Preparation of porous Al2O3 ceramics with enhanced properties by SLS using Al2O3 poly-hollow microspheres (PHMs) coated with CaSiO3 sintering additive

The mechanical properties of porous ceramics prepared by poly-hollow microspheres (PHMs) is usually low because of the weak bonding between different ceramic PHMs. In this paper, CaSiO₃ were coated to the surface of Al₂O₃ PHMs through co-precipitation method as sintering additive to improve the properties of Al₂O₃ poly-hollow microsphere ceramics (Al₂O₃ PHM ceramics). The influence of different amount of CaCl₂ solution on properties of the Al₂O₃ PHM ceramics such as phase composition, microstructure, porosity and mechanical properties were studied. The porosity of the Al₂O₃ PHM ceramics decreased from 77.03% to 68.16% with the increase of CaCl₂ solution amount, while compressive strength increased 29 times from 0.29 MPa to 8.39 MPa. The addition of the CaSiO₃ could decrease the sintering temperature of Al₂O₃ PHM ceramics and significantly improve the mechanical properties of Al₂O₃ PHM ceramics, which is beneficial for preparing highly porous Al₂O₃ PHM ceramics with high mechanical properties and complex shapes.     

Selective laser sintering of porous Al2O3-based ceramics using both Al2O3 and SiO2 poly-hollow microspheres as raw materials

Porous Al₂O₃-based ceramics with improved mechanical strength and different pore size were fabricated using Al₂O₃ and SiO₂ poly-hollow microspheres (PHMs) as raw materials by selective laser sintering (SLS). The effects of different contents of SiO₂ PHMs on phase compositions, microstructures, mechanical properties and pore size distribution of the prepared ceramics were investigated. It is found that moderate content of SiO₂ PHMs (≤30 wt%) could work as a sintering additive, which could enhance the bonding necks between Al₂O₃ PHMs. When the content of SiO₂ PHMs increased from 0 wt% to 30 wt%, the compressive strength of Al₂O₃-based ceramics increased from 0.3 MPa to 4.0 MPa, and the porosity decreased from 77.0% to 65.0% with open pore size decreased from 52.0 μm to 38.3 μm. However, SiO₂ PHMs could provide pores by keeping its integrity when the content of SiO₂ PHMs increased to 40 wt%, which could result in the porosity increasing to 66.8% and pore size decreasing to 30.1 μm. Selective laser sintering of different kinds of ceramic PHMs is a feasible method to fabricate porous ceramics with complex shape, controllable pore size and improved properties.     

Near-zero-shrinkage Al2O3 ceramic foams with coral-like and hollow-sphere structures via selective laser sintering and reaction bonding

The large shrinkage that ceramics undergo during sintering is a severe challenge for high-performance porous ceramics. In this study, we report a powder-based selective laser sintering (SLS) approach to prepare Al₂O₃ ceramic foams with near-zero shrinkage, high porosity, and outstanding strength. The ceramic foams consist of specific coral-like and hollow-sphere structures derived from the raw Al₂O₃/Al composite powders via reaction bonding (RB). A near-zero shrinkage of 0.91 ± 0.15 % and a high porosity of 73.7 ± 0.2 % can be achieved based on the Kirkendall effect during the oxidation of Al particles. Meanwhile, the reinforced sintering necks and robust bond-bridge connections between hollow-sphere and coral-like structures result in a remarkable bending strength of 7.37 ± 0.37 MPa. This measured strength is more than six times higher than other fabricated samples from spherical Al₂O₃ powders, and the comprehensive performance of ceramic foams prepared by this novel SLS/RB strategy is exceptionally remarkable versus that via conventional forming methods.     

A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Direct ink writing of aluminum-phosphate-bonded Al2O3 ceramic with ultra-low dimensional shrinkage

Three-dimensional (3D) printing of ceramics has attracted increasing attention in various fields. However, the pyrolysis of organic components used for binding or polymerization in 3D printing commonly causes a large shrinkage (up to 30 %–40 %), high porosity, and cracking or deformation, severely limiting practical applications. In this study, 3D printing of Al₂O₃ ceramic architectures with ultra-low shrinkage is realized by introducing inorganic binder aluminum dihydrogen phosphate (Al(H₂PO₄)₃, AP) as a ceramic precursor. Compared to organic binders, the inorganic AP binder can undergo crystallization conversion, which reduces mass loss during sintering at high temperatures, resulting in low shrinkage. Moreover, AP can be used as a rheological modifier to regulate the printability of the ceramic ink for direct ink writing of Al₂O₃ ceramic architectures, such as wood-piled scaffolds, honeycomb structures, and tubes with high fidelity. The resultant Al₂O₃ structural ceramics sintered at 1250 °C exhibit good mechanical performance and structural integrity. Most importantly, the linear shrinkage of the printed ceramics is less than 5 %, which is several times lower than that of ceramics with organic binders. This study provides a viable strategy for fabricating high-performance ceramic architectures with good dimensional fidelity for practical applications.     

Direct foaming of Al2O3-based macroporous ceramics containing pre-foamed colloidal alumina,calcite and CAC suspension

Adding pre-foamed colloidal alumina to ultrastable Al₂O₃-stabilised foams can be a path towards partially counteracting the firing shrinkage of these materials and producing macroporous ceramics with smaller pores. Nevertheless, this system still presents a long setting time and high sintering-induced shrinkage, which hinders the production of larger samples and reduces its porosity. In the present work, it was observed that adding calcium aluminate cement suspension (CAC_S) and CaCO₃ (calcite) to the aforementioned system can speed up its solidification kinetics, improve its mechanical strength and reduce its shrinkage after firing, maintaining high porosity and smaller pore sizes. By using these raw materials, samples with an average pore size below 60 μm, total porosity above 70%, and a narrower pore size distribution were attained after thermal treatment at 1600 °C for 5h. Moreover, due to the in situ formation of calcium hexaluminate, their shrinkage after sintering was almost halved (from ~20% to 11%).     

A new Al2O3 porous ceramic prepared by addition of hollow spheres

A method for making porous ceramic prepared by adding hollow spheres was developed, and the resulting porous ceramic was named as hollow spheres ceramic. Water soluble epoxy resin was used as a gel former in the gelcasting process of the Al₂O₃ hollow sphere and Al₂O₃ powder, the porous ceramic porosity varies from 22.3 to 60.1 %. The influence of amount of Al₂O₃ hollow sphere and sintering temperature on the microstructure, compressive strength and thermal conductivity were investigated. With an increasing amount of hollow sphere in the matrix, the porosity increases, which leads to decreased bulk density, compressive strength and thermal conductivity. The compressive strength of the porous ceramics has a power law relation with the porosity, and the calculated power law index is 4.5. The equations of the relationship between porosity and thermal conductivity of porous ceramics are proposed. The thermal conductivity of samples with 60.1 % porosity is as low as 2.1 W/m k at room temperature.     

High-strength and corrosion-resistant Al2O3 ceramics with excellent closed-cell structure

As a lightweight refractory, porous Al₂O₃ ceramics are advocated in the iron and steel smelting industry because of their excellent resource-saving and low heat loss. However, the severely poor slag corrosion resistance and low mechanical strength caused by open pores shorten their service life. To solve this problem, Al₂O₃ ceramics with excellent closed-cell structure were fabricated by combining β-SiC pore-foaming and gel-casting techniques, and their pore structure and properties were tailored by tuning the content of β-SiC and sintering temperature. It is noted that the closed pores introduced in the dense Al₂O₃ matrix play a pivotal role in improving the corrosion resistance and mechanical strength while maintaining the lightweight. And the sample with closed porosity of 20.6% exhibited compressive strength of 640 MPa and flexural strength of 272 MPa. Meanwhile, its corrosion and penetration indices were at a low level, 6.3% and 54.8%, respectively.     

Preparation and characterisation of closed-pore Al2O3-MgAl2O4 refractory aggregate utilising superplasticity

ABSTRACT

A novel high closed porosity Al₂O₃-MgAl₂O₄ refractory aggregate has been successfully fabricated by utilising superplasticity with Al₂O₃ and MgO as raw materials, SiC as high temperature pore-forming agent. The effects of the addition amounts of MgO and SiC on porosity, sintering behaviours, phase composition, pore size distribution and microstructure of the refractory aggregate have been investigated. The formation mechanism of the closed pore in the refractory aggregate has been discussed. The results showed that the MgO can improve the superplastic deformation ability of Al₂O₃-based ceramic at high temperature. With the content of MgO and SiC increased, the closed porosity and the pore size increased. The oxidation of SiC improved the sinterability of materials at the initial stage of sintering, and then the released gases due to the further oxidation of SiC promoted the formation of closed pores by motivating the superplastic deformation ability of Al₂O₃-based materials.     

Fabrication and microstructure of porous SiC ceramics with Al2O3 and CeO2 as sintering additives

In the present study, porous silicon carbide ceramics were prepared via spark plasma sintering at relatively low temperatures using Al₂O₃ and CeO₂ as sintering additives. Sacrificial template was selected as the pore forming mechanism, and gelcasting was used to fix the slurry in a short time. The evolution process of the microstructures during different steps was observed by SEM. The influence of the sintering temperature and sintering additives on the shrinkage and porosity of the samples was studied. The microstructures of different samples were characterized, and the mechanical properties were also evaluated.     

High-performance Al2O3 ceramics prepared by DCC-HVCI via microwave heating

A novel and rapid fabrication method for Al₂O₃ ceramics by the DCC-HVCI method via microwave heating was proposed. Effects of microwave heating temperature on coagulation time, micromorphology, as well as performance of the green body and ceramic sample were studied. As the microwave heating temperature rises, the coagulation time gradually reduced and compressive strength of green sample decreased while relative density and flexural strength of ceramics rose at the beginning and then dropped. The 50 vol.% Al₂O₃ suspension was coagulated and demolded after treating at 60°C for 800 s by microwave heating. The compressive strength of green samples reached 1.12 ± 0.13 MPa. The relative density of Al₂O₃ ceramic samples reached 99.39%. And the flexural strength of Al₂O₃ ceramics reached 334.55 ± 26.41 MPa. The Weibull modulus of Al₂O₃ ceramics reached 19. In contrast with the ceramic samples heated through water bath, the ceramic samples treated through microwave possessed uniform microstructures. Microwave heating could reduce the coagulation time by 77%. Meanwhile, it could significantly raise the compressive strength of green bodies by 65%. Additionally, it could increase the flexural strength of ceramics by 30%.     

Indirect selective laser sintering of epoxy resin-Al2O3 ceramic powders combined with cold isostatic pressing

To fabricate Al₂O₃ ceramic components with complex shape, selective laser sintering (SLS) combined with cold isostatic pressing (CIP) process was used to consolidate Al₂O₃ powder with additive of epoxy resin E06 (ER06) and polyvinyl alcohol (PVA). The starting material preparation combined spray drying with mechanical mixing to formulate compound powder consisting of PVA (1.5 wt%), ER06 (8 wt%) and Al₂O₃ and provide a good fluidity for SLS. Experimental investigations were carried the shrinkage, relative density, bending strength of Al₂O₃-ER06 SLS specimens in order to optimize the laser sintering parameters. It was found that Al₂O₃-ER06 SLS specimens represented acceptable shrinkage, high density and bending strength when laser power, scanning speed, scanning space and layer thickness were, respectively, 21 W, 1600 mm/s, 100 μm and 150 μm. Following that, the SLS specimens were processed through CIP to eliminate the pores in green ceramics. Finally, the optimized SLS/CIP Al₂O₃ specimens were debinded, sintered to produce crack-free Al₂O₃ bodies. The final Al₂O₃ components achieved a relative high density of more than 92% after furnace sintering. The study shows a novel and promising approach to fabricate complex ceramic matrix and ceramic components via indirect SLS and CIP process.     

Effects of AlN inorganic binder on the properties of porous Si3N4 ceramics prepared by selective laser sintering

Porous Si₃N₄ ceramics are widely used in the aerospace field due to its lightweight, high-strength, and high wave transmission. Traditional manufacturing methods are difficult to fabricate complex structural and functional ceramic parts. In this paper, selective laser sintering (SLS) technology was applied to prepare porous Si₃N₄ ceramics using AlN as an inorganic binder. And the effects of AlN content on the properties of the obtained ceramic samples were explored. As the AlN content increased, nano-Al₂O₃ and nano-SiO₂ formed the eutectic liquid phase, enhancing the sintering densification and phase transformation of Si₃N₄ poly-hollow microspheres (PHMs). The island-like partial densification structures in Si₃N₄ green bodies increased. During the high-temperature sintering, the eutectic liquid phase partially transformed into the mullite phase or reacted with AlN and Si₃N₄ to form the Sialon phase. With the increase of AlN content, the fracture mode of Si₃N₄ ceramics changed from fracturing along PHMs to fracturing across PHMs. The bonding depth between PHMs increased and the connection between the grains was tighter, so the Si₃N₄ ceramics became denser. With the increase of AlN addition, the total porosity of the porous Si₃N₄ ceramics tended to decrease and the flexural strength gradually increased. When AlN content was 20 wt%, the total porosity and the flexural strength were 33.6% and 23.9 MPa, respectively. The addition of AlN inorganic binder was carried out to develop a novel way to prepare high-performance porous Si₃N₄ ceramics by SLS.     

Densification,flexural strength and dielectric properties of CaO-MgO-ZnO-SiO2/Al2O3 glass ceramics for LTCC applications

Novel glass ceramics for LTCC applications with high flexural strength can be achieved by CaO-MgO-ZnO-SiO₂(CMSZ) glass cofiring with Al₂O₃. The sintering shrinkage behavior, crystalline phases, mechanical and dielectric properties, and thermal expansion of the CMZS/Al₂O₃ glass ceramic were determined. The X-ray diffraction results revealed that multiphases (CaMgSi₂O₆, Al₂Ca(SiO₄)₂ and ZnAl₂O₄) formed in the sintering process of the CMZS/Al₂O₃ glass ceramic. The flexural strength of CMZS/Al₂O₃ glass ceramics first increases and then decreases with increasing Al₂O₃ content. The CMZS/Al₂O₃ glass ceramic with 50 wt % Al₂O₃ sintered at 890 °C for 2 h achieved the best performance, with a maximum flexural strength of 256 MPa, dielectric constant (ε_r) of 7.89, dielectric loss (tan δ) of 3.41 × 10⁻³ (12 GHz), temperature coefficient of resonance frequency (τ_f) of −29 ppm/°C, and the CTE value of 7.93 × 10⁻⁶/°C.     

Fabrication of Al2O3-SiO2 ceramics through combined selective laser sintering and SiO2-sol infiltration

In this study, different posttreatment methods, including silica-sol infiltration (SI), vacuum silica-sol infiltration (vSI), debinding (DB), and pressure-less sintering (PS), were combined with selective laser sintering (SLS) to fabricate Al₂O₃-SiO₂ ceramics. The macro-morphology and microstructure of sample fabricated under different laser processing parameters and posttreatment process were investigated. Results show that the geometric dimension accuracy and surface quality of the final samples can be effectively improved with appropriate SLS parameters and posttreatment. The optimal SLS processing parameters are determined to be 0.15 mm, 10 W, 0.1 mm, and 1500 mm/s for the hatch spacing, laser power, layer thickness, and scanning speed, respectively. The SLS/DB/vIN/FS samples have the smallest linear shrinkage ratio (<1%), the least warpage degree (<3%), and the best surface quality (surface altitude difference <170 μm). Mullite, quartz, corundum, and cristobalite composed the phases of the sample, of which cristobalite came largely from the infiltrated silica-sol. Since a higher amount of silica-sol infiltrated into the sample under SLS/DB/vIN/FS process, more cristobalite phase formed in the pore of the sample during sintering, which avoided excessive microstructure shrinkage during sintering and ensured the high geometric accuracy and surface quality of the final sample.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

Microstructural comparison of porous oxide ceramics from the system Al2O3–ZrO2 prepared with starch as a pore-forming agent

In this paper we show examples of microstructures of porous oxide ceramics prepared by traditional slip casting (TSC) and starch consolidation casting (SCC) and present results obtained using different microstructural characterization techniques; Archimedes method (open and total porosity), shrinkage measurement, mercury intrusion porosimetry (pore size distribution) and microscopic methods – optical microscopy with microscopic image analysis (pore size distribution) and scanning electron microscopy (detailed investigation of the local microstructure). In particular, microstructures are compared for porous ceramics from the system Al₂O₃–ZrO₂ prepared with rice and corn starch. It is shown that maximum values of the total porosity of porous ceramics prepared with starch as a pore-forming agent were approx. 50%. A major finding by using SEM with respect to starch-produced porous ceramics is the existence of pore fillings in the form of small sintered ceramic shell inside the pores, as a result of starch granule shrinkage during the drying and burn-out steps.     

Effects of fine grains and sintering additives on stereolithography additive manufactured Al2O3 ceramic

Al₂O₃ ceramics are fabricated by stereolithography based additive manufacturing in present reports. To improve the densification and performance of Al₂O₃ ceramic, the introduction of fine grains or sintering additives has been studied by traditional fabrication techniques. However, no research has focused on the effects of adding fine grains and sintering additives on the stereolithography additive manufactured Al₂O₃ ceramic. In this study, both fine grains and sintering additives were added firstly, and then the effects of fine grains and sintering additives on the relative density, microstructure, mechanical properties, and physical properties of the stereolithography additive manufactured Al₂O₃ ceramics were investigated. Finally, defect-free Al₂O₃ ceramic lattice structures with high precise and high compressive strength were manufactured.     

Porous YbB6 Ceramics Prepared by In Situ Reaction between Yb2O3 and B4C Combined with Partial Sintering

Porous YbB₆ ceramic, a member of the ultrahigh‐temperature (UHT) family, is successfully prepared from Yb₂O₃ and B₄C powders by in situ synthesis combined with partial sintering method. Due to the fact that pores can be produced using the gases such as B₂O₃ and CO generated in reaction between Yb₂O₃ and B₄C, phase‐pure porous YbB₆ ceramics are obtained after sintering the Yb₂O₃/B₄C green bodies at 1750°C for 2 h in a flowing argon atmosphere under ambient pressure without addition of pore‐forming agent. Using this new and simple method, the porosity and volume shrinkage of porous YbB₆ ceramics are controllable by changing the green density. The prepared porous YbB₆ ceramic has homogeneous pore structure with very narrow pore diameter distribution. Furthermore, the porous YbB₆ possesses high compressive strength of ~21.34 MPa when the porosity is ~58.7% and the density is ~2.27 g/cm³. The combination of these favorable properties renders porous YbB₆ ceramic being a light‐weight structural and functional component for UHT applications.