A vacuum super insulation for temperatures up to 1073 K has been developed using an opacified powder mixture with high-density (rho= 180 kg m(-3)) expanded perlite as base material. To analyze radiative transfer, the massspecific extinction coefficient e of expanded perlite and various opacifiers (SiC, B4C, FeTiO3, TiO2) has been determined by FTIR spectrometry. For reduction of radiative transfer, a SiC powder with mean grain diameter of 2 mu m-4 mu m and bulk density of 870 kg m(-3) has been identified as most suitable opacifier, also under economic aspects. Subsequently, six homogeneous powder mixtures with weight fraction w of SiC between 0% and 60% have been prepared. In order to determine the optimum w, guarded hot plate (GHP) measurements under high vacuum conditions (p < 0.1 hPa) and at 673 K mean temperature have been performed on these mixtures, yielding a minimum effective thermal conductivity of lambda(e) = 13.10(-3) W m(-1) K-1 for w = 40% and p = 263 kg m(-3). This mean temperature corresponds to an insulation application with 950 K hot-side and ambient temperature cold-side. For the optimum mixture with w = 40%, e has likewise been determined with FTIR spectrometry. Furthermore, lambda(e) has been measured with the GHP method also at mean temperatures between 373 K and 873 K, where excellent values between 4.10 -3 W m(-1) K-1 and 20.10(-3) W m(-1) K-1 have been obtained. For the evaluation of these measurements, two different methods have been applied in order to separate radiative thermal conductivity. lambda(r) and solid thermal conductivity lambda(s): first the common plot of lambda(e) versus T-3, and second an improved method, which includes the temperature dependencies of e and lambda(s). Expanded perlite, opacified with SiC, has proven as a highly efficient and economic vacuum super insulation material for industrial applications up to 1073 K.

Zusammenfassung Die Effizienzsteigerung moderner Gasturbinen erfordert die stetige Anhebung der Betriebstemperatur. Die derzeitigen Brenngastemperaturen liegen mit über 1400 °C signifikant über der kritischen Temperatur der verwendeten Turbinenstähle. Zur Gewährleistung der Betriebssicherheit werden die Turbinenschaufeln neben Aktivkühlung durch Beschichtung mit thermischen Schutzschichten, sogenannten thermal barrier coatings (TBC), geschützt. Da es sich bei den TBC um Keramikschichten handelt, ist für die Erhöhung der Haftfestigkeit das Aufbringen eines Haftvermittlers (Verbindungsschicht) notwendig. Da die Eigenschaften dünner Schichten stark von den Eigenschaften des Bulkmaterials abweichen können und zudem von der Herstellungsmethode beeinflusst werden, ist eine Untersuchung der thermischen und infrarot-optischen Eigenschaften der tatsächlichen Schichtstrukturen unumgänglich, insbesondere im Hochtemperaturbereich. Hierfür wurden Proben des reinen Trägerstahls, des Trägerstahls mit Haftvermittlerschicht und des kompletten Schichtsystems aus Trägerstahl, Haftvermittlerschicht und Wärmedämmschicht verschiedener Dicken hergestellt und mittels Laser-Flash-Methode untersucht. Die Auswertung erfolgte dabei analytisch, ausgehend von der Trägerstahl-Einschichtprobe, über die Zweischicht- und Dreischichtsysteme. Vervollständigt wurden diese Untersuchungen durch infrarot-optische Charakterisierungen, mit denen sich die Wärmeausbreitung durch die Schichtsysteme beschreiben lässt. Zusammen mit den Laser-Flash Messungen erlaubt dies eine spätere Quantifizierung der einzelnen, bei Keramiken auftretenden, Wärmetransportmechanismen.

Due to their special combination of nanostructure and high porosity, aerogels are key materials for high performance thermal insulation. However, measuring reliable thermal conductivity values, which are essential for material’s optimization and as product parameter, is a challenging task in the case of aerogels. Experimentally derived thermal conductivity values for aerogels are so far more or less influenced by the experimental set-up and the experimental conditions and have to be carefully discussed. Here we present results of an intercomparison on thermal conductivity measurements performed for a polyurethane aerogel produced on a pilot scale as stiff panels by BASF. Prior to the intercomparison, the material was checked for reproducibility in production and homogeneity. The dependence of thermal conductivity on atmospheric pressure and temperature was also determined. We discuss the results submitted by 12 participants with respect to the different experimental techniques applied and identify experimental parameters with severe impact on the resulting thermal conductivities. The derived mean values for the thermal conductivity at 20 °C, 40 °C and 60 °C were related with relative uncertainties in the range from 0.6 % to 0.9 %. The dependence of the derived thermal conductivity values on the geographical location of the participating laboratory and the atmospheric weather conditions could be clearly observed and the precision of the results could be significantly improved by correcting for these effects. The values had to be partly corrected up to 2.5 %.

Increasing the efficiency of modern gas turbines requires the increase of operating temperature. Current fuel gas temperatures above 1400 degrees C significantly exceed critical temperatures of the turbine steels used. To ensure operational safety, the turbine blades are actively cooled and also protected by protective layers, the so-called thermal barrier coatings (TBC). Since the TBC are ceramic layers, an adhesion promoter (bond coat) must be applied to increase the adhesive strength. Since properties of thin layers can differ from properties of the bulk material and are also influenced by the manufacturing method, an investigation of the thermal and infrared-optical properties of the actual layer is essential, especially in the high temperature range. For this purpose, samples of the pure carrier steel, the carrier steel with bond coat and the complete layer system of carrier steel, bond coat and TBC of various thicknesses were produced and examined using the laser flash method. The evaluation was carried out analytically, starting from the single-layer sample, via the two-layer and three-layer systems. These investigations were completed by infrared-optical characterizations, allowing the description of the heat transport through the layer systems. These measurements allow a future quantification of the individual heat transport mechanisms occurring in ceramics.

Thermal energy storage with phase change materials (PCM) provides high storage capacities in small temperature intervals. For the design of storage systems, the enthalpy curve of the used PCM has to be known with high precision. The T-History method has evolved to a widely used method for the measurement of the enthalpy as a function of temperature of PCM because of its simplicity and the advantage to be able to investigate larger sample volumes than typically used for differential scanning calorimeters. In order to ensure isothermal specimen during the measurement a thermal insulation is often mounted around the sample holder, but this insulation material is not considered in the evaluation model. In this work a new evaluation model for insulated T-History setups is developed by an analytical heat balance. This model is validated by numerical simulations of insulated T-History measurements and experimentally. By the use of the new model for the evaluation of the enthalpy a significant increase of accuracy can be achieved.

The storage of latent heat in phase change materials (PCM) is of great interest in many applications, for example in building applications. However, there is no standard method for the determination of the thermophysical properties of application-sized PCM specimens, i.e., specimens with sizes around 100 ml to 1000 ml. In order to close this metrological gap, a commercially available heat flow meter was modified to perform enthalpy measurements. The feasibility of this method was proven by performing comparative measurements on a stainless steel specimen using both the standard method DSC and the modified heat flow meter. Furthermore, measurements on a gypsum board with microencapsulated PCM were performed with the heat flow meter in order to determine the enthalpy. The coincidence with literature values is within ±4% which demonstrates that this method is a good choice for performing measurements on application-sized PCM specimens.

Even though Differential Scanning Calorimetry (DSC) is a common method for the determination of specific heat capacity, there is also need for methods suitable for a larger specimen size in the range of several cubic centimeters instead of micro liters. For phase change materials or compounds which show specimen size dependent thermophysical caloric properties below a critical specimen volume, e.g. sub cooling effects, the larger specimen volumes are absolutely essential. The Longitudinal Heat Flow Method is a well-known steady state method to measure the thermal conductivity of medium sized solid samples. With a modification of the measurement procedure to a transient temperature step at the top and bottom end of the stack it is now possible to determine the heat capacity of a specimen in a defined temperature interval. An apparent heat flux is determined during the transient heating phase, which can be evaluated with a novel analytical model. Numerical simulations of this new measurement method are validated by experimental investigation of different materials.

The temperature of high voltage equipment is often the limiting factor when transmitting electrical energy because the electrical insulation can get severely aged when the temperature is exceeding a certain limit. Hence, cooling has to be improved or heat generation must be reduced to avoid damage of the insulation. In this paper a new method was examined by investigating electrically insulating phase change materials which are able to store latent heat during a phase change from the solid to liquid state in times of high energy demand. To verify the electrically insulating properties of paraffins, one class of phase change materials, a special test cell was designed allowing the determination of breakdown voltage of phase change materials. The measurements on one paraffin sample proved the promising electrical insulating properties and it was shown that the breakdown voltage in the liquid state is comparable to that of mineral oil (transformer oil). With this contribution the first step was done towards the use of phase change materials for application in high voltage equipment.

Even though Differential Scanning Calorimetry (DSC) is a well established method for the determination of specific heat capacity of various materials, there is also need for methods suitable for larger specimen sizes in the range of several cubic centimeters instead of micro liters. For phase change materials, i.e. a class of materials used for technical heat storage applications, or compounds which show size dependent thermophysical caloric properties below a critical specimen volume, e.g. subcooling effects, the larger specimen volumes are absolutely essential. The Longitudinal Guarded Heat Flow Method is a well-known steady state method to measure the thermal conductivity of medium sized solid samples. With a modification of the measurement procedure to a transient temperature step at the top and bottom end of the sample it is possible to determine the heat capacity of a specimen in a defined temperature interval. An apparent heat flux is determined during the transient heating phase. Numerical simulations of this new procedure are presented and discussed. The simulations indicate that, due to the transient nature of the technique, a correction in respect to the heat capacity of the reference specimens has to be applied to the measurement data. For this task a novel analytical method is provided. A correction factor is introduced which only depends on the geometry of the experimental setup. This analytical method is validated by numerical simulations. The results show good agreement and recommend the proposed method for the practical use.

A round-robin test between three institutes was performed on a paraffin phase-change material (PCM) in the context of the German quality association for phase-change materials. The aim of the quality association is to define quality and test specifications for PCMs and to award certificates for successfully tested materials. To ensure the reproducibility and comparability of the measurements performed at different institutes using different measuring methods, a round-robin test was performed. The sample was unknown. The four methods used by the three participating institutes in the round-robin test were differential scanning calorimetry, Calvet calorimetry and three-layer calorimetry. Additionally, T-history measurements were made. The aim of the measurements was the determination of the enthalpy as a function of temperature. The results achieved following defined test specifications are in excellent agreement.

A new method for evaluation of the specific heat capacity in the temperature regime between 77K and 330K using laser-flash calorimetry is presented. Usually, laser-flash calorimetry is accomplished by performing an additional laser-flash measurement on a reference specimen with a known specific heat capacity and by comparing the maximum rear-side temperatures rises. In this study, the calibration is achieved by comparison of the rear-side temperature rise to specific-heat-capacity data determined by other methods in an adjacent temperature regime. Subsequently, the thus yielded proportional factor is used for the evaluation of the specific heat capacity from laser-flash measurements at temperatures where no specific-heat-capacity data are available. The reliability of this method is shown by performing measurements on a material with known specific heat capacity, aluminum oxide. Furthermore, the specific heat capacity and thermal conductivity of borosilicate crown glass (BK7) was determined experimentally.

The thermophysical properties of an amorphous, multinary precursor derived Si/B/N/C ceramic with the approximate composition of SiBN3C were determined experimentally. Thermal diffusivity a(T) was determined from 77 to 300 K with an alternative laser-flash setup using the resistance change of a thin (circa 100 nm) gold strip as fast temperature probe. From 300 to 873 K a standard laser-flash setup was used. Specific heat capacity c(p)(T) was determined by temperature modulated differential scanning calorimetry (MDSC) in the temperature range between 190 and 773K. Using the relation lambda(T) = a(T).c(p)(T).rho with the specimen density of rho = (1561 +/- 47) kg m(-3), which was considered to be temperature-independent, the thermal conductivity lambda(T) was calculated. The analysis yielded values for the thermal conductivity at room temperature of lambda(296 K) = (0.490 +/- 0.042)W m(-1) K-1, and at the highest investigated temperature of lambda(773 K) = (0.706 +/- 0.063)W m(-1) K-1. Furthermore, space-resolved laser-flash measurements with a lateral resolution of about 1.5 mm were performed at 300K to examine local inhomogeneities on the specimen. Within the given measurement uncertainties, no inhomogeneities could be found. (C) 2011 Elsevier B.V. All rights reserved.

In diesem Verfahren zur Verbesserung von mehrkomponentigen Funktionsmaterialien mit einem Transportprozess als wesentlicher Funktion wird durch den Einsatz von Verdrangungskorpern der fur die Funktionsteilchen, welche fur den Transportprozess verantwortlich sind, zur Verfugung stehende Raum reduziert. Durch die Verdrangungskorper kann ein enger Kontakt zwischen den Transportteilchen erzwungen werden, ihre Form optimal ausgerichtet werden, sowie bei vorhandener Anisotropie eine optimale Ausrichtung erreicht werden. Somit wird durch Uberschreiten der Perkolationsschwelle bereits bei sehr geringer Dichte an Transportteilchen ein entscheidender Sprung des geforderten Transportphanomens beobachtet. Insbesondere wird ein Kompositmaterial zur Warmespeicherung zur Verfugung gestellt, welches sowohl eine hohe Energiespeicherdichte als auch eine hohe Fahigkeit Warme und falls notwendig Stoffe zu transportieren aufweist. Durch den Einsatz von Verdrangungskorpern wird der zur Verfugung stehende Raum reduziert, welcher fur die Warme transportierenden Teilchen (wie z. B. Grafit- oder Metallpartikel) zur Verfugung steht.

The determination of reliable thermal conductivity values of insulation materials at high temperatures is important for target-oriented material research, further improvement of products and quality management. However, there is a lack of reference materials for high temperature thermal conductivity measurements which are needed to ensure and improve good measurement practise. In order to investigate porous calcium silicate as reference material for temperatures up to 1100K, the German Thermophysics Working Group within GEFTA initiated an intercomparison of thermal conductivity measurements on a commercially available calcium silicate insulation material with seven participating laboratories. Stationary and instationary measurement methods were used to determine the effective total thermal conductivity of the calcium silicate material in the temperature range from 300K to 1100K. The derived weighted mean value of the thermal conductivity increases from 0.0846Wm−1K−1 at 300K to 0.173Wm−1K−1 at 1100K. Within the same temperature interval the relative uncertainty increases from 3.5% to 7%. The investigated product is commercially available and it could be therefore used in the daily laboratory work as reference material for thermal conductivity measurements at high temperatures.

The adoption of the popular laser-flash method at temperatures far below 300 K is restricted by the weak signal-to-noise ratio and the limited spectral bandwidth of the commonly used mercury cadmium tellurite (MCT) infrared (IR) detector used as a non-contacting temperature probe. In this work, a different approach to measure the temperature rise in pulse heating experiments is described and evaluated. This method utilizes the change of the temperature-dependent electrical resistance of a thin strip of sputtered gold for the detection of a temperature rise as it was proposed by Kogure et al. The main advantage of this method at lower temperatures is the significantly higher signal-to-noise ratio compared to the commonly used IR detectors. A newly developed laser-flash apparatus using this detection method for the determination of the thermal diffusivity in the temperature range from 80 K to 300 K is presented. To test the accuracy of the new detection method, the thermal diffusivity of a borosilicate crown glass (BK7) specimen at 300 K was determined and compared to results derived with a MCT detector. Good agreement of the derived thermal diffusivity values within 3 % was found. The thermal diffusivity of BK7 and polycrystalline aluminum nitride (AlN) was measured at temperatures between 80 K and 300 K by a laser-flash method to test the functionality of the apparatus. Finally, the thermal conductivity was calculated using values for the specific heat capacity determined by temperature modulated differential scanning calorimetry (MDSC). Comparisons with literature data confirm the reliability of the experimental setup.

Carbon aerogels, monolithic porous carbons derived via pyrolysis of porous organic precursors synthesized via the sol–gel route, are excellent materials for high-temperature thermal insulation applications both in vacuum and inert gas atmospheres. Measurements at 1773K reveal for the aerogels investigated thermal conductivities of 0.09W · m−1 · K−1 in vacuum and 0.12W · m−1 · K−1 in 0.1MPa argon atmosphere. Analysis of the different contributions to the overall thermal transport in the carbon aerogels shows that the heat transfer via the solid phase dominates the thermal conductivity even at high temperatures. This is due to the fact that the radiative heat transfer is strongly suppressed as a consequence of a high infrared extinction coefficient and the gaseous contribution is reduced since the average pore diameter of about 600nm is limiting the mean free path of the gas molecules in the pores at high temperatures. Based on the thermal conductivity data detected up to 1773K as well as specific extinction coefficients determined via infrared-optical measurements, the thermal conductivity can be extrapolated to 2773K yielding a value of only 0.14W· m−1 · K−1 in vacuum.