Abstract The effective thermal conductivity of two evacuated expanded perlite powders has been measured at temperatures between 295 K and 1073 K. Since conduction via the gas phase is suppressed, thermal transport occurs only via solid conduction and thermal radiation. Due to thermal contact resistances between the powder particles, solid conduction is very small and radiative heat transport dominates, especially at high temperatures. Applying the guarded-hot-plate (GHP) method to optically thick specimens, the true effective thermal conductivity λ e f f , which is the sum of solid thermal conductivity λ s and radiative thermal conductivity λ r from the diffusion model, has been measured. After plotting λ e f f as a function of third power of absolute temperature and calculating the regression line, λ s is obtained from the intercept of the line, and the extinction coefficient for thermal radiation E is determined from its slope. The resulting values are λ s = (6.4 ± 1.5) ∙ 10−3 W m−1 K−1 and E = (1600 ± 40) m−1 for the first perlite powder and λ s = (3.1 ± 1.3) ∙ 10−3 W m−1 K−1 and E = (5700 ± 350) m−1 for the second. With an effective thermal conductivity below 0.01 W m−1 K−1 up to a mean sample temperature of 473 K, the second material is suitable to realize an economic evacuated powder insulation for medium-temperature applications up to approximately 673 K at the hot side. Both materials have also been investigated with the transient-hot-wire (THW) method. This technique has the advantage of shorter measurement time, but underestimates the effective thermal conductivity according to numerical calculations from literature, especially for samples with 1000 m−1 ≤ E ≤ 10 000 m−1. This leads to an apparent extinction coefficient E a p p > E from the λ e f f vs. T 3 plot. Using the same procedure as above, the THW measurements deliver λ s = (3.9 ± 2.7) ∙ 10−3 W m−1 K−1 for the first perlite powder and λ s = (2.2 ± 1.4) ∙ 10−3 W m−1 K−1 for the second, which agrees with the values obtained from the GHP method. The apparent extinction coefficients are E a p p = (2170 ± 110) m−1 and E a p p = (6730 ± 370) m−1, which corresponds to an overestimation by 35.6% and 18.1%, respectively. Both results are in good agreement with the numerical calculations from literature, which have now been verified experimentally for the first time. Because such calculations can in principle be used to correct experimental THW data, it is possible to extend the applicability of the THW method to materials with 1000 m−1 ≤ E ≤ 10 000 m−1, i.e. near the limit of radiation diffusion.

中文翻译：

---

在 295 K 和 1073 K 之间的温度下用防护热板和瞬态热线法测量的真空膨胀珍珠岩的热导率

摘要 在 295 K 和 1073 K 之间的温度下测量了两种真空膨胀珍珠岩粉末的有效热导率。由于气相传导受到抑制，热传递仅通过固体传导和热辐射发生。由于粉末颗粒之间的热接触电阻，固体传导非常小，辐射热传输占主导地位，尤其是在高温下。将防护热板 (GHP) 方法应用于光学厚样品，测量了真实有效热导率 λ eff ，它是来自扩散模型的固体热导率 λ s 和辐射热导率 λ r 的总和。在绘制 λ eff 作为绝对温度的三次方函数并计算回归线后，从线的截距获得 λ s，热辐射的消光系数 E 由其斜率确定。对于第一种珍珠岩粉末，所得值为 λ s = (6.4 ± 1.5) ∙ 10−3 W m−1 K−1 和 E = (1600 ± 40) m−1，λ s = (3.1 ± 1.3) ∙ 10 -3 W m-1 K-1 和 E = (5700 ± 350) m-1 秒。第二种材料的有效热导率低于 0.01 W m-1 K-1，平均样品温度高达 473 K，适用于实现经济的真空粉末绝缘，适用于热侧高达约 673 K 的中温应用. 还使用瞬态热线 (THW) 方法对这两种材料进行了研究。该技术具有测量时间短的优点，但根据文献的数值计算低估了有效热导率，尤其是对于 1000 m-1 ≤ E ≤ 10 000 m-1 的样品。这导致从 λ eff 与 T 3 图中的明显消光系数 E app > E 。使用与上述相同的程序，THW 测量得出第一种珍珠岩粉末的 λ s = (3.9 ± 2.7) ∙ 10−3 W m−1 K−1 和 λ s = (2.2 ± 1.4) ∙ 10−3 W m -1 K-1 为秒，这与从 GHP 方法获得的值一致。表观消光系数为 E app = (2170 ± 110) m-1 和 E app = (6730 ± 370) m-1，分别对应高估了 35.6% 和 18.1%。这两个结果都与文献中的数值计算非常吻合，现在已经首次通过实验验证。由于此类计算原则上可用于校正实验 THW 数据，因此可以将 THW 方法的适用性扩展到 1000 m-1 ≤ E ≤ 10 000 m-1 的材料，即