利用太阳能多晶硅片生产过程中产生的固体废弃物合成莫来石晶须

以太阳能多晶硅片生产过程中产生的废弃石英坩埚及废弃超细氧化铝磨料为原料,以硫酸钠、硫酸钾复合盐作为熔盐体系制备莫来石晶须.同时,采用XRD,SEM等测试手段研究了不同温度下所合成粉体的物相及形貌分析.结果表明,在1100℃高火保温3h制备出了直径为0.1～2μm,长径比高达50～60的莫来石晶须,其形成机理属于熔解-沉淀反应机制.     

硫酸铝和氧化硅在硫酸钠熔盐中合成莫来石的动力学参数计算

以硫酸铝和氧化硅为原料,在硫酸钠熔盐中合成莫来石粉体.通过研究在不同温度下所合成粉体的物相组成和显微形貌,认为莫来石在800℃生成,在1 000℃可合成出物相组成单一的莫来石粉体,在1 200℃时莫来石分解.采用热分析动力学研究方法,按照热重分析曲线和Kissingger方程,计算出了在硫酸钠熔盐中合成莫来石反应的动力学参数:指前因子A为6.39×1037;表观活化能Ea为913.5kJ/mol;反应级数n近似为1.     

硫酸钾熔盐中合成莫来石晶须的形态和生长机理

以SiO2和Al2(SO4)3.18H2O为原料,在K2SO4熔盐介质中,分别于800℃、900℃、1000℃、1100℃和1200℃下保温3h,经溶解、分离、烘干后得到莫来石粉体,利用XRD和SEM对合成粉体的相组成和形貌进行了表征。研究结果表明:合成的莫来石粉体由纳米尺度的束状莫来石晶须组成;在900℃时开始形成莫来石,但仍有石英相存在;在1000℃时石英相完全消失,合成了高纯的莫来石粉体;当温度超过1100℃时,合成的莫来石开始分解。由此得出合理的合成反应温度为1000℃左右。同时,还根据液-固体系中晶体生长的基本理论,对束状莫来石晶须的形成机理进行了探讨。     

在硫酸钠熔盐中合成莫来石的热力学研究

以硫酸铝和二氧化硅为原料,以硫酸钠为介质合成莫来石粉体.利用差示扫描量热法研究了合成过程中的热量变化.由理论数据与实验数据分别经过计算得到的反应热变化基本一致,从而确定了合成反应的温度范围应在950～1 000℃之间.利用X射线衍射仪和场致发射扫描电子显微镜等手段对合成粉体的相组成、结构和形貌进行了研究.结果表明:在1 000℃保温3 h合成的莫来石粉体基本无其他相存在,其粉体呈针状晶须,直径约50～100 nm,长度约3～8 um.该方法工艺简单,合成温度较其它方法低200～400℃.     

原位合成莫来石晶须增强氧化铝基陶瓷

采用原位反应合成法研制了自生莫来石晶须增强氧化铝基陶瓷材料。研究了莫来石晶须的形成过程、微观形貌、化学成分以及复合材料的显微结构和力学性能。结果表明,添加适量的AlF3可以促进原位反应合成进程和莫来石晶须的生成。莫来石晶须的直径为0．2～1.0μm,长径比为8～30,可以显著提高复合材料的强度、韧性和抗热震性。     

原位合成莫来石晶须及其微观结构的研究

本文通过以高岭土、工业氢氧化铝为原料,添加适量AlF3和V2O5,在高温下原位合成了莫来石晶须,采用扫描电镜（SEM）研究了合成的莫来石晶须的微观形貌,探讨了莫来石晶须的生长机理,并讨论了影响莫来石晶须生长的相关因素.在综合考虑影响莫来石晶须生长的相关因素后,制备出晶须多、长径比大、分布均匀的莫来石晶须样品.     

邛崃高岭土熔盐法制备莫来石晶须

莫来石晶须为针状的莫来石单晶,其力学性能优于多晶莫来石,是一种优异的复合材料增韧补强剂。研究了以四川邛崃高岭土为原料,采用熔盐法制备莫来石晶须;利用XRD、SEM和XRF等手段对合成晶须的相组成和形貌进行了表征;高龄土、硫酸铝和硫酸钠的质量比例为1:4:5,在900℃煅烧2h制备了平均直径为100nm、长径比约为30的莫来石晶须。     

硼酸镁晶须的合成研究

报道了硼酸镁晶须的合成研究结果。硼酸镁晶须是一种性能价格比高的晶须产品，利用熔盐法合成出长10—50μm、直径0．5—2μm的硼酸镁晶须。还就硼酸镁晶须增强复合材料的研究和硼酸镁晶须工业化的前景作了展望。     

自生晶须强韧化莫来石材料的研究

采用溶胶－凝胶法制备Al2O3-SiO粉料,并通过二步加热处理工艺制备自生晶须莫来石材料。研究结果表明,凝胶胶化时间、干凝胶预烧温度和粉料粒度以及生坯密度对自生晶须有重要影响,自生晶须对莫来石材料有明显的强韧化作用。     

Catalyzed synthesis of rhombohedral boron nitride in sodium chloride molten salt

Although a variety of methodologies/techniques have been used to prepare h-BN powders with different sizes and purities, only a few methods are reported to synthesize r-BN. In this work, highly crystalline r-BN with a purity of 94 wt% was successfully synthesized in sodium chloride molten salt using Na₂B₄O₇ and Mg powders as starting materials at 1000 °C in nitrogen atmosphere. The sodium (Na) produced by the reaction of Mg and Na₂B₄O₇ has a positive effect on the formation r-BN in the molten salt. The effect of Na as a crystallization promoter to produce crystallized r-BN was demonstrated by heating a mixture of t-BN and Na at 800–1200 °C. The formation, dissolution and evaporation of Na in the melt was discussed. The influence of synthesis temperature on the phase composition and morphology of the final products in the melt was also investigated. The possible formation mechanism of r-BN is proposed.     

Mechanistic investigation into the electrolytic formation of iron from iron(III) oxide in molten sodium hydroxide

Iron(III) oxide tablets were electrolytically reduced to iron in molten sodium hydroxide at 530 °C and recovered to produce iron with 2 wt.% oxygen suitable for re-melting. The cell was operated at 1.7 V and an inert nickel anode was used. The thermodynamics and mechanism of the process was also investigated. By controlling the activity of sodium oxide in the melt, the cell could be operated below the decomposition voltage of the electrolyte with the net sequence of events being the ionization of oxygen, its subsequent transport to the anode and discharge leaving behind iron at the cathode. A reduction time of 1 h was achieved for a 1 g oxide tablet (close to the theoretical reduction time predicted by Faraday’s laws) at a current density of 520 mA cm⁻² with iron phase yields of ∼90 wt.%. The energy consumption was 2.8 kWh kg⁻¹.     

The cathode process in sodium chloride melts containing sulphate

The effect of addition of sulphate to sodium chloride and sodium fluoride melts was investigated by chronopotentiometry in the temperature range 820-1000 °C. It was found that the numerical value of the chronopotentiometric term jτ^1/2/c⁰ increased with increasing cathodic current density. This behaviour was explained by chemical decomposition of sodium sulphate into SO₃ and Na₂O, followed by electrochemical reduction of SO₃. The decomposition of sodium sulphate is enhanced with increasing temperature, resulting in an increase in the chronopotentiometric transition time. Numerical simulation of this electrode process supports this explanation of the electrode process. It is not a classical so-called CE mechanism when a chemical reaction precedes the electrochemical reduction as described in the literature, because the formation of SO₃ is suppressed by the presence of Na₂O in the melt.     

Leaching kinetics of ulexite in sodium hydrogen sulphate solutions

The aim of this study was to investigate the dissolution kinetics of ulexite in sodium hydrogen sulphate solutions in a mechanical agitation system and to declare an alternative reactant to produce boric acid. Reaction temperature, concentration of sodium hydrogen sulphate solutions, stirring speed, solid/liquid ratio and ulexite particle size were selected as parameters of the dissolution rate of ulexite. The experimental results were successfully correlated by linear regression using Statistica program. Dissolution curves were evaluated in order to test shrinking core models for solid-fluid systems. It was observed that increase in the reaction temperature and decrease in the solid/liquid ratio cause an increase in the dissolution rate of ulexite. The dissolution extent is highly increased with increase in the stirring speed rate between 100 and 700 rpm experimental conditions. The activation energy was found to be 36.4 kJ/mol. The leaching of ulexite was controlled by diffusion through the ash or product layer. The rate expression associated with the dissolution rate of ulexite depending on the parameters chosen may be summarized as: 1–3(1 − X)^2/3 + 2(1 − X) = 6.17 × C^0.97 × W^1.17 × D^−1.72 × (S/L)^−0.66 × e^(−36.4/RT)·t.     

A single-source molten salt synthesis of rod-shaped Na2Ti6O13 crystals

As one of the most potential negative electrode materials, Na₂Ti₆O₁₃ is expected to play an important role in the area of high-performance battery. In this work, we have developed an easy, efficient and controllable method to prepare rod-shaped Na₂Ti₆O₁₃ crystals. This approach utilized a single-source molten salt strategy and only needed to sinter a special precursor synthesized from an aqueous solution containing H₃BO₃ and (NH₄)₂TiF₆ in presence of sodium salts. The component and shape of precursor crystals can be tuned by adjusting the reagent concentration and reaction temperature. By sintering precursor crystals in air at 900 °C for 30 min, Na₂Ti₆O₁₃ with high crystallinity and purity can be obtained. X-ray diffraction and scanning electron micrographs results of different sintering times show that the sintering process can be divided into two steps. Firstly, the precursor crystals are converted to TiO₂ (anatase) nano-particles and amorphous sodium salts. Subsequently, molten salt reaction occurs between amorphous sodium salts and TiO₂ and forms rod-shaped Na₂Ti₆O₁₃ crystals.     

Electrochemical insertion of sodium into graphite in molten sodium fluoride at 1025 °C

The electrochemical insertion of sodium into graphite was studied in molten sodium fluoride at 1025 °C. The results obtained evidenced two mechanisms for sodium insertion into graphite: sodium intercalation between the graphite layers and sodium sorption into the porosity of the material. Subsequent internal rearrangement of inserted sodium occurred, via transference from the pores towards the intercalation sites. In addition, the intercalation compound was found to undergo a fast decomposition process (k = 2.55 × 10⁻⁹ mol s⁻¹). X-ray diffraction analysis was used to confirm the formation of a high stage compound (Na_0.1C₈), the composition of which was consistent with compositions observed in the case of chemical vapor and electrochemical insertion of sodium, during experiments in the sodium perchlorate-ethylene cabonate electrolyte.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Properties and mechanism of mullite whisker toughened ceramics

High-strength ceramics were prepared from high alumina fly ash (HAFA) and activated alumina as raw materials with magnesia as a sintering additive. The growth kinetics and influence mechanism of secondary mullite whiskers were investigated. Meanwhile, the effects of the Al₂O₃/SiO₂ mass ratio (A/S) and the amount of magnesia on the content and morphology of mullite in the green body were investigated, so as to emphasize the effect of the liquid phase in the sintering process on the growth of secondary mullite whiskers. The results showed that the aspect ratio of secondary mullite whiskers increased significantly after adding activated alumina to increase the A/S ratio of raw materials. When 30 wt% activated alumina was added, the mullite content increased by 5.39%, and the whisker length increased from 1.36 μm to 4.18 μm. The addition of magnesia improved the liquid phase formed during the sintering process and the K value method was used to determine the sintering liquid phase content under various conditions. It was observed that increasing the magnesia level by 1 wt% could raise the liquid phase content by 5–7%. When the total liquid content of the system was 30–40%, the growth activation energy in the diameter direction of the whisker reduced significantly, promoting the growth of secondary mullite whiskers along the C axis. The morphology of mullite gradually developed from fibrous to long columnar crystal, making it combine more densely with the green body matrix. Furthermore, the staggered long columnar mullite crystal structure changes the fracture mode of ceramics from intergranular to transgranular fracture, which fully uses the high mechanical strength of mullite. As a result, the fracture energy and strength of ceramics are significantly improved.