Equiaxed β–Si3N4 ceramics prepared by rapid reaction‐bonding and post‐sintering using TiO2–Y2O3–Al2O3 additives

Sintered reaction‐bonded Si₃N₄ ceramics with equiaxed microstructure were prepared with TiO₂–Y₂O₃–Al₂O₃ additions by rapid nitridation at 1400°C for 2 hours and subsequent post‐sintering at 1850°C for 2 hours under N₂ pressure of 3 MPa. It was found that α–Si₃N₄, β–Si₃N₄, Si₂N₂O, and TiN phases were formed by rapid nitridation of Si powders with single TiO₂ additives. However, the combination of TiO₂ and Y₂O₃–Al₂O₃ additives led to the formation of 100% β–Si₃N₄ phase from the nitridation of Si powders at such low temperature (1400°C), and the removal of Si₂N₂O phase. As a result, dense β–Si₃N₄ ceramics with equiaxed microstructure were obtained after post‐sintering at high temperature.     

Influence of sintering additives on densification kinetics and high‐temperature strength in γ‐Y2Si2O7 ceramics

Pressureless sintering of pure γ‐Y₂Si₂O₇ powders that had been synthesized by a solid‐liquid reaction method using Y₂O₃ and SiO₂ powders with Li₂O, MgO, and Al₂O₃ additives was reported. The sintering kinetics of γ‐Y₂Si₂O₇ powders was analyzed to track details of densification evolution. Apparent activation energies of the densification of γ‐Y₂Si₂O₇ powders were reported for the first time, which was 57.1, 96.6, and 100.2 kJ/mol for the powders with Li₂O, MgO, and Al₂O₃ additives, respectively, indicating that Li₂O could promote the densification behavior effectively. The flexural strengths as a function of temperature for the γ‐Y₂Si₂O₇ ceramics with different additives were also investigated. The degradation of high‐temperature flexural strength was mainly ascribed to the softening of grain‐boundary glassy phase. γ‐Y₂Si₂O₇ specimens fabricated using the powders with MgO or Al₂O₃ additives exhibited better high‐temperature mechanical properties.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Low‐Cost Silicon Nitride from β‐Silicon Nitride Powder and by Low‐Temperature Sintering

Aiming to manufacture low‐cost silicon nitride components, a low‐cost β powder was chosen as a raw powder and low‐temperature sintering at 1550–1600°C under atmospheric pressure nitrogen was carried out. The silicon nitride from β powder with 5 wt% Y₂O₃ and 5 wt% MgAl₂O₄ additives and sintered at 1600°C for 8 h was successfully densified, and it exhibited moderate strength and toughness of 553 MPa ± 22 and 3.5 MPa m^1/2, respectively. The results indicate that the low‐temperature sintering of the low‐cost β powder has a potential to reduce cost of components.     

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.     

Synthesis and growth mechanism of single crystal β‐Si3N4 particles with a quasi‐spherical morphology

Single‐crystal β‐Si₃N₄ particles with a quasi‐spherical morphology were synthesized via an efficient carbothermal reduction‐nitridation (CRN) strategy. The β‐Si₃N₄ particles synthesized under an N₂ pressure of 0.3 MPa, at 1450°C and with 10 mol% unique CaF₂ additives showed good dispersity and an average size of about 650 nm. X‐ray photoelectron spectroscopy analysis revealed that there was no SiC or Si–C–N compounds in the β‐Si₃N₄ products. Selected‐area electron‐diffraction pattern and high‐resolution image indicated single crystalline structure of the typical β‐Si₃N₄ particles without an obvious amorphous oxidation layer on the surface. The growth mechanism of the quasi‐spherical β‐Si₃N₄ particles was proposed based on the transmission electron microscopy and energy dispersive X‐ray spectroscopy characterization, which was helpful for controllable synthesis of β‐Si₃N₄ particles by CRN method. Owing to the quasi‐spherical morphology, good dispersity, high purity, and single‐crystal structure, the submicro‐sized β‐Si₃N₄ particles were promising fillers for preparing resin‐based composites with high thermal conductivity.     

Sintering and Dielectric Properties of Li2O–B2O3–Al2O3–SiO2 Glass‐Added (Ca0.7Sr0.3O)1.03(Ti0.1Zr0.9)O2 for Copper Electrode

When 1.5 wt% of Li₂O–B₂O₃–SiO₂ and 1.5 wt% of Li₂O–B₂O₃–Al₂O₃ glass‐added (Ca_0.7Sr_0.3O)_1.03(Ti_0.1Zr_0.9)O₂ batch was ball milled for 10~30 h followed by sintering at 950°C in flowing N₂‐10%H₂ atmosphere, an apparent density of approximately 4.5 g/cm³, a dielectric constant of approximately 26, and a quality factor of roughly about 3300 GHz were demonstrated. A prolonged ball mill time thereafter significantly decreased both of the dielectric properties because of the enhanced reduction of the specimens during sintering. The apparent evidence of a material reaction between the dielectric material and the copper electrode was not observed.     

High thermal conductivity silicon nitride ceramics prepared by pressureless sintering with ternary sintering additives

Silicon nitride ceramics were pressureless sintered at low temperature using ternary sintering additives (TiO₂, MgO and Y₂O₃), and the effects of sintering aids on thermal conductivity and mechanical properties were studied. TiO₂–Y₂O₃–MgO sintering additives will react with the surface silica present on the silicon nitride particles to form a low melting temperature liquid phase which allows liquid phase sintering to occur and densification of the Si₃N₄. The highest flexural strength was 791(±20) MPa with 12 wt% additives sintered at 1780°C for 2 hours, comparable to the samples prepared by gas pressure sintering. Fracture toughness of all the specimens was higher than 7.2 MPa·m^1/2 as the sintering temperature was increased to 1810°C. Thermal conductivity was improved by prolonging the dwelling time and adopting the annealing process. The highest thermal conductivity of 74 W/(m∙K) was achieved with 9 wt% sintering additives sintered at 1810°C with 4 hours holding followed by postannealing.     

Effects of Al2O3–Y2O3/Yb2O3 additives on microstructures and mechanical properties of silicon nitride ceramics prepared by hot-pressing sintering

Silicon nitride (Si₃N₄) ceramics doped with two different sintering additive systems (Al₂O₃–Y₂O₃ and Al₂O₃–Yb₂O₃) were prepared by hot-pressing sintering at 1800℃ for 2 h and 30 MPa. The microstructures, nano-indentation test, and mechanical properties of the as-prepared Si₃N₄ ceramics were systematically investigated. The X-ray diffraction analyses of the as-prepared Si₃N₄ ceramics doped with the two sintering additives showed a large number of phase transformations of α-Si₃N₄ to β-Si₃N₄. Grain size distributions and aspect ratios as well as their effects on mechanical properties are presented in this study. The specimen doped with the Al₂O₃–Yb₂O₃ sintering additive has a larger aspect ratio and higher fracture toughness, while the Vickers hardness is relatively lower. It can be seen from the nano-indentation tests that the stronger the elastic deformation ability of the specimens, the higher the fracture toughness. At the same time, the mechanical properties are greatly enhanced by specific interlocking microstructures formed by the high aspect ratio β-Si₃N₄ grains. In addition, the density, relative density, and flexural strength of the as-prepared Si₃N₄ ceramics doped with Al₂O₃–Y₂O₃ were 3.25 g/cm³, 99.9%, and 1053 ± 53 MPa, respectively. When Al₂O₃–Yb₂O₃ additives were introduced, the above properties reached 3.33 g/cm³, 99.9%, and 1150 ± 106 MPa, respectively. It reveals that microstructure control and mechanical property optimization for Si₃N₄ ceramics are feasible by tailoring sintering additives.     

Effect of Sintering Additives on Relative Density and Li‐ion Conductivity of Nb‐Doped Li7La3ZrO12 Solid Electrolyte

Lithium ion conductors with garnet‐type structure are promising candidates for applications in all solid‐state lithium ion batteries, because these materials present a high chemical stability against Li metal and a rather high Li⁺ conductivity (10^?3–10^?4 S/cm). Producing densified Li‐ion conductors by lowering sintering temperature is an important issue, which can achieve high Li conductivity in garnet oxide by preventing the evaporation of lithium and a good Li‐ion conduction in grain boundary between garnet oxides. In this study, we concentrate on the use of sintering additives to enhance densification and microstructure of Li₇La₃ZrNbO₁₂ at sintering temperature of 900°C. Glasses in the LiO₂‐B₂O₃‐SiO₂‐CaO‐Al₂O₃ (LBSCA) and BaO‐B₂O₃‐SiO₂‐CaO‐Al₂O₃ (BBSCA) system with low softening temperature (<700°C) were used to modify the grain‐boundary resistance during sintering process. Lithium compounds with low melting point (<850°C) such as LiF, Li₂CO₃, and LiOH were also studied to improve the rearrangement of grains during the initial and middle stages of sintering. Among these sintering additives, LBSCA and BBSCA were proved to be better sintering additives at reducing the porosity of the pellets and improving connectivity between the grains. Glass additives produced relative densities of 85–92%, whereas those of lithium compounds were 62–77%. Li₇La₃ZrNbO₁₂ sintered with 4 wt% of LBSCA at 900°C for 10 h achieved a rather high relative density of 85% and total Li‐ion conductivity of 0.8 × 10^?4 S/cm at room temperature (30°C).     

Thermal and mechanical properties of Si3N4 ceramics obtained via two-step sintering

Si₃N₄ ceramics were prepared using novel two-step sintering method by mixing α-Si₃N₄ as raw material with nanoscale Y₂O₃–MgO via Y(NO₃)₃ and Mg(NO₃)₂ solutions. Si₃N₄ composite powders with in situ uniformly distributed Y₂O₃–MgO were obtained through solid–liquid (SL) mixing route. Two-step sintering method consisted of pre-deoxidization at low temperature via volatilization of in situ-formed MgSiO₃ and densification at high temperature. Variations in O, Y, and Mg contents in Si₃N₄–Y₂O₃–MgO during first sintering step are discussed. O and Mg contents decreased with increasing temperature because SiO₂ on Si₃N₄ surface reacted with MgO to form low-melting-point MgSiO₃ compound, which is prone to volatilize at high temperature. By contrast, Y content hardly changed due to high-temperature stability of Y–Si–O–N quaternary compound. In the second sintering step, skeleton body was densified, and the formation of Y₂Si₃O₃N₄ secondary phase occurred simultaneously. Two-step sintered Si₃N₄ ceramics had lower total oxygen content (1.85 wt%) than one-step sintered Si₃N₄ ceramics (2.51 wt%). Therefore, flexural strength (812 MPa), thermal conductivity (92.1 W/m·K), and fracture toughness (7.6 MPa?m^1/2) of Si₃N₄ ceramics prepared via two-step sintering increased by 28.7%, 16.9%, and 31.6%, respectively, compared with those of one-step sintered Si₃N₄ ceramics.     

Enhanced thermal conductivity in Si3N4 ceramics by carbonizing polydopamine coatings

To enhance the thermal conductivity of Si₃N₄, a polydopamine (PDA) coating was creatively introduced into ceramics sintered with different additive contents through a two-step sintering process consisting of a first treatment at 1500 °C for 8 h followed by 12 h at 1900 °C under 1 MPa nitrogen pressure. After the first-step sintering, the PDA-coated sample exhibited a higher elimination effect of the liquid phase and an increase in the N/O ratio, which allowed the additives to directly interact with the Si₃N₄ grains, resulting in a microstructure with more and larger rod-shaped grains. After the second-step sintering, the densified samples of the PDA-coating attained slightly coarser rod-shaped grains, lower O content, higher N/O ratio and peak intensity (XRD) of the Y₂Si₃O₃N₄ phase, thicker grain boundary film, and secondary phases mainly residing in multigrain junctions. Consequently, the thermal conductivity of all the PDA-coated samples typically showed a 10–12% increment in comparison to the PDA-free samples for each additive content.     

Low‐Temperature Sintering and Microwave Dielectric Properties of CaMoO4‐Based Temperature Stable LTCC Material

The CaMoO₄–xY₂O₃–xLi₂O ceramics were prepared by the solid‐state reaction method. The sintering behavior, phase evolution, microstructure, and microwave dielectric properties were investigated. CaMoO₄ solid solution was obtained when x = 0.030, and two‐phase system including tetragonal CaMoO₄ phase and cubic Y₂O₃ phase formed when 0.066 ≤ x ≤ 1.417. A temperature stable CaMoO₄‐based microwave dielectric ceramic with ultralow sintering temperature (775°C) was obtained in the CaMoO₄–xY₂O₃–xLi₂O system when x = 0.306, which showed good microwave dielectric properties with a low permittivity of 9.5, a high Q_f value of 63 240 GHz, and a near‐zero temperature coefficient of resonant frequency of +7.2 ppm/°C.     

Effects of Impurity Iron Content on Characteristics of Sintered Reaction‐Bonded Silicon Nitride

The effects of impurity iron content on characteristics of sintered reaction‐bonded silicon nitrides were examined by adding iron powder to a high purity raw Si powder. Powder compacts of the raw Si powder doped with 2 mol% Y₂O₃ and 5 mol% MgSiN₂ as sintering additives and Fe as impurity (0 mass%, 0.1 mass%, 1.0 mass% and 5.0 mass%) were nitrided at 1400°C for 8 h under a N₂ pressure of 0.1 MPa, followed by post‐sintering at 1900°C for 6 h under a N₂ pressure of 0.9 MPa. All the SRBSN (Sintered Reaction‐Bonded Silicon Nitride) specimens had about the same 4‐point bending strength of 730–770 MPa. The fracture toughness of the specimens was gradually decreased with increasing Fe additive amount due to the inhibition of development of rodlike β‐Si₃N₄ grains by SiFe_x particles formed during nitridation process. The thermal conductivity was also decreased with an increase in Fe amount. It seems that the increasing oxygen in grain‐boundary phase caused by the oxidation of Fe during milling resulted in the increase in lattice oxygen of β‐Si₃N₄ grains, which caused phonon scattering and thereby decreased thermal conductivity of β‐Si₃N₄. There was little change in the dielectric breakdown strength of the specimens: 24, 22, 22, and 21 kV/mm for the specimens without Fe, and with 0.1 mass%, 1.0 mass% and 5.0 mass% Fe, respectively. The surface resistivity of the specimens with 0 mass%, 0.1 mass% and 1.0 mass% Fe was in the range of 10¹³ Ω, but the specimen with 5 mass% Fe was about one order lower than the others.     

Fabrication and properties of pressure-sintered reaction-bonded Si3N4 ceramics with addition of Eu2O3–MgO–Y2O3

Dense pressure-sintered reaction-bonded Si₃N₄ (PSRBSN) ceramics were obtained by a hot-press sintering method. Precursor Si powders were prepared with Eu₂O₃–MgO–Y₂O₃ sintering additive. The addition of Eu₂O₃–MgO–Y₂O₃ was shown to promote full nitridation of the Si powder. The nitrided Si₃N₄ particles had an equiaxial morphology, without whisker formation, after the Si powders doped with Eu₂O₃–MgO–Y₂O₃ were nitrided at 1400 °C for 2 h. After hot pressing, the relative density, Vickers hardness, flexural strength, and fracture toughness of the PSRBSN ceramics, with 5 wt% Eu₂O₃ doping, were 98.3 ± 0.2%, 17.8 ± 0.8 GPa, 697.0 ± 67.0 MPa, and 7.3 ± 0.3 MPa m^1/2, respectively. The thermal conductivity was 73.6 ± 0.2 W m^?1 K^?1, significantly higher than the counterpart without Eu₂O₃ doping, or with ZrO₂ doping by conventional methods.     

Non-additive sintering of Si2N2O ceramic with enhanced high-temperature strength,oxidation resistance,and dielectric properties

The high-temperature mechanical and dielectric properties of Si₂N₂O ceramics are often limited by the introduction of a sintering aid. Herein, dense Si₂N₂O was prepared at 1700 °C by hot-pressing oxidized amorphous Si₃N₄ powder without sintering additives. A homogeneous network with short-range order and a SiN₃O structure was formed in the oxidized amorphous Si₃N₄ powder during the hot-pressing process. Si₂N₂O crystals preferentially nucleated at positions within the SiN₃O structure and grew into rod-like and plate-like grains. Fully dense ceramics with mainly crystalline Si₂N₂O and some residual amorphous phases were obtained. The as-prepared Si₂N₂O possessed a good flexural strength of 311 ± 14.9 MPa at 1400 °C, oxidation resistance at 1500 °C, and a low dielectric loss tangent of less than 5 × 10⁻³ at 1000 °C.     

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si₃N₄) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La₂O₃, Sm₂O₃, Y₂O₃, and Lu₂O₃) on the phase compositions, development of Si₃N₄ grains and flexural strength (especially high-temperature flexural strength) were researched. Si₃N₄ ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si₃N₄ ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.     

Synthesis of Single‐Phase β‐Yb2Si2O7 and Properties of Its Sintered Bulk

Single‐phase β‐Yb₂Si₂O₇ was synthesized by solid‐state reaction using Yb₂O₃ and SiO₂ gel. SiO₂ gel significantly decreased the synthesis temperature and shortened the holding time. Bulk Yb₂Si₂O₇ was obtained by pressureless sintering. Grain size, relative density (92.9%), and flexural strength [(182.3 ± 2.0) MPa] were enhanced as the sintering temperature increased and equiaxed grains were obtained with an average grain size of approximately 3 μm. Bulk Yb₂Si₂O₇ possessed a suitable thermal expansion coefficient [(4.64 ± 0.01) × 10^?6/K] between 473 and 1573 K, and the thermal conductivities at 300 and 1400 K were 4.31 and 2.27 W/m·K, respectively.     

Silicon Oxynitride Ceramics Prepared by Plasma Activated Sintering of Nanosized Amorphous Silicon Nitride Powder without Additives

Si₂N₂O ceramics were prepared by plasma activated sintering using nanosized amorphous Si₃N₄ powder without sintering additives within a temperature range of 1400°C–1600°C in vacuum. A mixed Si–N_4?n–O_n (n = 0, 1…4) amorphous structure was formed in the process of sintering, and Si₂N₂O crystals were nucleated where the local structure was similar with Si₂N₂O. After sintering at 1600°C, the Si₂N₂O ceramic was composed of elongated plate‐like Si₂N₂O grains and amorphous phase. The Si₂N₂O grains showed a width of less than 100 nm and a very high aspect ratio.     

Fabrication of highly dense Si3N4 via record low-content additive system for low-temperature pressureless sintering

Dense Si₃N₄ ceramics were fabricated by pressureless sintering at a low temperature of 1650°C with a short holding period of 1 h under a nitrogen atmosphere. The role of ternary oxide additives (Y₂O₃–MgO–Al₂O₃, Y₂O₃–MgO–SiO₂, Y₂O₃–MgO–ZrO₂) on the phase, microstructure, and mechanical properties of Si₃N₄ was examined. Only 5 wt.% of Y₂O₃–MgO–Al₂O₃ additive was sufficient to achieve >98% of theoretical density with remarkably high biaxial strength (∼1200 MPa) and prominent hardness (∼15.5 GPa). Among the three additives used, Y₂O₃–MgO–Al₂O₃ displayed the finest grain diameter (0.54 μm), whereas Y₂O₃–MgO–ZrO₂ produced the largest average grain diameter (∼0.95 μm); the influence was seen on their mechanical properties. The low additive content Si₃N₄ system is expected to have superior high-temperature properties compared to the system with high additive content. This study shows a cost-effective fabrication of highly dense Si₃N₄ with excellent mechanical properties.