Determination of Heat Conductivity and Thermal Diffusivity of Waste Glass Melter Feed: Extension to High Temperatures

The heat conductivity (λ) and the thermal diffusivity (a) of reacting glass batch, or melter feed, control the heat flux into and within the cold cap, a layer of reacting material floating on the pool of molten glass in an all‐electric continuous waste glass melter. After previously estimating λ of melter feed at temperatures up to 680°C, we focus in this work on the λ(T) function at T > 680°C, at which the feed material becomes foamy. We used a customized experimental setup consisting of a large cylindrical crucible with an assembly of thermocouples, which monitored the evolution of the temperature field while the crucible with feed was heated at a constant rate from room temperature up to 1100°C. Approximating measured temperature profiles by polynomial functions, we used the energy equation to estimate the λ(T) approximation function, which we subsequently optimized using the finite‐volume method combined with least‐squares analysis. The heat conductivity increased as the temperature increased until the feed began to expand into foam, at which point the conductivity dropped. It began to increase again as the foam turned into a bubble‐free glassmelt. We discuss the implications of this behavior for the mathematical modeling of the cold cap.     

Thermal Diffusivity/Conductivity and Specific Heat of Mullite-Zirconia-Silicon Carbide Whisker Composites

The thermal diffusivity of mullite-ZrO₂-SiC_w materials was determined from 25° to 1000°C for composites in which the ZrO₂ had different amounts of monoclinic and tetragonal phase, and varying SiC _w content. At 25°C the thermal diffusivity of the matrix materials increased with increasing amounts of monoclinic phase, and little or no anisotropy was seen. Composites with SiC _w additions showed significant increases in thermal diffusivity and anisotropy as compared with the matrix materials. With increasing temperature, thermal diffusivities of the matrices and their respective composites decreased, and at 1000°C differences were small. Specific heat was determined from 25° to 700°C and thermal conductivity values calculated. Specific heat increased with temperature and was not composition dependent, except for one sample. Thermal conductivity values as a function of temperature followed the same trends as the thermal diffusivity values.     

Transient Hot-Strip Method for Simultaneous Determination of Thermal Conductivity and Thermal Diffusivity of Refractory Materials

The transient hot-strip method has been used for determining thermal conductivity and thermal diffusivity of a refractory brick. The technique is based on recording the transient temperature increase of a 25-μm-thick, 8-mm-wide, and 70-mm-long iron strip clamped between two sample halves and heated with a constant direct current. The method is accurate to within 5% in the thermal conductivity and to within 10% in the thermal diffusivity in the temperature range 20° to 700°C.     

Thermal Diffusivity/Conductivity of Alumina—Silicon Carbide Composites

Thermal diffusivity and conductivity values for several Al₂O₃-SiC whisker composites were determined. The thermal diffusivity values spanned the range from 373 to 1473 K, and thermal conductivity data wre obtained between 305 and 365 K. The thermal diffusivity decreased with increasing temperature and increased with SiC-whisker content. An estimate of the thermal conductivity of the whiskers was obtained from the direct thermal conductivity measurements, but attempts to derive whisker conductivity values from the thermal diffusivity data were not successful because the laser flash method lacks the required accuracy and precision. Specimens were subjected to two different thermal quench experiments to investigate the effect of thermal history on diffusivity. In the most severe case, multiple 1073- to 373-K quenches, radial cracks were observed in the test specimens; however, there was no change in diffusivity. The lack of sensitivity to thermal cycling appears to be related to the sample size.     

Phase Transformation of Metastable ZnSnO3 Upon Thermal Decomposition by In‐Situ Temperature‐Dependent Raman Spectroscopy

Temperature‐dependent in‐situ Raman spectroscopy is used to investigate the phase transformation of zinc metastannate (ZnSnO₃) to zinc orthostannate (Zn₂SnO₄) induced upon annealing in the ambient. ZnSnO₃ microcubes (MCs) were synthesized at room temperature using a simple aqueous synthesis process, followed by characterization using electron microscopy, X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). Annealing of the ZnSnO₃ MCs was carried out up to 1000°C, while recording the Raman spectra in‐situ at regular intervals. Phase transformation from metastannate to orthostannate was found to begin around 500°C with an activation energy of ~0.965 eV followed by the recrystallization into the inverse spinel orthostannate phase at ~750°C. Results from this study provide detailed understanding of the phase transformation behavior of perovskite ZnSnO₃ to inverse spinel Zn₂SnO₄ upon thermal annealing.     

Simultaneous Determination of Thermal Conductivity and Specific Heat for Refractory Materials

The present work describes a transient method for determining thermal properties of refractory materials. Specific to this technique are (1) the experimental measurement of heat flux on the heated surface and (2) a method of data analysis based on the numerical techniques of nonlinear least squares and finite difference. Several mathematical formulations are proposed to minimize computing time requirements. Silica molding sand is used in two examples to illustrate the applicability of the technique to refractory materials. Owing to the measurement of heat flux on the heated surface, both thermal conductivity k and volumetric specific heat C can be determined if their temperature dependencies are not considered. In the case when temperature-dependent thermal properties are considered, it is found that the inherent accuracy of the experiment does not allow the determination of both temperature-variable k and C at the same time. It is shown that the present method has advantages in the ease and directness of the required experiment and in greater computational efficiency. Thermal properties are reported for a bonded silica and an unbonded silica sand.     

Temperature‐ and Atmosphere‐Dependent Defect Chemistry Model of SnO2 Nanocrystalline Film

By combining the concept of defect chemistry and the small‐polaron hopping conduction model, the present work takes an intensively considering of the electron conduction mechanism in the nonstoichiometric SnO₂ nanocrystalline film. The temperature‐dependent and atmosphere‐dependent relationship between the electrical conductivity and the defect reaction is outlined. To investigate the influence of temperature and atmosphere on the electrical properties of the SnO₂ nanocrystalline film, a temperature‐programmed system integrated with the high‐throughput screening platform of gas‐sensing materials (HTSP‐GM) is developed as the test tool in this work. With this platform, the temperature‐dependent conductivity of SnO₂ nanocrystalline film in different atmosphere (dry air, nitrogen, and formaldehyde) was conducted. A good fit between the theoretical deductions and experimental results is achieved.     

Thermal Conductivity/Diffusivity of Silicon Carbide Whisker Reinforced Mullite

The thermal conductivity and diffusivity of silicon carbide whisker reinforced mullite was shown to increase with whisker content. This effect was much greater for vapor-liquid-solid (VLS) whiskers than for rice-hull (RH) whiskers. This suggests that the thermal conductivity for the VLS whiskers was significantly higher than for the RH whiskers. Due to preferred orientation of the whiskers, thermal conductivity and diffusivity of the composite samples exhibited significant anisotropy.     

Melter Feed Reactions at T ≤ 700°C for Nuclear Waste Vitrification

To understand feed‐to‐glass conversion for the vitrification of nuclear waste, we investigated batch reactions and phase transitions in a simulated nuclear waste glass melter feed heated at 5 K/min up to 700°C using optical microscopy, scanning electron microscopy with energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. To determine the content and composition of leachable phases, we performed leaching tests; the leachates were analyzed by inductively coupled plasma atomic emission spectroscopy. By 400°C, gibbsite and sodium borates lost water and converted to amorphous phase, whereas other metallic hydroxides dehydrated to oxides. Between 400°C and 700°C, carbonates decomposed before 500°C; amorphous aluminum oxide and calcium oxide reacted with the sodium borate and formed the more durable amorphous borate phase along with intermediate crystalline products; above 500°C, quartz began to dissolve, and hematite started to convert to trevorite.     

Strain‐Dependent Plasticity Evolution of Window Glass

How the applied strain can affect the plasticity evolution of window glass was systematically explored through a series of nanoindentations with various sharp indenters. It was revealed that, as the strain increases, the contribution of shear flow to total plasticity becomes larger, whereas that of densification gets smaller. The results are discussed in terms of the sequence in which each mechanism plays and the detailed mechanism of shear flow.     

A Method of Determination of Concentration Dependent Effective Moisture Diffusivity

ABSTRACT

A method based on Fourier series solution to Fick's diffusion equation has been proposed to evaluate effective diffusivity (D) as a function of moisture content in agricultural materials undergoing shrinkage during drying process. The shrinkage kinetics of the particulate was used to correlate its instantaneous size (spherical equivalent diameter) as a function of material moisture content A computer program was used to evaluate D based on shrinkage kinetics and experimental drying data and relate it to moisture content. The method was used to obtain moisture diffusivity data for thin layer drying of grape and corn.     

Temperature Dependence of the Thermal Diffusivity/Conductivity of Aluminum Nitride

The temperature dependence of the thermal conductivity of a polycrystalline AIN was measured using the flash-diffusivity technique over a range of experimental conditions. Thermal conductivity data from room temperature to 300°C, obtained with attenuated laser pulse energies to minimize the specimen temperature increase, were found to be inversely proportional to the absolute temperature, as expected from theory. For high pulse energies, the experimental data deviated significantly from expected behavior. This latter effect is offered as an explanation for the anomalously low temperature dependence for the thermal conductivity reported in the literature.     

Arc-Imaging Technique for Measuring High-Temperature Thermal Conductivity and Diffusivity of Refractory Oxides

An imaging technique was developed for measuring the thermal conductivity and diffusivity of small disks of refractory oxides at temperatures ranging from 1500°K to the melting point of the material. One face of the sample is exposed to the radiant heat flux of an arc-imaging furnace. A shutter instantaneously cuts off incident radiation, and the cooling of the specimen is recorded with a motion-picture camera. The flux re-radiated by the nonirradiated face of the sample is measured with a thermopile. Thermal conductivity and temperatures in the bulk of the sample are determined by computational analysis. Experimental data are fitted to computed cooling-temperature curves to determine thermal diffusivity. This technique was applied to pure and stabilized ZrO₂, Y₂O₃, and ThO₂.     

Thermal Diffusivity/Conductivity of Magnesium Oxide/Silicon Carbide Composites

SiC-particle-reinforced MgO composites have been fabricated by hot pressing, and the thermal diffusivities of the composites measured in the temperature range 200–1000°C using a laser flash technique. The thermal conductivity of the composites was calculated by multiplying the diffusivity with density and with heat capacity. The Eshelby inclusion model has been examined, and an equation suitable for particulate composites with porosity has been derived using the multiphase Eshelby model. The model also considers the interfacial thermal condition. Good agreement was obtained between the predictions and the experimental results of the thermal conductivity of the composites, even for various levels of porosity in the composites. Crystal defects, observed in the composites, influenced the thermal conductivity, resulting in a deviation from isothermal interfacial condition. This was reflected in the interfacial thermal parameter,β used in the modeling, and the predicted value of β was in the range of 3–10, depending on the thermal conductivity of SiC used for the calculations.     

A Method for Determining Bulk Density,Material Density,and Porosity of Melter Feed During Nuclear Waste Vitrification

Glassmelting efficiency largely depends on heat transfer to reacting glass batch (melter feed), which in turn is influenced by the bulk density (ρ_b) and porosity (?) of the reacting feed as functions of temperature (T). Neither ρ_b(T) nor ?(T) functions are readily accessible from direct measurements. For the determination of ρ_b, we monitored the profile area of heated feed pellets and calculated the pellet volume using numerical integration. For the determination of ?, we measured the material density of feeds quenched at various stages of conversion via pycnometry and then computed the feed density at heat‐treatment temperature using thermal expansion values of basic feed constituents.     

Specimen Size Effect of the Thermal Diffusivity/Conductivity of Aluminum Nitride

It has been generally found that the thermal diffusivity/conductivity of AIN measured by the laser-flash technique decreases with decreasing specimen size. The results of this study indicate that much of this effect can be attributed to the relatively large temperature rises for thin specimens, especially for high-energy laser pulses, and results from the combination of the noninearity of the commonly used type of IR detector and the strongly negative temperature dependence of the thermal diffusivity of AIN. The experimental results indicate that for reliable data, the specimen temperature rise at thermal equilirium should be kept to less than 1°C by using specimens with a thickness near 3 mm in combination with keeping the pulse energy to a reasonable minimum by attenuation of the laser beam by passing it through an aqueous solution of CuSO₄.     

Neutron Diffraction Measurements and Micromechanical Modelling of Temperature‐Dependent Variations in TATB Lattice Parameters

Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic‐bonded explosive (PBX) formulations. TATB‐based explosives exhibit irreversible volume expansion (“ratchet growth”) when thermally cycled. A theoretical understanding of the relationship between anisotropy of the crystal, crystal orientation distribution (texture) of polycrystalline aggregates, and the intergranular interactions leading to this irreversible growth is necessary to accurately develop physics‐based predictive models for TATB‐based PBXs under various thermal environments. In this work, TATB lattice parameters were measured using neutron diffraction during thermal cycling of loose powder and a pressed pellet. The measured lattice parameters help clarify conflicting reports in the literature as these new results are more consistent with one set of previous results than another. The lattice parameters of pressed TATB were also measured as a function of temperature, showing some differences from the powder. This data is used along with anisotropic single‐crystal stiffness moduli reported in the literature to model the nominal stresses associated with intergranular constraints during thermal expansion. The texture of both specimens were characterized and the pressed pellet exhibits preferential orientation of (001) poles along the pressing direction, whereas no preferred orientation was found for the loose powder. Finally, thermal strains for single‐crystal TATB computed from lattice parameter data for the powder is input to a self‐consistent micromechanical model, which predicts the lattice parameters of the constrained TATB crystals within the pellet. The agreement of these model results with the diffraction data obtained from the pellet is discussed along with future directions of research.     

热管技术在玻璃熔窑尾气余热回收中的应用

热管技术是一门新型传热技术,它是一种利用封闭在管内的物质反复进行物理相变(蒸发与凝结)来传递热量的一种高效能传热元件,以热管元件制成的换热器称为热管换热器,主要形式有蒸汽发生器、水加热器、空气预热器等。从20世纪80年代开始,热管技术在中国逐渐推广应用,20余年来,得到迅猛发展。现已广泛应用于石化、化肥、冶金、电力等行业余热利用,为相关企业带来了可观经济效益。     

Temperature‐Dependent Deformation and Dislocation Density in SrTiO3 (001) Single Crystals

This study evaluates the change of flow stress as related to dislocation density in SrTiO₃ single crystals in order to provide guidance for later electrical studies. The key parameters varied are temperature and loading rate during the deformation. It is found that in <100>‐oriented SrTiO₃ single crystals, the dislocation density is enhanced by plastic deformation, more so at higher temperature as compared to room temperature. The experimental approach of quantifying the dislocation density through a determination of ex situ X‐ray diffraction rocking curves was successfully applied over the upper temperatures region of the lower temperature ductility zone for strontium titanate, i.e., in the so‐called “A‐regime”. For 1.0% deformed samples deformed at 300°C, a fourfold increase in dislocation density to 1.4 × 10¹³ m^?1 was found as compared to the nondeformed state (3.7 × 10¹² m^?1). Cross‐section techniques confirmed that the observed dislocation densities measured at the surfaces were identical to those seen in the core of the crystals. The use of rapid changes in loading rate provided an estimate for activation volume of the dislocation core for both 25°C and 300°C.