Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 2, Refractories and Ceramics with Porosity Exceeding 30%

Effective thermophysical properties of ceramic materials (mainly insulating materials) with porosity (II) >30% are reviewed. Nonmonotonic pressure and temperature dependences of the effective thermal conductivity (X) are analyzed, based on the ceramic microstructure (pores, cracks, and grain boundaries present in many industrial refractories) and several heat-transfer mechanisms in composite multiphase materials. These mechanisms include heat conduction in solid and gas phases, thermal radiation, gas convection, and the mechanism originating from intrapore chemical conversion processes accompanied by gas emission. For high temperatures, λ of porous insulations is governed by thermal radiation. Contact-heat-barrier resistances play a less-important role in highly porous ceramics than in their dense counterparts. This underlies a weaker pressure dependence at low temperatures (<500°C) of λ of the majority of industrial insulating materials than in dense materials possessing microcracks and small pores in the grain-boundary region. For high gas pressure, λ of porous insulating materials is governed by free convective-gas motion. For low gas pressures (normally <1 kPa), where heat transfer in pores occurs in the free-molecular regime, X is controlled by the pressure-dependent mean free path of gas molecules in pores. A classification of the porous material structure and thermophysical properties is proposed, based on the geometric model described in Part 1 of this series.     

Gas Pressure and Temperature Dependences of Thermal Conductivity of Porous Ceramic Materials: Part 1, Refractories and Ceramics with Porosity below 30%

A review of experimental results and theoretical models for thermal conductivities of ceramic materials with porosity less than 30% is given. It is shown that the abnormal non-monotonic pressure and temperature dependences of thermal conductivity arise from the effects of microcracks and porous grain boundaries, characterizing many industrial refractories, and from the competitive influences of classical and novel mechanisms of heat transfer in composite multiphase materials. The latter mechanisms include segregation and surface diffusion of impurities and defects in crystal structure, and the mechanism arising from chemical conversion and gas emission, occurring within pores of ceramic materials. A fractal model of porous materials' structure is proposed and used for analysis, explanation, and classification of thermophysical properties of ceramic materials measured in various thermodynamic conditions.     

Effect of microporous aggregates and spinel powder on fracture behavior of magnesia-based refractories

The influences of microporous aggregates and spinel powder on the properties and fracture behavior of magnesia-based refractories were investigated by the three-point bending test and wedge splitting test with the digital image correlation method. With microporous aggregates instead of dense ones, lower thermal conductivity, higher cold modulus of rupture and compressive strength were observed for lightweight magnesia-based refractories. Besides, the results indicate that the strengthened interlocking interface between microporous aggregates and matrix in lightweight magnesia refractories decreased the proportion of crack propagation along the aggregate/matrix interface (P_AM). This reduced the tortuosity of crack propagation as well as increased the brittleness. With the addition of spinel powder in the matrix, the pregenerated microcracks by thermal mismatch increased the P_AM, which increased the tortuosity of crack propagation, improved fracture energy and reduced the brittleness. Lightweight magnesia spinel refractories merely showed a slightly higher brittleness than dense ones.     

Microcracking of high zirconia refractories after t → m phase transition during cooling: An EBSD study

High zirconia refractories are composed of a zirconia skeleton surrounded by an intergranular glassy phase. In these materials, zirconia undergoes up to two successive phase transitions during the manufacturing process, c → t then t → m. This leads, after complete cooling, to the formation of microcracks.Preliminary observations have enabled to identify the mechanism mostly responsible for the observed microcracking. In particular, SEM imaging emphasizes the link between the positions of cracks and the presence of distinct crystallographic domains.Thus, our work focuses on the arrangement of the monoclinic and tetragonal domains in zirconia dendrites. The assessment by XRD of the thermal expansion coefficients of zirconia at the lattice scale and the analysis of EBSD maps show that cracking is produced by the thermal expansion mismatch between groups of crystallographic variants. The further reconstruction of both cubic and tetragonal - in the case of a presence of monoclinic zirconia at room temperature - parent grains enables to determine the impact of each transition on the final microstructure and the generated microcracking.     

Thermal conductivity of hydrated silica-gel

The temperature of points inside a cylindrical packed bed of silica-gel grains is measured under steady-state and transient radial heat flow conditions, for different pressures and water contents of the silica. The stationary results are used to calculate the thermal conductivity of the silica packed beds. The transient results are used to find the thermal diffusivity of the beds using a numerical solution of the heat conduction equation. The conductivity of the particle material can be calculated using the Bauer and Schluender model for the heat transfer inside the granular bed. The thermal conductivity of the packed bed is found to increase with pressure at pressures below 10 kPa. The thermal conductivity of the bed, and of the solid particles, is found to vary linearly with the water content of the silica-gel.     

Adding combustible liquids to fireclay bodies to regulate the structure and increase spalling resistance

Conclusions A simple method has been discovered and investigated to control the structure of fireclay refractories with the aim of increasing their spalling resistance by creating microcracks on the surface of the grains of grog at the boundary with the bond.The development of microcracks is attained by first coating the grains of grog with a hydrophobic organic liquid (before mixing grog with bond clay). This increases the spalling resistance of fireclay refractories made of plastic and semidry bodies 1.5–2.0 times.When such additives are introduced into the bond part of the batch, the microcracks are situated, not around the grains, but in the main mass and are preferentially oriented parallel to each other. Under these conditions the spalling resistance of the material deteriorates.A reduction in the magnitudes of a number of properties (linear thermal expansion, elasticity, strength, Poisson Ratio, and thermal conductivity) takes place with both methods of treating fireclay bodies with organic liquids (covering the grog-grains or adding it to the bond). This suggests the prevailing positive influence of the structural nature (microcracks at the surface of the grains of grog) on the spalling resistance of the firebrick refractories.It is shown that with an increase in porosity of the fireclay refractories, their spalling resistance increases (to a given level) if the pores are localized chiefly at the surface of the grains of grog and bond; it is reduced when the pores are on the bonding mass of the refractory.     

Role of Interfacial Carbon layer in the Thermal Diffusivity/Conductivity of Silicon Carbide Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon-coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion-limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.     

Thermal insulation characteristics of roll forming alumina ball material

Rolling ceramic thermal insulation balls have advantages of low cost, large output and easy control of particle size, so it is likely to become the main raw material for 3D printing in the future, but there is little research on its thermal insulation. In this study, we used three kinds of rolling aluminum oxide balls as raw materials to obtain single-granularity-level and multi-granularity-level bulk materials. And the effects of temperature, particle size, and thermal fatigue times on the thermal conductivity of the samples were analyzed. Additionally, the experimental results were verified by FloEFD heat conduction simulation software using finite analysis method to analyze their heat conduction characteristics. With the increase of temperature from 400 °C to 1500 °C, the thermal conductivity of single-granularity-level and multi-granularity-level bulk materials increased linearly. The thermal conductivity of single-granularity-level bulk materials have no direct relationship with the particle size, and the thermal conductivity of multi-granularity-level materials with small particle size difference was a bit lower than that of materials with large particle size difference, and a bit higher than that of materials with single-granularity-level. The simulation results showed that the main reason for the above phenomenon was that the point contact between particles played a dominating role in the heat transfer process. When the contact area increased, the thermal conductivity increased obviously, and the thermal conductivity with the increasing of temperature decreased in a quadratic curve. The improved model considering the shrinkage could improve accuracy of simulation results. Heat flux at the surface contact area was 10.19 times higher than that of the point contact and 15.10 times higher than that of the solid-gas contact at 400 °C. Therefore, reducing the surface contact area and increasing the porosity could significantly reduce the thermal conductivity of the materials.     

Thermal Shock Resistance of Zirconia with 15 Mole % Titanium

The thermal shock resistance of zirconia with 15 mole % titanium prepared either by cold-pressing and vacuum-sintering or by vacuum hot-pressing was determined by radially quenching disks with thermally insulated faces under various conditions of heat transfer. The thermal shock resistance of calcia-stabilized zirconia disks was determined for comparison. For quenches from below the transformation temperature range of zirconia, the thermal shock resistance of zirconia with 15 mole % titanium was much better than that of calcia-stabilized zirconia, but for quenches from above the transformation range it was slightly inferior. The thermal shock resistance of zirconia with 15 mole % titanium is fairly insensitive to the methods of manufacture used in this investigation. In an attempt to identify the physical properties responsible for the improved thermal shock resistance of zirconia with 15 mole % titanium, the heat capacity, thermal expansion, modulus of rupture, modulus of elasticity, and thermal conductivity as functions of temperature were determined.     

Eulerian–Eulerian simulation of heat transfer between a gas–solid fluidized bed and an immersed tube-bank with horizontal tubes

A two dimensional Eulerian–Eulerian simulation of tube-to-bed heat transfer is carried out for a cold gas fluidized bed with immersed horizontal tubes. The horizontal tubes are modelled as obstacles with square cross section in the numerical model. Simulations are performed for two gas velocities exceeding the minimum fluidisation velocity by 0.2 and 0.6 m/s and two operating pressures of 0.1 and 1.6 MPa. Local instantaneous and time averaged heat transfer coefficients are monitored at four different positions around the tube and compared against experimental data reported in literature. The effect of constitutive equations for the solid phase thermal conductivity on heat transfer is investigated and a fundamental approach to modelling the solid phase thermal conductivity is implemented in the present work. Significant improvements in the agreement between the predicted and measured local instantaneous heat transfer coefficients are observed in the present study as compared to the previous works in which the local instantaneous heat transfer coefficients were overpredicted. The local time averaged heat transfer coefficients are within 20% of the measured values at the atmospheric pressure. In contrast, underprediction of the time averaged heat transfer coefficient is observed at the higher pressure.     

Über den Einfluss des Wärmeleitvermögens der Rohrwand auf den umfangsgemittelten Wärmeüberganskoeffizienten beim. Sieden im horizontalen VerdampferrohrThe influence of the circumferential heat conduction on the perimeter-averaged heat transfer coefficient at flow boiling in horizontal tubes

The different heat transfer coefficients of gas and liquid produce a temperature distribution along the perimeter in horizontal evaporator tubes. This temperature distribution, and thus the perimeter-averaged heat transfer coefficient is influenced considerably by the thermal conductivity, the wall thickness and the diameter of the tube. With measured local heat transfer coefficients and wetting limits at flow boiling of argon and nitrogen, perimeter-averaged heat transfer coefficients were calculated, taking as parameters the thermal conductivity, the wall thickness and the diameter of the tube. The results show that, by varying the values for these parameters, perimeter-averaged heat transfer coefficients can differ by several hundred per cent.     

Role of Interfacial Gaseous Heat Transfer in the Transverse Thermal Conductivity of a Uniaxial Carbon Fiber-Reinforced Aluminoborosilicate Glass

The transverse thermal conductivity of an aluminoborosilicate glass uniaxially reinforced with carbon fibers was found to be lower under near-vacuum than in nitrogen, whereas no such difference was found for the longitudinal thermal conductivity. This effect was attributed to the existence of an interfacial gap resulting from the thermal expansion mismatch between the matrix and fibers. The presence of this gap permits the gaseous environment access to the fiber-matrix interface and thereby contributes to the interfacial heat transfer. Its presence does not affect the longitudinal thermal conductivity, however, because the gap is aligned parallel to the fibers and, therefore, the direction of heat flow. Analysis of the experimental data indicates that, in nitrogen at atmospheric pressure, the gaseous conductance constitutes about one-third of the total interfacial conductance.     

Thermal conductivity of magnesite refractories in the range 500–1800°C

Conclusions An experimental setup was developed for studying the thermal conductivity of refractories up to 2300°C on the hot face of the specimen.In the average temperature range of 500–1800°C a study was made of the thermal conductivity of magnesite refractories of different porosity. The experimental data obtained satisfactorily agree with well-known literature and calculated values for the thermal conductivity coefficients.Translated from Ogneupory, No. 1, pp. 17–21, January, 1972.     

应用马丁─侯状态方程计算含氨气体混合物的物性参数

本文将马丁—侯状态方程用于计算气体混合物的恒压热容、粘度和导热系数．含氨气体混合物的恒压热容为理想气体混合物热容与真实气体混合物剩余热容之和，根据剩余粘度法计算粘度，剩余导热系数法计算导热系数．三种压力，不同温度条件下气体混合物恒压热容、粘度和导热系数的计算结果表明：计算值与实验值相吻合．并分别拟合了合成氨生产三种工艺含氨气体混合物恒压热容、粘度、导热系数的计算式。     

A study of the effect of structure on the thermal stability of fireclay refractories

Conclusions A study of the thermal stability of refractories has demonstrated a regularity in the change in thermal shock resistance with the reorientation of microcracks in the structure of the material.The values of the thermal stability of fireclay refractories, obtained by the disruptive temperature gradient method, are in qualitative agreement with the results of the commercial testing of bottom ware having an increased concentration of microcracks oriented along the boundaries of the coarse fireclay grains.Determination of the thermal stability of inhomogeneous refractories using known calculated criteria must be made by considering the structural properties of the material.Translated from Ogneupory, No.4, pp. 56–59, April, 1969.     

Thermal Expansion Coefficients of Unidirectional Fiber-Reinforced Ceramics

The influence of internal stresses, due to the thermomechanical mismatch between the fiber and the matrix, on the thermal expansion behavior of unidirectional fiber-reinforced ceramics is considered. Using the composite cylinder model, the effective thermal expansion coefficients of the composite are calculated from the total strains, which consist of the strains due to temperature changes and the strains induced by the presence of internal stresses. The results reveal that when the fiber and the matrix have the same elastic constants, the rule of mixtures approach can be used to obtain the thermal expansion coefficients of the composite, as observed in previous analytical solutions. Also, for the case of low volume fractions of fibers with Young's moduli much larger than those of matrices, and the thermal expansion coefficients lower than those of matrices, the transverse thermal expansion coefficient of the composite is higher than that of either the fiber or the matrix. However, unlike previous studies, the present analysis provides a physical basis for this phenomenon in terms of the internal thermal stress state within the composite.     

Characterisation and properties of low-conductivity microporous magnesia based aggregates with in-situ intergranular spinel phases

Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al₂O₃. Intergranular MgAl₂O₄ phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al₂O₃ presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m^?1 k^?1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.     

The use of drying experiments in the study of the effective thermal conductivity in a solid containing a multicomponent liquid mixture

The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures has been studied. To achieve this, the liquid composition, liquid content and temperature distributions have been measured in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side. A quasi-steady state reached at high source temperatures permits to calculate the total heat flux from temperatures measured on the surface and the gas stream. The simulations performed and compared with experimental data made it possible to estimate the adjusting geometric parameter of Krischer's model for the effective thermal conductivity. The effective thermal conductivity has been widely studied for two-phase systems, mostly with regard to thermal insulation elements. The calculation of this transport parameter includes the contribution to heat transfer of the evaporation–diffusion–condensation mechanism undergone by the multicomponent mixture. The influence of liquid composition and temperature on the thermal conductivity due to the evaporation–diffusion–condensation mechanism and the effective thermal conductivity is described. The results reveal that in this case the resistance to heat transfer seems to correspond to a parallel arrangement between the phases.     

Thermal expansion of ceramics containing aluminum titanate

Conclusions The investigation of the thermal expansion hysteresis facilitates, by relatively simple and available means, the obtaining of information about the presence in the ceramic material of active microcracks, that is, those capable of reverse collapse, and about their behavior during thermal cycling of the specimen. The presence of the expansion hysteresis in ceramics containing aluminum titanate exerts a considerable influence on their thermal behavior in working conditions. During thermal cycling of such ceramics in a relatively narrow temperature range (for example, from 700 to 900°C) no changes occur in the microcracked structure, and in this case the ceramic operates in the region of elastic deformations. However, with the expansion of the thermal cycling range brittle failure occurs in the intergrain bonds established at the higher temperature, which leads to the formation, after each cycle of heating and cooling, of a new system of microcracks, which reduce the strength of the bonds between the grains of the ceramic, and in the final account break it up.Translated from Ogneupory, No. 8, pp. 45–49, August, 1988.     

A phenomenological model for heat transfer in freeboard of fluidized beds

A phenomenological model was developed to predict heat transfer to tubes located in the freeboard region of gas fluidized beds. The model is concerned with the conductive/convective mechanism of heat transfer. For high temperature applications, an additional contribution by thermal radiation would need to be incorporated. The model considers that the tube surface experiences alternating contact with a dense emulsion phase and a lean void phase. Contributions by dense and lean phases are represented by transient conduction and convection mechanisms, respectively. Particletube contact information was obtained experimentally for a wide range of operating conditions at room temperature and pressure. Predictions of the model were compared with measured heat transfer coefficients. Over a 20-fold range in magnitudes of heat transfer coefficients, the model successfully predicted the measured values with an average deviation of 44 percent.