Low‐temperature synthesis of ultrafine TiB2 nanopowders by molten‐salt assisted borothermal reduction

The synthesis of TiB₂ nanopowders arouses considerable interests due to its importance for implementing the extensive applications of TiB₂ ceramic. Herein, the high‐purity ultrafine TiB₂ nanopowders were successfully synthesized via a molten salt assisted borothermal reduction technique at a relatively low temperature of 1173 K using TiO₂ and B powders as precursors within a KCl/NaCl salt. The results showed that the as‐obtained TiB₂ nanopowders possessed a polycrystallinity structure, and their specific surface area and equivalent average particle size were 33.18 m²/g and 40 nm, respectively. This study provides a new low temperature synthesis technique of TiB₂ nanopowders.     

Synthesis of Ti0.2Zr0.8B2 solid‐solution nanopowders by molten salt assisted borothermal reduction

High‐purity Ti_0.2Zr_0.8B₂ solid‐solution nanopowders were successfully synthesized via a molten salt assisted borothermal reduction at 1323‐1373 K using ZrO₂, TiO₂ and amorphous B as starting materials. The Ti_0.2Zr_0.8B₂ solid‐solution nanopowders synthesized at 1323 K show the largest specific surface area of 12.24 m²/g and the lowest equivalent average particle size of 86 nm. Meanwhile, they exhibit the high compositional uniformity and the good single‐crystal hexagonal structure. This study provides a new method to synthesize the high‐purity solid‐solution nanopowders of the transition‐metal borides.     

Synthesis of TaB2 powders by borothermal reduction

Borothermal reduction processes of Ta₂O₅ with boron under vacuum were investigated. Ta₂O₅ reacted with boron to form various borides (TaB₂, Ta₃B₄, and TaB), depending on the boron/Ta₂O₅ molar ratio and temperature. In order to prepare pure TaB₂ powders, two routes were developed. The first route was one‐step heat treatment at 1550°C. With boron/Ta₂O₅ molar ratio of 9.0, pure TaB₂ powders with strong agglomeration were synthesized by the first route, and the particle size and oxygen content were 0.7 μm and 0.9 wt%, respectively. The second route consisted of two‐step heat treatment at 800°C and 1550°C plus intermediate water washing. With lower boron/Ta₂O₅ molar ratio of 8.2, pure TaB₂ powders with less agglomeration and more uniform distribution were synthesized by the second route, and the particle size and oxygen content were 0.8 μm and 0.8 wt%, respectively. Moreover, the particle size similarity of TaB₂ powders by the two routes suggested that byproduct boron oxides, which were previously reported as the most important factor in promoting the coarsening of ZrB₂ powders by borothermal reduction, did not lead to the significant coarsening of TaB₂ powders.     

Low‐temperature preparation of titanium diboride fine powder via magnesiothermic reduction in molten salt

Phase pure titanium diboride (TiB₂) powder of 100‐200 nm was synthesized from TiO₂ and B₂O₃ using a molten‐salt‐assisted magnesiothermic reduction technique. The effects of salt type, Mg amount, reaction temperature, and TiO₂ raw materials on the synthesis process were examined and the relevant reaction mechanisms discussed. Among the three chloride salts (NaCl, KCl, and MgCl₂), MgCl₂ showed the best accelerating‐effect. To synthesize phase pure TiB₂, 20 mol% excessive Mg had to be used to compensate for the evaporation loss of Mg. Particle shape and size of raw material TiO₂ showed little effect on the formation of TiB₂ and its shape and size, suggesting that relatively cheaper and coarser TiO₂ raw materials could be used for low‐temperature synthesis of TiB₂ fine particles. The “dissolution‐precipitation” mechanism governed the overall molten salt synthesis process.     

Low‐Temperature Rapid Synthesis of Rod‐Like ZrB2 Powders by Molten‐Salt and Microwave Co‐Assisted Carbothermal Reduction

A novel molten‐salt and microwave coassisted carbothermal reduction (termed as MSM‐CTR) method was developed to prepare ZrB₂ powders from raw materials of ZrO₂, B₄C, and amorphous carbon. The results indicated that the carbothermal reduction reaction for synthesizing ZrB₂ was initiated at the temperature as low as 1150°C, and phase pure ZrB₂ powders were obtained after only 20 min at 1200°C, which were significantly milder than that of the conventional CTR method as well as the modified CTR method even using active metal as additional reducing agents. More interestingly, the as‐obtained ZrB₂ powders consisted of well‐defined single‐crystalline nanorods, which had diameters of 40–80 nm and high aspect ratios of >10. These results demonstrated that the MSM‐CTR is a simple and efficient route for preparation of high‐quality ZrB₂ powders.     

Molten salt synthesis of BaTiO3 nanorods: Dielectric,optical properties,and structural characterizations

Single-crystalline perovskite BaTiO₃ (BTO) nanorods were synthesized by (BaTi₂O₅-template) molten salt synthesis (MSS) methods. The effects of process parameters (eg, calcination temperature, reaction time, molten salt content, as well as different phase structures of TiO₂ and barium oxides) on the formation of BTO nanorods and their structural characteristics were systematically investigated. The BTO nanorods synthesized at 800°C for 5 hours by MSS method with the molar salt ratios of BaCO₃:TiO₂ (anatase):NaCl:KCl equal to 1:1:60:60, exhibited smooth and clean surfaces through their lengths. Their diameters were in the range of 130-200 nm with average length of 5 μm. Large-scale amount of BTO nanorods was synthesized by BaTi₂O₅-template MSS method at 650°C with molar salt ratios of BaC₂O₄:BaTi₂O₅ (template):NaCl:KCl equal to 1:1:60:60. The BTO nanorods successfully retained the one-dimensional morphology of the BaTi₂O₅ templates and their lengths were in the range of 5-10 μm with an average diameter of ~300 nm. The single-crystalline nature of individual BTO nanorod was revealed by its selected area electron diffraction patterns and high-resolution TEM images. The BTO nanorods exhibited good optical properties with optical bandgaps in the range of 2.5-2.6 eV. Such optical bandgaps make the present BTO nanorods promising candidates for ferroelectric photovoltaic devices. Dielectric properties of the BTO nanorods synthesized by MSS method were comparatively investigated with the BTO nanorods synthesized by the BaTi₂O₅-template MSS method. It is found that the later BTO nanorods exhibit better dielectric properties and their synthesized temperature is also much lower than the former ones.     

熔盐辅助碳诱导Ti3AlC2分解制备碳化钛纳米片

二维过渡金属碳化物因其独特的性质而成为极具吸引力的纳米材料。以炭黑和Ti₃AlC₂粉体为原料,以期通过在熔盐环境下实现碳还原Ti₃AlC₂合成碳化钛（TiC）纳米片。研究结果表明,炭黑可以诱导Ti₃AlC₂分解,随着加热温度的升高,Ti₃AlC₂的分解程度逐渐加剧。Ti₃AlC₂在低温（1 000~1 100 ℃）下可以严重分解为Al、Ti、Ti_2.9Al_2.1和TiC,Al和Ti可与微量氧反应生成少量的TiO₂和Al₂O₃,生成的TiC为纳米颗粒,而引入熔盐是确保TiC纳米片形成的必要条件。通过优化制备工艺得到合成高含量TiC纳米片的适宜条件：炭黑与Ti₃AlC₂物质的量比为1.05∶1,氯化钠与氯化钾的质量比为1∶1,盐的总质量与炭黑和Ti₃AlC₂混合物的质量比为2∶1;氩气保护,1 000 ℃保温2 h。通过熔盐热处理得到以TiC为主并含有少量Al₂O₃的试样。产物中TiC的形貌为细小的纳米片,厚度约为10 nm、长度约为170 nm。     

立方相二氧化钕纳米棒的制备与表征

以硝酸钕和柠檬酸为原料,采用均相沉淀法在柠檬酸溶液中合成出棒状纳米Nd(Cit)络合物,对该络合物进行热处理得到二氧化钕纳米棒。采用X射线衍射和透射电子显微镜对其进行了表征。结果表明,棒状纳米Nd(Cit)络合物在空气中于600℃氧化处理可得到多晶NdO2纳米棒;将该棒状纳米络合物在NaCl熔盐中于900℃热处理可得到单晶NdO2纳米棒。高分辨透射电子显微镜观察表明:所合成的NdO2纳米棒沿[220]方向择优生长。     

Role of OH− in the low temperature hydrothermal synthesis of ZnO nanorods

BACKGROUND: ZnO nanorods, which have a wide range of applications, were grown on a Si substrate by low temperature hydrothermal synthesis. An understanding of the reaction mechanism of ZnO nanorods is crucial to control their growth kinetics. Therefore, the effect of OH^? concentration in a zinc sulfate solution on the growth of ZnO nanorods was investigated in order to better understand the growth mechanism of ZnO nanorods. RESULTS: The growth rate and diameter of ZnO nanorods were increased by increasing the pH of the zinc sulfate solution from 10.4 to 10.6, and the highest growth rate, 850 nm h^?1, was observed when the nanorods were grown in a solution with a pH of 10.6. However, further increase in the pH of the solution decreased the growth rate, due to the simultaneous dissolution of ZnO nanorods by OH^?. The amount of OH^? consumed by the dissolution of ZnO and other subreactions was minimized in a solution with a pH of 10.6. The intensity of c‐axis (0001) orientation was the highest in the solution with a pH of 10.6. CONCLUSION: It is concluded that the concentration of OH^? plays a critical role in the hydrothermal synthesis of ZnO nanorods, and that the suppression of OH^? subreactions increases the growth rate of ZnO nanorods. From the changes in the length and diameter of ZnO nanorods with synthesis time, it is suggested that the nucleation of ZnO nanocrystals occurs in the first 30 min, from which the growth of nanorods then proceeds. Copyright © 2007 Society of Chemical Industry     

低温熔盐法制备纳米钨酸钴

以钨酸钠和氯化钴为原料,采用硝酸锂-硝酸钠熔盐法制备出纳米钨酸钴粉体,考察了合成温度对粉体相结构、形貌和光吸收性能的影响,初步探讨了合成机理。结果表明:熔盐与钨酸钴前驱体质量比为6∶1,在210,270,340℃分别反应8 h,可以制备出具有钨锰铁矿结构的单斜相纳米钨酸钴;随着温度的升高,粉体结晶程度提高,平均粒径依次增加,在紫外区和可见光区对光呈现出不同程度的吸收;熔盐条件下,粉体的生长过程由扩散机制控制。低温熔盐法具有合成温度低、操作简单、易于规模化生产等优点。     

The extraction of potassium from K-feldspar ore by low temperature molten salt method

The low temperature molten salt method was used to extract potassium from K-feldspar ore, and some related factors including mass ratio between NaNO₃, NaOH, H₂O and K-feldspar ore, particle size of K-feldspar ore, reaction temperature and time were investigated, respectively. In addition, the optimum condition for this method was determined by a series of condition experiments. What was more, the K-feldspar ore and the leach residue after reaction based on the above optimum condition were analyzed by XRD, SEM and EDS, separately. The results of which indicated that the mechanism of extraction of potassium for this method was according to the ion exchange reaction between sodium ion and potassium ion, and the extraction ratio of potassium had an obvious improvement than that of traditional methods, which could reach up to 96.25%. Therefore, this method can be a feasible solution to extract potassium from K-feldspar ore for its low energy consumption and high efficiency.     

Low temperature synthesis of ZrB2-SiC powders by molten salt magnesiothermic reduction and their oxidation resistance

Herein, we prepare phase-pure ZrB₂-SiC composite powders by molten-salt-mediated reduction of ZrSiO₄/B₂O₃/activated carbon mixtures with Mg, showing that the phase composition and morphology of the above composites is influenced by firing temperature, B:Zr and C:Si molar ratios, and the amount of excess Mg. Notably, phase-pure ZrB₂-SiC powder with a ZrB₂:SiC weight ratio of ~75:25 could be obtained by 3-h firing at 1200?°C, i.e., at a temperature lower than that used for conventional carbothermal reduction by at least 200?°C. As-prepared ZrB₂-SiC composites exhibited grain sizes of several microns and comprised SiC nanoparticles well distributed in the ZrB₂ matrix. Finally, the oxidation activation energies of the prepared ZrB₂ and ZrB₂-SiC powders were determined as 326 and 381?kJ/mol, respectively, which demonstrated that the introduction of SiC improved the oxidation resistance of monolithic ZrB₂.     

Fabrication and characterization of ternary La1‐xCexB6 by a novel molten salt synthesis route

A novel molten salt synthesis route has been developed to prepare ternary La_1‐xCe_xB₆ (x = 0.2, 0.4, 0.6, 0.8), using LaCl₃, CeCl₃, and NaBH₄ as reactants and eutectic LiCl/KCl salt as molten salt medium. All the synthesized ternary La_1‐xCe_xB₆ are single‐phase solid solution, and their lattice constant decreases linearly with increasing x. Morphology, microstructure, and elements composition of La_0.6Ce_0.4B₆ which is a typical representative of the series of La_1‐xCe_xB₆ are analyzed in detail.     

金纳米棒合成与自组装的研究进展

与球形金颗粒相比,各向异性的金纳米棒同时具有化学和光学上的各向异性,其更为特殊的表面等离子共振（SPR）特性和基于表面SPR的强吸收和发光特性,在材料科学和生物医学领域中存在着巨大的应用前景。本文主要评述了金纳米棒合成与组装的最新研究进展,具体内容包括：金纳米棒的合成、模板诱导的金纳米棒的自组装、表面张力诱导的金纳米棒的自组装及应用。     

Synthesis of ultra‐fine TaB2 nano powders by liquid phase method

Ultra‐fine TaB₂ powders were synthesized by a liquid phase method using tantalum ethoxide, boric acid and sucrose as the sources of tantalum, boron, and carbon. The TaB₂ precursor powders is a Ta–B–C–O network system, which were heat‐treated at lower temperature (1500°C) in normal argon atmosphere to obtain the TaB₂ powders. XRD confirms the presence of only hexagonal TaB₂, while EDS and XPS spectrums confirm the composition and element chemical states of TaB₂. The TEM images show a platelet shape of the TaB₂ powder and the monocrystal SAED pattern confirms the presence of hexagonal TaB₂. Particle size distribution curves show that particle size of the TaB₂ powders distributes in the range of 30‐160 nm, whose mean particle size is 106 nm.     

Seed crystal of modified potassium sodium niobate prepared by simple molten salt synthesis

Recently, an interesting phenomenon has been reported in (K,Na)NbO₃ (KNN)‐based ceramics. That is the growth of huge grains (10‐30 mm) which is a kind of abnormal grain growth (AGG). It was also interesting that the huge grains showed the stack of plate‐type grains. Using one of those compositions, therefore, seed plates were synthesized by not topochemical reaction but simple molten salt synthesis (SMSS) which was a simple‐and‐cheap process. The calcined K_0.4925Na_0.4925Ba_0.0165Nb_0.998O_3.004 (KNBN) powders and KF powders were heat‐treated at 1050°C‐1150°C. The seed crystals showed the (100)_C‐oriented structure. The size of seed plates was smaller than 500 μm and one layer of a thin plate‐type grain might show the thickness of 50 nm. They might be useful as the seeds at RTGG or TGG process.     

Low temperature synthesis of nano-crystalline gadolinium titanate by molten salt route

Nano-crystalline Gadolinium Titanate (Gd₂Ti₂O₇) powder was successfully synthesized by “Single Step Molten Salt Technique”. LiCl–KCl eutectic mixture was used as a molten medium for the reaction. The duration of the synthesis was 10 h. Stoichiometric proportion of the reactants were mixed in an (LiCl–KCl) eutectic medium and treated at 750 °C in an electrical resistance furnace. Single phase Gadolinium Titanate compound was obtained by the thermal process. The synthesized powders were characterized using XRD, FT-IR, UV, EDAX and XPS analyses. The morphology of the powder was examined using SEM and TEM techniques. From the above studies, it has been concluded that pure crystalline Gadolinium Titanate powders can be synthesized via low temperature molten salt process.     

活性炭在室温熔盐电解质中的电容特性

制备了热稳定性高、电化学性能良好的室温熔盐高氯酸锂(LiClO4)-1,3-氮氧杂环戊-2-酮(OZO),对其作为电解质材料在活性炭电化学电容器中的应用进行了研究.恒流充放电和循环伏安测试表明,活性炭电极与该类室温熔盐电解质表现出良好的电化学兼容性,比电容达到55.2 F/g,并具有良好的循环性能,循环充放电760次后容量损失小于5%.结合室温熔盐不宜燃、不挥发等特性,表明其是超级电容器非常有前景的新型电解液.     

片状氧化铝的熔盐法制备关键参数

高径厚比片状Al₂O₃在高端珠光颜料片状基材应用方面潜力巨大,但是其制备技术为国外所垄断,开发片状Al₂O₃商用生产技术迫在眉睫。熔盐法是制备片状Al₂O₃的理想方法,目前尚无系统研究熔盐法关键工艺参数对片状Al₂O₃影响规律的报道。本文通过片状Al₂O₃的形成机理,系统研究了TiO₂添加剂、Na₃P₃O₉添加剂、煅烧温度和熔盐用量4种关键工艺参数对制备片状Al₂O₃的影响规律,并进一步优化制备工艺,提出最优工艺参数。利用扫描电子显微镜和X射线衍射仪分析Al₂O₃的形貌、粒度和物相。结果表明：采用熔融盐（Na₂SO₄-K₂SO₄）添加NaCO₃为凝胶剂,再加入质量分数分别为3.0%的Na₃P₃O₉和2.0%的TiO₂为添加剂,在煅烧温度为1200℃、保温时间5h时,所制得的六角片状Al₂O₃粉体具有优异的品质,其平均粒径约为4μm,厚度为50～200nm,径厚比为20～80。     

Low-temperature molten salt synthesis of high-entropy carbide nanopowders

Synthesis of the powders is critical for achieving the extensive applications of high-entropy carbides (HECs). Previously reported studies focus mainly on the high-temperature (>2000 K) synthesis of HEC micro/submicropowder, while the low-temperature synthesis of HEC nanopowders is rarely studied. Herein we reported the low-temperature synthesis of HEC nanopowders, namely (Ta_0.25Nb_0.25Ti_0.25V_0.25)C (HEC-1), via molten salt synthesis for the first time. The synthesis possibility of HEC-1 nanopowders was first theoretically demonstrated by analyzing lattice size difference and chemical reaction thermodynamics based on the first-principle calculations, and then the angular HEC-1 nanopowders were successfully synthesized via molten salt synthesis at 1573 K. The as-synthesized nanopowders possessed the single-crystal rock-salt structure of metal carbides and high compositional uniformity from nanoscale to microscale. In addition, their formation mechanism was well interpreted by a classical molten salt-assisted growth.