Influence of α-Si3N4 coarse powder on densification,microstructure, mechanical properties,and thermal behavior of silicon nitride ceramics

Silicon nitride (Si₃N₄) ceramics, with different ratios of fine and coarse α-Si₃N₄ powders, were prepared by spark plasma sintering (SPS) and heat treatment. Further, the influence of coarse α-Si₃N₄ powder on densification, microstructure, mechanical properties, and thermal behavior of Si₃N₄ ceramics was systematically investigated. Compared with fine particles, coarse particles exhibit a slower phase transition rate and remain intact until the end of SPS. The remaining large-sized grains of coarse α-Si₃N₄ induce extensive growth of neighboring β-Si₃N₄ grains and promote the development of large elongated grains. Noteworthy, an appropriate number of large elongated grains distributed among fine-grained matrix forms bimodal microstructural distribution, which is conducive to superior flexural strength. Herein, Si₃N₄ ceramics with flexural strength of 861.34 MPa and thermal conductivity of 65.76 W m⁻¹ K⁻¹ were obtained after the addition of 40 wt% coarse α-Si₃N₄ powder.     

Mechanical properties and thermal conductivity of Si3N4 ceramics with YF3 and MgO as sintering additives

Si₃N₄ ceramic was densified by hot pressing sintering at 1750 °C for 1 h under the uniaxial pressure of 20 MPa in N₂ atmosphere with YF₃ and MgO as sintering additives. The thermal conductivities of SN-YF specimen were both higher than that of Si₃N₄ ceramic sintered with Y₂O₃ and MgO before and after annealing treatment. The grain size and aspect ratio in SN-YF specimen were both bigger than those in the SN-YO specimen, which was beneficial for the creation of high thermal conductive path. On the other hand, the improvement of thermal conductivity by the addition of YF₃ might be attributed to the reduction of the grain boundary phase due to the evaporation of SiF₄, and the resultant reduction of the lattice oxygen due to the reduction of SiO₂ in the grain boundary phase. The {0001} direction of grains had the probability of growing along the hot pressing direction in the SN-YF specimen, which was beneficial for the improvement of thermal conductivity while the {0001} direction grew along the X₀-Y₀ plane in the SN-YO specimen. The mechanical properties of SN-YF specimen were comparable to those of SN-YO specimen.     

The effect of gelcasting parameters on microstructural optimization of porous Si3N4 ceramics

In this article we are reporting the optimization of gelcasting process which enabled us to manufacture silicon nitride bodies with around 30–37% porosity and 182–250?MPa flexural strength. Owing to their potential applications mainly in biomedical and aerospace, porous ${\text{Si}}_3{\text{N}}_4$ ceramics have gained ever increasing interests. However, the main challenge has been ensuring their high strength while maintaining high porosity. ${\text{Si}}_3{\text{N}}_4$ bodies were prepared via a controlled gelcasting followed by pressureless sintering in a coke bed. Monomers including acrylamide (AM) and $\text{N},{\text{N}}^{\text{'}}$ -methylenebisacrylamide (MBAM) were employed to formulate the primary slurry. Sub-micron ${\text{Si}}_3{\text{N}}_4$ powder of 35?vol% was used as solid loading where ammonium persulfate (APS) and $\text{N},\text{N},{\text{N}}^{\text{'}},{\text{N}}^{\text{'}}$ -tetramethylethylenediamine (TEMED) were added to initiate and catalyze polymerization reactions. Sintering was carried out at 1650?°C for 2–6?h and at 1750?°C for 2?h where 6?wt% mixture of ${\text{Al}}_2{\text{O}}_3?{\text{Y}}_2{\text{O}}_3$ was included as a sintering aid. Phase characterization and microstructural evolution were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively; while mechanical evaluation was based on bending test. It was found that by optimizing the viscosity of slurry and the idle time of gelation, a well distribution of high porosity structure is developed. The high strength of the sintered bodies is related to a high volume and unique microstructure of fine and elongated β- ${\text{Si}}_3{\text{N}}_4$ grains evolved during optimized sintering conditions. At temperatures above 1650?°C and sintering times of more than 4?h, the strength is decreased due to coarsening of β- ${\text{Si}}_3{\text{N}}_4$ grains, however.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Microwave dielectric properties and thermal conductivities of low-temperature sintered (Na1-xKx)2MoO4 (x ≤ 0.2) ceramics

The Mo-based glass-free spinel-type structure of the (Na_1-xK_x)₂MoO₄ (x = 0.0, 0.1, and 0.2) ceramic series was prepared using the traditional solid-state method at the low sintering temperature (<650 °C). The microwave dielectric properties of the (Na_1-xK_x)₂MoO₄ series were determined in terms of phase compositions, crystal structure (via XRD), and microstructure analysis (via FE-SEM and EDS). The results revealed that the double-phase (cubic and orthorhombic) formation plays a significant role in the entire (Na_1-xK_x)₂MoO₄ series. It exhibits excellent dielectric properties: dielectric constant ε_r = 4 (1 GHz)/3.77 (15 GHz), tangent loss tan δ = 8.3 × 10^?2 (1 GHz)/7 × 10^?3 (15 GHz; Q × f = 2143 GHz), temperature coefficient of frequency (TCF) τ_f = ?6.45 ppm/°C, and room temperature thermal conductivity (κ) = 1.76 W/(m.K) for x = 0.1 at a sintering temperature of 575 °C. These make the (Na_1-xK_x)₂MoO₄ ceramic series a potential candidate for low-temperature co-fired ceramic (LTCC) substrate applications (as used in antennas) for high-speed data communications.     

Influence of injection temperature and pressure on the properties of alumina parts fabricated by low pressure injection molding (LPIM)

The quality of ceramics parts made by powder injection molding (PIM) method is influenced by a range of factors such as powder and binder characteristics, rheological behavior of feedstock, molding parameters and debinding and sintering conditions. In this study, to optimize the molding parameters, the effect of injection temperature and pressure on the properties of alumina ceramics in the LPIM process were thoroughly studied. Experimental tests were conducted on alumina feedstock with 60 vol% powder. Injection molding was carried out at temperatures and pressures of 70–100 °C and 0.1–0.6 MPa respectively. Results showed that increase in injection temperature and pressure and the resulting increase in flow rate leads to the formation of void which impairs the properties of molded parts. The SEM studies showed that injection at temperature of 100 °C results in evaporation of binder components. From the processing point of view, the temperature of 80 °C and pressure of 0.6 MPa seems to be the most suitable condition for injection molding. In addition, the effects of sintering conditions (temperature and time) on the microstructure and mechanical properties are discussed. The best final properties were found using injection molding under the above stated conditions, thermal debinding and sintering at 1700 °C during 3 h.     

Effects of YH2 addition on pressureless sintered AlN ceramics

A new type of non-oxide sintering additive of YH₂ was introduced for the fabrication of AlN ceramics with high thermal conductivity and flexural strength. The effects of YH₂ addition (0–5 wt%) on the phase composition, densification, microstructure, thermal conductivity and flexural strength of pressureless sintered AlN ceramics were investigated and compared with those Y₂O₃-added samples (1–5 wt%). The addition of 1 wt% YH₂ led to an in-situ reduction reaction with oxygen impurities, the formation of Y₂O₃ and finally the formation of yttrium aluminate, which in turn improved densification and microstructure. A high flexural strength (408.69 ± 28.23 MPa) was achieved. The addition of 3 wt% YH₂ increased the average grain size and purified the lattice. All these effects are believed to help achieve a high thermal conductivity of 184.82 ± 1.75 W·m^?1·K^?1. Although the thermal conductivity was close to the value of 3 wt% Y₂O₃-added sample, its strength was much increased to 381.53 ± 43.41 MPa. Meanwhile, it demonstrated a good combination of the thermal conductivity and flexural strength than the values reported in some literature. However, further increasing the YH₂ addition to 5 wt% resulted in a high N/O ratio that inhibited the densification behavior of AlN ceramics. The current study showed that AlN ceramics with excellent thermal and mechanical properties could be obtained by the introduction of a suitable YH₂ additive.     

Secondary phases,microstructures and properties of AlN ceramics sintered by adding nitrate sintering additives

AlN ceramics were sintered at a temperature range from 1650 to 1800°C through adding the Ca and Y nitrate sintering additives. Secondary phases, microstructures and properties of the AlN ceramics were studied. When the AlN ceramics are sintered at 1650 or 1675°C, CaO and Y₂O₃ from the sintering additives react with Al₂O₃ in the AlN powder to generate CaAl₄O₇ and Y₃Al₅O₁₂. Part of Y₃Al₅O₁₂ reacts with CaO and Al₂O₃ to form CaYAl₃O₇ at 1700°C. At 1800°C, CaYAl₃O₇ decomposes into CaAl₄O₇ and Y₃Al₅O₁₂. Finally, CaAl₄O₇ volatilises and only Y₃Al₅O₁₂ remains. As the sintering temperature increases, the AlN grains grow continuously and the bending strength and thermal conductivity of the AlN ceramics increase first and then decrease. The AlN ceramics sintered at 1700°C are fully dense and have the highest bending strength and thermal conductivity of 373·7 MPa and 136·7 W m^?1 K^?1 in this work.     

新型氧化铝耐磨材料

以工业煅烧氧化铝为主要原料,通过同结构设计和各类掺杂物加入,使氧化铝试样在１５００℃得到烧结。结果表明：掺杂物的复合加入有效地降低了氧化铝的烧结温度,抑制了晶粒生长,形成细晶结构,从而提高了材料耐磨性能。     

氧化铝陶瓷烧结助剂研究概述

文章综述了近二十多年来国内外氧化铝陶瓷低温烧结助剂体系的研究状况及未来可供研究的方向。     

Effect of ZrO2 content on the microstructure and flexural strength of Al2O3–ZrO2 composites with the stored failure energy-fragmentation relations

High flexural strength is an important mechanical property for a ceramic armor component to withstand high tensile stresses and protect its structural integrity against multiple hits. Also, larger fragments are required in fragmentation as larger fractured parts are harder to leave of the way for the penetrator and cause more abrasion and higher penetration resistance. In this study, the effect of different ZrO₂ content (0, 0.5, 1, 3, 5, 10, 20 vol%) on the flexural strength of Al₂O₃–ZrO₂ composites was investigated with relationship of the stored failure energy-crack length to evaluate the fragmentation behavior under possible impact conditions. Monotonic equibiaxial flexural strength test was used to measure the fracture strength. The highest strength was obtained for 20 vol% ZrO₂ containing composite as 435 ± 78 MPa, ~ 24% increase in comparison with the pure Al₂O₃. The transformation of tetragonal to monoclinic phase occurred during the strength test in the 10 and 20 vol% ZrO₂ content composites. 20 vol% ZrO₂ containing composite had the smallest total crack length accompanying the largest fragment size for a given fracture energy among all the composites due to the stress-induced transformation of ZrO₂ consumes energy that results in decreasing effective crack driving energy required for the crack branching.     

Influences of B2O3/CuO additions on the sintering behavior,microstructure and microwave dielectric properties of 6Nd[(Zn0.7Co0.3)0.5Ti0.5]O3–4(Na0.5Nd0.5)TiO3 ceramics

B₂O₃ and CuO were codoped into 6Nd[(Zn_0.7Co_0.3)_0.5Ti_0.5]O₃–4(Na_0.5Nd_0.5)TiO₃ (abbreviated as 6NZCT–4NNT) ceramics as sintering aids. The influences of the sintering aids on the sintering characteristics, microstructure and microwave dielectric properties of the 6NZCT–4NNT ceramics were systematically investigated as a function of the proportion of B₂O₃ and CuO. Codoping could greatly reduce the sintering temperature from 1410 °C to 1150 °C, indicating that B₂O₃/CuO are good sintering aids for 6NZCT–4NNT ceramics. The B₂O₃/CuO sintering aids had no significant impact on the phase purity of the investigated ceramics, even though a solid solution was formed due to Cu²⁺ ion substitution. However, they had evident influences on the surface morphology and grain size. The average grain size was enlarged with increasing amounts of CuO in the B₂O₃/CuO sintering aids. Remarkable deterioration of the microwave dielectric properties for 6NZCT-4NNT ceramics was not observed when codoping an appropriate amount of B₂O₃ and CuO. The 6NZCT–4NNT ceramics codoped with 2.0 mol% B₂O₃ and 2.0 mol% CuO sintered at 1150 °C for 3 h exhibited a homogeneous microstructure and promising microwave dielectric properties: an appropriate dielectric constant (ε_r = 49.37), a high quality factor (QF = 47,295 GHz), and a near-zero temperature coefficient of resonant frequency (TCF = +0.9 ppm/^°C).     

Influence of processing parameters on thermal conductivity of uranium dioxide pellets prepared by spark plasma sintering

High density uranium dioxide (UO₂) pellets with grain sizes between 0.9 μm and 9 μm were produced by spark plasma sintering (SPS). A systematic study was performed by varying the sintering temperature between 750 °C and 1450 °C and hold time between 0.5 min and 20 min to obtain UO₂ pellets with a range of theoretical densities (TD) and grain sizes. The microstructure development in terms of grain size, density and porosity distribution was investigated. The oxygen/uranium (O/U) ratio of the resulting pellets was found to decrease after SPS. The thermal conductivity of UO₂ pellets increased with the theoretical density but the grain size in the investigated range had no significant influence. The measured thermal conductivity values up to 900 °C were consistent with the reported literature for conventionally sintered UO₂ pellets. The benefits of using SPS over the conventional sintering of UO₂ are summarized.