常压烧结制备氧化钨陶瓷靶材的致密化研究

采用常压固相烧结工艺,制备出高纯度、高致密度的氧化钨靶材。考察了粉体粒度、成型压强、烧结温度和保温时间等对靶材致密度的影响。测试结果表明,以粒度0.27μm的粉体为原料,成型压强为60MPa,烧结温度为1200℃,保温时间为1h的条件下,可以制备出高致密度的氧化钨靶材,其组成为高纯的单斜晶相。     

通过凝胶注模成型和无压烧结制备碳化硅陶瓷

以四甲基氢氧化铵为分散剂,糊精为碳源,通过静电稳定作用,制备了高固相含量、分散良好的碳化硅陶瓷浆料。以水溶性N,N–二甲基丙烯酰胺为单体,N,N’–亚甲基双丙烯酰胺为交联剂,采用实验室开发的偶氮[2–(2–咪唑啉–2–基)]丙烷HCl引发体系,在45～50℃引发单体聚合,制备出水基凝胶注模碳化硅素坯,素坯的相对密度达58%,抗弯强度大于40MPa。进一步通过无压烧结制备相对密度高于98%,硬度达28GPa,强度达530 MPa的SiC陶瓷。对素坯和SiC陶瓷的微结构和力学性能进行了测试和表征。结果表明:采用糊精作为碳源可以提高凝胶注模浆料的分散性,避免凝胶过程中的碳阻聚问题,有利于制备出高性能的碳化硅陶瓷材料。     

Processing Low‐Oxide ZrB2 Ceramics with High Strength Using Boron Carbide and Spark Plasma Sintering

A phase diagram‐assisted powder processing approach is shown to produce low‐oxygen (0.06 wt%O) ZrB₂ ceramics using minimal B₄C additions (0.25 wt%) and spark plasma sintering. Scanning electron microscopy and scanning transmission electron microscopy with elemental spectroscopy are used to identify “trash collector” oxides. These “trash collector” oxides are composed of manufacturer metal powder impurities that form discreet oxide particles due to the absence of standard Zr–B oxides found in high oxygen samples. A preliminary Zr–B–C–O quaternary thermodynamic database developed as a part of this work was used to calculate the ZrO₂–B₄C pseudobinary phase diagram and ZrB₂–ZrO₂–B₄C pseudoternary phase diagrams. We use the calculated equilibrium phase diagrams to characterize the oxide impurities and show the direct reaction path that allows for the formation of ZrB₂ with an oxygen content of 0.06 wt%, fine grains (3.3 μm) and superior mechanical properties (flexural strength of 660 MPa).     

无压烧结Al2O3/ZrSiO4复相陶瓷的研究

以Al2O3颗粒为增强体,MgO-Y2O3为烧结助剂,采用无压烧结方法,在组分优化的基础上,制备的锆英石基复相陶瓷室温抗弯强度和断裂韧性分别可达371 MPa和3.4 MPa·m1/2;采用XRD、SEM分析样品的相组成和显微结构,结果显示:确定ZrSiO4为主要晶相,另外还有少量Al2O3和ZrO2的存在;确定复相陶瓷的强韧化是由Al2O3颗粒引发的裂纹偏转、微裂纹增韧和由ZrSiO4分解而来的ZrO2相变增韧共同作用而实现的,断裂方式主要为穿晶断裂.     

低温无压烧结氮化硅陶瓷的相变及致密化研究

采用氧化铝（Al2 O3）和氧化钇（Y2 O3）为烧结助剂,利用无压烧结工艺在低温下制备氮化硅陶瓷材料。利用XRD和SEM等着重研究了无压烧结氮化硅陶瓷低温阶段时的物相组成及其致密化。结果表明：当添加剂含量为10％,烧结温度高于1430℃时,α→β相转变较快;当烧结温度达到1510℃时,α相全部转变为β相。     

Freeze casting for near‐net‐shaping of dense zirconium diboride ceramics

Zirconium diboride (ZrB₂) was formed into dense complex shapes using freeze casting as a near‐net‐shaping technique. Aqueous‐based formulations were compared with nonaqueous (cyclohexane) based formulations in terms of rheological behavior, particle packing in the green body, sintered density, macroscale porosity, and cracking. The influence of particle solids concentration and freezing rate was investigated. The aqueous formulations were found to be deficient in that they produced macroscale porosity that could not be eliminated during sintering resulting in low density and large pores in the final shaped objects. The nonaqueous‐based system was able to produce complex shaped objects with significantly reduced macroscale porosity. The higher concentration of solids in the nonaqueous‐based formulations was primarily responsible for the reduced macroscale porosity and enabled higher sintered densities (up to 90%‐91.5% of theoretical density for fast freezing). The microstructure of the ZrB₂ formed at fast freezing rates and high solids content typically had isolated pores in the order of 5‐10 μm in size, mainly found along grain boundaries (grain sizes between 20 and 50 μm). Although this rapid freezing produced denser components, it tended to produce objects with internal cracks. When slower freezing rates were used, intricate complex shaped objects could be produced without cracks but their density was only between 65% and 80% of theoretical density.     

Pressureless Sintering and Properties of Titanium Diboride Ceramics Containing Chromium and Iron

The simultaneous addition of 0.5 wt% Cr and Fe was found to enhance the densification of TiB₂. The densities of specimens that were sintered for 2 h at 1800° and 1900°C were 97.6% and 98.8% of the theoretical value, respectively. The mechanical properties of the specimen sintered at 1800°C, which had a strength of 506 MPa and a fracture toughness of 6.16 MPa·m^1/2, were much better than those observed in the specimen sintered at 1900°C.     

Fabrication of ZrB2 ceramics by reactive hot pressing of ZrB and B

Zirconium diboride (ZrB₂) ceramics were prepared by reactive hot pressing of ZrB+B powder mixture. Formation of a transient liquid due to eutectic reaction of ZrB₂+Zr→L_eu(ZrB2+Zr) at 1661°C following peritectic decomposition of 2ZrB=ZrB₂+Zr at 1250°C during heating up of the ZrB+B mixture facilitated densification. The liquid phase was subsequently eliminated via reaction of B with Zr in the eutectic liquid L_eu(ZrB2+Zr) to result in a dense ZrB₂ ceramic. Full density was reached after reactive hot pressing at 1900°C under 30 MPa for 1 h. The ZrB₂ ceramic had a refined microstructure consisting of grains of <1.5 μm in size and relatively good Vickers hardness (21 ± 2 GPa) and flexural strength (595 ± 63 MPa).     

α-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering

Dense α-SiAlON ceramics were obtained by pressureless sintering of green compacts prepared using slip casting. The rheological properties of the reaction SiAlON suspension were optimized to achieve a high degree of dispersion with a high solids volume fraction, which resulted in homogeneous and relatively dense green bodies with high sintering ability, which could be densified by pressureless sintering at 1750°C for 2 h. The sintered samples revealed a high degree of uniformity and almost fully dense microstructures that consisted of many small, elongated grains homogeneously dispersed in the fracture surfaces, which had properties comparable with those of other SiAlONs obtained using hot pressing.     

无压烧结氮化硅陶瓷的物理性能研究

以α-Si₃N₄粉末为原料,Y₂O₃和MgAl₂O₄体系为烧结助剂,采用无压烧结方式,研究了烧结温度、保温时间、烧结助剂含量以及各组分配比对氮化硅致密化及力学性能的影响。结果表明:以Y₂O₃和MgAl₂O₄为烧结助剂体系,氮化硅陶瓷在烧结温度为1 600 ℃,保温时间为4 h,烧结助剂含量为12.5%(质量分数),Y₂O₃和MgAl₂O₄质量比为1∶1时,综合性能最好;氮化硅陶瓷显气孔率为0.21%,相对密度为98.10%,抗弯强度为598 MPa,维氏硬度为15.55 GPa。     

无压烧结碳化硅复合材料的制备与性能研究

以部分碳化钛为增强相投入到碳化硅基体材料中,并投入微量炭黑和碳化硼为烧结活化剂,利用无压固相烧结技术制造了碳化硅基陶瓷复合材料。评测了其力学性能,凭借扫描电镜(SEM)观测了试样的断口形貌与表观形貌,并探讨了其氧化行为。结果表明:在碳化硅中投加部分碳化钛,对复合材料的力学性能有非常大地益处,于9 wt%时达到顶峰,弯曲强度497 MPa,相对密度98.9%,断裂韧性4.79 MPa·m^1/2。复合材料的显微组织构造紧致密实,TiC颗粒在SiC材料中的离散作用而激发的钉扎效果和裂纹偏移转向为其主要的增韧原理。在设定的氧化条件下(1200℃保温2 h),试样表面形成了一层较为致密并可以弱化氧化进程的氧化膜层。     

常压烧结制备AlN-W复合微波衰减材料及其性能研究

以氮化铝、钨为原料,在氮气气氛下,采用无压烧结方式制备了AlN-W复合微波衰减材料.采用扫描电子显微镜、矢量网络分析仪及激光导热仪对样品的微观结构、介电性能及热导率进行测试分析.结果表明:随着烧结温度的增加,AlN-W复合陶瓷的介电常数逐渐增大,介电损耗变化不大.随着烧结助剂添加量的增加,AlN-W复合陶瓷的介电常数和介电损耗相应增大.随着W含量的增加,AlN-W陶瓷的介电常数呈增加的趋势,介电损耗逐渐降低,当W添加量为60％时损耗几乎为零.随着W颗粒粒径的增加,AlN-W复合陶瓷的介电常数变化不大,介电损耗逐渐降低.由此可通过调节烧结温度、衰减剂含量、衰减剂粒径及烧结助剂的添加量,来制备符合实际需要的复合微波衰减材料.1750℃下制备的A1 N-40％W复合陶瓷热导率为91 W/m·K.初步分析了AlN-W复合陶瓷的微波衰减机理.     

Pressureless ultrafast sintering of near-net-shaped superhard isotropic B4C/rGO composites with Ti-Al additives

Superhard composites of B₄C reinforced with randomly-oriented reduced graphene oxide (rGO) nanoplatelets are manufactured by a near-net-shape fabrication route based on three successive steps. Firstly, aqueous colloidal processing is used for the environmentally-friendly preparation of a semi-concentrated multi-component slurry (B₄C as main component, Ti-Al as sintering additive, and rGO as toughening reinforcement), whose suitability for wet shaping is demonstrated by rheological measurements. Secondly, slip casting is used to produce robust green parts with shapes on demand and microstructures free of macro- and micro-defects. And thirdly, pressureless spark-plasma sintering (PSPS) is used for the ultrafast and energy-efficient densification of the green parts with shape retention. Measurements of shrinkage and hardness, as well as the microstructural observations, are used to identify suitable PSPS temperatures leading to obtaining isotropic B₄C/rGO composites that are superhard and almost twice as tough as the monolithic B₄C ceramics.     

无压烧结硼化锆基ZrB2-SiC复相陶瓷的结构与性能

以钇铝石榴石-YAG为烧结助剂,通过无压烧结制备了ZrB2-SiC复相陶瓷。研究了烧结助剂含量对烧结材料力学性能和显微结构的影响,材料的显微结构由扫描电镜SEM及其能谱分析EDS测定。研究结果表明,烧结助剂(YAG)和原料中的杂质形成玻璃相填充在晶界上,显著促进了硼化锆基ZrB2-SiC复相陶瓷的致密化。     

Tape Casting, Pressureless Sintering, and Grain Growth in Ti3SiC2 Compacts

In this work we demonstrate that fine Ti₃SiC₂ powders can be tape-cast and/or cold-pressed and pressureless-sintered in Ar- or Si-rich atmospheres to produce fully dense, oriented microstructures in which the basal planes are parallel to the surfaces. Carbon- and/or Si-rich environments suppress grain growth. In the case of the tape casting, the C-residue from binder burnout results in small core grains relative to the surface grains that can grow significantly. When sintering in high Si activities, titanium silicide phases form at the grain boundaries that slow grain growth. Annealing the latter in Ar at 1600°C, for extended periods (30 h), rids the samples of these grain-boundary phases, which in turn results in grain growth. The advantage of the latter process is that the final grain size distribution is more uniform from surface to bulk.     

In situ hot-pressed ZrB2-based composite with polycarbosilane-derived SiC: Microstructure and mechanical behavior

In this study, a homogenously dispersed finer SiC particles-containing ZrB₂ composite was prepared using nanosized polycarbosilane (PCS) particles-containing ZrB₂ mixture powder, followed by hot pressing. The microstructure of the resulting composite was characterized by field-emission scanning electron microscopy and transmission electron microscopy. The composite microstructure comprised finer equiaxed ZrB₂ and SiC grains. The mechanical behavior of the composite was evaluated using four-point bending test at different temperatures between room temperature (RT) and 1600°C. The results show that the composite exhibited only linear deformation behavior prior fracture at or below 1500°C. However, a trace quantity of nonlinear deformation was observed at 1600°C. In addition, the flexural strength of the composite decreased as the temperature increased from RT to 1200°C, then the strength increased as the temperature raised to 1400°C. Subsequently, the flexural strength remained almost the constant between 1400°C and 1600°C, with a strength of ~760 MPa.     

Dense m-Li2ZrO3 formed by aqueous slip casting technique: Colloidal and rheological characterization

An aqueous colloidal processing method was proposed to prepare m-Li₂ZrO₃ bodies, with high green density and a homogenous microstructure, employing a pressureless forming technique. For this purpose, the preparation conditions of m-Li₂ZrO₃ aqueous suspensions were optimized considering colloidal processing variables, such as milling time (0–40 min), dispersant concentration (1–10 wt% of the solids content) and maximum solid loading (41–74 wt%). Particle size distributions and steady-state flow curves of the slips were analyzed, together with phase composition and microstructure of the ceramic bodies. The results show that after planetary ball milling for 30 min, the aqueous suspension with 63.1 wt% of m-Li₂ZrO₃, and a dispersant content equal to 10 wt% of the solid loading, yielded green bodies with bulk densities close to 66% TD by slip casting. Then, after the heat treatment process at 1100 °C for 12 h, bulk densities close to 88% TD were achieved. It was found that density values and microstructure of the green and sintered products bear a direct relationship to the particle size and the consistency of the slips.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: II, Effects of Sintering Additives and Green Body Infiltration

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part II of the study, the effects of Y₂O₃/MgO sintering additives and green body infiltration on densification behavior and microstructure development are reported. Both sintering additives and green body infiltration resulted in enhanced densification. However, the infiltration approach was more effective for samples with high SiC whisker concentrations. Samples with 27 vol% whiskers could be pressureless sintered to ∼93% relative density and ∼3% open porosity. Fracture toughness values and microstructural features (e.g., grain size) for the infiltrated samples remained approximately the same as observed in the uninfiltrated samples.     

Electrochemical corrosion behavior and surface modification of ZrB2 in hydrofluoric acid aqueous solution

The electrochemical corrosion behavior in an open system of zirconium diboride (ZrB₂) was studied in hydrofluoric acid aqueous solution at room temperature. Based on analysis of the intermediate products of corrosion, a corrosion mechanism for ZrB₂ is suggested. The morphology evolution of anodic ZrB₂ is investigated. The density of the hexagonal‐pyramid grain array is dictated by current density, and the time dictates the length of the hexagonal pyramid grains. The microstructure of the surface can be controlled by the current density, which demonstrates that the surface wettability of ZrB₂ can be modified by tailoring the current density.     

Achieving fabrication of highly transparent Y2O3 ceramics via air pre-sintering by deionization treatment of suspension

An easy albeit quite effective deionization suspension treatment was adopted to alleviate the detrimental effects related to the hydrolysis of Y₂O₃ in an aqueous medium. Fabrication of highly transparent Y₂O₃ ceramics with a fine grain size via air pre-sintering and post–hot isostatic pressing (HIP) treatment without using any sintering additive was achieved using the treated suspensions. The hydrolysis issue of Y₂O₃ powder in an aqueous medium was effectively alleviated by using deionization treatment, and a well-dispersed suspension with a low concentration of dissolved Y³⁺ species was obtained. The dispersed suspensions were consolidated by the centrifugal casting method, and the green bodies derived from the suspension of 35.0 vol% solid loading showed an improved homogeneity with a relative density of 52.1%. Fully dense Y₂O₃ transparent ceramic with high transparency was obtained by pre-sintering consolidated green compacts at a low temperature of 1400°C for 16 h in air followed by a post-HIP treatment at 1550°C for 2 h under 200 MPa pressure. The sample had a fine average grain size of 690 nm. The in-line transmittance of the sample reached 83.3% and 81.8% at 1100 nm and 800 nm, respectively, very close to the theoretical values of Y₂O₃.