In Situ Fabrication of B4C–NbB2 Eutectic Composites by Spark–Plasma Sintering

This contribution reports, for the first time, the sintering, microstructure and properties of in situ synthesized/consolidated eutectic composites by spark–plasma sintering (SPS). SPS involves a number of processing parameters that may be used to tailor the composition of eutectic composites. It was shown that pressure may be used as a means of controlling the eutectic formation. By changing the pressure, temperature and composition, we propose a processing route that results in the in situ formation of strengthened eutectic composites, consisting of a matrix (B₄C, and B₄C–NbB₂ composite) and also containing regularly distributed whiskers of NbB₂. The composites with the eutectic composition of 35 mol.% NbB₂ obtained by SPS exhibit a high Vickers hardness (26–27 GPa) and indentation fracture toughness (~6 MPa·m^1/2).     

High‐Strength B4C–TaB2 Eutectic Composites Obtained via In Situ by Spark Plasma Sintering

The in situ synthesis/consolidation of B₄C–TaB₂ eutectic composites by spark plasma sintering (SPS) is reported. Samples for the evaluation of bending strength were cut from specimens with diameters of 30 mm. The sample prepared for the three‐point flexural strength test had fibers of tantalum diboride with diameter of 1.3 ± 0.4 μm distributed in the B₄C matrix, thereby reducing composites brittleness and yielding an indentation fracture toughness of up to 4.5 MPa·m^1/2. Furthermore, the Vickers hardness of B₄C–TaB₂ eutectics formed by SPS was as high as 26 GPa at an indentation load of 9.8 N. The flexural strength of the B₄C–TaB₂ system has been reported for the first time. Some steps were identified in the load–displacement curve, suggesting that micro‐ and macrocracking occurred during the flexural test. Ceramic composites with a eutectic structure exhibited a room‐temperature strength of 430 ± 25 MPa. Compared with other eutectic composites of boron carbide with transition‐metal diborides, room‐temperature strength the B₄C–TaB₂ was 40% higher than that of B₄C–TiB₂ ceramics, demonstrating advantage of the in situ synthesis/consolidation of eutectic composites by SPS.     

Long‐Range Ordered Structure of Ti–B–C–N in a TiB2–TiCxN1−x Eutectic Composite

A TiB₂–TiC_xN_1?x quasi‐binary eutectic composite, with a rod‐like faceted texture and a long‐range ordered structure of Ti–B–C–N, was prepared by the arc‐melting of TiB₂, TiC, and TiN powders. Hexagonal single‐crystalline TiC_xN_1?x rods were grown in a single‐crystalline TiB₂ matrix with a crystal orientation relationship of TiB₂ //TiC_xN_1?x and TiB₂ [0001]//TiC_xN_1?x [111]. A long‐range ordered structure of Ti–B–C–N was formed by the intermixing of the coherent interplanar spacings of seven TiB₂ (0001) and nine TiC_xN_1?x (111) planes.     

Microstructure and mechanical properties of B4C–TiB2–SiC composites toughened by composite structural toughening phases

B₄C–TiB₂–SiC composites toughened by composite structural toughening phases, which are the units of (TiB₂–SiC) composite, were fabricated through reactive hot pressing with B₄C, TiC, and Si as raw materials. The units of (TiB₂–SiC) composite with the size of 10‐20 μm are composed of interlocking TiB₂ and SiC with the size of 1‐5 μm. The addition of TiC and Si can effectively promote the sintering of B₄C ceramics. The relative densities of all the B₄C composites with different contents of TiB₂ and SiC are close to completely dense (98.9%‐99.4%), thereby resulting in superior hardness (33.1‐36.2 GPa). With the increase in the content of TiB₂ and SiC, the already improved fracture toughness of the B₄C composite continuously increases (5.3‐6.5 MPa·m^1/2), but the flexure strength initially increases and then decreases. When cracks cross the units of the (TiB₂–SiC) composite, the cracks deflect along the interior boundary of TiB₂ and SiC inside the units. As the crack growth path is lengthened, the crack propagation direction is changed, thereby consuming more crack extension energy. The cumulative contributions improve the fracture toughness of the B₄C composite. Therefore, the composite structural toughening units of the (TiB₂–SiC) composite play an important role in reinforcing the fracture toughness of the composites.     

Spark Plasma Sintering of Superhard B4C–ZrB2 Ceramics by Carbide Boronizing

A carbide boronizing method was first developed to produce dense boron carbide‐ zirconium diboride (“B₄C”–ZrB₂) composites from zirconium carbide (ZrC) and amorphous boron powders (B) by Spark Plasma Sintering at 1800°C–2000°C. The stoichiometry of “B₄C” could be tailored by changing initial boron content, which also has an influence on the processing. The self‐propagating high‐temperature synthesis could be ignited by 1 mol ZrC and 6 mol B at around 1240°C, whereas it was suppressed at a level of 10 mol B. B₈C–ZrB₂ ceramics sintered at 1800°C with 1 mole ZrC and 10 mole B exhibited super high hardness (40.36 GPa at 2.94 N and 33.4 GPa at 9.8 N). The primary reason for the unusual high hardness of B₈C–ZrB₂ ceramics was considered to be the formation of nano‐sized ZrB₂ grains.     

ZrB2–SiC–G Composite Prepared by Spark Plasma Sintering of In‐Situ Synthesized ZrB2–SiC–C Composite Powders

To avoid introduction of milling media during ball‐milling process and ensure uniform distribution of SiC and graphite in ZrB₂ matrix, ultrafine ZrB₂–SiC–C composite powders were in‐situ synthesized using inorganic–organic hybrid precursors of Zr(OPr)₄, Si(OC₂H₅)₄, H₃BO₃, and excessive C₆H₁₄O₆ as source of zirconium, silicon, boron, and carbon, respectively. To inhabit grain growth, the ZrB₂–SiC–C composite powders were densified by spark plasma sintering (SPS) at 1950°C for 10 min with the heating rate of 100°C/min. The precursor powders were investigated by thermogravimetric analysis–differential scanning calorimetry and Fourier transform infrared spectroscopy. The ceramic powders were analyzed by X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscopy. The lamellar substance was found and determined as graphite nanosheet by scanning electron microscopy, Raman spectrum, and X‐ray diffraction. The SiC grains and graphite nanosheets distributed in ZrB₂ matrix uniformly and the grain sizes of ZrB₂ and SiC were about 5 μm and 2 μm, respectively. The carbon converted into graphite nanosheets under high temperature during the process of SPS. The presence of graphite nanosheets alters the load‐displacement curves in the fracture process of ZrB₂–SiC–G composite. A novel way was explored to prepare ZrB₂–SiC–G composite by SPS of in‐situ synthesized ZrB₂–SiC–C composite powders.     

Plasma Arc Welding of TiB2–20 vol% TiC

Ceramics consisting of titanium diboride with titanium carbide additions were fusion welded to produce continuous joins. A welding current of 135 A, welding speed of 8 cm/min, and plasma flow rate of 0.75 L/min were combined with a preheat temperature of ~1450°C to fusion weld coupons of TiB₂ containing 20 vol% TiC with a thickness of 4 mm. The resulting fusion zone (FZ) was 3.9 mm deep at the center of the joint and 10.4 mm wide. During cooling of the melt pool, four distinct regions of crystal growth and nucleation were observed due to thermal gradients. Regions at the top and bottom of the FZ exhibited smaller TiB₂ crystals due to higher nucleation rates whereas regions in the middle of the FZ showed higher growth rates, with TiB₂ crystals up to 1.2 mm in length. Thermal gradients also affected cooling of the eutectic phase, causing a cellular structure to appear in the cooled eutectic. Plasma arc welding was a viable method for joining diboride‐based ceramics.     

Oxidation of TiB2 Powders below 900°C

Oxidation studies have been performed on titanium dibor-ide (TiB₂) powders, from room temperature up to 900°C. The studies were performed at low partial pressures of oxygen, at 10 and 0. 05 ppm of O₂ in argon, simulating furnace atmosphere with flowing neutral gas of medium and high purity, and in air. It has been found that titanium borate (TiBO₃) is formed in these processes. It has also been revealed that the oxidation process starts below 400°C and is reversible in this low temperature range from 100° to 400°C.     

TiB2对放电等离子烧结法合成Ti3AlC2/TiB2复合材料性能和结构的影响

放电等离子烧结合成了Ti_3AlC_2/TiB_2复合材料,对其进行了密度、硬度、相含量、断裂韧性和弯曲强度以及微观结构的测试,比较系统地研究了TiB_2对Ti_3AlC_2/TiB_2复合材料性能和结构的影响。实验结果表明:在Ti_3AlC_2中添加适量的TiB_2,可以在断裂韧性略有降低的情况下,得到高硬度和高弯曲强度的致密的Ti_3AlC_2/TiB_2复合材料。     

Synthesis and Characterization of Hydroxyapatite–Wollastonite Composite Powders by Sol–Gel Processing

Composites of hydroxyapatite–wollastonite were synthesized by a sol–gel route using calcium acetate and triethyl phosphate as precursors of hydroxyapatite and high-purity natural wollastonite added in ratios of 20, 50, and 80 wt%. These composites were characterized by thermal analysis, X-ray diffraction, FT-IR and Raman spectroscopy, and scanning and transmission electron microscopic techniques. Formation of hydroxyapatite occurs at a relatively low temperature, about 420°C, accompanied by calcium carbonate; wollastonite remains unreacted. The composites were purified by heat treatment to a higher temperature and washed with hydrochloric acid and distilled water, to produce B-type carbonated hydroxyapatite–wollastonite composites as final products.     

Competition between densification and microstructure development during spark plasma sintering of B4C–Eu2O3

The densification of nonoxide ceramics has been a known challenge in the field of engineering ceramics. The amount and type of sinter‐aid together with sintering conditions significantly influence the densification behavior and microstructure in nonoxide ceramics. In this perspective, the present work reports the use of Eu₂O₃ sinter‐aid and spark plasma sintering towards the densification of B₄C. The densification is largely influenced by the solid‐state sintering reactions during heating to 1900°C. Based on the careful analysis of the heat‐treated powder mixture (B₄C–Eu₂O₃) and sintered compacts, the competitive reaction pathways are proposed to rationalize the formation of EuB₆ as dominant microstructural phase. An array of distinctive morphological features, including intragranular and intergranular EuB₆ phase as well as characteristic defect structures (asymmetric twins, stacking faults and threaded dislocations) are observed within dense B₄C matrix. An attempt has been made to explain the competition between microstructure development and densification.     

NaB5C–B5/C Composite Ceramics Prepared by Reaction Sintering in Na Vapor

Reaction sintering of sodium carbaboride (NaB₅C) was performed by heating compacts of mixtures of amorphous boron (B) and carbon black (C) powders (B/C molar ratio, 5/1) at 1173 K in Na vapor. The ceramics obtained from the compacts of B₅/C powder mixed with a ball mill (compact density, 1.67 Mg/m³) were the composites of NaB₅C (74 mass%) and unreacted B₅/C (26 mass%). The bulk density and bending strength of the composite ceramics were 2.04 ± 0.03 Mg/m³ and 320.9 ± 10.4 MPa, respectively. Transmission electron microscope observation revealed that nanometer‐size amorphous grains of B or C were included in the matrix of NaB₅C.     

TiB2 Powders Synthesis by Borothermal Reduction in TiO2 Under Vacuum

TiB₂ powders were synthesized by borothermal reduction in nanoscale TiO₂ with boron under vacuum. Reaction processes were investigated, and the effect of by‐product B₂O₃ was evaluated. Results showed that TiO₂ was firstly reduced by boron to form TiBO₃ and Ti₂O₃, and then to produce TiB₂ and B₂O₃ with increasing temperature. The reaction processes of TiB₂ powders synthesis included two‐step reduction in TiO₂ by boron and the removal of B₂O₃. The presence of B₂O₃, which was previously reported as the most important factor in promoting the coarsening of ZrB₂ and HfB₂ powders by borothermal reduction, did not lead to significant coarsening of TiB₂ powders. Due to the minor effect of B₂O₃, TiB₂ powders with small particle size and low oxygen content could be prepared by direct heat treatment of TiO₂ and boron at 1550°C under vacuum for 1 h. The particle size and oxygen content of synthesized TiB₂ powders were ~0.9 μm and ~1.7 wt%, respectively.     

Spheroidization of Titanium Carbide Powders by Induction Thermal Plasma Processing

Highly spherical particles of titanium carbide (TiC) have been produced by in-flight heat processing of irregularly shaped TiC powders in an aerosol reactor under argon-hydrogen and argon-helium induction thermal plasma. The spherical powders obtained by the plasma treatment consist of unagglomerated and uniform particles with mean diameters between 25 and 28.5 μm, which is smaller than the original TiC particle mean diameters (29.5 μm) because of partial evaporation of the particles during the plasma treatment. The spheroidization ratio of the treated TiC powders increases with the increase of hydrogen flow rate in plasma gases and the reduction of powder feeding carrier gas flow rate. Under certain processing conditions, the TiC powders have been completely spheroidized. The morphology and structure of individual spherical particles were examined and their formation mechanism was discussed based on calculation of heat transfer kinetics of the particles in the thermal plasma.     

Preparation of B4C composites toughened by TiB2-SiC agglomerates

High-performance B₄C composites toughened by TiB₂-SiC agglomerates were fabricated via reactive hot pressing with B₄C, TiC and Si as raw materials. The TiB₂-SiC composite serves as a composite toughening phase formed in the B₄C matrix through an in situ reaction; its agglomerates are composed of interlocked TiB₂ and SiC, which can remarkably improve the toughness of the B₄C composites. The Vickers hardness, flexural strength and fracture toughness of the B₄C-TiB₂-SiC composite reached 35.18 ± 0.45 GPa, 567 ± 14 MPa, and 6.38 ± 0.18 MPa m^1/2 respectively. The special toughening structure of the TiB₂-SiC composite introduced into B₄C ceramics was evaluated for the first time in this study.     

Studies on the C–H/C–D Exchange in Imidazolium (Pilocarpinium) and in 2- and 4-Hydroximinomethyl Pyridinium Compounds

The H–D exchange of methylene protons in a side-chain when situated between a positively charged N-atom (α–CH₂) and a ketone function was studied in various pilocarpinium (N-methyl imidazolium) and hydroximinomethyl pyridiniums using NMR measurements. Structural factors which determine the course of the exchange are reported, and the role of the solvent was examined. Mass-spectra of nondeuteriated and deuteriated 4-hydroximinomethyl-1-phenacyl-pyridinium chloride confirmed the fast exchange of the α–CH_a. An alternative H–D exchange of the C₂–H in the imidazolium moiety of certain pilocarpinium compounds without a ketone in the N-chain was detected. Mechanisms for the two types of exchanges involving an ylid as intermediate and attack of a deuteron were proposed.     

TiB2-BN-AlN复相陶瓷的结构与性能研究

采用不同TiB2、BN、AlN体积配比制备TiB2-BN-AlN复相陶瓷,研究了TiB2、BN、AlN含量的变化对复相陶瓷烧结致密度、组织结构和性能的影响.结果表明:随着BN含量的增加,TiB2-BN-AlN复相陶瓷的致密度和抗弯强度降低,电阻率升高.另一方面,在空气中AlN颗粒表面易水解产生AlOOH和NH3,在热压烧结过程中,AlOOH会分解成Al2O3,同时Al2O3与AlN发生反应生成AlON,使得物质在AlN 晶粒之间扩散,从而提高复相陶瓷的致密度.     

壳核式Al2O3-Y2O3 /ZrB2复合粉体的放电等离子烧结行为研究

ZrB2具有优良的物理特性和化学稳定性而应用于许多领域,但是ZrB2难以烧结致密和在高温条件下容易被氧化.为了充分发挥ZrB2的优点,改善ZrB2的缺点,本文采用共沉淀法制备壳核式Al2O3-Y2O3 /ZrB2复合粉体,通过放电等离子烧结法制备高致密的ZrB2-YAG陶瓷.在相同实验条件下得到的纯ZrB2陶瓷的相对密度都很低,在85%以下.通过此种方法加入少量YAG后,烧结密度得到明显提高,随YAG添加量的增加相对密度增加.添加量超过30wt%后,可以达到几乎完全致密的ZrB2-YAG陶瓷.     

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.     

SiO2/g-C3N4复合粉体制备及其光催化性能

以St?ber法制备出的二氧化硅(SiO2)微球和三聚氰胺为原料,两者按一定质量比混合后得到前驱体,通过煅烧该前驱体可成功获得SiO2/g-C3 N4复合粉体.利用XRD、SEM、UV-Vis和BET等表征手段对获得的复合粉体进行物相组成、形貌、可见光吸收性能以及比表面积大小等进行分析和测试.结果表明,改进的St?be...