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However, those methods are limited to large sample sizes and low thermal conductivities. Materials with thermal conductivity values above 25 W•m-1•K-1 can generally not be analyzed using these traditional methods. Furthermore, those methods are typically very time consuming, with tests between room temperature and 1000°C generally requiring measurement times of several days.
Flash methods, on the other hand, are non-contact measurement techniques and can handle high thermal conductivity materials without any difficulties.1 Flash methods are absolute methods for the determination of thermal diffusivity. Modern instruments often allow additional measurement of the specific heat-and therefore the determination of the thermal conductivity-without further tests. However, the flash method has traditionally been somewhat limited to small sample sizes and homogeneous materials, so it has generally not been employed for inhomogeneous coarse ceramics.
Various improvements and modifications have been made to the flash method since its introduction by Parker et al.1 New evaluation models were developed that account for heat loss2 and finite pulse effects.3 Some analysis methods are already available that simultaneously take both of these into account in an optimum way.4,5 New developments in the field of detectors and electronics carried out in recent years allow for using this technique for larger samples as well. Therefore, the flash technique can now be used for inhomogeneous materials such as coarse ceramics or refractory materials.
ConceptIn a flash measurement, the front side of a small, usually disk-shaped plane-parallel sample is heated by a short light (laser) pulse. The temperature increases induced in a small surface layer of the sample by the laser diffuse through the sample and lead to a small temperature increase of the entire sample, including the back surface. This temperature rise on the rear surface is measured vs. time using an infrared detector.
By placing the sample into a tube furnace, temperature-dependent measurements can also easily be realized. Modern flash systems6 are fully automated top-loading devices equipped with a flash source (e.g., Nd:YAG lasers) on the bottom and an infrared detector on top (see Figure 1). However, other flash systems are equipped with an external laser connected to the measurement part by glass fiber optics.*
*Netzsch LFA 427
Testing and ResultsThe measurements on the oxide ceramics and the magnesia-carbon refractory materials presented in this work were carried out using two laser flash systems.† Several small samples of the same material were measured to check the homogeneity of the material and the reproducibility of the method.
The analyzed materials came from different sources. A sillimanite ceramic material containing approximately 75% alumina and 25% silica was measured. From a larger block, three samples were prepared and measured with a laser flash device up to 1200°C. A low-conductivity refractory brick (major contents were silica, magnesia and alumina) was also analyzed. Again, three samples were prepared from a larger brick and measured up to 800°C. The sample structure can be seen in Figure 2. Positions 1, 2 and 3 indicate the positions where the samples were prepared.
† NETZSCH models LFA 427 and LFA 457 MicroFlash
All tests were conducted in a dynamic argon atmosphere. The tests on the sillimanite samples were carried out between room temperature and 1200°C, in steps of approximately 300 K. Three samples of 12.7 mm in diameter and approximately 3 mm high were prepared from the original brick and measured. The measurements on the low-conductivity silica-magnesia-alumina materials were done at room temperature, 300°C, 600°C and 800°C. Tests were carried out on three samples of 12.7 mm in diameter and approximately 3 mm high.
The temperature dependence of the density was not considered because this influence is generally small (within a few percent) for such materials. Depicted in Figure 3 are the measured thermal diffusivity and specific heat of the first sillimanite sample. Also shown is the thermal conductivity calculated from the measured results and a room temperature bulk density of 2.542 g•cm-3.
Thermal conductivity decreases vs. temperature up to 600°C, which is what can be expected for a pure phonon-conducting material.8 At higher temperatures, the values increase again. This is mainly due to the influence of the radiation heat transfer. Compared to other ceramic materials,8 the results show only slight temperature dependence. All thermal conductivity results are between 2 and 3 W•m-1•K-1. The weak temperature dependence, as well as the low thermal conductivity results, are typical for such alumino-silicate ceramics.
The low thermal conductivity, as well as the temperature dependence, measured on the first sample was found for all three samples. All results were within 5%, which is within the general uncertainty range of the instrument for direct thermal conductivity determinations.
The measured specific heat increased vs. temperature. As can be expected from the composition, the results for the specific heat were close to the results measured for the sillimanite material. The thermal diffusivity decreased slightly with temperature up to 600°C. Above this temperature, the results increased again. The thermal diffusivity results were low, which is attributable to the composition of the material and the high porosity. The results at room temperature were around 0.3 mm2•s-1. The calculated thermal conductivity increased vs. temperature over the entire temperature range. The values were between 0.15 and 0.25 W•m-1•K-1. These values are typical for porous silica ceramics.8
The thermal conductivity values of the three different samples of the porous silica-magnesia-alumina refractory ceramic showed a nearly linear increase vs. temperature between 300 and 800°C, while the room temperature results were slightly below the linear trend. For the three individual samples, slight deviations were detected (within ± 4%), which can be explained by the high degree of inhomogeneity of the material.
The decomposition of the binder yielded a slightly higher porosity and level of carbon black between the grains of the refractory. This resulted in an increased thermal resistance between the grains, causing a decrease in the effective thermal diffusivity of the entire material. Above 500°C, the thermal diffusivity was nearly constant.
A comparison of the thermal conductivity of all five of the measured magnesia-carbon refractories clearly showed that thermal conductivity decreased with decreasing carbon content (see Figure 6). Materials 2 and 4 were only measured at room temperature, 300°C and 1000°C. However, the results for these samples fit quite well with the results for the other materials tested.
Fast and Reliable MeasurementsLaser flash systems are fast and reliable instruments for the determination of thermophysical properties such as specific heat, thermal diffusivity and thermal conductivity of homogeneous ceramic materials. By using larger sample sizes or analyzing different samples of the same brick, the method can easily be extended to inhomogeneous materials such as coarse ceramics or refractories. The results achieved and discussed here for a sillimanite material, a silica-magnesia-alumina brick and magnesia-carbon refractories have proved that the non-contact flash method can be used for such materials.
For additional information regarding the laser flash technique, contact NETZSCH-Gerätebau GmbH, Wittelsbacherstrasse 42, 95100 Selb, Germany; e-mail firstname.lastname@example.org; or visit www.netzsch.com.