also be done in parallel to a thermal analysis measurement in which, as required to calculate the characteristic curves, the sensor plate is also heated. If the characteristic curve deviates from a predetermined characteristic curve by more than a maximum tolerable amount, for example due to extreme signs of aging, this is preferably indicated optically or acoustically, so that the user of the device is informed in this respect.

The Seebeck coefficient is determined in the above manner for several temperature values. An additional sensor can be used to determine the (average) temperature of the sensor plate. However, this additional sensor can be saved in that the average of the two temperature values supplied by the second temperature sensors is formed.

The high sensitivity of the differential thermal analysis device according to the invention is thus brought about by special semiconductor physical effects of the semiconductor material used at least for the coupling zones, these effects being inextricably linked to the specific temperature characteristic of the Seebeck coefficient. In the case of a typical ribbon cable, which is predominantly present in the most efficient thermoelectric semiconductors, the Seebeck coefficient rises steadily with temperature before it decreases again when the intrinsic cable is inserted and drops to very low values. Within a limited temperature interval of a maximum of a few 100 K, the advantage of an extremely high sensitivity can thus be exploited with the differential thermal analysis device according to the invention.

In order to be able to use the measuring principle of differential thermal analysis broadly in practice, the differential thermal analysis device should have an essentially uniformly high sensitivity (and temperature resolution) over a wide temperature range, that is to say a uniformly large thermal voltage. Based on the current state of knowledge, it has always been assumed that this requirement can only be met by metallic sensor plates ^' . ^' Although these systems have low values of the Seebeck coefficient, they rise to high temperatures or stabilize at medium values. In addition, the known metallic sensors are well structured, due to the high charge carrier densities of the metals insensitive to impurities and phase stable. The latter is particularly important for such a sensitive measuring system as the differential thermal analysis. With the invention, namely the use of thermoelectric semiconductor materials with the possibility of determining the temperature dependency of the Seebeck coefficient of the sensor plate or the active sensor element of a differential thermal analysis device that changes as a result of aging of the semiconductor material, however, thermal analysis systems with a high level can also be used Realize sensitivity over a wide temperature range. According to the invention, the solution to this problem is that the sensor plate has a combination of several different semiconductor materials with the highest sensitivity (thermal force) in different temperature ranges. The characteristic temperature at which the Seebeck coefficient of semiconducting materials begins to fall is determined by the doping concentration and the width of the band gap in the semiconductor. The band gap is a material-specific parameter and varies from material to material. For most standard semiconductors, the doping concentration can be set in a defined manner over several powers of ten. In this way, the onset of self-conduction and thus the maximum of the Seebeck coefficient can be practically "programmed" for a given temperature. By combining only two or three specially matched semiconductor materials, the temperature range relevant for practical measuring systems can already be covered.

The above-described further development of the differential thermal analysis device according to the invention aimed at increasing the temperature range can be implemented in two fundamentally different ways. Both variants have in common that the sensor plate has several pairs, each of a measurement sample and a reference sample coupling zone, the two coupling zones within a pair of the same Semiconductor material with the same thermoelectric properties exist, but the semiconductor materials or their thermoelectric properties differ from pair to pair. The coupling zones of each pair are arranged centrally symmetrically on the sensor plate, with the heat output being fed in by the heat source in the center of symmetry of the sensor plate or centrally - symmetrically from the outside. In the first implementation variant, the individual coupling zones are spaced apart from one another to such an extent that several identical measurement samples and several identical reference samples are used. Each measurement sample or reference sample is thus thermally coupled to a single coupling zone. All coupling zones are provided with temperature sensors, in particular thermocouples, the output voltages of which are fed to the evaluation unit. The evaluation unit now selects the coupling zone pair for the difference measurement whose associated temperature sensors deliver the greatest output voltages.

The structure of the sensor plate described above makes it necessary that several identical measurement samples are measured or examined simultaneously. Depending on the test sample, this can be problematic, especially when the amount of substance to be examined is limited. For such a case, it makes sense to concentrate different semiconductor materials in a common coupling zone, these individual coupling sub-zones being arranged so closely adjacent to one another that they are all in thermal contact with a measurement sample or a reference sample at the same time. In this embodiment of the sensor plate, too, the individual coupling partial zones are arranged in a centrally symmetrical manner. A circular area is an example of a geometrical shape for a coupling zone. In the previously described configuration of the sensor plate with pro To this extent, it is expedient to cover whole-numbered partial areas of the circular area with different semiconductor materials, the individual partial areas being electrically insulated from one another.

For the sensitivity of the measurement, it is necessary if the sensor plate has a thermal resistance concentration around the individual coupling zones. Too large thermal resistances, however, have a disadvantageous effect on the time constant of the overall system, which in principle increases as the thermal resistance of the sensor plate increases. By choosing suitable materials, a compromise can be found here that, with reasonable time constants, allows sufficiently large thermal resistances to develop around the coupling zones, which in turn has a positive effect on the sensitivity in that the heat flow from the coupling zones to the rest of the sensor plate is impeded and is thereby affected forms an increased temperature difference between the coupling zone and the main mass of the sensor plate.

An increase in the thermal resistance around the coupling zones can be realized constructively by reducing the amount of material in the area between the coupling zones and the rest of the sensor plate. This in turn can be achieved by reducing the thickness of the sensor plate in the connection area between the coupling zones and the main mass of the sensor plate and / or by introducing cutouts in this connection area. The recesses can be made by an etching technique, by laser cutting, mechanical grinding or the like. known material processing techniques can be realized. In order to reduce the thermal resistance between the coupling zones and the measurement or reference samples, it is expedient to apply a metal coating to the coupling zones. Ni, Au, Mo, Pt, Al or alloys thereof are particularly suitable for this, the thickness of the metal coating generally being between 0.1 and 500 μm. In order to suppress interference signals due to phase changes between the semiconducting material of the coupling zones and the metallization, it is expedient to arrange a protective layer between the semiconducting material and the metal coating, which is preferably an oxide. To protect against external contamination of the semiconducting materials of the sensor plate, it is advisable to provide it with a non-conductive, chemically inert coating, in particular Si0 ₂ , SiN, Al ₂ 0 ₃ .

Various solution concepts are available for the construction of the sensor plate provided with coupling zones made of thermoelectric semiconducting ceramic or monocrystalline or polycrystalline semiconductor material as the active material. For example, it is possible for the sensor plate to be made of metal or another base material with good thermal conductivity, a disk or piece of semiconducting thermoelectric material being inserted, welded, soldered or otherwise fixed into the plate for each coupling zone to be created. The at least two coupling zones are electrically connected to one another via the base material, so that the required temperature difference measurement can be carried out using the Seebeck effect by means of the thermocouples. As an alternative to the structure mentioned above, it makes sense to manufacture the sensor plate from insulating or electrically poorly conductive material (for example oxide ceramic, sapphire, mica, lightly doped semiconductors) the sensor plate carries one or more layers or conductor tracks made of ceramic or crystalline thermoelectric semiconductors, which form the coupling zones and connect them electrically. Finally, it is also conceivable to manufacture the sensor plate from electrically insulating or poorly conductive material with good thermal conductivity, the electrical contacts between the highly sensitive coupling zones, which are arranged in the above manner on or in the sensor plate, by means of an applied metallization or otherwise metallic connections is made.

In particular, the following materials are used as the semiconducting thermoelectric material:

Si ₁ _ _x Ge _x with x between 0 and 1 (inclusive), doped with P, Ga, As or B or other additives, in particular with charge carrier densities between 10 ¹⁵ and 10 ²¹ cm ^"3 ,

FeSi ₂ doped with Co, Al, Mn, Ni or Cr, their combinations or other elements, so that the charge carrier density assumes values between 10 ¹⁵ and 10 ²¹ cm ^"3 , - Bi ₂ (Te ₁ _ _x Se _x ) ₃ with x between 0 and 1 (inclusive), so doped (e.g. with J, Cl, Sn, or the like or by means of

Te or Se excess) that the charge carrier density

Assumes values between 10 ¹⁵ and 10 ²¹ cm ^"3 ,

(Bi ^ Sb ₂ Te ₃ with x between 0 and 1 (inclusive), doped (e.g. with Pb, Ge, or the like or using an excess of Bi or Sb) such that the charge carrier density values between 10 ¹⁵ and 10 assumes ²¹ cm ^"3 , and semiconductors which, by suitable doping, exhibit interference band line effects (" hopping conductivity ") or polaron effects, and which are therefore able to measure the thermal voltage over a wide temperature range. areas to increase. The materials AlB ₁₂ , FeSi ₂ and B ₁ _ _X C _X should be mentioned here in particular.

The currently largest market for thermal analysis devices is quality control in the material manufacturing and processing industry. The polymer range should be mentioned here in particular, where polymerisation behavior is tested in temperature ranges up to 400 ° C. There are established devices for this market; some with automatic sample changer. The achievable sensitivities of today's disc measuring systems are sufficient here. Nevertheless, an improvement can be achieved by the differential thermal analysis device according to the invention, since the increased sensitivity enables a higher heating rate and thus shortens the cycle time for the entire measurement.

A new market for thermal analysis devices has been growing for two years. This is the area of food control in which the quality of fats, starches, proteins etc. is assessed based on their melting, cross-linking or denaturing behavior. This usually happens at temperatures below 150 ° C. In this case, the heat effects during networking or conversions are so low that they cannot be resolved with current disc measuring systems. For this reason, the more sensitive but slow cup systems had to be used up to now.

This is arguably the largest market for the differential thermal analysis device according to the invention. An estimate has shown that an increase in sensitivity by a factor of 4 compared to the conventional disk measuring systems is sufficient to meet all the requirements required. This system is already sensitive enough, fast enough and automatable. Exemplary embodiments of the invention are explained in more detail below with reference to the figures. In detail show:

Fig. 1 is a schematic representation of the structure of a differential thermal analysis device

Use of a semiconductor sensor plate according to a first exemplary embodiment with measurement-related updating of the temperature profile of the Seebeck coefficient of the semiconductor material to record and correct its aging processes,

2 shows a plan view of a sensor plate according to a second exemplary embodiment of the invention with a heat resistance concentration implemented in a constructive manner around the measurement sample and reference sample positions,

Fig. 3 is a schematic representation of a differential thermal analysis device according to another

Embodiment of the invention, in which the sensor plate has several reference sample and measurement sample positions with different semiconducting materials.

4 shows a plan view of the sensor plate of the device according to FIG. 3,

Fig. 5 is a plan view from above of the sensor plate of a further differential thermal analysis device and

6 is a bottom view of the sensor plate according to FIG. 5.

1 shows a first exemplary embodiment of a differential thermal analysis device 10 in principle poses. This device 10 has a heat source 12, the heat of which is fed via a thermally coupled rod 14 or an outer covering, capsule or the like to a disk-shaped sensor plate 16. The sensor plate 16 is thermally coupled to the rod 14, the rod 14 being connected to the center of the sensor plate 16 or at its periphery. A coupling zone 18 for a measurement sample 20 and a coupling zone 22 for a reference sample 24 are formed on the sensor plate 16. These two coupling zones 18, 22 are diametrically opposed to one another and thus centrally symmetrical to the center of the sensor plate 16. The measurement sample 20 and the reference sample 24 are accommodated in crucibles 26, 28 which are in good thermal contact on the coupling zones 18, 22. The sensor plate 16 consists consistently of a ceramic, single or polycrystalline thermoelectric semiconducting material, which is in particular one of the materials mentioned in the introduction to the description. On the underside of the sensor plate 16 in the area of

Coupling zones 18, 22 are thermocouples 30, 32, which are realized by connecting electrical supply lines 34, 36 to the sensor plate 16. The material of these leads 34, 36 is different from the material of the sensor plate 16. The usual metallic thermocouple materials are used as the material for the leads 34, 36. The supply lines 34, 36 lead to an evaluation unit 38, in which the voltage difference between the two supply lines 34, 36 that is established due to the temperatures of the coupling zones 18, 22 is evaluated. The result is displayed on a display device 40 or stored on a control computer.

As indicated at 42 and 44, the sensor plate 16 has metallizations in the area of its coupling zones 18, 22, which serve to improve the heat transfer from the sensor plate 16 to the crucibles 26, 28. Between these metallization coatings 42, 44 and the sensor plate 16, a (for example, electrically insulating) protective layer, in particular made of an oxide, can be arranged in order to suppress interference signals by phase conversions. In the remaining area outside the coupling zones 18, 22, the sensor plate is provided with a protective layer 46 made of, for example, SiO ₂ , SiN, Al ₂ O ₃ .

During the measuring operation of the device 10, the sensor plate 16 is heated by the heat source 12. The heat flow runs from the heat source 12 via the rod 14 to the center of the sensor plate 16, from where it is radially evenly discharged to the outside. To carry out quick measurements, it is advisable if the thermal resistance is only low. On the other hand, in order to prevent heat flow from the coupling zones 18, 22 to the remaining area of the sensor plate 16, care should be taken to ensure that the thermal resistance around the coupling zones 18, 22 is increased.

Due to the different behavior of measurement sample 20 and reference sample 24, the coupling zones 18 and 22 assigned to them will heat up to different degrees. This temperature difference is detected via the differential thermocouple (DTE) 34, 16, 36 and converted into a measuring voltage, which is evaluated in the evaluation unit 38.

The differential thermal analysis device 10 shown in FIG. 1 is provided with a self-calibration of the temperature profile of the Seebeck coefficient of the semiconductor material of the sensor plate 10. For this purpose, the device 10 has two further thermocouples 34-60-64 and 36-62-66 Temperature sensors, which are electrically connected to the evaluation unit 38 via feed lines 34, 64 and 36, 66. The material of the supply lines 64 and 66 is different from the material of the supply lines 34 and 36, the connections of the supply lines 34 and 64 on the one hand and 36 and 66 on the other hand being directly adjacent to the sensor plate 16. The voltage differences occurring between the supply lines 34 and 64 on the one hand and 36 and 66 on the other hand are converted into absolute temperatures T _x and T ₂ in evaluation circuits 68, 70 which are arranged within the evaluation unit 38. As before, the voltage difference between the leads 34 and 36 is measured. Since the Seebeck coefficient can be calculated from the quotient of this voltage difference and the difference between the temperatures T ₂ and T ₂ , it is now possible to determine the associated Seebeck coefficient for any temperature value. In this way, the temperature profile of the Seebeck coefficient of the semiconductor material of the sensor plate 16 can be measured again at any time, as a result of which changes due to signs of aging of the semiconductor material can be taken into account for subsequent thermal analysis measurements. The average of the two measured temperatures T _x and T _{2 is} taken as the temperature assigned to the calculated Seebeck coefficient. The support points of the Seebeck coefficient over temperature determined in this way create a compensation curve which forms the characteristic curve required for later thermal analysis measurements.

The characteristic curve can be recorded both in the heating and in the cooling mode of the device 10. In parallel to the self-calibration, the time constant can also be recorded in the device 10 in order in relation to the determined value of the heating rate Device 10 to be adapted, whereby a minimal measurement time is achieved.

With the differential thermal analysis device 10 described here, the previously customary metallic sensor plates can be replaced by a semiconducting ceramic with high thermal voltages such as suitably doped Si, Ge, Si-Ge, FeSi ₂ or material based on Bi ₂ Te ₃ . These materials have thermal stresses that are more than an order of magnitude higher than those of the usual metallic thermocouple materials. Similar or even higher values of thermal conductivity are achieved, which results in design variants with an extremely short time constant. Overall, the sensitivity of a disc measuring system can easily be increased by an order of magnitude by using a semiconducting thermoelectric sensor plate without the design having to be changed significantly.

Even if semiconductor technology today can reproducibly adjust the composition and doping so that the sensor properties can be specified within narrow tolerances, frequent checking of the method is necessary, since slow aging processes generally take place. These difficulties when using ceramic thermoelectric materials can be overcome by the modified construction of the device according to FIG. 1 and the consequent application of the calibration method.

The constructional change of the measuring system consists essentially in the fact that instead of a single thermocouple line at each measuring point a pair of thermocouples is attached. Absolute temperature differences can be read out via both pairs in a regular calibration procedure a current sensor characteristic curve can be recorded continuously. Since all thermal analysis systems are now computerized, the constant updating of the conversion function is no longer a practical problem.

The measurement and evaluation method further development consists in the synchronous recording of two measuring voltages instead of one. In addition to the actual measured value, the DSC curve, the absolute temperature difference between the sample and reference position is recorded simultaneously with the associated thermo-voltage measured value. The current temperature profile of the thermal force of the sensor can be calculated therefrom by means of standard mathematical methods of the compensation calculation after the end of the temperature measurement series (in sections or over the entire temperature range). The decisive advantage of this procedure lies in its methodical simplicity and in the low additional equipment expenditure.

It is essential for the feasibility that the long-term change in the sensor characteristic can be completely described by the long-term behavior of the Seebeck coefficient, since the thermal conductivity of semiconductors with a very low electrical charge carrier concentration is practically not dependent on their amount.

FIG. 2 shows a top view of an alternative embodiment of the sensor plate 16 of FIG. 1. In FIG. 2, this sensor plate is designated 16 '. Insofar as the parts of the sensor plate 16 'correspond to those of the sensor plate 16 of FIG. 1, they are provided with the same reference symbols in FIG. 2.

In the alternative configuration of the sensor plate 16 ', the coupling resistance is increased to increase the thermal resistance. Zones 18, 22 each have a specially designed connection area 48, in which a plurality of continuous recesses 50 with radially extending webs 52 are made in the sensor plate 16 '. The coupling zones 18, 22 are therefore mechanically and electrically connected to the rest of the sensor plate 16 'via the connecting webs 52. The connecting webs 52 also have a smaller thickness than the sensor plate 16 'in its remaining area. This spoke-like construction leads to an increased thermal resistance around the coupling zones 18, 22, which increases the measuring sensitivity of the sensor plate 16 '.

3 shows a basic illustration of a differential thermal analysis device 10 "according to a further exemplary embodiment of the invention. Here, too, it applies that those parts which correspond to the differential thermal analysis device 10 of FIG. 1 may be double-crossed with the same reference symbols The sensor plate 16 "of the thermal analysis device 10" has a total of three coupling zones 18a ", 18b" and 18c "for three measuring samples and three coupling zones 22a", 22b "and 22c" for three reference samples. The measuring samples and the reference samples must All pairs of measurement sample and reference sample coupling zones 18a ", 22a", 18b ", 22b", 18c ", 22c" are diametrically opposed to each other and thus central - symmetrical to the center of the sensor plate 16 "or mirror-symmetrical to one Line of symmetry arranged. Around the coupling zones are the connection areas labeled 48 ", in which the sensor plate 16" has an increased thermal resistance.

For the coupling zones 18a "-18c" and 22a "-22c" pairs are identical semiconducting materials and from pair to

Pair of different semiconducting materials or Semiconductor materials with different thermo-electrical properties are used. For example, the coupling zones 18a "and 22a" have a first semiconductor material, the coupling zones 18b "and 22b" have a second semiconductor material and the coupling zones 18c "and 22c" have a third semiconductor material, which are either due to their semiconductor material or because of their doping (with respect to the Doping material and / or the doping concentration) differentiate. A thermocouple 30a "-30c" and 32a "-32c" is assigned to each coupling zone. These thermocouples are connected in pairs to the evaluation unit 38 ″.

Provided that there are identical measurement samples on the coupling zones 18a "-18c" and identical reference samples on the coupling zones 22a "-22c", there is the same temperature difference between each pair of coupling zones 18a ", 22a" -18c ", 22c" on. However, since different thermoelectric semiconductor materials have been used for the coupling zones of these three pairs, voltages of different sizes result between the respectively assigned thermocouples 30a ", 32a" -30c ", 32c". This is because the maximum of the temperature profile of the Seebeck coefficient of the three thermoelectric semiconducting materials selected for the coupling zones is at different temperatures. The evaluation unit 38 "selects the largest of the three (measuring) voltages supplied to it for the evaluation and carries out its calculations on the basis of this largest measuring voltage, which gives a uniformly maximum sensitivity of the sensor plate 16" over a wide temperature range.

A further exemplary embodiment of a sensor plate 16 ″ ″ with several different thermoelectric semiconducting materials is shown in FIGS. 5 and 6. It also applies here that those parts of the sensor plate 16 '''which correspond to those parts of the sensor plate 16 of FIG. 1 are identified by the same reference numerals, but with three lines.

As with the sensor plate 16 ″, the sensor plate 16 ″ also has a plurality of measurement sample coupling zones 18a ″ ″ -18d ″ ″ and a plurality of reference sample coupling zones 22a ″ ″ -22d ″ ″, which are pair-symmetrically central to the center of the Sensor plate 16 ″ ″. In contrast to sensor plate 16 ″, only one measurement sample position and one reference sample position are provided for sensor plate 16 ″ ″. In other words, a crucible for the measurement sample contacts all four measurement sample coupling zones 18a '''-18d''', while the crucible for the reference sample contacts all reference sample coupling zones 22a '''-22d''''. As can be seen from FIGS. 5 and 6 results, the individual coupling zones are designed in the manner of quarter circle surfaces, but are arranged at a distance from one another (see column 54 in FIGS. 5 and 6). Each coupling zone is in turn provided with a thermocouple 30a '''-30d''' and 32a '''-32d''', the feed lines of which are fed in pairs to the evaluation unit. Since the measurement sample and reference sample coupling zones consist in pairs of identical semiconductor material and from pair to pair of different semiconductor materials, different voltage differences U1-U4 are established at the thermocouples assigned to the coupling zones of a pair. The evaluation unit selects the largest of the voltage differences for evaluation purposes. In this way, it is ensured that maximum voltage differences can be used across the entire temperature range of interest, which means that the sensor plate 16 '''has a high sensitivity over the entire temperature range of interest. Instead of a centrally symmetrical arrangement of the coupling zones, an arrangement symmetrical to an axis of symmetry is also possible. This applies to all of the exemplary embodiments described here.

As shown in Figs. 5 and 6, the measurement sample coupling zones and the reference sample coupling zones are connected by electrically insulating, highly heat-conducting ceramic disks 56 (generally disks made of electrically insulating material which is thermally well coupled to the coupling zones), which are each arranged in the middle of the coupling zones. The crucibles for the measurement sample and the reference sample are placed on these ceramic disks 56.

The on the basis of FIGS. 5 and 6 described embodiment of the sensor plate 16 '' 'is particularly suitable when the same semiconductor base material, but with different doping is used for the coupling zones. Because of the proximity of the arrangement of the coupling zones, it is in fact expedient if they show the same temperature expansion behavior. This is best guaranteed if the base materials are the same and differ only slightly in the doping. In this way, the requirement that the sensor plate 16 '' 'should not be destroyed due to thermal stresses is satisfied.

Claims

EXPECTATIONS

1. Differential thermal analysis device for the metrological examination or determination of a parameter of a measurement sample, in particular the phase transition or the specific heat of the measurement sample, with a heat source (12), a sensor plate (16, 16 'thermally coupled to the heat source (12), 16 ", 16 '''), on which a measurement sample coupling zone (18, 18', 18", 18 ''') for the thermal coupling of a measurement sample (20) to be measured and a reference sample coupling zone (22,22', 22 ", 22 ''') are designed for the thermal coupling of a reference sample (24) with a behavior which is known with regard to the parameter to be examined or measured, the sensor plate having a ceramic or a coupling in or in at least within its measuring sample and reference sample coupling zones has polycrystalline thermoelectric semiconductor material, two first thermosensors (32, 32 ', 32'',32''') each formed as thermocouples, which are connected to the coupling ones (18, 22, 18 ', 22', 18 ", 22", 18 ''',22''') of the sensor plate (16, 16 ', 16 ", 16''') are connected and the temperature difference output voltage difference between them, two second temperature sensors (60, 62) for determining the temperatures of the coupling zones (18, 22, 18 ', 22', 18 ", 22", 18 '", 22'") of the sensor plate ( 16,16 ', 16 ", 16'") and an evaluation unit (38,38 "), which with the thermocouples (30,32,30 ', 32', 30", 32 ", 30 ''',32''') and the second temperature sensors (60, 62) is connected, the evaluation unit

(38.38 ") during a heating and / or cooling of the sensor plate (16, 16 ', 16", 16' '') caused by the heat source (12), the voltage difference between the two thermocouples (30,32 , 30 ', 32', 30 ", 32", 30 '", 32'") and the temperature values of the two second temperature sensors (60, 62), based on the temperature measurement values of the second thermal sensors (60, 62) Temperature difference and the voltage difference determined the Seebeck coefficient of the semiconductor material for the temperature of the sensor plate (16, 16 ', 16 ", 16"') at the respective times of inquiry, a characteristic of the Seebeck coefficient from the calculated values for the Seebeck coefficient above the temperature of the sensor plate (16, 16 ', 16 ", 16"') and carries out a subsequent thermal analysis taking into account the measured characteristic of the Seebeck coefficient.

Apparatus according to claim 1, characterized in that the temperature of the sensor plate (16, 16 ', 16 ", 16' ''), which is assigned a calculated value for the Seebeck coefficient, is the average of the two of the second temperature sensors (60 , 62) delivered temperature values.

Apparatus according to claim 1 or 2, characterized in that the evaluation unit (38) the calculated characteristic curves of the temperature profile of the Seebeck coefficient with a default characteristic curve compares and generates an output signal if a calculated characteristic curve deviates from the specified characteristic curve by more than a dimension which can be predetermined with regard to its absolute size and its sign.

Device according to one of Claims 1 to 3, characterized in that the thermoelectric semiconductor material is Si ^ Ge ,, doped with P, Ga, As or B in a charge carrier density of between 10 ¹⁵ and 10 ²¹ l / cm ³ , where x is between 0 and 1, including.

Device according to one of Claims 1 to 3, characterized in that the thermoelectric semiconductor material is FeSi ₂ doped with Co, Al, Mn, Ni or Cr or combinations thereof in a charge carrier density between 10 ¹⁵ and 10 ²¹ l / cm ³ .

Device according to one of Claims 1 to 3, characterized in that the thermoelectric semiconductor material is Bi ₂ (Te- ^ Se ₃ ⁾ doped with Te or Se in a charge carrier density of between 10 ¹⁵ and 10 ²¹ l / cm ³ , where x is between 0 and 1 , including, lies.

Device according to one of claims 1 to 3, characterized in that the thermoelectric semiconductor material with Bi or Sb in a charge carrier density between 10 ¹⁵ and 10 ²¹ l / cm ³ doped (Bi _x - _x Sb _x ) ₂ Te ₃ , where x between 0 and 1, inclusive.

Device according to one of claims 1 to 3, characterized in that the thermoelectric semiconductor material has interference band conduction effects or polaron effects caused by doping and in particular AlB ₁₂ , FeSi ₂ and B- ^ C _x with x between 0 and 1, inclusive, consists.

9. Device according to one of claims 1 to 8, characterized in that the entire sensor plate (16,16 ') consists of the thermoelectric semiconductor material.

10. Device according to one of claims 1 to 8, characterized in that the sensor plate (16, 16 ', 16 ", 16' '') has a carrier plate with which the measurement sample and reference sample coupling zones

(18,22,18 ', 22', 18 ", 22", 18 '' ', 22' ") are thermally connected from the thermoelectric semiconductor material.

11. Device according to one of claims 1 to 10, characterized in that the temperature sensors

(30,32,30 ', 32', 30 ", 32", 30 '", 32'") are designed as thermocouples, each with the measurement sample and reference sample coupling zones

(18, 22, 18 ', 22', 18 ", 22", 18 '' ', 22' '') connected supply line (34, 36) from one of the thermoelectric semiconductor material of the coupling zones

(18, 22, 18 ', 22', 18 ", 22", 18 "', 22'") have different material, the temperature difference between the measurement sample coupling zone and the reference sample coupling zone (18, 22, 18 ', 22 ', 18 ", 22", 18' '', 22 '' ') as the voltage difference between the two leads leading to the thermocouples

(34.36) is present.

12. The device according to one of claims 1 to 11, characterized in that the sensor plate (16, 16 ', 16 ", 16''') in the surrounding area around the coupling zones (18, 22, 18 ', 22', 18", 22 ", 18 ''',22''') around has an increased thermal resistance.

13. The apparatus according to claim 12, characterized in that the increased thermal resistance constructively by reducing the amount of material in the surrounding area (48) of the coupling zones (18, 22, 18 ', 22', 18 ", 22", 18 '' ', 22' '') is realized.

14. The apparatus according to claim 13, characterized in that the surrounding area (48) has recesses and / or dilutions (52).

15. Device according to one of claims 1 to 14, characterized in that the sensor plate (16 ", 16 '' ') several pairs of measurement sample and reference sample coupling zones (18", 22 ", 18' '', 22 '' '), wherein each pair of semiconductor material with different thermoelectric properties consists of each measurement sample and reference sample coupling zone (18 ", 22", 18' '', 22 '' ') a first temperature sensor (30 ", 32", 30 '' ', 32' ''), which are all electrically connected to the evaluation unit (38 "), and that the evaluation unit (38") is the difference temperature based on the largest

Output signals of the temperature sensors (30 ", 32", 30 "", 32 "") assigned to a pair of measurement sample and reference sample coupling zones (18 ", 22", 18 "", 22 "") are determined.

16. The apparatus according to claim 15, characterized in that each coupling zone (18 ", 22") is provided for thermal coupling with a measurement or reference sample (20, 24).

17. The apparatus according to claim 15, characterized in that several, in particular all measurement sample coupling zones (18 ''') and several, in particular all reference sample coupling zones (22''') are provided together for thermal coupling with a measurement sample (20) or a reference sample (24).

18. Device according to one of claims 15 to 17, characterized in that the coupling zone pairs (18 ", 22", 18 '' ', 22' '') consist of different semiconductor materials.

19. Device according to one of claims 15 to 17, characterized in that the coupling zone pairs (18 ", 22", 18 '' ', 22' '') consist of the same semiconductor material, but have different doping and / or doping charge carrier densities.

20. Device according to one of claims 1 to 19, characterized in that the sensor plate (16, 16 ', 16 ", 16' '') in the coupling zones (18, 22, 18 ', 22', 18", 22 " , 18 '' ', 22' '') is provided with a metal coating (42, 44).

21. The apparatus according to claim 20, characterized in that between the metal coating (42,44) and the semiconductor material of the coupling zones (18, 22, 18 ', 22', 18 ", 22", 18 '' ', 22' '' ) an electrically insulating layer is arranged.