Si3N4粉粒度对振荡压力烧结Si3N4陶瓷的影响

为了提高Si₃N₄陶瓷的烧结致密度,采用振荡压力烧结工艺分别在1 745和1 775℃制备了Si₃N₄陶瓷,主要研究了Si₃N₄粉的粒度（平均粒径分别为0.4、2.0、2.3μm）对Si₃N₄陶瓷的显微结构和性能的影响。结果显示：1)在两种温度的振荡压力烧结工艺下,由三种不同粒度的Si₃N₄粉制备的Si₃N₄陶瓷的相对密度都很大,为99.65%～99.86%,彼此相差很小。2)由平均粒径为0.2μm的Si₃N₄粉在1 745℃烧结制备的试样的微观结构最均匀,其β-Si₃N₄晶粒平均长径比、抗弯强度和维氏硬度均最大,分别达到5.0、（1 364±65） MPa和（15.72±0.8） GPa;由平均粒径为2.3μm的Si₃     

Formation mechanism of elongated β–Si3N4 crystals in Fe–Si3N4 composite via flash combustion

Elongated β–Si₃N₄ crystals have a significant influence on the mechanical property of Fe–Si₃N₄ composite. In this paper, the formation mechanism of elongated β–Si₃N₄ crystals in Fe–Si₃N₄ composite was investigated. During the preparation process, β–Si₃N₄ crystals developed in a spiral and layer growth mechanism in the dense areas. They kept growing from the dense areas and formed radially distributed elongated crystals with hexagonal prismatic morphology as time went on. As for the formation mechanism, the (100) crystal plane of β–Si₃N₄ from Si-N-O melt is mainly the vicinal crystal planes growth with different angles from the (100) crystal plane. At the later stage, the crystallization and the diffusion forces in Si-N-O molten phase decreased. However, the short range diffusion remained active and resulted in the gradient distribution of N content near the boundary. With the temperature decreasing, the disappearance of the short range diffusion implied the end of the crystallization process of the elongated β–Si₃N₄ crystals.     

Sn/Si3N4复合镀层中纳米Si3N4含量测定方法的比较

采用电熔解法溶解Sn/Si3N4复合镀层,以紫外可见分光光度法测定了镀层中纳米Si3N4的含量,并与传统酸溶解及重量法对比。结果表明,电解法在10 min内使镀层完全溶解且在测定波长448 nm处纳米Si3N4溶液的吸光度值与其浓度在10～200 mg/L范围保持良好的线性关系,该方法简单,省时,误差小,回收率98.5%～100%。     

Microstructure and reaction phases in Si3N4/Si3N4 joint brazed with Cu–Pd–Ti filler alloy

Si₃N₄ ceramic was self-jointed using a filler alloy of Cu–Pd–Ti, and the microstructure of the joint was analyzed. By using a filler alloy of Cu76.5Pd8.5Ti15 (at.%), a high quality Si₃N₄/Si₃N₄ joint was obtained by brazing at 1100–1200 °C for 30 min under a pressure of 2 × 10⁻³ MPa. The microstructure of the Si₃N₄/Si₃N₄ joint which was observed by EPMA, XRD and TEM, and the results indicated that a reaction layer of TiN existed at the interface between Si₃N₄ ceramic and filler alloy. The center of the joint was Cu base solid solution containing Pd, and some reaction phases of TiN, PdTiSi and Pd₂Si found in the Cu [Pd] solid solution.     

Synthesizing high α-phase Si3N4 powders containing sintering additives

Fine grain α-phase silicon nitride (Si₃N₄) ceramic powders were produced via carbothermic reduction of colloidal SiO₂, which contained pre-mixed additives of sintering aids primarily consisting of oxides such as MgO and Y₂O₃. The powders that were pre-mixed in the starting reactants were chosen based on the final powder composition and on type and amount of the secondary phases desired for sintering. After synthesizing, powder properties were examined using standard characterization techniques (XRD, SEM, BET, etc). This technique of ceramic synthesis has advantages in providing nitride-based ceramic powders, which contain secondary in situ phases that are distributed as sintering additives. Silicon nitride ceramic powders synthesized using this method might therefore be readily sintered because the homogeneously distributed sintering additives were present in the starting materials. In this work, the processing parameters are described in terms of the synthesis conditions.     

用硅灰合成无碳Si3N4/SiC纳米粉末

用机械与同步热激活法首次从废硅灰中合成无碳Si3N4/SiC纳米粉末。通过这种新方法可在1 465℃形成晶粒尺寸小至45nm的Si3N4/SiC纳米粉末。为了合成无碳Si3N4/SiC纳米粉末,研究了两种不同方法移除纳米粉末中游离碳的有效性。这两种方法分别使用氢气和空气。研究发现,虽然使用氢气时Si3N4和SiC是稳定的,但是这种方法不是很有效。相比之下,使用空气移除游离碳是有效的。游离碳除去后立即停止使用空气,Si3N4/SiC纳米粉末也会有少量被氧化。本文提供了明确的途径将硅灰有效合成无碳Si3N4/SiC纳米粉末。     

SiC原料颗粒级配对Si3N4结合SiC复合陶瓷材料的影响

采用SiC粉和Si粉高温氮化反应烧结制备Si3N4结合SiC复合陶瓷材料。研究四种SiC原料粉体(0-1mm、74μm、44μm、和0.5μm)中三种不同粒度不同含量颗粒级配对Si3N4结合SiC复合陶瓷材料的影响。通过X射线衍射仪和扫描电子显微镜对试样的物相和显微结构进行表征,并对试样的耐压强度等力学性能进行测试。结果表明:采用三种SiC较细粉体颗级配且如下组成:74μm的含量为5 wt%,44μm的含量为10 wt%,0.5μm的含量为35 wt%,所制备的Si3N4结合SiC陶瓷材料的基本烧结性能较好,其体积密度为2.43g/cm3,耐压强度为324MPa。     

ZrSiO4对Si3N4结合SiC制品强度和抗氧化性的影响

研究了硅酸锆(ZrSiO     

Shock wave-material interaction in ZrB2–SiC based ultra high temperature ceramics for hypersonic applications

We investigate the thermochemical stability of ZrB₂–SiC based multiphase ceramics to hypersonic aerothermodynamic conditions in free piston shock tube with an objective to understand quantitatively the role of thermal shock and pressure. The developed ceramics sustained impulsive thermomechanical shock, under reflected shock pressure of 6.5 MPa and reflected shock temperature of 4160 K in dissociated oxygen, without structural failure. The conjugate heat transfer analysis predicts the surface temperature of ZrB₂–SiC to reach a maximum of 693 and 865 K, for ZrB₂–SiC–Ti. The transient shock-material response is characterized by surface oxidation of the investigated ceramics, when exposed to high enthalpy gaseous environment, as a consequence of the interaction with ultrafast-heated (10⁶ K/s) gas for ~5 ms. Spectroscopic and structural characterization reveals that addition of Ti improves thermomechanical shock resistance, which is attributed to the assemblage of refractory phases. Taken together, ZrB₂–SiC–Ti based multiphase ceramics exhibit favorable shock-material response under impulse loading.     

B4C对Si3N4/Si2N2O结合SiC材料抗热震性能的影响

首先以Si粉、SiO₂微粉为原料,先在700℃空气气氛处理,然后在1 400℃氮气气氛下合成Si₂N₂O,研究了B₄C添加量(外加质量分数分别为0、1.0%、2.0%、3.0%、4.0%)对Si₂N₂O合成效果的影响。然后根据B₄C最优加入量,先在700℃空气气氛保温5 h,然后在1 400℃氮气气氛保温5 h制备了Si₃N₄/Si₂N₂O结合SiC试样。采用1 300℃风冷5次后试样的抗折强度保持率评价其抗热震性,分析了热震前后试样的物相组成和显微结构。结果表明：1)合成Si₂N₂O的B₄C最优添加量为3%(w);在700℃空气处理时,B₄C优先和气氛中O₂反应生成液相B₂O₃,为1 400℃氮...     

Microstructure evolution of Si3N4/Si3N4 joint brazed with Ag–Cu–Ti + SiCp composite filler

The Si₃N₄ ceramic was brazed by Ag–Cu–Ti + SiCp composite filler (p = particle) prepared by mechanical mixing. Effects of the content of Ti and SiC particles on microstructure of the joint were investigated. A reliable Si₃N₄/Si₃N₄ joint was achieved by using Ag–Cu–Ti + SiCp composite filler at 1173 K for 10 min. A continuous and compact reaction layer, with a suitable thickness, forms at the Si₃N₄/braze interface. The SiC particles react with Ti in the brazing layers, forming Ti₃SiC₂ thin layers around the SiC particles themselves and Ti₅Si₃ small particles in the Ag[Cu] and Cu[Ag] based solid solution. The higher content of SiC particles in the filler (≥10 vol%) depresses interfacial bonding strength between the Si₃N₄ substrate and composite brazing layer due to the thinner reaction layer and the bad fluidity of the filler. The Ti₃SiC₂ → TiC + Ti₅Si₃ reaction occurs when Ti concentration around SiC particles in the filler increases.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

低温热压制备高硬度、高韧性Si3N4–SiCw–ZrB2陶瓷

通常低温热压烧结的Si₃N₄陶瓷具有较高的硬度和较低的断裂韧性;而高温热压烧结的Si₃N₄陶瓷具有较低的硬度和较高的断裂韧性。为了获得高硬度、高韧性Si₃N₄陶瓷,添加20%SiC_w（SiC晶须,体积分数）和2.5%ZrB2,在1 500℃低温热压制备了Si₃N₄基陶瓷,开展其相组成、致密度、显微结构和力学性能研究,并与1 800℃高温热压烧结Si₃N₄进行了对比研究。结果表明：SiC_w的引入阻碍了Si₃N₄低温致密化,致密度从97.9%降低到92.9%,Vickers硬度从20.5 GPa降低到16.4 GPa,断裂韧性从2.9 MPa·m^1/2增加到3.4 MPa·m^1/2。同步引入SiC_w和ZrB₂     

Fabrication and wear behaviors of graded Si3N4 ceramics by the combination of two-step sintering and β-Si3N4 seeds

Graded Si₃N₄ ceramics with sandwich-like microstructure were fabricated by the combination of hot-pressing, spark plasma sintering and β-Si₃N₄ seeds. Phase compositions, microstructures, mechanical properties, and wear behaviors were investigated. Main α-Si₃N₄ phase were detected in the outer layers, and only β-Si₃N₄ phase were observed in the inner layers. The outer layer with ultra-fine equiaxed grains were well bonded to the inner layer with a distinct bimodal grain size distribution. Vickers hardness of outer layer (～21.2?GPa) was much higher than that of inner layer (～16.1?GPa), whereas fracture toughness of outer layer (～3.5?MPa?m^1/2) was much lower than that of inner layer (～5.9?MPa?m^1/2), indicative of the hard surface and tough core. Due to the ultra-fine microstructure and high hardness of outer layer, the graded Si₃N₄ ceramics exhibited superior wear resistance with low wear rate.     

Si3N4粉体表面化学分析及表面改性

Si3N4陶瓷是一种重要的结构陶瓷,本文着重阐述了Si3N4粉体的表面分析方法,以及为了提高固相体积分数而进行的Si3N4粉体表面改性方面的进展.     

Elastic–plastic analysis of ultrafine-grained Si2N2O–Si3N4 composites by nanoindentation and finite element simulation

Ultrafine-grained Si₂N₂O–Si₃N₄ composites are fabricated by hot-press sintering of amorphous nano-sized silicon nitride powders at 1600, 1650, and 1700 °C with nano-sized Al₂O₃ and Y₂O₃ as additives. Sintered materials of increasing average grain sizes of 280, 360, and 480 nm were obtained with increasing sintering temperature. Hardness and elastic modulus are tested by nanoindentation. Finite element simulations of the nanoindentation are performed to study the elastic and plastic mechanical properties based on the elastic modulus and P–h curve that were obtained through the nanoindentation tests. A theoretical method is proposed for calculating the stress–strain relationship of brittle ceramic materials based on the experimental nanoindentation data. Several relevant coefficients in the theoretical calculation formula are determined by comparing the calculation and simulation results.     

Joining of Si3N4 to Si3N4 using a AuPd(Co,Ni)–V filler alloy and the interfacial reactions

Au38.0–Pd28.0–Co18.0–Ni7.0–V9.0 (in wt%) alloy was designed as a filler for joining Si₃N₄. The filler alloy showed a contact angle of 77.2° on Si₃N₄ ceramic at 1473 K. The Si₃N₄/Si₃N₄ joint brazed with the rapidly-solidified filler foils at 1443 K for 10 min exhibits an average three-point bend strength of 320.7 MPa at room temperature and the strength values are 217.9 MPa and 102.9 MPa at 1073 K and 1173 K respectively. The interfacial reaction products were composed of V₂N and Pd₂Si, and the elements Co and Ni in the brazing alloy did not participate in the interfacial reactions. The coarse-network-like distribution of refractory Pd₂Si compound within the Au–Pd–Co–Ni alloy matrix throughout the joint contributes to the stable high-temperature joint strengths.     

Dielectric properties of Cf–Si3N4 sandwich composites prepared by gelcasting

C_f–Si₃N₄ sandwich composites were prepared by gelcasting using α-Si₃N₄ powder, SiC-coated carbon fibers and sintering additives as starting materials. The microstructure and composition, dielectric properties of C_f–Si₃N₄ sandwich composites were investigated. SEM and EDS analysis results reveal that the SiC interphase could effectively overcome incompatibility between carbon fiber and silicon nitride matrix under the condition of pressure-less sintering at 1700 °C. The investigation of microwave absorbing property reveals that, compared with the Si₃N₄ ceramics, both the real (ε?$\epsilon ?$) and imaginary (ε??$\epsilon ??$) permittivity of C_f–Si₃N₄ sandwich composites show strong frequency dispersion characteristics at X-band. Microwave absorption ability of the C_f–Si₃N₄ sandwich composites are significantly enhanced compared with pure Si₃N₄ ceramic, and the reflection loss gradually decreases from −3.5 dB to −14.4 dB with the increase of frequency, while the pure Si₃N₄ ceramic keeps at −0.1 dB. Particularly, the relationship between permittivity of C_f–Si₃N₄ sandwich composites and frequency at X-band has been established through an equivalent RC circuit model. Results showed that both ε?$\epsilon ?$ and ωε??$\omega \epsilon ??$ are inversely proportional to the frequency square ω²${\omega }^2$, and the predicted results agree quite well with the measured data.