Porosity Of Ceramics Research Articles

Influence of casting-warm pressing process on the thermal conductivity of micro–nano-Al2O3 substrate

Alumina substrates are increasingly used for high-power integrated circuits due to their high thermal conductivity, low thermal expansion coefficient, and excellent insulation properties. However, pores in the green tape from the tape casting process reduce the thermal conductivity and permittivity of the sintered ceramic substrate. Researchers have attempted to minimize ceramic porosity with chemical additives or by sintering pure alumina at temperatures above 1650 °C, but these methods often degrade thermal conductivity or quality of substrate evenness. This study proposes a low-cost casting-warm pressing process to densify pure alumina ceramic substrates using micro–nano-mixed alumina powders and sintering at relatively low temperatures. The results indicate that the relative density of the pure alumina ceramic substrate prepared at 1500 °C is 93%, a 4.4% improvement over the tape casting process. Additionally, the thermal conductivity of the alumina substrate from the casting-warm pressing process reaches 15.89 W/(m K), which is 1.4 times higher than that of the tape casting process. Microstructure analysis shows that the casting-warm pressing process with micro- and nano-multi-scale mixed alumina powders forms a novel thermal conduction enhancement mechanism. Large particles in the green tape overlap, while small particles fill the spaces between the large particles. The connected micrometer-sized particle skeletons form high thermal conduction net channels in the substrate, improving the thermal conductivity for heat transfer.

MoO3 and AlF3 effect on the properties of porous mullite ceramics with needle‐like microstructure

AbstractPorous mullite ceramics were synthesized via ceramic processing using a local industrial kaolin, alumina, and with AlF3 and MoO3 precursor additives to catalyze the formation of pores and acicular mullite grains. The textural properties, crystalline phases, microstructures, and mechanical properties of the obtained samples were comprehensively analyzed and compared with a reference mullite formulated without additives. The obtained ceramics have a well‐defined porous microstructure, with a porosity of about 50%, and a pore size distribution in the 0.1–2 µm range. Ceramics that involve the addition of MoO3 present well‐defined acicular mullite grains, with slightly higher porosity, larger pores, and better mechanical resistance compared to those processed using AlF3.To correlate the ceramic porosity with its mechanical properties, a simple model of spherical pores was proposed to assess the flexural strength of the dense ceramic. It was found that the inclusion of additives promoting needle‐like microstructures increases the mechanical resistance up to four times the value determined for the ceramic without additives (flexural strength up to 390 MPa for zero‐porosity extrapolation). These results, together with the refractoriness of mullite, allow inferring the potential applications of the developed materials as structural ceramics with relatively low density and for filtering applications.

The Influence of Hot Isostatic Pressing on the Phase Composition and Porosity of Plasma-Sprayed Ceramics of Pure Alumina

The Influence of Hot Isostatic Pressing on the Phase Composition and Porosity of Plasma-Sprayed Ceramics of Pure Alumina

Investigations on porous silicon nitride ceramics prepared by the gel-casting method

Abstract Due to their special structure and many excellent properties, porous Si3N4 ceramics have a wide range of potential applications as a new structural and functional integrated material. In the present study, porous Si3N4 ceramics with excellent bending properties and satisfactory porosity were prepared by the gel-casting method. The variations in porosity, bending strength, fracture morphology and phase composition of the porous Si3N4 ceramics with different weight fractions of La2O3–MgO sintering additive were investigated. The results showed that the density of porous Si3N4 ceramics increased linearly with the increase of sintering additive content. When the content of sintering additive increased from 2.5 wt.% to 10 wt.%, the porosity of porous ceramics decreased from 30 % to 10 %. With the increase of sintering additive content, the bending strength of the samples first increased and then decreased. When the sintering additive content was 5.0 wt.%, the average bending strength of the samples reached a maximum of 240.88 MPa, and scanning electron microscopy and X-ray diffractometry results showed that β-Si3N4 particles with well-developed pores and a maximum aspect ratio of 5.8 were formed in the porous silicon nitride ceramics. This study provides a reference for the preparation of porous Si3N4 ceramics with good bending properties and porosity.

Investigation of Excitation Modes in Al2O3-YAG Phosphor Ceramics for High-Power Laser-Driven Lighting

The performance of laser-driven white light can vary significantly depending on the excitation mode. To attain desirable application performance of color converters in laser lighting, the excitation mode must be selected carefully. In this work, to understand the effect of the excitation mode on the performance of color converters, a detailed study was conducted on the heat dispersion, luminescence, and color quality of Al2O3-YAG:Ce phosphor ceramics in both transmissive and reflective modes. The relationship between ceramic porosity and scattering performance has also been analyzed by studying the beam expansion ratio. The results demonstrate that the phosphor ceramics in reflective mode exhibit superior heat dissipation capabilities and can withstand higher power laser excitation compared to the transmissive mode, while the light uniformity is relatively poorer due to the specular reflection. Therefore, in situations where a uniform light is not necessary, reflective color converters are more suitable for achieving superior luminance and luminous efficacy. Finally, a laser-driven white light device with a luminous flux of 1206 lm, a luminous efficacy of 241.2 lm W−1, and a CCT of 5749 K has been realized using Al2O3-YAG:Ce phosphor ceramic in reflective mode, which can be applied for automotive headlights.

Clinker High-Hollow Ceramic Stones: Prospects for Technology and Application

A substantiation is given for the prospects of using clinker large-format ceramic stones of increased hollowness with a water absorption of less than 3% in construction. It has been shown that due to the high strength of the ceramic material itself (more than 100–150 MPa), ceram-ic stones will have the necessary strength – more than 10–15 MPa in terms of compressive strength. Due to the increased void content with as many rows of voids as possible per 100 mm length of the ceramic stone and a smaller thickness of the internal walls, the stones will have reduced thermal conductivity. Due to the low porosity of clinker ceram-ics as a material and the use of appropriate masonry mortars, masonry made of clinker stones will be guaranteed not to be vapor permeable, which will significantly increase the service life of buildings. When us-ing certain technological techniques, namely the application of cham-fers, relief, engobes on the front faces, large-format high-hollow clinker stones can also play the role of facing products, which will significantly increase their consumer attractiveness. With an increase in the hollow-ness of stones, the costs of mass preparation, drying and firing of products are proportionally reduced, which significantly reduces their cost. It has been shown that the “ideal” raw material for producing clinker large-format ceramic stones can be stone-like clay raw materials – argillite-like clays, argillites, shales and their transitional varieties. When grinding them and preparing molding masses, due to the for-mation of a certain grain composition and the introduction of corrective microadditives, it is possible to obtain molding masses with optimal pre-firing technological properties and a clinker shard with water ab-sorption of up to 3% and a compressive strength of up to 200 using one raw material. MPa, bending strength up to 50 MPa.

Vat photopolymerization of dual-scale porosity alumina honeycombs using phase-separable camphene porogen dissolved in binary photopolymer blends

Dual-scale porosity ceramics comprised of macro-scale pore networks surrounded by porous frameworks manufactured by additive manufacturing (AM) can offer significantly enhanced functions compared to conventional porous ceramics. However, exploring new ways of more tightly controlling micro-scale porous structures for AM still remains challenging. Here, we propose a novel type of vat photopolymerization using camphene as a phase-separable porogen dissolved in binary photopolymer blends for manufacturing dual-scale porosity alumina honeycombs. More specifically, solid camphene with various contents (40 vol%, 50 vol%, and 60 vol%) could be completely dissolved in blends of triethylene glycol dimethacrylate (TEGDMA) and polyethylene glycol diacrylate (PEGDA) used as solvent and anti-solvent, respectively, at room temperature and then crystallize dendritically at ∼ 5 °C via phase separation. After which, thin layers of phase-separated alumina suspensions could be selectively photopolymerized by a digital light processing (DLP) engine to construct 3-dimensionally interconnected macro-scale pore channels. In addition, micro-scale pores could be created in alumina frameworks by removing camphene-rich crystals by freeze-drying. This new approach enabled manufacturing of well-defined macro-scale honeycomb structures with tailored micro-scale porosities. As the camphene content increased from 40 vol% to 60 vol%, micro-scale porosity increased from 39.1 ± 0.5 vol% to 57.5 ± 0.6 vol%, and thus the overall porosity increased from 58.5 ± 0.1 vol% to 68.8 ± 0.3 vol%, while the compressive strength decreased from 355.6 ± 67.1 MPa to 97.6 ± 10.2 MPa.

Beyond Scanning Electron Microscopy: Comprehensive Pore Analysis in Transparent Ceramics Using Optical Microscopy

Developing an effective method of quantifying defects in the bulk of transparent ceramics is a challenging task that could facilitate their widespread use as a substitute for single crystals. Conventionally, SEM analysis is used to examine the microstructure but it is limited to the material surface. On the other hand, optical transmittance assesses material quality, but does not provide information on the size and concentration of defects. In this study, we illustrate the use of a digital optical microscope for the non-destructive, precise, and rapid analysis of residual porosity in transparent ceramics. YAG-based ceramics doped with Yb have been selected for this study because they are used as laser gain media, an application that requires virtually defect-free components. Different production processes were used to produce YAG samples, and the digital optical microscope analysis was used to compare them. This analysis was shown to be effective and precise to measure the size and concentration of the residual pores. In addition, the comparison of samples obtained with different production processes showed that the size and distribution of the residual porosity is affected by the drying step of the powders before shaping by pressing, as well as by the sintering aids used to ease the densification. It also showed that the transmittance is influenced by both the total volume and the concentration of the pores.

Effect of La2O3 adding on Properties of SiC Porous Ceramic Support

In this paper, 80-90 grains of SiC as the main raw material, doped La2O3 into the binder composed of potassium feldspar, kaolinite and quartz, and then graphite and activated carbon as pore-making agents to prepare silicon carbide porous ceramic carrier. The effects of doped La2O3 content on the bending strength, porosity and filtration pressure drop of porous SiC ceramics were studied. The results show that the formation of mullite phase and the bending strength of SiC porous ceramics can be improved by adding proper amount of La2O3. Under the sintering temperature of 1300°C for 90 minutes, when the content of La2O3 in the binder is 2.5wt%, the comprehensive performance is the best, the porosity is 21.5%, the bending strength is 53.5MPa, and the filtration pressure drop is low.

Mechanical properties and forming conditions optimization of SiC ceramics using Camellia oleifera shell as carbon source

AbstractUsing Camellia oleifera shell as raw materials to preparative SiC ceramics by warm‐press forming and sintering is proposed in this paper. Warm‐press forming before preparing for the C. oleifera shell‐based SiC ceramics is beneficial to obtain the internal uniform and suitable pore structure of the cylindrical sample. Thermogravimetric analysis with differential scanning calorimetry was applied to analyze the pyrolysis behavior of C. oleiefra shell and phenolic resin. The scanning electron microscope and X‐ray diffraction were used to investigate the properties of C. oleifera shell‐based SiC ceramics. Then, the response surface method was applied to establish a multivariate prediction model based on the process parameters of warm‐press forming. And the regression equations of porosity and bending strength of COS‐SiC are obtained. The result demonstrated that the predicted and experimental values can be in good agreement for the model. By comparing the F‐value, it is found that the forming temperature and pressure during the process of warm‐press forming have a more significant influence on the porosity and bending strength of SiC ceramics. The optimization process parameters were obtained as follows: forming temperature 153°C, pressure 35 MPa, and holding time 30 min.

Effect of BPO4-B2O3 Composite Additive on Crystallization and Sintering of Silica Ceramics

BPO4-B2O3 composite additive enhanced silica ceramics were successfully prepared by solid phase sintering. The effects of BPO4-B2O3 addition and sintering temperature on the microstructure and phase composition and mechanical properties of silica ceramics were analysed by scanning electron microscopy and X-ray diffraction techniques. The results showed that at 1200°-1400°C, BPO4-B2O3 composite additive formed a glass phase in the sample, filled the pores, and improved the sintering densification of the sample. In addition, the addition of 9 wt% BPO4-B2O3 successfully inhibited the crystallization of fused silica, which significantly improved the mechanical strength of silica ceramics. After sintering at 1200°C for 3 h, the apparent porosity of silica ceramics with 9 wt% BPO4-B2O3 was only 1.33%, and the cold modulus of rupture reached a maximum of 60.99 MPa, which was about 35 times that of pure silica ceramics. It is shown that adding BPO4-B2O3 composite additive provides a feasible method for preparing silica ceramics with good mechanical properties.

A review of pore-forming agents on the structures, porosities, and mechanical properties of porous ceramics

<p>This review article provided a thorough examination of porous ceramic materials, concentrating on production, characteristics, and the involvement of pore-forming agents. The primary objective of this research was to evaluate the effects of various ceramic materials and pore-forming agents on the structure, porosity, and mechanical characteristics of porous ceramics. The study's scope included a thorough investigation of key sources of literature, such as academic publications, review articles, and industry reports, to provide a comprehensive understanding of porous ceramic technology. According to the literature review, the selection of ceramic material and pore-forming agents has a significant influence on the pore size distribution, porosity, and mechanical strength of porous ceramics. Various manufacturing methods, including foaming, sintering, and sol-gel procedures, were explored in terms of their influence on porous ceramic microstructure and characteristics. Furthermore, the study emphasized the need to optimize processing settings and select pore-forming agents to obtain the necessary qualities in porous ceramic materials. Overall, this review is useful for researchers, engineers, and practitioners who desire to learn more about porous ceramic manufacturing, characteristics, and applications.</p>

Near-net-size porous mullite ceramics with tuneable sintering shrinkage for thermal insulation applications

The microstructure and properties of porous ceramics are severely affected by volume shrinkage during the sintering process. Therefore, reducing sintering shrinkage while maintaining the properties of porous ceramics is a serious challenge. Herein, near-net-size porous mullite ceramics with tuneable sintering shrinkage were prepared by the foam-gelcasting method using kyanite as raw material and aluminum fluoride and aluminum powders as additives through the volume expansion generated by the sintering reaction. The effects of Al powder content on the sintering shrinkage, porosity, compressive strength and thermal conductivity of porous mullite ceramics were investigated. The results show that the sintering shrinkage gradually decreases with increasing Al powder content, and expansion is even produced. When the Al powder content was 5 wt%, the sintering shrinkage of porous mullite ceramics was −0.21 %, with porosity of 84.2 %, compressive strength of 2.45 ± 0.15 MPa, and thermal conductivity of 0.19 W/(m·K) at room temperature. The results of the high-temperature fire resistance test showed that the temperature difference between the front and back sides of the sample under the flame gun reached 800 °C, indicating good thermal insulation performance. The near-net-size sintering process of porous ceramics can effectively reduce the subsequent processing cost, open new paths for their production, and broaden the application fields of porous mullite ceramics.

Influence of MgO-Al2O3 hollow sphere content on the microstructure and mechanical properties of calcium hexaluminate porous ceramics

ABSTRACT Calcium hexaluminate (CA6) porous ceramics were prepared by foaming and pore-forming agent methods, with Al2O3 and calcium aluminate cement (CAC) as raw materials and magnesium aluminum spinel hollow spheres (MASHSs) as pore-forming agent. The effects of the amount of magnesium aluminum spinel hollow sphere on the phase composition, bulk density, apparent porosity, flexural strength, microstructure, and thermal conductivity of CA6-based porous ceramics were systematically studied. The pores of such porous ceramics are mainly formed by the enhanced amount of MASHSs and the generation of CA6 phase. The results demonstrate that with the elevation of MASHSs content, the microstructure of the samples transforms from a uniform distribution of closed pores to an interconnected micro-pore arrangement. Notably, the fracture paths transition from cohesive to interfacial interactions between MASHSs. This intricate interplay culminates in a remarkable outcome – our ceramics exhibit thermal conductivities ranging from 0.210 to 0.451 W/(m/k), an augmented gas permeability of 25.96 × 10−11m2, and exceptional compressive strengths reaching up to 22.3 MPa. Overall, this study provides insights into the influence of MASHSs content on the microstructure and thermal-mechanical properties of porous materials, which may contribute to the development of advanced ceramic materials with tailored properties.

Coextrusion of Clay-Based Composites: Using a Multi-Material Approach to Achieve Gradient Porosity in 3D-Printed Ceramics

3D printing of ceramics has started gaining traction in architecture over the past decades. However, many existing paste-based extrusion techniques have not yet been adapted or made feasible in ceramics. A notable example is coextrusion, a common approach to extruding multiple materials simultaneously when 3D-printing thermoplastics or concrete. In this study, coextrusion was utilized to enable multi-material 3D printing of ceramic elements, aiming to achieve functionally graded porosities at an architectural scale. The research presented in this paper was carried out in two consecutive phases: (1) The development of hardware components, such as distinct material mixtures and a dual extruder setup including a custom nozzle, along with software environments suitable for printing gradient materials. (2) Material experiments including material testing and the production of exemplary prototypes. Among the various potential applications discussed, the developed coextrusion method for clay-based composites was utilized to fabricate ceramic objects with varying material properties. This was achieved by introducing a combustible as a variable additive while printing, resulting in a gradient porosity in the object after firing. The research’s originality can be summarized as the development of clay-based material mixtures encompassing porosity agents for 3D printing, along with comprehensive material-specific printing parameter settings for various compositions, which collectively enable the successful creation of functionally graded architectural building elements. These studies are expected to broaden the scope of 3D-printed clay in architecture, as it allows for performance optimization in terms of structural performance, insulation, humidity regulation, water absorption and acoustics.

Fabrication of broadband wave-transparent Si3N4 ceramics with octet-truss lattice structure by vat photopolymerization 3D printing technology

Vat photopolymerization 3D printing of Si3N4 ceramics has attracted great attention recently. To overcome the low curing depth limitation caused by α-Si3N4, β-Si3N4 with low absorbance and large particle size was selected in this paper. After printing, the effects of pressureless sintering temperatures on mechanical and dielectric properties were systematically investigated. At 1800 °C, the real permittivity of Si3N4 ceramics was 7.37, and the maximum bending strength was 367 ± 75 MPa. Considering the application of Si3N4 ceramics in radomes/windows, the dielectric constant of Si3N4 ceramics should be further reduced. Therefore, the octet-truss lattice structure was introduced to optimize the porosity of ceramics. As the porosity increased from 25% to 36%, the real permittivity decreased from 5.56 to 4.42, and the bending strength ranged from 98 ± 7–62 ± 13 MPa. It shows that vat photopolymerization 3D printing can be a promising technology for the fabrication of wave-transparent Si3N4 ceramics.

Microstructure and properties of porous Si3N4 ceramics fabricated by digital light processing combined with spark plasma sintering

Sintering process is an important factor affecting the properties of porous Si3N4 ceramics. However, there are few studies on the properties of porous Si3N4 ceramics fabricated by combining digital light processing (DLP) technology and spark plasma sintering (SPS) technology. In this study, a method to prepare porous Si3N4 ceramics by combining DLP with SPS technology was proposed, and ceramic samples were successfully prepared. The effect of sintering temperature and holding time of SPS technology on the microstructure and properties of porous Si3N4 ceramics prepared by DLP technology was discussed. With the increase of sintering temperature from 1600 °C to 1700 °C, the conversion content of α-Si3N4 phase to β-Si3N4 phase increased from 19.19% to 76.72%. The fracture mode of porous Si3N4 ceramics gradually transformed from intergranular to transgranular. The porosity of Si3N4 ceramics gradually decreased from 40.34 ± 4.9% to 30.54 ± 2.7% with opposite trend in bulk density. And the flexural strength increased from 109.1 ± 7.6 MPa to 249.5 ± 4.0 MPa. When the sintering temperature was 1700 °C, the content of β-Si3N4 increased first and then remained essentially constant with the increase of holding time from 5 min to 15 min. The porosity increased from 30.54 ± 2.7% to 35.15 ± 2.5%, while the variation in bulk density were opposite. And the flexural strength severely decreased from 249.5 ± 4.0 MPa to 126.1 ± 7.2 MPa. This research provides valuable insights into the DLP-forming ceramic materials sintered by SPS technology.

Thermal shock resistant 3D printed ceramics reinforced with MgAl2O4 shell structure

The demand for the swirl nozzle with enhanced temperature resistance and lightweight properties is increasing as the thrust-to-weight ratio of aero-engines rises. The Al2O3 ceramic swirl nozzle can maintain high strength in a hostile environment of high temperature and severe corrosion, while also meeting the requirements of aircraft to enhance efficiency and decrease weight. However, Al2O3 ceramics are limited in their application for aerospace components due to their poor thermal shock resistance (TSR) stemming from their inherent brittleness. This work reported an innovative design and fabrication strategy based on photopolymerization 3D printing technology to realize the three-dimensional shell structure through element interdiffusion and nanoscale stacking of the reinforced phase. With this strategy, a novel type of the new dual-structure Al2O3 ceramic composed of MgAl2O4 shell structure and matrix could be constructed in situ. The nano-sized MgAl2O4 caused a crack passivation effect after the thermal shock, which could improve the strength and TSR of 3D-printed Al2O3 ceramic. In addition, the effects of MgO content and sintering temperature on sintering behavior, flexural strength, porosity, and TSR of Al2O3 ceramics manufactured by digital light processing (DLP) processing were systematically studied. The optimum overall performance of Al2O3 ceramics was obtained at the sintering temperature of 1550 °C and the MgO content of 1.0 wt.%, with a maximum flexural strength of 111.929 MPa and a critical temperature difference of 374.24 °C for TSR. Based on the above research, an aero-engine swirl nozzle with high thermal shock resistance has been successfully prepared by ceramic 3D printing technology, which enhances high-temperature resistance and promotes lightweight design in aero-engine.

Controllable preparation and properties of gel-casting porous mullite ceramics via lignin fiber assisted pore formation method

The preparation of high-strength porous mullite via gel-casting has emerged as a hot topic, but balancing high strength and high porosity remains challenging. Herein, we successfully prepared porous mullite with uniform pore size and high strength using natural lignin fibers as the pore-forming agent via gel-casting. Moreover, we investigated the properties of mullite slurries, as well as green bodies and sintered ceramics in details. The strong water transport capacity of the lignin fibers enabled the homogeneous dispersion of gel system and improved the flowability of slurry. The flexural strength of green body was significantly increased to 16.07 MPa by optimizing the lignin fiber content. Porous mullite ceramics sintered at 1400 °C exhibited a homogeneous microstructure with average pore size of 5.62 μm. Furthermore, mullite ceramics had a porosity of 64.73% at a total solid loading of 55 wt% and a lignin fiber content of 10 wt%, whereas the compressive strength remains high at 11.32 MPa. Permeability tests confirmed the presence of interconnected pores, its coefficients k1 and k2 increases with the total porosity of ceramics. This work provides new insights into designing the preparation of porous mullite ceramics with high performance.

Infiltration of silica slurry into silica-based ceramic cores prepared by selective laser sintering based on pre-sintering

The silica-based ceramic core is an important mold for manufacturing hollow blades. The preparation of silica-based ceramic cores by combining selective laser sintering (SLS) and vacuum infiltration (VI) technologies is a promising approach. To enhance infiltration effect, pre-sintering was introduced to obtain the bodies with high porosity and hydrophilicity. The infiltrated silica content increased from 29.58 wt% for samples infiltrated once to 68.91 wt% for samples infiltrated three times. The increase of silica content in the matrix reduced the porosity of final ceramics, thus improving the room-temperature flexural strength. The infiltrated silica gradually transformed into cristobalite during the high-temperature test, which improved the high-temperature flexural strength and creep resistance. When the final-sintering temperature was 1225 °C, room-temperature and high-temperature flexural strength reached 21.17 MPa and 21.21 MPa, respectively, due to the enhanced sintering necks and low porosity. These results indicate that our work provides a promising route to fabricate silica-based ceramic cores by SLS technology.