沉淀法合成HA粉末过程中球磨处理的作用

采用Ca(OH)2/H3PO4体系的化学沉淀法合成羟基磷灰石粉末,并对反应产物进行球磨处理,以促进合成反应的进行。应用X射线衍射(XRD)、扫描电镜(SEM)、粉末粒度分析等测试方法对HA的组成、晶化过程、颗粒形貌和粒度分布进行表征。研究结果表明,增加球磨处理后得到粉末的XRD图中β-TCP和CaO峰消失,煅烧过程中粉末更易发生晶化,粉末粒度分布变窄,集中在0.3μm～0.5μm。因此球磨处理促进Ca(OH)2/H3PO4体系液固反应进行的程度,提高了羟基磷灰石粉末的纯度,减小粉末粒度并缩小其粒度分布范围。     

碳化硼微粉的低温合成

以聚乙烯醇和硼酸为原料,首先合成聚乙烯醇硼酸酯前驱物凝胶,然后将前驱物热解及碳热还原制备碳化硼粉末。考察了聚乙烯醇与硼酸的物质的量比,前驱物热解温度,碳热还原温度以及还原时间等因素对碳化硼合成的影响。采用IR、化学分析、XRD、离心粒度分析、SEM等方法对中间物及产物进行了表征,确定了中间物及产物的组成、物相、粒度分布及形貌。研究结果表明：前驱物合成的适宜原料配比是n(聚乙烯醇)∶n(硼酸)=4∶1;前驱物在600 ℃下热解2 h,在1 300 ℃下碳热还原2 h,得到粒径为10 μm左右的碳化硼微粉。     

碳热还原氮化合成TiN的研究

以锐钛矿(中位径0.38μm)、金红石(中位径4.58μm)和鳞片石墨(粒度<0.15mm)、炭黑(平均粒度0.02μm)、可膨胀石墨(粒度<0.15mm)为原料,固定配比nC∶nTiO2为5∶1时分别组成不同的原料组合,并以锐钛矿和鳞片石墨为原料,改变配比nC∶nTiO2分别为3∶1、4∶1、5∶1、6∶1、7∶1和8∶1进行配料,在管式电炉、流动N2中分别于1300℃和1400℃制备了TiN,并进行了合成产物的氧化脱碳试验;采用XRD测定TiN的特征峰(d200=0.212nm)强度,以表征TiN的合成率,研究了原料粒度、反应物活性、反应温度等因素对TiN合成率的影响。结果表明:选用粒度较细或晶格活性大的原料,提高反应温度,均有利于提高TiN粉末的合成率;合成TiN粉末的最佳原料组合是可膨胀石墨和锐钛矿;以鳞片石墨和锐钛矿为原料时,其配比为nC∶nTiO2=6∶1时TiN合成率最高;合成产物中均含有一定量的碳,采用普通的加热氧化法不能除碳,其原因是TiN的氧化温度低于石墨的。     

铈锆氧化物粉末制备工艺及其优化

用共沉淀法，氨水为沉淀剂，利用正交试验法对合成CeO2-ZrO2-La2O3粉末的工艺进行优化，结果得出：以100g／L混合料液为原料，聚乙二醇6000为分散剂，700℃煅烧可得粒度均匀，平均粒径约3μm的混合粉末。经激光粒度仪、TG、比表面积仪对优化合成的CeO2-ZrO2-La2O3复合氧化物粉末的粒度、热稳定性、比表面积进行了测试，证明CeO2-ZrO2-La2O3复合氧化物粉末具有粒度分布均匀，比表面积大的优良性能。     

大颗粒氧化铈粉体的合成及粒度控制

采用正交试验法,研究了用草酸溶液作沉淀剂,直接沉淀合成大颗粒二氧化铈粉末,并对影响氧化铈粉末粒度的各种因素进行了详细研究。结果表明,适当降低硝酸铈质量浓度可以有效控制氧化铈粉末的粒度;同时沉淀剂浓度、料液酸度、煅烧温度均对氧化铈粉末的粒度有较大影响,得出了制备粒度为30-40μm的大颗粒氧化铈粉体的最佳工艺条件为：硝酸铈质量浓度70 g/L,料液酸度0.5 mol/L,草酸沉淀剂的质量浓度100 g/L,煅烧温度900-950℃。按此工艺参数可以直接合成粒度为38.521μm的氧化铈粉末。     

反应温度对碳热还原法合成SiC-B4C复合粉末烧失率和物相组成的影响

以硼酸、硅溶胶、片状石墨为原料,采用碳热还原法制备SiC-B_4C复合粉末。研究了反应温度对SiC-B_4C复合粉末的烧失率和物相组成的影响。采用X射线衍射仪(XRD)、激光粒度分析仪(LPSA)对实验样品的物相组成、粒度大小及其分布进行分析。结果表明:在反应温度达到1500℃以上时,采用碳热还原法可以成功合成出SiC-B_4C复合粉末。合成SiC-B_4C复合粉末的最适温度与硼酸用量有关。当硼酸用量为理论配比时,合成SiC-B_4C复合粉末的适宜反应温度为1500℃;当硼酸过量30%时,合成Si C-B_4C复合粉末的适宜反应温度为1550℃。当硼酸过量30%时,在1550°C下合成的SiC-B_4C复合粉末中位粒径为0.14μm。     

超细氧化铈粉体的合成及粒度控制

为了一步合成超细氧化铈粉末，用碳酸氢铵溶液作沉淀剂，使用沉淀法合成氧化铈的前驱体，前驱体经低温烘干、高温煅烧后得到氧化铈超细粉末，同时研究了各种影响因素对氧化铈粉末粒度的影响。结果表明，加入一定量适当的分散剂可以有效控制氧化铈粉末的粒度；同时料液质量浓度、沉淀剂的质量浓度、煅烧温度、保温时间均对氧化铈粉末的粒度有较大影响。通过实验，给出了优化的工艺参数，按此工艺可以直接合成粒度为0．115μm的氧化铈粉末。     

亚微米氮化硅粉体的制备及表征

以自蔓延高温合成法合成的氮化硅粉料为原料,用搅拌球磨的方法对其进行超细粉碎,测试不同球磨工艺条件下粉料的粒度、形貌和XRD谱图,探讨不同球磨工艺参数对粉料粒度、形貌及晶格类型的影响,最终确定合适的搅拌球磨工艺参数。优化工艺参数:粉料与溶剂的质量比为1∶1,粉料与磨介的质量比为1∶3,最佳研磨时间为8~10 h。在此条件下,可将中位径为5μm左右的氮化硅粉料用搅拌球磨的方法制备得到中位经为0.719μm的粉料。     

LaBaCoFeO5+δ-Ce0.8Sm0.2O1.9复合阴极的电化学性能研究

采用固相反应法合成中温固体氧化物燃料电池的LaBaCoFeO5+δ阴极粉末,研究不同煅烧温度对晶体结构的影响.将等量的LaBaCoFeO5+δ和Ce0.8 Sm0.2 O1.9电解质粉末通过机械混合和煅烧制备成LaBaCoFeO5+δ-Ce0.8 Sm0.2 O1.9复合阴极粉末.研究了复合阴极粉末的化学相容性、粒度分布、热膨胀和电化学性能.结果表明,LaBaCoFeO5+δ固相反应的最佳温度为1200℃,LaBaCoFeO5+δ和Ce0.8 Sm0.2 O1.9之间没有发生明显的反应,复合阴极粉末的中位径D50为2.441μm.LaBaCoFeO5+δ-Ce0.8 Sm0.2 O1.9复合阴极比LaBaCoFeO5+δ阴极组成的单电池在800℃的极化电阻下降了约48.7％,而最大输出功率密度提高了约82.5％,表现出更好的电化学性能.     

Zr-B体系自蔓延高温合成ZrB2陶瓷粉末

采用自蔓延高温合成(self-propagating high-temperature synthesis,SHS)技术制备了ZrB2陶瓷粉末,研究了Zr-B体系中Zr粉粒度对SHS反应的影响规律。采用XRD分析粉末的相组成,用SEM观察粉末的显微结构。研究结果表明：Zr粉粒度各为150,50,38μm的体系SHS产物均为单相的ZrB2粉末,粒度为50μm的Zr粉体系SHS产物中ZrB2含量为98．95％;38μm和50μmZr粉体系燃烧速率分别为最大和最小;150μm和50μmZr粉体系燃烧温度分别是最高和最低。SEM分析表明：SHS产物颗粒基本上为圆形或椭圆形的晶粒,颗粒尺寸也比较均匀,粒径大约在1～5μm左右。     

The influence of polyelectrolyte on the synthesis of B4C/BN nanocomposite powders via sol-gel method

In the present study, B₄C-BN nanocomposite powders were synthesized by using the sol-gel method. To investigate the effects of polyelectrolyte on phase content, particle size, and final morphology of synthesized powders different amounts of ammonium polycarboxylate were used as a gel dispersing agent and a nitrogen source. Highly crystalline, sub-micron/micron-sized boron carbide particles with varying morphologies including polyhedral-equiaxed, belt-like, needle-like, and complex-shaped hierarchical structures were produced from the polymeric gel containing glycerine, tartaric acid, and citric acid as carbon sources, and boric acid as boron source. With the addition of ammonium polycarboxylate as a polymeric gel network modifier, nanocomposite powders containing micron-sized polyhedral-equiaxed boron carbide particles and boron nitride nanoflakes were obtained. The results indicated that the particles dimensions, crystallinity, and B₄C to BN phase ratio of the synthesized powders are directly related to the preliminary formation of borate-ammonium and/or amine complexes in the polymeric gel. The SEM inspections revealed that the size of boron carbide particles tends to increase from 2 μm up to 40 μm as a function of ammonium polycarboxylate content. It was also observed that the average size and thickness of boron nitride nanoflakes within the range of 80 nm to 3 μm and 10–150 nm, respectively. B₄C/BN nanocomposite powders were synthesized with up to 32% BN content using a 43 wt% ammonium polycarboxylate additive.     

Pressureless Sintering of Zirconium Diboride: Particle Size and Additive Effects

Zirconium diboride (ZrB₂) was densified by pressureless sintering using <4-wt% boron carbide and/or carbon as sintering aids. As-received ZrB₂ with an average particle size of ∼2 μm could be sintered to ∼100% density at 1900°C using a combination of boron carbide and carbon to react with and remove the surface oxide impurities. Even though particle size reduction increased the oxygen content of the powders from ∼0.9 wt% for the as-received powder to ∼2.0 wt%, the reduction in particle size enhanced the sinterability of the powder. Attrition-milled ZrB₂ with an average particle size of <0.5 μm was sintered to nearly full density at 1850°C using either boron carbide or a combination of boride carbide and carbon. Regardless of the starting particle size, densification of ZrB₂ was not possible without the removal of oxygen-based impurities on the particle surfaces by a chemical reaction.     

Processing factors influencing the free carbon contents in boron carbide powder by rapid carbothermal reduction

High quality boron carbide powder without free carbon is desired for many applications. In this study, the factors that influence free carbon content in boron carbide powders synthesized by rapid carbothermal reduction reaction were evaluated. The dominant factors affecting free carbon contents in boron carbide powder were reaction temperature, precursor homogeneity, the particle size of reactants, and excess boron reactant amount. The reaction temperature at 1850 °C was sufficient to synthesize boron carbide with low free carbon content. Depending on process conditions, precursor homogeneity was also affected by the calcination temperature and time. Smaller particle size of reactants contributed to less carbon content and more uniformity in synthesized boron carbide. Excess boric acid effectively compensated for B₂O₃ volatilization. In the optimal sample, using 80 mol% excess nano boric acid and calcined at 500 °C, the free carbon in the synthesized boron carbide was negligible (0.048 wt.%).     

Synthesis of sub-micro sized high purity zirconium diboride powder through carbothermal and borothermal reduction method

Sub-micro sized zirconium diboride (ZrB₂) powders were successfully prepared via the boro/carbothermal reduction method using zirconium oxide and boron carbide as the primary raw materials. The prepared mixtures were thermally reacted at 1250 °C for 1 h. The optimized composition range containing the lowest oxide and carbide impurity, which was 0.14% of oxygen and 0.3% of carbon contents, was determined using crystallographic and elemental analysis. The particle size was reduced from 5 µm to 245 nm by the addition of B₄C as a reductant within a composition range that maintained the highest purity. The morphology changed from faceted to angular hexagonal bar-like with a simultaneous growth in particle size. Changes in the particle structure were a result of the existing liquid B₂O₃ phase during the reaction. The 245-nm particles contained 12.1% oxygen content and 16.2% oxygen content for the 5-μm particle in the circumstance in which limited oxides could be produced.     

Synthesis and characterization of submicron silicon carbide powders with silicon and phenolic resin

A novel three-step process is used to fabricate submicron silicon carbide powders in this paper. The commercially available silicon powders and phenolic resin are used as raw materials. In the first step, precursor powders are produced by coating each silicon powder with phenolic resin shell. Then, precursor powders are converted into carbonized powders by decomposing the phenolic resin shell. The submicron silicon carbide powders are formed in the reaction of silicon with carbon during the third step of thermal treatment. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and thermogravimetric (TG) analyses are employed to characterize the microstructure, phase composition and free carbon content. It is found that the sintered powders consist of β-SiC with less than 0.2 wt.% of free carbon. The particle size of the obtained silicon carbide powders varies from 0.1 to 0.4 μm and the mean particle size is 0.2 μm. The silicon carbide formation mechanism of this method is based on the liquid-solid reaction between liquid silicon and carbon derived from phenolic resin. The heat generated during the reaction leads to great thermal stress in silicon carbide shell, which plays an important role in its fragmenting into submicron powders.     

Effect of particle size on the mechanical properties of reaction bonded boron carbide ceramics

In the present communication, effect of boron carbide particle size on the mechanical properties such as hardness, fracture toughness and flexural strength of reaction bonded boron carbide (RBBC) ceramics were investigated. RBBC composites were produced by the reactive infiltration of molten silicon into porous preform containing boron carbide and free carbon. Boron carbide powders with mean particle size of 18.65 μm, 33.70 μm and 63.35 μm were chosen for the RBBC composites. The experimental results show that hardness increases from 1261.70±64.74 kg/mm² to 1674.90±100.00 kg/mm² and fracture toughness drops from 5.76±0.26 MPa m^1/2 to 3.4±0.37 MPa m^1/2. However, flexural strength decreases from 403.41±5.70 MPa to 256.15±25.05 MPa with the increase in particle size. Indentation induced cracks in RBBC are mainly median type and number of cracks increase with the increase of starting particle size.     

纯B4C和掺碳B4C的烧结机制

研究了中位粒径为0．42μm的纯B4C和掺碳B4C的烧结致密化过程。根据烧结温度和保温时间对线收缩率的影响。得出了它们的烧结动力学方程;由特征指数n值对比研究了它们的烧结致密机制。纯B4C的烧结致密机制为体扩散和晶界扩散,而掺碳B4C的烧结机制主要为晶界扩散,因此,掺碳对B4C起到了活化烧结的作用,在2160℃烧结45min,掺碳B4C烧结后相对密度大于90％,掺入的碳除了固溶于B4C晶格中之外,其它均以游离石墨形式存在,不形成新相。掺碳还导致B4C晶粒尺寸大大减小。     

Optimization of the decarbonizing ratio of boron carbide powders by reverse flotation using response surface methodology

Free carbon is the main impurity in boron carbide and has many side effects on the quality of boron carbide. In this study, reverse flotation was used for the first time to remove free carbon in boron carbide. The response surface methodology was utilized to optimize the reverse flotation factors, and the samples were analyzed by X-ray diffraction, scanning electron microscope, laser particle size analyzer, and chemical analysis. The study results reveal that the main factors affecting the decarbonization ratio were slurry concentration, collector dosage, foaming agent dosage and pH value. Furthermore, the results also show that reverse flotation could be applied effectively to the removal of free carbon in boron carbide. Slurry concentration of 25.14%, collector dosage of 567.9 g/t, foaming agent dosage of 199.32 g/t and pH value of 8.4 were found to be the best conditions. Under the optimal conditions, the decarbonization ratio is 84.23%. Mass ratio of free carbon in boron carbide reduced from 2.98 to 0.47.     

Modification of commercial boron carbide powder using Rapid Carbothermal Reduction

Non-uniform morphology and existence of free carbon are two main problems for commercial boron carbide powders. This work proposes a method for eliminating free carbon and changing the morphology of commercial powders using Rapid Carbothermal Reduction (RCR) process. Free carbon is eliminated from commercial boron carbide powders and morphology is evolved to less angular shapes with limited particle size growth. Commercial and modified powders were densified by Spark Plasma Sintering at 1900°C with 0, 5, and 20 minutes dwell. Despite the particle size growth, modified boron carbide powders reached >99% TD with shorter dwell times compared with commercial starting powders. Improved microhardness observed with dense modified samples as a result of enhanced morphology and increased twinning.     

Densification and characterization of rapid carbothermal synthesized boron carbide

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.