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.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

Preparation and properties of porous alumina with inter-locked platelets structure

Porous alumina ceramics with alumina platelets was prepared by vapor-solid reaction sintering of AlOF mesophase gas by the reaction of HF and Al₂O₃. The effect of heating treatment temperatures on porosity, the formation of inter-locked platelets structure and compressive strength of porous alumina ceramics was determined by Archimedes' method, XRD, SEM and compressive tests. The results indicated that after heating at temperatures from 1300 °C to 1600 °C, the porosity of alumina ceramics decreased from 61.6% to 48.4%. Increasing the heating treatment temperature was beneficial to form inter-locked structure between alumina platelets. The maximum compressive strength of porous ceramics with porosity of 48.4% can reach 29.8 MPa heated at 1600 °C; this strength was attributed to the strong bonding between the alumina platelets.     

Promotion of the alginate gelling method for preparing Al2O3 honeycomb ceramics

In this work, Al₂O₃ honeycomb ceramics with unidirectionally aligned channels were fabricated by the ionotropic gelation process of alginate/Al₂O₃ suspensions. By heating the bottom of the suspension container during the gelation step, the heat energy conducted upward from foot to the top surface of the slurry, which has shortened the gelation time from 48 to 10?h and improved efficiency dramatically. Meanwhile, as the heating temperature increased from 25 to 40°C, the porosity of Al₂O₃ honeycomb ceramics remained unchangeable with the pore size decreasing from 163 to 79?μm for the increasing opportunities for forming capillaries in the primary membrane. By the integrated effect of unidirectional pore channels and dense pore walls, both the compressive strength and water permeability of the sintered samples were higher than those of Al₂O₃ foam ceramics.     

Mechanical and dielectric properties of reduced graphene oxide nanosheets/alumina composite ceramics

Reduced graphene oxide (rGO) nanosheets/alumina (Al₂O₃) composite ceramics were fabricated by hot-pressing sintering. The density, porosity, microhardness, flexural strength and complex permittivity were investigated to study their mechanical and dielectric properties. The results revealed that the rGO nanosheets were uniformly distributed in the Al₂O₃ matrix and that the composite ceramics were highly dense at 3.67–3.99 g/cm³. Due to low rGO hardness and elevated porosity, the microhardness exhibits a decreasing trend as the rGO content increases. The flexural strength first increased and then decreased with the escalation of rGO content, and the highest strength of 313.75 MPa was obtained at 3 wt%, increasing by 37.61% relative to that of the hot-pressing sintered Al₂O₃ ceramic. Owing to the enhanced interfacial polarization, dipole polarization, polarization relaxation loss and conductance loss, the real part and imaginary part of complex permittivity increase from 10.40 to 52.73 and from 0.08 to 28.86 as the rGO content rose from 0 wt% to 4 wt%, respectively.     

Effect of different contents of zirconia and yttrium oxide on microstructure and properties of high alumina ceramics

The effects of zirconia and yttrium oxide addition on microstructure, bulk density, microhardness, flexural strength, and wear resistance of high alumina ceramics (>97 wt% Al₂O₃, MSA ceramics) composed of MgO–SiO₂–Al₂O₃ system have been investigated. The results show that the addition of zirconia makes the mechanical properties and wear properties of ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂ (MSAZ ceramics) system have been greatly improved compared with MSA ceramics. In addition, the ceramics composed of MgO–SiO₂–Al₂O₃–ZrO₂–Y₂O₃ (MSAZY ceramics) system have better mechanical properties and wear properties than MSAZ ceramics. With the contents of zirconia and yttrium oxide increase, the bulk density, microhardness, and flexural strength of MSAZ and MSAZY ceramics increased at first and then decreased. However, the wear rate shows the opposite. When 0.4 wt% ZrO₂ and 0.6 wt% Y₂O₃ were added to the matrix, the wear rate of MSAZY ceramics reached a minimum of 0.042%, and the wear resistance was improved by about 73.8% compared with MSA ceramics with a wear rate of 0.16%. In addition, the optimum additions of zirconia and yttria are 0.4% and 0.6%, respectively.     

Cyclic thermal shock resistance of h-BN composite ceramics with La2O3–Al2O3–SiO2 addition

The effects of La₂O₃–Al₂O₃–SiO₂ addition on the thermal conductivity, coefficient of thermal expansion (CTE), Young's modulus and cyclic thermal shock resistance of hot-pressed h-BN composite ceramics were investigated. The samples were heated to 1000 °C and then quenched to room temperature with 1–50 cycles, and the residual flexural strength was used to evaluate cyclic thermal shock resistance. h-BN composite ceramics containing 10 vol% La₂O₃–Al₂O₃ and 20 vol% SiO₂ addition exhibited the highest flexural strength, thermal conductivity and relatively low CTE, which were beneficial to the excellent thermal shock resistance. In addition, the viscous amorphous phase of ternary La₂O₃–Al₂O₃–SiO₂ system could accommodate and relax thermal stress contributing to the high thermal shock resistance. Therefore, the residual flexural strength still maintained the value of 234.3 MPa (86.9% of initial strength) after 50 cycles of thermal shock.     

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.     

Strong and tough laminated ceramics prepared from ceramic foams

Here, we present a novel strategy to prepare laminated ceramics by combining the ceramic foams and hot-pressing sintering. Al₂O₃ and ZrO₂ ceramic foams prepared by the particle-stabilized foaming method was cut into thin slices and then directly laminated and hot-pressing sintered. Al₂O₃/ZrO₂ laminated ceramics with various structures were prepared. Compared with the slices prepared by conventional process, ceramic foams can easily regulate the thickness of laminate to resemble the nacre-like structure. In addition, the grain in the ceramic foams have lower activity and shrinkage rate, thereby weakening the residual tensile internal stress caused by grain coarsening and differences in coefficient of thermal expansion. The effects of layer number and thickness ratio on residual stress and the structure-activity relationship between mechanical properties and microstructure were investigated. The fracture toughness, flexural strength, and work of fracture of the optimal Al₂O₃/ZrO₂ laminated ceramics are 8.2 ± 1.3 MPa·m^1/2, 356 ± 59 MPa, and 216 J·m^?2, respectively.     

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.     

Influence of laser surface texturing on the flexural strength of Al2O3 and Si3N4

Laser surface texturing (LST) is well known to be capable of improving the tribological performance and reducing the friction of ceramic surfaces. However, the influence of LST on the flexural strength of ceramics has rarely been researched.In this study, we examine the influence of LST on high purity (> 99.5 wt%) dense-sintered fine Al₂O₃ and hot-pressed fine Si₃N₄ with polished and laser-textured surfaces based on the biaxial ball-on-3-balls (B3B) test. A heat transfer simulation of the LST process is performed to understand the occurrence of residual stress. In addition, the B3B strength of Al₂O₃ and Si₃N₄ texture groups is calculated by adapting the previous formula based on the finite element (FE) simulation. Subsequently, the stress distribution in the FE simulation is used to calculate the effective volume and effective surface to study the size effect on both ceramics. It is found that LST improves the strength of Al₂O₃ and Si₃N₄ due to two reasons: it induces compressive residual stress on the tensile-loaded surface of ceramic specimens; more importantly, it reduces the effective volume and effective surface remarkably, thus improving the component strength significantly.     

Effect of sintering time on the microstructure and stability of Al2O3–ZrO2 composite powders under microwave-assisted sintering

The function of ceramic coating is closely related to the construction technology and the quality of ceramic powders. Generally, Al₂O₃–ZrO₂ powders are rapidly sprayed on the material surface at high temperatures to obtain better performance. Improving the quality of Al₂O₃–ZrO₂ powders can make them more widely used in ceramic coating. In this paper, microwave sintering was used to enhance the sintering process of the powders, and the effect of sintering time on the microstructure, properties, and stability of Al₂O₃–ZrO₂ powders was investigated. The results proved that microwave heating could improve the crystallinity and stability of the samples. At 900 °C, the tetragonal phase content in samples with different sintering times were 63.05%, 63.25%, 62.39%, and 63.22%, respectively. The average particle sizes obtained by Gaussian fitting are 1.04 μm, 0.83 μm, 0.88 μm, 0.86 μm, respectively. The Gaussian fitting particle size data was consistent with the normal distribution. Compared with the particle size of raw material (1.10 μm), the particles were refined, and the dispersion effect was noticeable. Therefore, the best sintering time for microwave sintering Al₂O₃ stabilized zirconia was 2 h. This paper aims to provide reasonable data support for improving the preparation of high-quality Al₂O₃-PSZ ceramic powders and to guide the industrial production of Al₂O₃-PSZ powders.     

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.     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

A new gelcasting using Isobam both as dispersant and monomer

Gelcasting is a kind of colloidal processing with many attractive advantages for fabricating ceramics. In this study, a new and simple gelcasting system with only two additives was investigated using Isobam (a copolymer of isobutylene and maleic anhydride) both as dispersant and monomer, and Tetraethylenepentamine (TEPA) as crosslinker. The gelation studies in solutions showed the reaction between Isobam and TEPA was an acylation reaction which was controlled by temperature and concentration. The rheological behavior and samples’ properties of 50 vol% Al₂O₃ slurries with different Isobam contents were studied. With 8 wt% of Isobam by water, the dried green bodies had relatively density of 56.3% and flexural strength of 25.7 ± 2.2 MPa. After sintering at 1530 °C for 3 h, the relatively density and flexural strength of sintered ceramics were 98.7% and 416 ± 19 MPa respectively. The SEM and macroscopic results showed that dried green bodies and sintered ceramics with homogeneous and defect–free morphology were obtained by using Isobam–TEPA system.     

Low thermal conductivity in porous SiC–SiO2–Al2O3–TiO2 ceramics induced by multiphase thermal resistance

The introduction of multiple heterogeneous interfaces in a ceramic is an efficient way to increase its thermal resistance. Novel porous SiC–SiO₂–Al₂O₃–TiO₂ (SSAT) ceramics were fabricated to achieve multiple heterogeneous interfaces by sintering equal volumes of SiC, SiO₂, Al₂O₃, and TiO₂ compacted powders with polysiloxane as a bonding phase and carbon as a template at 600 °C in air. The porosity could be controlled between 66% and 74% by adjusting the amounts of polysiloxane and the carbon template. The lowest thermal conductivity (0.059 W/(m·K) at 74% porosity) obtained in this study is an order of magnitude lower than those (0.2–1.3 W/(m·K)) of porous monolithic SiC, SiO₂, Al₂O₃, and TiO₂ ceramics at an equivalent porosity. The typical specific compressive strength value of the porous SSAT ceramics at 74% porosity was 3.2 MPa cm³/g.     

0.73ZrTi2O6–0.27MgNb2O6 microwave dielectric ceramics modified by Al2O3 addition

0.73ZrTi₂O₆–0.27MgNb₂O₆ ceramics with various Al₂O₃ contents (0‐2.0 wt%) were prepared by conventional ceramic route. The effects of Al₂O₃ on the phase composition, microstructure, conductivity, and microwave dielectric properties were systematically investigated. The coexistence of a disordered α–PbO₂‐type phase and a rutile second phase was found in all compact ceramics with low Al₂O₃ contents (x = 0, 0.5, and 1.0 wt%), while a corundum phase was detected when Al₂O₃ additive increased to 1.5 and 2.0 wt% based on X‐ray diffraction results. With the addition of Al₂O₃, the decreased grain size of the matrix phase was observed using field‐emission scanning electron microscope, accompanied with increased resistivity and band‐gap energy. Additionally, Al₂O₃ additives efficiently improved the quality factor of the ceramics. After sintering at 1360°C for 3 hours, the ceramic with 1.0 wt% Al₂O₃ exhibited excellent microwave dielectric properties: a dielectric constant of 43.8, a quality factor of 33 900 GHz (at 6.6 GHz), and a near‐zero temperature coefficient of resonant frequency (3.1 ppm/^°C).     

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.