Thermal diffusivity of reinforced plastics at elevated temperatures

Results of measurements of the thermal diffusivity of precoked asbestos, glass, and carbon Textolite composites in the temperature range from 900° to 2800°K are reported. The nature of the temperature dependence of the thermal diffusivity and of the dependence of the thermal diffusivity on the composition of the materials is explained.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 17, No. 5, pp. 841–846, November, 1969.     

Thermal expansion of β-FeSi2 at low temperatures

We present the temperature dependence of the lattice constants (a, b, c) of β-FeSi₂ single crystals at low temperatures. a showed the largest temperature dependence of 0.14% and the relationship of a>c>b did not change at 8-300 K. The linear thermal expansion coefficients α showed remarkable anisotropy. α along the a-axis (αa) was much larger than αb and αc, and showed negative thermal expansion at temperatures below 60 K. From these results, we estimated the temperature dependence of the lattice mismatches at β-FeSi₂/Si heterojunctions.     

Thermal expansion at low temperatures of glass-ceramics and glasses

The linear thermal expansion coefficient, α, has been measured from 2 to 32 K and from 55 to 90 K for a machineable glass-ceramic, an ‘ultra-low expansion’ titanium silicate glass (Corning ULEE), and ceramic glasses (Cer-Vit and Zerodur), and for glassy carbon. α is negative for the ultra-low expansion materials below 100 K, as for pure vitreous silica. Comparative data are reported for α-quartz, α-cristobalite, common opal, and vitreous silica.     

Thermal expansion of Cr,Mo and W at low temperatures

Linear expansion coefficients have been measured for Cr, Mo and W from 2–30 K, 55–90 K, and near room temperature. At low temperatures, the lattice contributions for Mo and W, although small, are determined to better than 10% giving respective limiting values of the lattice Grüneisen parameter, γ₀, of 1.3 and 1.35 compared with γ(283 K) = 1.61; their electronic components are very different, giving γ_e = d In $\text{N(E}{}_{\mathrm{<mtext>F</mtext>}}\text{)}\text{d}$ In V = 1.1 (Mo) and 0.3 (W). For Cr, the ‘electronic’ term is large and negative at low temperatures, giving γ_em = ?9; anomalies occur in α (T) at 124 and 311 K.     

Thermal expansion of rhodium,iridium, and palladium at low temperatures

Coefficients (α) of linear thermal expansion of Rh, Ir, and Pd are reported to be respectively 8.45, 6.65, and 11.78×10^?6 ° K^?1 at 238°K, and 3.50, 3.43, and 6.21×10^?6 °K^?1 at 75°K. At temperatures below 10°K, α may be represented by $$\begin{gathered} 10^{10} \alpha = 20{\rm T} + 0.052{\rm T}^3 (Rh) \hfill \\ 10^{10} \alpha = 9{\rm T} + 0.070{\rm T}^3 (Ir) \hfill \\ 10^{10} \alpha = 40.5{\rm T} + 0.435{\rm T}^3 (Pd) \hfill \\ \end{gathered} $$ TheT andT ³terms are identifiable with electron and lattice vibrational components, respectively. Corresponding Grüneisen parameters are γ (electron)≈2.8, 2.7, and 2.22 for Rh, Ir, and Pd, and γ ₀ (lattice)≈2.0, 2.3, and 2.25.     

Thermal expansion of copper,silver, and gold at low temperatures

Improvements have been made in a differential dilatometer using the three-terminal capacitance detector. The dilatometer is of copper and has been calibrated from 1.5–34 K in an extended series of observations using silicon and lithium fluoride as low-expansion reference materials. The expansion of silver and gold samples has been measured relative to the dilatometer, while the calibrations themselves have been used to determine the expansion of copper relative to the reference materials. Analyses of six sets of observations indicate that below 12 K the linear expansion coefficient α of copper is represented by $$10^{10} \alpha = (2.1_5 \pm 0.1){\rm T} + (0.284 \mp 0.005){\rm T}^3 + (5 \pm 3) \times 10^{ - 5} T^5 K^{ - 1} $$ corresponding to respective electronic and lattice Grüneisen parameters γ _e =0.9 ₃ and γ ₀ =γ ₁ =1.78. Measurements on oxygen-free silver yield $$10^{10} \alpha = (1.9 \pm 0.2){\rm T} + (1.14 \mp 0.03){\rm T}^3 + (2 \pm 2) \times 10^{ - 4} T^5 K^{ - 1} $$ below 7 K, whence γ _e ? 0.9 ₇ , γ ₀ =γ ₁ =2.23. By contrast, silver containing ca. 0.02 at. % oxygen showed a much larger expansion at the lowest temperatures: below 7 K, 10 ¹⁰ α ～ 7T+1.19T ³ . We have not been able to obtain an unambiguous representation for gold, but find a reasonable fit below 7 K to be $$10^{10} \alpha \simeq (1 \pm 0.5){\rm T} + (2.44 \mp 0.05){\rm T}^3 - (5 \pm 1) \times 10^{ - 3} T^5 K^{ - 1} $$ with γ ₁ ? 2.94 and γ _e ? 0.7 (free-electron value).     

Thermal and mechanical properties of high-strength structural steel HSA800 at elevated temperatures

The temperature dependent material properties of structural steel are important for fire resistant (FR) design and fire simulation of steel structures. In this study, the material tests were conducted for a high-strength steel developed in Korea, namely HSA800, to determine thermal and mechanical properties at elevated temperatures up to 1000 °C. Then test results were compared with current design models – ASCE, Eurocode 3 and AISC to verify which design models are fit well to the HSA800 at the elevated temperature. The thermal properties test results show that the specific heat and thermal strain at elevated temperatures agreed well with those predicted by current design models but thermal conductivity was slightly less than the design models. In the mechanical properties test results, the ASCE model was adequate for predicting yield strength of HSA800. Eurocode 3 and AISC models were unconservative for predicting yield strength but conservative for elastic moduli. The stress–strain relationships of HSA800 at elevated temperatures were developed using Ramberg–Osgood model and they agreed well with the test results up to 2% strain.     

Fatigue response of APC-2 composite laminates at elevated temperatures

The response of thermoplastic AS-4/PEEK composite laminates of two lay-ups, such as cross-ply and quasi-isotropy, subjected to tension–tension (T–T) fatigue loading at elevated temperatures was investigated. It is found that the ultimate strength of cross-ply laminate is higher than that of quasi-isotropic laminate at various temperatures, so does the fatigue strength. However, the slope of normalized stress vs. cycles curves in the quasi-isotropic laminates is higher than that of the cross-ply laminates at elevated temperatures. Finally, the simple semi-empirical predictive models in statistical analysis and multiple regressions are proposed and provided for design and application purposes.     

火灾下平行钢丝束温度膨胀及高温蠕变试验研究

为研究开放空间拉索的火灾响应,该文考虑火源的空间辐射理论和拉索表面的环境换热边界,结合内部空腔辐射、接触传导、间隙导热三大基本传热理论,建立开放火灾下拉索腔体传热计算方法,并通过试验结果验证数值分析模型的准确性,分析预应力拉索在不同的内部传热方式、不同的包裹环境和不同的风环境下截面的温度场和应力场的时空分布特征,以及截面的轴力和弯矩的内力时变特征。结果表明：进行拉索的火灾响应分析时,考虑拉索的完整腔体传热模型可以比较准确地计算开放火灾下拉索截面的瞬态温度分布,随着曝火时间的增加,拉索截面的温度场和应力场分布呈现反对称状态且从二次分布特征向线性分布特征逼近,拉索截面会出现轴力损失和弯矩效应。相比于完整腔体传热模型,圆钢传热模型的截面应力分布均匀、轴力损失偏大且弯矩效应很小,空腔辐射模型的截面应力分布过于集中、弯矩效应偏大且轴力损失偏小。烟气包裹环境会加剧拉索截面的轴力损失和削弱拉索截面的弯矩效应。迎风环境会严重加剧拉索截面的轴力损失和弯矩效应。该文的研究成果可为索结构的抗火设计与防护提供理论依据。

     

Mechanical behaviour of MgO-partially stabilized ZrO2 ceramics at elevated temperatures

Transformation toughened partially stabilized zirconia ceramics containing magnesia exhibit quite high fracture toughness (K _lc 8 MPa m^1/2) at temperatures of up to 500° C. The observed temperature dependences of the toughness and the fracture strength are consistent with that of the transformation behaviour. The high toughness of these materials results in a significant reduction in the sensitivity of the flexure strength to crack size increases. Exposure of these materials at 1000° C for prolonged periods results in flexure strength changes associated with the generation of the monoclinic phase by tetragonal precipitate destabilization and eutectoid decomposition. However, when exposed at 500° C, neither the phase contents nor the flexure strength are altered for exposures of up to 1000 h.     

Performance of metakaolin concrete at elevated temperatures

An experimental investigation was conducted to evaluate the performance of metakaolin (MK) concrete at elevated temperatures up to 800 °C. Eight normal and high strength concrete (HSC) mixes incorporating 0%, 5%, 10% and 20% MK were prepared. The residual compressive strength, chloride-ion penetration, porosity and average pore sizes were measured and compared with silica fume (SF), fly ash (FA) and pure ordinary Portland cement (OPC) concretes. It was found that after an increase in compressive strength at 200 °C, the MK concrete suffered a more severe loss of compressive strength and permeability-related durability than the corresponding SF, FA and OPC concretes at higher temperatures. Explosive spalling was observed in both normal and high strength MK concretes and the frequency increased with higher MK contents.     

Solid lubricants for applications at elevated temperatures

Relative motion between mating surfaces at elevated temperatures often causes substantial material degradation due to friction and wear. Conventionally, solid lubricants have been used to reduce wear damage and friction drag under extreme conditions where liquid lubricants do not function properly. The recent trend towards higher operating temperatures in advanced power generating systems, i.e. turbomachinery, gas turbines, and hot adiabatic diesel engines, has imposed severe limitations on the currently available solid lubricants. The unusually aggressive conditions in these systems phased out most conventional solid lubricants and gave impetus to the search for more efficient materials. This paper discusses the lubricating characteristics of four different groups of materials known to provide lubricity under elevated temperature conditions. These groups are polymers, laminar solids, metal fluorides and metal oxides. Polymer lubricants are efficient lubricants within the range from room temperature to about 300 °C. Laminar solids extend that range to about 450 °C. Graphite, also a laminar solid, is an exception since it can offer excellent lubricity beyond 450 °C in the form of gaseous oxidation products. Stable fluorides and metal oxides are useful lubricants between 500 and 1000 °C, though their performance is rather poor at lower temperatures.     

MoSi2 diffusion barrier for the passivation of copper at elevated temperatures

Sputter deposited MoSi₂ coatings (200 and 400 nm thick) on copper have been studied in an attempt to prevent or at least reduce the oxidation of copper. Samples were exposed to an air ambient at temperatures ranging from 600–850 °C for up to 15 min. Sputter-deposited MoSi₂ was amorphous upon deposition and crystallized on annealing. Silicon from the MoSi₂ was found to diffuse into the copper causing the MoSi₂ to transform to lower suicides. The primary oxidation product for MoSi₂-coated samples was CuO (with small amounts of Cu₂O), which is in contrast to uncoated copper where Cu₂O is the main oxidation product. The amount of copper consumed by oxidation, for a 200 nm MoSi₂ barrier relative to uncoated copper, was reduced by 140 times at 600 °C and 30 times at 800 °C. A 400 nm MoSi₂ coating yielded an improvement of 420 times at 600°C, and 200 times at 850 °C. For the 400 nm barrier exposed to air for 15 min, this corresponds to a 35 nm CuO layer at 600°C and a 300 nm thick oxide layer at 850 °C.     

Absorption of 9.6-micron CO2 laser radiation by CO2 at elevated temperatures

Absorption of 9.6-micron CO2 laser radiation by CO2 at temperatures between 296 and 625 K has been measured at a pressure of 200 Torr. Experimental results for the R1O-R26 and P1O-P28 transitions have been obtained and compared with computed values of absorption. The relative optical broadening coefficients due to He and N2 have been measured on the R16-R22 and P16-P22 transitions over the same temperature range.     

Photoconductivity of ZnTe thin films at elevated temperatures

Photoconductivity of thermally evaporated ZnTe thin films was studied at different elevated temperatures. A gap type cell configuration with Al electrodes on glass substrates was used. The conductivity was found to obey two distinct conduction mechanisms within the region of applied fields. At low fields the photoconduction is ohmic and at high fields it is of Poole-Frenkel type. With increase of ambient temperatures, the Poole-Frenkel conductivity regions were found to extend to lower fields. The temperature dependence of dark conductivity also was found to be of similar nature. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.     

Infra-red studies of vitreous silica at elevated temperatures

The purpose of this paper is to present a new approach to the problem of structure of the ordered regions in X-ray amorphous silica glass. The problem is open because of the differences in scientific opinion as to which polymorph of silica is the true crystalline model of vitreous silica. Infra-red absorption spectra of cristobalite and silica glass taken at temperatures ranging from room temperature to 500° C were used to identify a characteristic low-high temperature polymorphic transition relaxed in X-ray amorphous silica after heating at 210° C for at least 10 h. The infra-red spectrum of silica glass from various sources was identified as the spectrum of high-cristobalite, the spectrum after the polymorphic transition in silica glass is identical with that of low-cristobalite. The results give a strong support to the high-cristobalite/cubic/model of normal silica glass, with ordered regions in metastable form at room temperature.