Synthesis and Characterization of Nanocrystalline Niobium Nitride Powders

Nanocrystalline NbN powder was synthesized by the direct nitridation of amorphous Nb₂O₅ powder with high BET surface area. X-ray diffractometry analysis indicated that the cubic-phase NbN powder could be obtained by nitridation at 650°–800°C for 3–8 h. Transmission electron microscopy images showed that the particle sizes were in the range of 15–40 nm. The effect of the nitridation temperature and holding time on the powder properties was discussed.     

Consolidation of Nano-Sized TiN Powders by Spark Plasma Sintering

Nano-sized TiN powders with an average particle size of 19 nm were synthesized via a new method, reduction–nitridation reaction in liquid ammonia. A consolidation procedure using spark plasma sintering (SPS) was used, and a dense TiN ceramic (>98% of theoretical) with mean grain size of 100–150 nm was obtained at 1380°C. The influence of sintering temperature on grain growth and microstructural evolution was also discussed.     

低温放电等离子烧结法制备氮化硅陶瓷

分别以MgO-Al2O3或MgO-AlPO4作为烧结助剂,采用放电等离子体低温快速烧结方法制备了主相为α相的Si3N4陶瓷材料.采用X射线衍射和扫描电子显微镜分析了样品的物相组成和显微结构;研究了烧结助剂及其含量、烧结温度对陶瓷样品的相对密度与力学性能的影响.结果表明:当采用4%质量分数,下同)MgO-4%Al2O3烧...     

Densification of Nanocrystalline Yttria by Low Temperature Spark Plasma Sintering

We investigated the densification of undoped, nanocrystalline yttria (Y₂O₃) powder by spark plasma sintering (SPS) at sintering temperatures between 650°C and 1050°C at a heating rate of 10°C/min and an applied stress of 83 MPa. In spite of the low sinterability of the undoped Y₂O₃, a remarkable densification of the powder started at about 600°C, and a theoretical density of more than 97% was achieved at a sintering temperature of 850°C with a grain size of about 500 nm. The low temperature SPS is effective for fabricating dense Y₂O₃ polycrystals.     

Photoluminescence Properties of Nanocrystalline ZnO Ceramics Prepared by Pressureless Sintering and Spark Plasma Sintering

Nanocrystalline ZnO ceramics with grain sizes of ∼100 nm were prepared by pressureless sintering at 800°C for 2 h and spark plasma sintering (SPS) at 550°C for 2 min, respectively. Excellent green emission properties were obtained in the ZnO ceramic prepared by the SPS process and in the pressureless-sintered ZnO ceramic prepared at 1000°C for 2 h, which are attributed to the vacuum ambience of the SPS process and the sublimeness of the interstitial Zn at >900°C in air, respectively.     

Field-Assisted Sintering of Nanocrystalline Titanium Nitride

Nanosized TiN powder was densified via field-assisted sintering at temperatures of 1150°–1350°C and a pressure of 66 MPa under vacuum. A maximum relative density of ∼97% and a maximum mean grain size of 150–200 nm were obtained. Densification and microstructural evolution have been discussed, in terms of superplasticity and electric-field effects.     

Phase Transitions in Nanocrystalline Barium Titanate Ceramics Prepared by Spark Plasma Sintering

The bulk dense nanocrystalline BaTiO₃ (BT) ceramics ranging from 20 to 100 nm have been successfully prepared by the spark plasma sintering (SPS) method. Raman spectra and X-ray diffraction were used in combination with electron microscopy to study the evolution of lattice structure and phase transformation behavior with grain growth from nanoscale to micrometer scale for BT ceramics. The results reveal that the SPS technique provides exceptional opportunity to compact ceramics to full density with nanograin size. It is also demonstrated that all structural modifications in nanocrystalline BT and low-symmetry structures still exist in 20 nm nanograin BT ceramics. The ferroelectric properties of crystalline structures were investigated by scanning force microscopy in piezoresponse mode. Piezoelectric hysteresis loop was recorded, demonstrating that 20 nm BT ceramics has a remanent polarization and is switchable by an electric field. Thus, if a critical grain size exists for ferroelectricity, it is less than 20 nm for polycrystalline BT ceramics.     

烧结温度对放电等离子烧结氮化硅陶瓷显微结构和力学性能的影响

以亚微米级氮化硅为原料、Al_2O_3–Y_2O_3为烧结助剂,利用放电等离子烧结(spark plasma sintering,SPS)烧结技术制备氮化硅陶瓷。用X射线衍射和扫描电子显微镜对试样的物相组成和显微结构进行分析,研究了烧结温度对氮化硅陶瓷力学性能和显微结构的影响。结果表明,采用SPS烧结技术可在较低温度下获得致密度较高、综合力学性能较好的β相氮化硅陶瓷。随着烧结温度的提高,样品致密度、抗弯强度、断裂韧性均不断增大,在1 550℃时,其抗弯强度和断裂韧性分别达到973.74 MPa和8.23 MPa?m1/2。在1 550℃以下,陶瓷样品中β相氮化硅含量可达到98%,显微结构均匀,晶粒发育良好、呈长柱状,晶间紧密连接,晶间气孔较少。继续升高温度,部分晶粒发生异常长大,产生了更多的显微孔洞,抗弯强度急剧下降。     

Carbothermal Synthesis of Nanocrystalline Aluminum Nitride Powders

A new precursor technique for the carbothermal synthesis of nanocrystalline AlN powder has been developed. A precursor that contains an intimate mixture of nanocrystalline Al₂O₃ and carbon has been synthesized by using a chemical pyrophoric reaction. The formation of AlN starts at 1473 K, and complete conversion has been observed at temperatures >1673 K. The synthesized AlN particles are nanocrystalline (<100 nm) in size.     

Spark Plasma Sintering of Alumina

A systematic study of various spark plasma sintering (SPS) parameters, namely temperature, holding time, heating rate, pressure, and pulse sequence, was conducted to investigate their effect on the densification, grain-growth kinetics, hardness, and fracture toughness of a commercially available submicrometer-sized Al₂O₃ powder. The obtained experimental data clearly show that the SPS process enhances both densification and grain growth. Thus, Al₂O₃ could be fully densified at a much lower temperature (1150°C), within a much shorter time (minutes), than in more conventional sintering processes. It is suggested that the densification is enhanced in the initial part of the sintering cycle by a local spark-discharge process in the vicinity of contacting particles, and that both grain-boundary diffusion and grain-boundary migration are enhanced by the electrical field originating from the pulsed direct current used for heating the sample. Both the diffusion and the migration that promote the grain growth were found to be strongly dependent on temperature, implying that it is possible to retain the original fine-grained structure in fully densified bodies by avoiding a too high sintering temperature. Hardness values in the range 21–22 GPa and fracture toughness values of 3.5 ± 0.5 MPa·m^1/2 were found for the compacts containing submicrometer-sized Al₂O₃ grains.     

Synthesis of New Nanocrystalline Titanium–Niobium Oxynitride Powders

New titanium–niobium oxynitride (Ti_{1− z} Nb _z O _x N _y ) powders were synthesized by ammonolysis of nanosized TiO₂/Nb₂O₅ composite powders at 700°–900°C for 5 h. The products were characterized by X-ray diffraction (XRD), chemical analysis, and transmission electron microscopy. The results indicated that the as-synthesized powders were pure cubic structures with sizes of 30–60 nm. With increasing value of z , XRD peaks of Ti_{1− z} Nb _z O _x N _y powders tended to shift toward low 2θ angle and the cell parameter showed a linear increase.     

放电等离子烧结氮化硅陶瓷刀具材料的摩擦磨损性能

研究了放电等离子烧结氮化硅陶瓷刀具材料(91wt％ α-Si3N4 +5wt％ MgSiN2 +3wt％ Y2O3 +1wt％ CeO2)在干滑动条件下分别与氮化硅陶瓷和轴承钢GCr15的摩擦磨损性能.结果 表明:Si3N4/Si3N4滑动副的摩擦系数和磨损率随着滑动速度的增加而减小,且不同滑动速度下的磨损表面均有明显的犁削特征和剥落,其磨损形式主要为磨粒磨损;Si3N4/GCr15滑动副的摩擦系数和磨损率随着负载的增加先减小后增大,其磨损机理为氧化磨损和粘着磨损.由于氮化硅球的高硬度和轴承钢的相对低硬度,相同条件下Si3N4/Si3N4滑动副的磨损率均大于Si3N4/GCr15.     

Neck Formation and Self-Adjusting Mechanism of Neck Growth of Conducting Powders in Spark Plasma Sintering

By using spherical Cu powders as the conducting sintering material, the microstructures of sintered powder particles at different stages in the process of spark plasma sintering (SPS) have been investigated. Theoretical analyses are proposed to quantify the effects of the pulsed direct current on the neck formation and the neck growth of conducting powders. It is found that there is a considerable inhomogeneous distribution of the temperature increase from the particle-contacting surface to the center of the particle when the pulsed current passes through. The temperature at the particle-contacting surface may reach the boiling point of the material, which results in neck formation at relatively low-sintering temperatures through a process of local melting and rapid solidification. The neck growth depends on the local distribution of the current intensity, which is determined by the competition between the neck cross-sectional area and the electrical resistivity increasing with the temperature. Accordingly, we propose that the coarsening of necks follows a "self-adjusting" mechanism, which is likely to be the essential reason for the homogeneous distributions of neck sizes and sizes of fine grains formed in the neck zones during the SPS process.     

Effects of Prior Annealing on the Spark Plasma Sintering of Nanostructured Y2O3 Powders

The effects produced by annealing Y₂O₃ nanopowders on their spark plasma sintering (SPS) behavior are systematically investigated in this work. It is found that the annealed powders display higher sinterability with respect to the as‐received ones. Indeed, the maximum densification level reached from pristine powders is about 97.5%, whereas density decreases when further increasing either the sintering temperature or the dwell time. In contrast, the density of SPS products obtained from pretreated powder monotonically increases with temperature and processing time, thus leading to fully dense materials in 30 min at 1050°C and 60 MPa. Correspondingly, it is found that the annealing treatment markedly inhibits grain coarsening during SPS. Thus, dense translucent samples with grain size below 100 nm can be attained from annealed powders. On the other hand, white‐opaque specimens with significantly coarser microstructures (up to 1‐μm‐sized grains) are obtained when pristine powders are directly processed under the same sintering conditions. Furthermore, it is observed that the annealing treatment of SPS samples in air allows for graphite contamination removal, whereas no improvement in term of light transmittance is produced.     

Reactive Spark Plasma Sintering of rhenium diboride

The simultaneous synthesis and densification of rhenium diboride is investigated starting from Re and B as reactants by using the Spark Plasma Sintering (SPS) apparatus. It is shown that SPS represents an effective technique to synthesize ReB₂ bulk samples with high purity and density. In particular, a dense product with traces of secondary phases (Re₇B₃) is obtained in 35 min of total processing time by applying a maximum temperature of 1600 °C and a mechanical pressure of 20 MPa.     

Synthesis of Ultrafine Hafnium Diboride Powders Using Solution‐Based Processing and Spark Plasma Sintering

Ultrafine hafnium diboride (HfB₂) powders were synthesized by the boro/carborthermal reduction process. Fine‐scale mixing of the reactants was achieved by solution‐based processing using hafnium oxychloride (HfOCl₂·8H₂O) and phenolic resin as the precursor of HfO₂ and carbon respectively. The heat treatment was completed at a temperature range 1300–1500°C for 1h using spark plasma sintering (SPS) apparatus. The crystallite sizes of the synthesized powders were small (<500 nm) and the oxygen content was low (0.85 wt%). The grain growth of HfB₂ could be effectively suppressed using SPS due to the fast heating rate. The effects of temperature and holding time on the synthesis of ultrafine HfB₂ powders were discussed.     

Spark Plasma Sintering of Bismuth Titanate Ceramics

Spark plasma sintering (SPS) was used to fabricate bismuth titanate (Bi₄Ti₃O₁₂) ceramics. The densification, microstructure development and dielectric properties were investigated. It was found that the densification process was greatly enhanced during SPS. The sintering temperature was 200°C lower and the microstructure was much finer than that of the pressureless sintered ceramics, and dense compacts with a high density of over 99% were obtained at a wide temperature range of 800°–1100°C. Dielectric property measurement indicated that the volatilization of Bi³⁺ was greatly restrained during SPS, resulting in an unprecedented low dielectric loss for pure Bi₄Ti₃O₁₂ ceramics.     

Synthesis of Niobium(V)-Stabilized Tetragonal Zirconia Nanocrystalline Powders

The polymer precursor method is very useful to prepare Nb⁵⁺-stabilized nanocrystalline powders of t -ZrO₂. The precursor solution is composed of zirconium oxalate, niobium tartrate, and poly(vinyl alcohol), which help to form a network matrix to disperse the metal ions homogeneously. Nb⁵⁺ is an effective agent to stabilize t -ZrO₂, and ease of formation of the tetragonal phase increases with increasing dopant concentration. Thermal stability of t -phase is found up to 1700°C having 15 mol% Nb⁵⁺, prepared at 600°C with particle sizes of 35 ± 5 nm.     

Nanocrystalline Aluminum Nitride: II, Sintering and Properties

Nanocrystalline aluminum nitride powders obtained from gas condensation and in situ nitridation in a forced-flow reactor have been sintered successfully without pressure or additives. The resulting densified pellets showed good thermal conductivity and low oxygen content. A comparison of the improvement in densification with micrometer-size and nanocrystalline yttria additives was undertaken and it was found that the nanocrystalline yttria decreased the sintering temperature significantly. Besides spherical nanoparticles, needle-shaped nanocrystalline aluminum nitride particles that resulted in highly textured compacts when hot pressed could also be produced.