An easy approach to obtain textured microstructure and transparent seed crystal prepared by simple molten salt synthesis in modified potassium sodium Niobate

Microstructural engineering in conjunction with compositional design has been used for tailoring the properties of piezoelectric materials. In piezoelectric ceramics, the plate-like particles are used for the seed materials in order to realize the textured microstructures. In this study, the seed crystals are synthesized by simple molten salt synthesis (SMSS), using a generalized rule for driving the abnormal grain growth in (K,Na)NbO₃ (KNN)-based ceramics. Furthermore, the textured ceramics are successfully produced in Ba-doped KNN materials, using the seed crystals which are grown by SMSS. Consequently, the textured ceramic shows the Lotgering factor of 92% and the piezoelectric constant (d₃₃) which is similar to that of a single crystal. It is suggested that the excess donor ratio must be considered to obtain the seed crystals (synthesized by SMSS) and textured microstructures in KNN-based ceramics.     

A composition design rule for crystal growth of centimeter scale by normal sintering process in modified potassium sodium niobate ceramics

It has been reported that single crystals could be grown by normal sintering process, without the addition of a seed, in (K,Na)NbO₃ (KNN)-based ceramics. The growth of huge grains (approximately 5–30 mm) is due to the donor effect on abnormal grain growth (AGG) in KNN-based ceramics. In this study, a composition design rule is suggested to obtain the large single crystal without the seed addition in KNN-based ceramics. In addition, it is also identified by the microstructure observation that the huge grains can be due to the donor effect on the abnormal grain growth which is found in perovskite materials. The donor ratio must be designed to be located at the range between 0.7% and 0.9% compared to that of Na⁺ ions, in order to obtain the large single crystals in KNN-based ceramics. This range of the donor ratio is narrower than that in BaTiO₃ (or SrTiO₃) ceramics.     

Low-temperature synthesis of tungsten diboride powders via a simple molten salt route

WB₂ powders were successfully prepared at 1000°C using NaCl–KCl salts as molten salts. Compared to the conventional solid-state reaction, the introduction of NaCl–KCl salts decreased the synthesis temperature of WB₂ phase, which was related to the faster diffusion rate of particles in the liquid state than in the solid-state. The effect of reaction temperature, time, and the amount of molten salts on the synthesis of WB₂ were studied in detail. The template formation mechanism was considered to have played an essential part in the synthesis process. Transmission electron microscope analysis showed that the as-synthesized WB₂ powders exhibited a hexagonal crystal structure. Furthermore, the oxidation protective performance of as-synthesized WB₂ powders was investigated by TG-DSC measurement.     

Preparation and characterization of potassium sodium niobate nanofibers by electrospinning

In this work, we describe the electrospinning of (K,Na)NbO₃ fibers and the effect of calcination temperature on the final phase composition. The envisaged application is for the fabrication of ferroelectric sensor hybrid materials. A solution of potassium acetate, sodium methoxide, and niobium ethoxide dissolved in methanol, acetylacetone, and acetic acid was mixed with polyvinylpyrrolidone (PVP) dissolved in methanol, producing a viscous solution for electrospinning. Confirmation that the proposed equation on the average diameter of fibers produced from high viscosity solutions was larger than that of a lower viscosity solution was made. A scanning electron microscopy (SEM) study showed the fibers to be cylindrical, smooth with diameters of around 400 nm and an aspect ratio >1000. The electrospun fibers were calcined from 700°C to 1050°C observing the fiber morphology. With increasing calcining temperature, the grain size increased. The calcined (K,Na)NbO₃ nanofibers were brittle and generally found to display the “necklace effect.”     

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.     

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。     

Preparation of alumina ceramic template by the molten salt method

ABSTRACT

Single-crystal alpha alumina (α-Al₂O₃) platelets were synthesised with aluminium sulphate in the study. Moreover, the effects of the heating temperature, holding time, and different power mixtures of γ-Al₂O₃ and sodium sulphate (Na₂SO₄) on the morphology of as-prepared α-Al₂O₃ platelets were explored. The crystalline phase evolution and morphology of the samples were characterised by X-ray diffraction and scanning electron microscopy. According to the further analysis results, most of α-Al₂O₃ platelets were hexagonal particles. After the power mixture of γ-Al₂O₃ and Na₂SO₄ (1:2) was heated at 1200°C for 1?h, the platelet size became relatively uniform with an average diameter of 3–5?μm and a thickness of 50–100?nm. This method is an easily controllable, simple, and green method to produce high-quality α-Al₂O₃ platelets.     

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.     

Preparation of Zr3Al3C5 ceramic powder by molten salt synthesis

Methods for synthesising Zr-based MAX phases powder generally require high temperature and product processing such as milling, which can introduce impurities and high cost. A new method to syntheszse Zr₃Al₃C₅ powder from Zr, Al, and C particles in NaCl‒KCl molten salt was reported. The effects of temperature, Al content, and type of carbon source on the formation of Al–Zr–C compounds were investigated. The results show that molten salt synthesis can be used to prepare Zr₃Al₃C₅ powder at a low temperature. When additional Al was added to yield a Zr:Al ratio of 2:9 and resin carbon was used as the carbon source, the purity of the prepared Zr₃Al₃C₅ reached 96%. ZrC is always present in the powder as an impurity, which is consistent with many commercial MAX phases.     

Sodium‐potassium niobate nanorods with various crystal structures and their application to nanogenerator

(Na_1?xK_x)NbO₃ (NKN) nanorods with an orthorhombic (OR) structure, a rhombohedral (RH) structure, or a polymorphic phase (PP) structure, which contains both OR and RH structures, are prepared. The presence of RH and PP NKN nanorods is explained by the existence of OH^? defects at the O^2? sites of the NKN nanorods. The PP NKN nanorods grown on a Nb⁵⁺‐doped SrTiO₃ substrate show the largest piezoelectric strain constant of 175 pm/V because they have more directions for dipole rotation than OR and RH NKN nanorods. Piezoelectric nanogenerators (NGs) are synthesized using composites consisting of NKN nanorods with various structures and polydimethylsiloxane. The largest open‐circuit output voltage is 35 V, and the short‐circuit current is 5.0 μA, which are obtained using the NG containing 0.7 g of PP NKN nanorods. Moreover, this NG shows a maximum output power of 16.5 μW for an external load of 10.0 MΩ.     

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。     

改性层状结晶二硅酸钠合成工艺研究

研究了用锂盐和溴化十六烷基三甲基铵（CTMAB）改性制备新型无磷洗涤助剂层状结晶二硅酸钠的方法,探讨了原料中的杂质对产品质量的影响及工艺的控制条件。最佳工艺条件：硅与CTMAB与锂物质的量比为40∶1∶2,液体硅酸钠中二氧化硅与氧化钠物质的量比为2.00±0.88,喷雾干燥后干粉含水低于12%（质量分数）,焙烧温度705-725℃,焙烧时间20-30 min,所得产品的钙离子交换能力大于370 mg/g,镁离子交换能力大于400 mg/g。实验表明,当原料满足一定的要求,控制好工艺条件,能够生产出符合要求的合格产品。     

熔盐电解法制备硼粉的研究

对熔盐电解法制备硼粉的方法进行了论述,比较了各熔盐体系的优点和缺点,得出了氯化钾或氯化钾和氟化钾混合物的熔盐体系最适宜。用冷却曲线法对KF-KCl-KBF4体系熔盐体系的初晶温度进行了研究,研究表明:KF-KCl-KBF4体系的初晶温度为1 023—1 033 K,即750—760℃之间,当电解温度高于初晶温度20—30℃时,电解效率最高。这就为熔盐电解法制备元素硼提供了依据。     

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。     

Low temperature synthesis of TaB2 nanorods by molten‐salt assisted borothermal reduction

TaB₂ powders were synthesized by a molten‐salt assisted borothermal reduction method at 900°C‐1000°C in flowing argon using Ta₂O₅ and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B₂O₃ and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB₂ powders exhibited a flow‐like shape assembled from nanorods grow along [001] direction or c‐axis. The morphology of the synthesized TaB₂ powders could be tailored by the amount of B₂O₃ or NaCl/KCl.     

Defect engineering of high‐performance potassium sodium niobate piezoelectric ceramics sintered in reducing atmosphere

Reducing atmosphere fired (K_0.5Na_0.5)NbO₃‐based ceramics open up an important opportunity for manufacturing lead‐free multilayer piezoelectric devices cofiring with the base metal inner electrode. In this study, the effects of sintering atmosphere on the piezoelectric, dielectric, and insulating properties in Sn‐doped (Na_0.52K_0.44Li_0.04)NbO₃ (KNN) were investigated. To establish the relationship between the defect structure and electrical properties, the thermally stimulated depolarization current (TSDC) technique was implemented. The electrical properties degrade severely when the pure KNN ceramics are sintered in reducing atmosphere, compared with the ceramics sintered in air. The reason is that the oxygen vacancy concentration in reduced fired ceramics is much higher than that in air fired ceramics. However, the 0.6 mol% Sn‐doped KNN ceramics sintered in reducing atmosphere exhibit comparable electrical properties to the air fired ceramics. From the TSDC analysis, the reducing atmosphere fired ceramics have approximately the same oxygen vacancy concentration as the air fired ceramics because B‐site substituted Sn works as an acceptor.     

熔盐法制备氧化镁粉体的研究

以氯化镁、碳酸钙、氯化锂为原料,采用熔盐法制备了氧化镁粉体。通过热重-差示扫描量热仪(TG-DSC)、X射线衍射仪(XRD)、场发射扫描电子显微镜(SEM)等手段对反应过程及产物进行了分析和表征,同时采用水解动力学分析法研究了煅烧温度和煅烧时间对氧化镁粉体活性的影响。TG-DSC分析表明:在650℃时碳酸钙可完全反应。XRD分析表明:在430℃保温3 h条件下进行热处理,产物中有氧化镁生成;在650℃保温3 h条件下进行热处理,产物经无水乙醇洗涤后,全部为氧化镁晶体,经SEM分析表明,所制备的氧化镁形貌为颗粒状,形状不规整,大小介于50~250 nm。水解动力学分析表明:当温度大于500℃、保温时间超过3 h后,随着热处理温度的升高和时间的延长,氧化镁粉体的活性下降。     

Molten salt synthesis,characterization, and formation mechanism of superfine (HfxZr1-x)B2 solid-solution powders

High-purity superfine (Hf_xZr_1-x)B₂ solid-solution powders with various chemical compositions (x = 0.2, 0.5, and 0.8) were successfully synthesized via molten salt synthesis technique at 1423 K using ZrO₂, HfO₂, and 20% excessive B as precursors within a KCl/NaCl molten salt for the first time. The results showed that the as-synthesized solid-solution powders exhibited the irregular polyhedral morphology with the average particle sizes of 170–185 nm and simultaneously possessed a single-crystalline hexagonal structure and good compositional uniformity from nanoscale to microscale. In addition, their formation mechanisms were well interpreted by analyzing the thermodynamics of the possible chemical reactions in the molten salt.     

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.     

Synthesis of Ti3SiC2 MAX phase powder through molten salt method

Titanium silicon carbide (Ti₃SiC₂) MAX phase powder was synthesized from elemental reactants using the molten salt synthesis (MSS) method. Optimum experimental parameters were also investigated to determine the purity and synthesis pathway of the Ti₃SiC₂ MAX phase. The results showed that Ti₃SiC₂ was not synthesized using carbon black as the carbon source in the starting materials because of the high quantity of TiC formed along with the TiSi₂ silicide phase. However, Ti₃SiC₂ was successfully synthesized in a relatively high purity (93%) at 1200°C for 2 h using graphite as the source of carbon because of the formation of TiC and Ti₅Si₃ intermediate phases. The Ti₅Si₃ silicide phase was found to play a crucial role in the formation of the Ti₃SiC₂ MAX phase using the MSS method. Moreover, applying a pressure of 150 MPa to the prepared samples and using the eutectic mixture of NaCl–KCl (molar ratio: 1:1) instead of NaCl also resulted in the higher formation of the Ti₃SiC₂ MAX phase. The formation mechanism of Ti₃SiC₂ was determined to be the reaction among Ti₅Si₃, TiC, and residual carbon through the template-growth and dissolution–precipitation mechanisms that occurred at different stages of the synthesis process.