Lightweight porous silica ceramics with ultra-low thermal conductivity and enhanced compressive strength

In recent years, the need for robust thermal protection for reusable spacecraft and vehicles has spurred strong demand for high-performance lightweight thermal insulation materials that exhibit high strength. Herein, we report silica porous ceramics prepared via the direct foaming technique with lightweight, ultra-low thermal conductivity and enhanced compressive strength. Silica particles (particle size: 500 nm and 2 μm) were used as the raw materials. The nano-sized silica particles were easily sintered, thereby improving the compressive strength of the ceramics, whereas the micro-sized silica particles maintained the pore structure integrity without deformation. The addition of nano-silica enhanced the compressive strength by 764% (from 0.039 to 0.337 MPa). In addition, the thermal conductivity of the ceramics was as low as 0.039 W m^?1 K^?1. Owing to these outstanding characteristics, these porous silica ceramics are expected to be employed as thermal insulation material in diverse fields, especially aerospace and space where weight is an important constraint.     

Lightweight porous silica-alumina ceramics with ultra-low thermal conductivity

Thermal protection has always been an important issue in the energy, environment and aerospace fields. Porous ceramics produced by the particle-stabilized foaming method have become a competitive material for thermal protection because of their low density and low thermal conductivity. However, the study of porous ceramics for composite systems using particle-stabilized foaming method was relatively rare. Here, silica-alumina composite porous ceramics were prepared by particle-stabilized foaming method, which was achieved by tailoring the surface charges of silica and alumina through adjustment of the pH. Porous ceramics exhibited porosity as high as 97.49% and thermal conductivity (25 °C) as low as 0.063 W m^?1 K^?1. The compressive strength of porous ceramics sintered at 1500 °C with a solid content of 30 wt% could reach 0.765 MPa. Based on the light weight and excellent thermal insulation properties, the composite porous ceramic could be used as a potential thermal insulation material in the spacecraft industry.     

Theoretical and experimental analyses of relationship between processing and thermal conductivity of SiC with oxide additives

This paper analyzes theoretically and experimentally the thermal conductivity of the SiC-oxide additive-pore system. In the developed 6 model structures, the thermal conductivity of an SiC compact (κ_b) with oxide was calculated as functions of the volume fractions of SiC, oxide additive and pores. The calculated κ_b decreases in the order of a continuous phase where the other two particulate phases are dispersed: SiC>oxide additive>pores. The measured κ_b values of SiC compacts hot-pressed with 4–50 mass% oxide additive (mixture of 33.3 mass% Al₂O₃-33.3 mass% Y₂O₃-33.3 mass% SiO₂) were well explained by the calculated κ_b in two types of oxide continuous phase models. The thermal conductivities for only SiC grains in SiC compacts hot-pressed with 4 mass% Al₂O₃, Y₂O₃, SiO₂, Al₂O₃-Y₂O₃, Y₂O₃-SiO₂ and Al₂O₃-Y₂O₃-SiO₂ at 1950 °C were also estimated theoretically in the developed two model structures using the measured κ_b (oxide continuous phase model and SiC continuous phase model). Based on the calculated results, the following key factors are identified to achieve a high κ_b: (1) high sintered density, (2) a small amount of oxide additive with a high thermal conductivity, (3) no dissolution of foreign atoms from a liquid phase into SiC grains during solidification process.     

Theoretical and experimental analyses of thermal properties of porous polycrystalline mullite

This paper examined experimentally and theoretically the thermal diffusibility (α), heat capacity (C_P1) at a constant pressure (1 atm, 101.33 kPa) and thermal conductivity (κ=C_P1α) for the porous mullite ceramics with 0–55% porosity in a wide temperature range from 298 to 1073 K. The change in the κ values with temperature or porosity for the porous mullite was similar to the temperature dependence or porosity dependence of the α values, which were greatly reduced by the air included in the pores. The κ values for the porous mullite were theoretically analyzed with two model structures of pore–dispersed mullite continuous phase system (A model) and mullite–dispersed pore continuous phase system (B model). The measured κ values at 0–23% porosity agreed well with the κ values calculated for model A structure. In the high porosity range from 33% to 55%, the measured κ values deviated from the κ curve calculated for model A structure and approached the κ value curve for model B structure with increasing porosity. The real microstructure of 30–60% porosity is equivalent to the mixed microstructure of model A and model B for the thermal conductivity measurement.     

The relationship between electric processing condition and microstructure in the solidification of multicomponent oxides

Systematic characterization for the relationship between the electropulse processing conditions and microstructure has been carried out in the present work. It is found that electropulse can refine porosity and alter grain morphology considerably. A processing with optimum electropulsing parameters can reduce over 75% volume fraction of porosity and more than 70% average pore diameter in comparison with that of reference sample without electric treatment. Electric current treatment promotes the growth of dendrites with smaller thickness of primary arms dendrite and prevents the liquid entrapping between the growing solid grains. The former is caused by the effect of electricity-enhanced kinetic mobility on the radius of curvature at the tip of dendrite. The latter is attributed to the effects of electric thermodynamics on the microstructural formation such as the enhanced connectivity of conductive phase. The microstructures obtained by electropulsing treatment are favourable for heat conduction, structural strength and crack prohibition. However, the excess pulse frequency and pulse width can generate unwanted heat to counteract electric effect. The research reveals the relationship between electric processing conditions and microstructure in a perfectly controversial solidification condition in oxide materials to that of the metals and alloys. The results confirm from the opposite side the validity of the pulsated solidification.     

Modelling and evaluating thermal conductivity of porous thermal barrier coatings at elevated temperatures

Thermal conductivity of various porous thermal barrier coatings (TBCs) used at elevated temperatures for gas turbines has been evaluated using the proposed six-phase model. These TBCs rely on microstructural properties and yield different types of porosities. This paper studies the thermal conductivity of TBCs based on microstructural features to evaluate the effect of different types of porosities on thermal conductivity. The first part of this paper investigates the microstructural characterization of various TBCs using image analysis (IA) technique. The second part of this paper evaluates the thermal conductivity using the image analysis. The volumetric fraction of porosities along with their orientation, shape and morphology, shows a considerable impact on the overall thermal conductivity of TBCs. The proposed six-phase model can predict thermal conductivity of porous TBCs with a good agreement with the measured values. The model results can help to better understand the effect of microstructural changes on thermal conductivity and can provide useful guide to fabricate TBCs with low thermal conductivity.     

Production of anorthite refractory insulating firebrick from mixtures of clay and recycled paper waste with sawdust addition

Production of porous anorthite refractory insulating firebricks from mixtures of two different clays (K244 clay and fireclay), recycled paper processing waste and sawdust addition are investigated. Suitability of alkali-containing-clay, low-alkali fireclay, pore-making paper waste and sawdust in the products was evaluated. Prepared slurry mixtures were shaped, dried and fired. Highly porous anorthite ceramics from the mixtures with up to 30% sawdust addition were successfully produced. Physical properties such as bulk density, apparent porosity, percent linear change were investigated as well as the mechanical strengths and thermal conductivity values of the samples. Thermal conductivities of the samples produced from fireclay and recycled paper waste decreased from 0.25 W/mK (1.12 g/cm³) to 0.13 W/mK (0.64 g/cm³) with decreasing density. Samples were stable at high temperatures up to 1100 °C, and their cold strength was sufficiently high. The porous anorthite ceramics produced in this study can be used for insulation in high temperature applications.     

Analyses of Young's modulus and thermal expansion coefficient of sintered porous alumina compacts

This paper reports the derivation of Young's modulus (E) and thermal expansion coefficient (TEC, β) for a sintered porous structure with open pores. The theoretical E is affected by the number of grains and the grain boundary area in sintered ceramics. The measured E–porosity relationship for porous alumina compacts were compared with the theoretical E values that were derived for the present open-pore structure and also for the dispersed (closed)-pore structure treated previously. With decreasing porosity (50% → 10%), the scattered E values showed a gradually increasing tendency, which were located between two theoretical curves for the open-pore structure. The sudden increase of E values in the porosity range from 10% to 0% was well explained by the theoretical dependence of E on porosity for the open- or closed-pore structure. The β values for the porous alumina structures were independent of porosity and close to the β values reported for fully dense alumina compacts. This result was in accordance with the theoretical β–porosity relationships for the open-pore and closed-pore structures.     

Processing and properties of silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity

Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity were prepared by sintering nano-SiC powder-carbon black template compacts at 600–1200 °C for 2 h in air. The microstructure of the silica-bonded porous nano-SiC ceramics consisted of SiC core/silica shell particles, a silica bonding phase, and hierarchical (meso/macro) pores. The porosity and thermal conductivity of the silica-bonded porous nano-SiC ceramics can be controlled in the ranges of 8.5–70.2 % and 0.057–2.575 Wm⁻¹ K⁻¹, respectively, by adjusting both, the sintering temperature and template content. Silica-bonded porous nano-SiC ceramics with extremely low thermal conductivity (0.057 Wm⁻¹ K⁻¹) were developed at a very low processing temperature (600 °C). The typical porosity, average pore size, compressive strength, and specific compressive strength of the porous nano-SiC ceramics were ∼70 %, 50 nm, 2.5 MPa, and 2.7 MPa·cm³/g, respectively. The silica-bonded porous nano-SiC ceramics were thermally stable up to 1000 °C in both air and argon atmospheres.     

Theoretical consideration of the effect of porosity on thermal conductivity of porous materials

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size R_p to the phonon mean free path for zero-porosity, l₀. The thermal conductivity for l₀/R_p > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l₀/R_p < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.     

Improvement of uncooled infrared imaging detector by using mesoporous silica as a thermal isolation layer

Thermal-insulation properties of ordered mesoporous silica could possibly improve the efficiency of uncooled infrared imaging detector by introducing a mesoporous silica layer between TiN absorber and Si₃N₄ passivation layer. IR absorption rate of 50% porosity mesoporous silica was simulated and the results showed that above 90% infrared absorptance could be achieved in 8–12 μm wavelength region. Thermal conductivity of mesoporous silica was measured by 3ω method. Finite element modeling was used to simulate thermal isolation effect of mesoporous silica inserted structure. The result shows that an effective thermal-insulation effect could be obtained with 150 nm thick mesoporous silica layer. Also according to the simulation result of time transient temperature variation, it was found that mesoporous silica layer for thermal isolation induced an increase of 30% of residual temperature on TiN absorber. The mesoporous silica film was found to be a good thermal isolation layer for microbolometer.     

Conductivity of porous materials with spheroidal pores

Models predicting the conductivity of porous materials with spheroidal insulating pores are summarized and a new model, based on our exponential relation, is proposed. Using the well-known single-inclusion solution for spheroids, Maxwell coefficients (“intrinsic conductivities”) are calculated in dependence of the pore aspect ratio for isotropic microstructures with randomly oriented spheroidal pores, and implemented into the three traditional effective medium approximations (Maxwell-type, self-consistent, differential) and our exponential relation. As expected, all models predict that prolate pore shape has a very small influence on the porosity dependence, while oblate pores affect the porosity dependence of conductivity significantly. However, the self-consistent predictions are linear and imply spurious percolation thresholds, whereas Maxwell-type and differential models (power-law relations) are known to provide predictions that are unrealistically high for the special case of spherical pore shape. Thus, our exponential relation seems to be currently the most suitable relation for implementing the single-inclusion solution for spheroids.     

Application of grits waste as a renewable carbonate material in manufacturing wall tiles

This work focuses on the reuse of grits waste, from cellulose industry, as a raw material to replace traditional carbonate material in ceramic wall tiles. Wall tile formulations bearing up to 15?wt% of the grits waste were prepared for replacement of calcareous. The tile manufacturing route consisted of dry powder granulation, uniaxial pressing, and firing at temperatures ranging from 1100?°C to 1180?°C by using a fast-firing cycle. The wall tile specimens were tested to determine their physical and mechanical properties (linear shrinkage, water absorption, apparent porosity, apparent density, breaking strength, and flexural strength). The firing behavior, phase transformations, and microstructure were evaluated by dilatometry, XRD, and SEM. The results showed that the fired wall tile specimens are composed of anorthite and quartz, as major mineral phases, and mullite as a minor phase. It was found that the grits waste had a positive influence on the properties and microstructure of the wall tile specimens. The results also revealed that the grits waste from cellulose industry could be used as a total replacement of traditional calcareous material in wall tile formulations.     

Thermal conductivity studies on Si/SiC ceramic composites

Ceramic heat exchangers are increasingly used in many nuclear power plants. Silicon carbide has been treated as a promising material for heat exchanger application since it has good thermal conductivity and corrosion resistance. In this work, four different types of Si/SiC ceramic composites were prepared by liquid silicon infiltration technique. Thermal conductivities of these ceramic composites at different temperatures are measured by the laser flash thermal conductivity method. Results show that the presence of free carbon and voids are notably affecting the thermal conductivity of these materials.     

Gel-combustion, characterization and processing of porous Ni-YSZ cermet for anodes of solid oxide fuel cells (SOFCs)

The NiO-YSZ powder was prepared by the combustion of nitrate-glycine gel, a novel and versatile technique to form the nano-composite. The phase purity of the as-formed powder was confirmed by XRD. Onset sintering temperature was measured and sintering of NiO-YSZ specimens was carried out at 1450 °C for different soaking times. Porosity of the specimens decreases with increasing soaking time. Ni-YSZ cermet with varying open porosities between 23 and 41% was obtained by the in situ reduction of NiO-YSZ. Electrical conductivity, thermal expansion coefficient, and SEM microstructure of the Ni-YSZ specimens possessing varying porosities were evaluated. The electrical conductivity decreased while thermal expansion coefficient remained essentially the same with increasing porosity. SEM results confirmed the presence of interconnected open pores.     

Representation of thermal conductivity of solid material with particulate inclusion

This paper reviewed the previously proposed models of thermal conductivity (κ) for a series and parallel flow of energy input in laminated composites. These two models were coupled to derive the thermal conductivity (κ_a) of material with simple cubic particulate inclusion. The derived equation depends upon κ₁ of inclusion, κ₂ of a continuous phase and volume fraction of inclusion. The size and shape factors of inclusion are cancelled during the derivation of κ_a. The newly constructed κ_a equation explains well the measured κ_a for AlN particle-dispersed SiO₂ system.     

Improvement on the phase stability,mechanical properties and thermal insulation of Y2O3-stabilized ZrO2 by Gd2O3 and Yb2O3 co-doping

Gd₂O₃ and Yb₂O₃ co-doped 3.5 mol% Y₂O₃–ZrO₂ and conventional 3.5 mol% Y₂O₃–ZrO₂ (YSZ) powders were synthesized by solid state reaction. The objective of this study was to improve the phase stability, mechanical properties and thermal insulation of YSZ. After heat treatment at 1500 °C for 10 h, 1 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (1Gd1Yb-YSZ) had higher resistance to destabilization of metastable tetragonal phase than YSZ. The hardness of 5 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (5Gd1Yb-YSZ) was higher than that of YSZ. Compared with YSZ, 1Gd1Yb-YSZ and 5Gd1Yb-YSZ exhibited lower thermal conductivity and shorter phonon mean free path. At 1300 °C, the thermal conductivity of 5Gd1Yb-YSZ was 1.23 W/m K, nearly 25% lower than that of YSZ (1.62 W/m K). Gd₂O₃ and Yb₂O₃ co-doped YSZ can be explored as a candidate material for thermal barrier coating applications.     

Effect of starch addition on microstructure and properties of highly porous alumina ceramics

Porous alumina ceramics with ultra-high porosity were prepared through combining the gel-casting process with the pore-forming agent technique. Porosity and pore size distribution of the sintered bulks were evaluated with and without adding starch, respectively. In particular, the influences of starch addition on the properties, including thermal conductivity and compressive strength were studied. It was found that the incorporation of starch increased the nominal solid loading in the suspension and subsequently promoted the particle packing efficiency. The porosity is raised with increasing starch content from 0 to 30 vol%, which brings the decrease in thermal conductivity, whereas the compressive strength isn't seriously degraded. The further higher starch addition (40 vol%), however, would deteriorate the performance of the alumina porous ceramics. It is believed that the appropriate starch amount (lower than 30 vol%), working as a pore-forming agent, suppresses the driving force of densification without affecting the connections of neighboring grains while excessive starch amount would lead to the collapse of the porous structure.     

Influence of ZrO2 and WO3 doping additives on the thermal properties of porous mullite ceramics

Porous mullite-corundum refractory ceramics were produced by a patented slurry slip casting method from compositions based on commercially available α-Al₂O₃ and γ-Al₂O₃, fused SiO₂ and kaolin. Pores were formed as a result of a chemical reaction of aluminium with water. The influence of usage of raw materials and doping additives such as micro-size ZrO₂ and WO₃ on the sintering temperature, formation of crystalline phases, linear thermal expansion, thermal conductivity and thermal shock resistance of mullite-corundum ceramic was studied. The best thermal shock resistance and, simultaneously, lower thermal conductivity was achieved for the samples doped with WO₃. This was due to the influence of micro-sized WO₃ on the change in γ-Al₂O₃ modification to α-Al₂O₃ and on the structure of mullite ceramics.     

Thermal and tensile properties of polysialate composites

Polysialate, or geopolymer, composites have gained interest due to their inherent high temperature resistance, low density and ease of manufacturing. These characteristics also suggest that polysialate composites have significant potential as materials in high temperature structures, although little is known about their thermal and mechanical properties. This study aimed to determine relevant thermal and mechanical properties over a representative temperature range. The results show that polysialate composites can exhibit stable thermal properties up to 1000 °C. Tensile properties up to 760 °C highlight a significant reduction in stiffness, but a retention of strength, at these temperatures. The thermal and mechanical results achieved provide strong evidence that polysialate composites can be suitable for use in high temperature structures, whilst subsequently providing an understanding of their limitations. In addition to this, the values ascertained also provide the data required for the design and modelling of next generation high temperature structures.