Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam-gelcasting

Excessive sintering shrinkage leads to severe deformation and cracking, affecting the microstructure and properties of porous ceramics. Therefore, reducing sintering shrinkage and achieving near-net-size forming is one of the effective ways to prepare high-performance porous ceramics. Herein, low-shrinkage porous mullite ceramics were prepared by foam-gelcasting using kyanite as raw material and aluminum fluoride (AlF₃) as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF₃ content on the shrinkage, porosity, compressive strength, and thermal conductivity of mullite-based porous ceramics were investigated. The results showed that with the increase of content, the sintering shrinkage decreased, the porosity increased, and mullite whiskers were produced. Porous mullite ceramics with 30 wt% AlF₃ content exhibited a whisker structure with the lowest shrinkage of 3.5%, porosity of 85.2%, compressive strength of 3.06 ± 0.51 MPa, and thermal conductivity of 0.23 W/(m·K) at room temperature. The temperature difference between the front and back sides of the sample reached 710°C under high temperature fire resistance test. The low sintering shrinkage preparation process effectively reduces the subsequent processing cost, which is significant for the preparation of high-performance porous ceramics.     

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.     

Facile processing of oriented macro-porous ceramics with high strength and low thermal conductivity

Unidirectionally oriented architectures demonstrate a notable efficiency in enhancing the properties of macro-porous materials, yet are difficult to construct in a time- and cost-effective fashion. Here a facile approach was exploited for fabricating oriented macro-porous ceramic materials by employing natural graphite flakes as a fugitive material and preferentially aligning the flakes within ceramic matrices using accumulative rolling technique. Flaky to near-ellipsoid shaped pores with a homogeneous distribution were created in macro-porous zirconia ceramics with their porosity and microstructural characteristics adjustable by controlling the additive amounts of graphite flakes. The resulting materials exhibited a good combination of properties with high compressive strength up to over 1.5 GPa, which exceeds those of most other porous zirconia ceramics with similar porosities, along with low thermal conductivity of 0.92–1.85 Wm^?1·K^?1. This study offers a simple means for developing new oriented macro-porous materials with enhanced properties, and may promote their application by allowing for easy mass production.     

Porous anorthite ceramics with ultra-low thermal conductivity

Porous anorthite ceramics with an ultra-low thermal conductivity of 0.018 W/m K have been fabricated by hydrous foam-gelcasting process and pressureless sintering method using γ-alumina, calcium carbonate and silica powders as raw materials. Microstructure and phase composition were analyzed by SEM and XRD respectively. Properties such as porosity, pore size distribution and thermal conductivity were measured. High porosity (69–91%) and low thermal conductivity (0.018–0.13 W/m K) were obtained after sintering samples with different catalyst additions at 1300–1450 °C. Porosity, pore size, pore structure and grain size had obvious effect on heat conduction, resulting in the low thermal conductivity. The experimental thermal conductivity data of porous anorthite ceramics were found to be fit well with the computed values derived from a universal model.     

Effects of porosity on electrical and thermal conductivities of porous SiC ceramics

The effects of porosity on the electrical and thermal conductivities of porous SiC ceramics, containing Y₂O₃–AlN additives, were investigated. The porosity of the porous SiC ceramic could be controlled in the range of 28–64 % by adjusting the sacrificial template (polymer microbead) content (0–30 wt%) and sintering temperature (1800–2000 °C). Both electrical and thermal conductivities of the porous SiC ceramics decreased, from 7.7 to 1.7 Ω⁻¹ cm⁻¹ and from 37.9 to 5.8 W/(m·K), respectively, with the increase in porosity from 30 to 63 %. The porous SiC ceramic with a coarser microstructure exhibited higher electrical and thermal conductivities than those of the ceramic with a finer microstructure at the equivalent porosity because of the smaller number of grain boundaries per unit volume. The decoupling of the electrical conductivity from the thermal conductivity was possible to some extent by adjusting the sintering temperature, i.e., microstructure, of the porous SiC ceramic.     

Fabrication of high thermal conductivity silicon nitride ceramics by pressureless sintering with MgO and Y2O3 as sintering additives

The fabrication of silicon nitride (Si₃N₄) ceramics with a high thermal conductivity was investigated by pressureless sintering at 1800 °C for 4 h in a nitrogen atmosphere with MgO and Y₂O₃ as sintering additives. The phase compositions, relative densities, microstructures, and thermal conductivities of the obtained Si₃N₄ ceramics were investigated systemically. It was found that at the optimal MgO/Y₂O₃ ratio of 3/6, the relative density and thermal conductivity of the obtained Si₃N₄ ceramic doped with 9 wt% sintering aids reached 98.2% and 71.51 W/(m·K), respectively. EDS element mapping showed the distributions of yttrium, magnesium and oxygen elements. The Si₃N₄ ceramics containing rod-like grains and grain boundaries were fabricated by focused ion beam technique. TEM observations revealed that magnesium existed as an amorphous phase and that yttrium produced a new secondary phase.     

Investigation on thermal conductivity and mechanical properties of Si3N4 ceramics via one-step sintering

High thermal conductivity Si₃N₄ ceramics were fabricated using a one-step method consisting of reaction-bonded Si₃N₄ (RBSN) and post-sintering. The influence of Si content on nitridation rate, β/(α+β) phase rate, thermal conductivity and mechanical properties was investigated in this work. It is of special interest to note that the thermal conductivity showed a tendency to increase first and then decrease with increasing Si content. This experimental result shows that the optimal thermal conductivity and fracture toughness were obtained to be 66 W (m K)^-1 and 12.0 MPa m^1/2, respectively. As a comparison, the nitridation rate and β/(α+β) phase rate in a static pressure nitriding system, i.e., 97% (MS10), 97% (MS15), 97% (MS20) and 8.3% (MS10), 8.3% (MS15), 8.9% (MS20), respectively, have obvious advantages over those in a flowing nitriding system, i.e., 91% (MS10), 91% (MS15), 93% (MS20) and 3.1% (MS10), 3.3% (MS15), 3.3% (MS20), respectively. Moreover, high lattice integrity of the β-Si₃N₄ phase was observed, which can effectively confine O atoms into the β-Si₃N₄ lattice using MgO as a sintering additive. This result indicates that one-step sintering can provide a new route to prepare Si₃N₄ ceramics with a good combination of thermal conductivity and mechanical properties.     

Fabrication of Si3N4 ceramics with high thermal conductivity and flexural strength via novel two-step gas-pressure sintering

Si₃N₄ ceramic substrates serving as heat dissipater and supporting component are required to have excellent thermal and mechanical properties. To prepare Si₃N₄ with desirable properties, a novel two-step gas-pressure sintering route including a pre-sintering step followed by a high-temperature sintering step was devised. The effects of pre-sintering temperature (1500 – 1600 °C) on the phase transformation, microstructure, thermal and mechanical properties of the samples were studied. The pre-sintering temperature played an important role in adjusting the Si₃N₄ particles’ rearrangement and α→β transformation rate. Furthermore, the densification process for the Si₃N₄ ceramics prepared via the two-step gas-pressure sintering was revealed. After sintered at 1525 °C for 3 h followed by a high-temperature sintering at 1850 °C for another 3 h, the prepared Si₃N₄ compact with a bimodal microstructure presented the highest thermal conductivity and flexural strength of 79.42 W·m^?1·K^?1 and 801 MPa, respectively, which holds great application prospects as ceramic substrates.     

Preparation of porous Y2SiO5 ceramics with high porosity and extremely low thermal conductivity for radome applications

It is well known that aqueous gel-casting is challenging to prepare high-porosity ceramics due to the considerable drying shrinkage, cracking, and deformation of green bodies during drying caused by the high surface tension of water. Porous Y₂SiO₅ ceramics with high porosity were prepared by introducing carbon fibers as a support material in the drying process of aqueous gel-casting to reduce shrinkage during drying. Burning out the carbon fibers after drying does not negatively affect the properties of the porous ceramic. As prepared green bodies by aqueous gel-casting have low shrinkages of 8.69%–6.81% during drying processes and high compressive strength of 13.73 ± 1.55–10.66～0.49 MPa. The higher compressive strength of the green body has a positive significance for processing porous ceramics into special-shaped structures. As prepared porous Y₂SiO₅ ceramics have high porosity of 73.94%–87.71%, lightweights of 1.16–0.55 g?cm³, extremely low thermal conductivities of 0.134 ± 0.006 to 0.051 ± 0.001 W?m^?1?k^?1, relatively low dielectric constants of 2.34–1.58, and tan δ are lower than 1.25 × 10^?3. Porous Y₂SiO₅ ceramics with excellent dielectric properties and thermal insulation properties meet the requirements of thermal insulation and wave transmission integration of radome materials. Aqueous gel-casting also enriches the preparation methods of high-porosity Y₂SiO₅ ceramics.     

Electric current assisted sintering of AlN ceramics: thermal conductivity and transparency

Abstract

Densification, thermal conductivity and transparency of aluminium nitride ceramics densified by electric current assisted sintering are reviewed. Aluminium nitride powders could be densified without any sintering additives by electric current assisted sintering. Sintering additives, Y₂O₃ or CaF₂, and others are effective for densification, increase in thermal conductivity and transparency. As a result of the reaction with the sintering additives and oxide on the surface of the AlN powder, a secondary phase forms between AlN grains, oxygen impurity between aluminium nitride grains decreases and thermal conductivity increases. These mechanisms are effective in the sintering of aluminium nitride ceramics densified by electric current assisted sintering, though sintering time is short, within 5 min. The secondary phase formation does not always give good effect for transparency.     

Rare-earth-tantalate high-entropy ceramics with sluggish grain growth and low thermal conductivity

A series of rare-earth-tantalate high-entropy ceramics ((5RE_0.2)Ta₃O₉, where RE = five elements chosen from La, Ce, Nd, Sm, Eu and Gd) were prepared by conventional sintering in air at 1500 ^°C for 10 h. The (5RE_0.2)Ta₃O₉ high-entropy ceramics exhibit an orthogonal structure and sluggish grain growth. No phase transition occurs in the test temperature of 25–1200 ^°C. The thermal conductivities of all (5RE_0.2)Ta₃O₉ ceramics are in the range of 1.14–1.98 W m^?1 K^?1 at a test temperature of 25–500 ^°C, approximately half of that of YSZ. The sample of (Gd_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)Ta₃O₉ exhibits a low glass-like thermal conductivity with a value of 1.14 W m^?1 K^?1 at 25 ^°C. The thermal expansion coefficient of (5RE_0.2)Ta₃O₉ ceramics ranges from 5.6 × 10^?6 to 7.8 × 10^?6 K^?1 at 25–800 ^°C, and their fracture toughness is high (3.09–6.78 MPa·m^1/2). The results above show that (5RE_0.2)Ta₃O₉ ceramics could be a promising candidate for thermal barrier coatings.     

Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity,high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.     

Balancing thermal conductivity and strength of hot-pressed AlN ceramics via pre-sintering and annealing process

The heat dissipation requirements of the new generation of semiconductors are placing higher demands on the overall performance of aluminum nitride (AlN) ceramics. This has led to an increasing emphasis on AlN ceramics that combine thermal conductivity and strength. AlN ceramics are often strengthened by hot-press sintering, but their thermal conductivity is typically modest. In this study, pre-sintering and annealing processes are introduced to optimize the thermal conductivity of hot-pressed AlN ceramics to avoid the detrimental effects of oxygen impurities in the AlN lattice. The effect of the oxide layer on the surface of commercial AlN particles is investigated, including density, phase composition, microstructure, and fracture behavior. The effect of annealing on the improvement of thermal conductivity and flexural strength is also verified. Electron paramagnetic resonance (EPR) analysis is performed to examine the concentration of intercrystalline defects under various conditions. Finally, AlN ceramics with a thermal conductivity of 204 W m⁻¹ K⁻¹ and a flexural strength of 376 MPa are obtained.     

Enhancing the thermal insulating properties of lanthanum zirconate porous ceramics via pore structure tailoring

The potentially useful role of lanthanum zirconate (La₂Zr₂O₇, LZO) porous bulk ceramics has been rarely explored thus far, much less the optimisation of its pore structure. In this study, LZO porous ceramics were successfully fabricated using a tert-butyl alcohol (TBA)-based gelcasting method, and the pore structures were tailored by varying the initial solid loading of the slurry. The as-prepared ceramics exhibited an interconnected pore structure with high porosity (67.9 %–84.2 %), low thermal conductivity (0.083–0.207 W/(m·K)), and relatively high compressive strength (1.56–7.89 MPa). The LZO porous ceramics with porosity of 84.2 % showed thermal conductivity as low as 0.083 W/(m·K) at room temperature and 0.141 W/(m·K) at 1200 °C, which is much lower than the counterparts fabricated from particle-stabilized foams owing to its unique pore structure with a smaller size, exhibiting better thermal insulating performance.     

Highly porous nano-SiC with very low thermal conductivity and excellent high temperature behavior

Highly porous nano-SiC is fabricated by partial sintering and decarburizing process using SiC nano-powders as starting materials and graphite flakes as pore forming agents. The prepared porous nano-SiC ceramics possess multiple pore structures, including well-distributed meso-pores in the skeleton and interconnected flakelike micro-pores. The samples prepared at 1800 °C have relatively low thermal conductivities of 5.61 ～ 0.25 W m^?1 K^?1 with porosities of 55.5–76.1%. While the samples sintered at 1500 °C with porosities between 54.0% to 76.3% show very low thermal conductivities of 0.74 ～ 0.14 W m^?1 K^?1, which is attributed to the integrated nano-scale phonon-scattering mechanisms and duplex pore structures. Porous nano-SiC ceramics also show good retention of elastic stiffness up to 1350 °C and low thermal conductivity at 1400 °C. Our results shed light on porous nano-SiC as a promising thermal insulator used in extreme thermal and chemical environments.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Preparation and characterization of eco-friendly and low-cost mullite-corundum foamed ceramics with low thermal conductivity

Mullite-corundum foamed ceramics were prepared by direct-foaming method using white clay and industrial alumina as raw materials, and calcium aluminate cement (CAC) as the binder. Effects of the calcium aluminate cement content on the phase compositions, microstructures and properties of the foamed ceramics were investigated through X-ray diffractometer (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS), etc. The results showed that with increasing the calcium aluminate cement content from 0 wt% to 8 wt%, the viscosities of the initial slurries decreased, the porosities of the samples improved, the thermal conductivities decreased, and the compressive strengths increased first and then decreased. The optimized sample with 4.0 wt% CAC content had a high porosity of 80.8%, a low bulk density of 0.56 g/cm³ and a low thermal conductivity of 0.232 W/(m·K) (800 °C). On the foundation of research results, the Gong equation (GE model) and a modified GE model were used to predict the thermal conductivity at 200 °C and 800 °C of the mullite-corundum foamed ceramics with different pore characteristics and phase compositions, respectively.     

Improved mechanical strength and thermal resistance of porous SiC ceramics with gradient pore sizes

Thermal insulation applications of porous SiC ceramics require low thermal conductivity and high mechanical strength. However, low thermal conductivity and high mechanical strength possess a trade-off relationship, because improving the mechanical strength requires decreasing the porosity, which increases the thermal conductivity. In this study, we established a new strategy for improving both the mechanical strengths and thermal resistances of porous SiC ceramics with micron-sized pores by applying a double-layer coating with successively decreasing pore sizes (submicron- and nano-sized pores). This resulted in a unique gradient pore structure. The double-layer coating increased the flexural strengths and decreased the thermal conductivities of the porous SiC ceramics by 24–70 % and 29–49 % depending on the porosity (48–62 %), improving both their mechanical strengths and thermal resistances. This strategy may be applicable to other porous ceramics for thermal insulation applications.     

Highly porous (La1/5Nd1/5Sm1/5Gd1/5Yb1/5)2Zr2O7 ceramics with ultra-low thermal conductivity

The high-entropy rare earth zirconate (La_1/5Nd_1/5Sm_1/5Gd_1/5Yb_1/5)₂Zr₂O₇ porous ceramics ((5RE_1/5)₂Zr₂O₇ PCs) were prepared using a foam-gel casting-freeze drying method combined with segmented calcination process. The results of SEM, TEM, and XRD analyses of the (5RE_1/5)₂Zr₂O₇ PCs indicated the formation of a defective fluorite crystal structure, with the rare earth elements homogeneously distributed. Meanwhile, the as-prepared (5RE_1/5)₂Zr₂O₇ PCs exhibited high porosity, low bulk density, low thermal conductivity, and relatively high compressive strength. Moreover, the high-temperature thermal conductivity of the samples was evaluated, and the results showed that the (5RE_1/5)₂Zr₂O₇ PCs maintain a thermal conductivity of 0.150 ± 0.002 W m^?1 K^?1 even at 1000 °C. The strategy used in this paper can be extended to the synthesis of other high-entropy porous ceramics with high porosity and low thermal conductivity, which is suitable for applications as thermal insulation materials.     

Effect of humidity on the thermal conductivity of porous zirconia ceramics

This study focuses on the role of the water content on the effective thermal conductivity of porous ceramics placed in different conditions of relative humidity. Fully stabilized zirconia samples with variation in the capacity to take up water were prepared by varying the temperature of the thermal treatment. The pore volume fraction of the dried samples decreases from 56% down to 30%. Thermal conductivity measurements were made on samples placed in a chamber where the relative humidity was fixed between 3% and 99%. For all samples, the experimental values of the effective thermal conductivity increase significantly with the water content. Experimental results agree closely to analytical predictions based on the upper limit of the Hashin and Shtrikman expressions for calculating the thermal conductivity of the pores (constituted by air and water) and Landauer's effective medium expression for calculating the effective thermal conductivity of the material.