Low-temperature synthesis of CdCu3Ti4O12 powders with high dielectric permittivities

The molten salt method was adopted in this work to synthesize CdCu₃Ti₄O₁₂ precursor powders and was found capable of notably reducing the sintering temperature. It was also found that the grain size gradually increases and the grain shape changes from spherical to cubic with increasing proportions of molten salt. The optimal sintering conditions were found to be at approximately 730?°C with the addition of 20% molten salt. After further sintering at 975?°C, the obtained perovskite ceramics exhibited a high dielectric constant at low frequencies.     

Effect of LiF dosage on morphology of ZrO2 prepared by the molten salt method

Zirconia nanorods and polyhedral particles were prepared using the molten-salt method. The effects of the LiF dosage on the ZrO₂ crystal morphology were studied using XRD combined with Rietveld refinement, FE-SEM combined with EDS, FTIR, Raman spectroscopy, and high-temperature microscopy. The results show that the ZrO₂ obtained with LiF is in the monoclinic phase. ZrO₂ nanorods were synthesized at low LiF dosages. Polyhedral ZrO₂ particles were synthesized, and the ZrO₂ crystal planes (100) and (200) were exposed to a high LiF dosage. LiF promoted the dissolution of ZrO₂ and was adsorbed onto the ZrO₂ crystal surface. This work provides a new strategy for controlling morphology and crystal surface exposure.     

Defect generation and morphology transformation mechanism of CeO2 particles prepared by molten salt method

Ceria is widely used in industrial fields due to its unique chemical properties. In this work, a series of CeO₂ particles with controllable morphology, size, and defect concentration were obtained by a simple molten salt method. The adjustment of temperature and molten salt concentration has a considerable effect on the morphology and particle size of the final CeO₂ particles while prolonging the holding time has little effect. Ion doping and reducing atmosphere calcination were used to regulate the defect concentration to improve the chemical activity of CeO₂ particles. SEM results show that the morphology of CeO₂ particles transforms from sphere to octahedron under the two treatments. The Rietveld refinement results and the XPS spectra indicate that increasing calcination temperature, reducing atmosphere calcination and ion doping are beneficial to improving the oxygen vacancies and Ce³⁺ concentration of CeO₂ samples, which are the reason for enhancing the photocatalytic activity of the samples. Moreover, the oriented attachment, agglomeration and merging of crystals formed by the decomposition of cerium precursors are the key to the growth of CeO₂ particles. Aggregates with exposed low-energy planes merge directly to form particles of various morphologies to maintain their own low energy.     

Preparation and microwave dielectric properties of Ca0.6La0.8/3(SnxTi1−x)O3 ceramics

Ca_0.6La_0.8/3(Sn_xTi_1−x)O₃ ceramics were prepared via a conventional solid state reaction method, and the effect of Sn doping on their crystal phase structure and microwave dielectric properties was investigated. Results showed that Sn doping could hinder the formation of the rutile TiO₂ detrimental phase of Ca_0.6La_0.8/3TiO₃ ceramic. Also, the Q×f₀ value was enhanced and the τ_f value was lowered by Sn doping. The best microwave dielectric properties, i. e. ε_r=113 and Q×f₀=8487 GHz were obtained for a Sn doping content of 0.02. The mechanism of the improved properties deriving by Sn doping is discussed.     

Crystal structure and microwave dielectric behaviors of scheelite structured (1-x)BiVO4-xLa2/3MoO4 (0.0 ≤ x ≤ 1.0) ceramics with ultra-low sintering temperature

In the present work, a novel (1-x)BiVO₄-xLa_2/3MoO₄ scheelite related solid solution ceramics were prepared via solid state reaction method. As revealed by X-ray diffraction data, the crystal structure changed continuously from monoclinic to tetragonal phase at x = 0.10 and then the tetragonal solid solution was kept in a wide composition range up to x = 0.70. Both the Raman and far infrared spectra supported this phenomenon. When x value reached 0.9, the ceramic sample was found to composed of both scheelite tetragonal and monoclinic La_2/3MoO₄ phases. Large microwave permittivity value between 68 ± 0.2–73 ± 0.3 can be achieved in compositions with 0.02 ≤ x ≤ 0.10 with high Qf value about 10,000 ± 500 GHz. This series of solid solution ceramics might be candidate for both dielectric resonator and low temperature co-fired ceramic technology.     

Crystal structure,impedance and broadband dielectric spectra of ordered scheelite-structured Bi(Sc1/3Mo2/3)O4 ceramic

Bi(Sc_1/3Mo_2/3)O₄ ceramics were prepared via solid state reaction method. It crystallized with an ordered scheelite-related structure (a?=?16.9821(9)?Å, b?=?11.6097(3)?Å, c?=?5.3099(3)?Å and β?=?104.649(2)°) with a space group C12/C1, in which Bi³⁺, Sc³⁺ and Mo⁶⁺ are ?8, ?6 and ?4 coordinated, respectively. Bi(Sc_1/3Mo_2/3)O₄ ceramics were densifiedat 915?°C, giving a permittivity (ε_r) ～24.4, quality factor (Qf, Q?=?1/dielectric loss, f?=?resonant frequency) ～48, 100?GHz and temperature coefficient of resonant frequency (TCF)?～??68?ppm/°C. Impedance spectroscopy revealed that there was only a bulk response for conductivity with activation energy (E_a) ～0.97?eV, suggesting the compound is electrically and chemically homogeneous. Wide band dielectric spectra were employed to study the dielectric response of Bi(Sc_1/3Mo_2/3)O₄ from 20?Hz to 30?THz. ε_r was stable from 20?Hz to the GHz region, in which only ionic and electron displacive polarization contributed to the?ε_r.     

Crystal structures and microwave dielectric properties of low-firing Ca5Zn4-xMgxV6O24 ceramics

Ca₅Zn_4-xMg_xV₆O₂₄ (x?=?0–3) microwave dielectric ceramics with low sintering temperature were synthesized via the conventional solid-state reaction. Effects of the substitution of Mg²⁺ for Zn²⁺ on crystal structures and microwave dielectric properties were investigated. XRD and Rietveld refinement showed the solid solution single phase formed when 0?≤?x?≤?2, but a few ZnO was observed when x?=?3. Meanwhile, the lattice parameters were found to decrease monotonously with Mg content increasing. The vibration modes of Raman were confirmed and the relationship with microwave dielectric properties was analyzed. Appropriate substitution of Mg²⁺ improved the packing fraction, the cation ordering degree, and the Y-site bond valence, contributing to high Q×f and low | τ_f |. However, the ε_r reduced with the increasing content of Mg²⁺ due to the decrease of ion polarizability. Finally, the best microwave dielectric properties were achieved at x?=?2 with ε_r =?11.0, Q?×?f?=?66,365?GHz (at 10.0?GHz), and τ_f =??80.4?ppm/°C.     

熔融盐电镀铝的研究进展

综述了无机熔融盐电镀铝的各种熔盐组成及相关参数。探讨了无机熔融盐电镀铝的反应机理和铝枝晶的抑制,并介绍了无机熔融盐体系电镀铝的应用和发展前景。     

Molten salt shielded synthesis and formation mechanism of Ti2AlC in NaCl–KCl medium

Herein, a highly crystalline Ti₂AlC was synthesized via the improved molten salt synthesis method called molten salt shielded synthesis. To achieve this goal, the mixture of Ti, Al, and graphite and KCl–NaCl eutectic composition salt was heated at 1000, 1050, and 1100 °C for 0.5, 1, and 1.5 h. The X-ray diffraction (XRD) patterns showed that the optimum condition for obtaining the more crystalline Ti₂AlC was achieved at 1100 °C for 1.5 h. Such phase identification, and transmission electron microscopy (TEM) images, proved that applying a protective carbon layer on the surface of salt led to inhibiting the diffusion of oxygen into the surface of the green pellet. As a result, the crystallinity of Ti₂AlC improved, while the content of undesirable compounds such as Al₂O₃ and Ti_xO_y decreased drastically. In order to shed light on the Ti₂AlC synthesis mechanism, differential thermal analysis (DTA) was employed. The DTA curve revealed that the Ti₂AlC formation completed in three levels. First, the partial dissolution of Ti in KCl–NaCl salt followed by a reaction with liquid Al resulted in the TiAl formation. Next, Ti(II) reacted in-situ on the surface of graphite that resulted in the non-stoichiometric TiC (TiC_1-x) formation, and, at last in a reaction between TiAl and TiC_1-x, Ti₂AlC phase formation took place at 940 °C.     

Surface modification of LiMn2O4 cathode with LaCoO3 by a molten salt method for lithium ion batteries

Spinel LiMn₂O₄ is a promising cathode due to its advantages of low-cost, nontoxicity and thermal stability. However, the dissolution of manganese and the phase transformation induce the rapid capacity fade. Surface coating is an effective method to improve its electrochemical performance. In this work, spinel LiMn₂O₄ modified with perovskite LaCoO₃ was prepared using a novel molten salt method. The resulted samples were characterized by X-ray diffraction (XRD), transmission/scanning electron microscopy (TEM/SEM), Fourier transformation infrared (FT-IR), Raman, and X-ray photoelectronic spectroscopy. The content of Mn³⁺ increased with the LaCoO₃ coating accompanied by the increased concentration of oxygen vacancy. LiMn₂O₄ modified with 2% LaCoO₃ shows a higher capacity and cycling stability than others at 0.2 C, while the cathode with 4% LaCoO₃ shows the best rate performance at a larger current at 2 and 5 C. This enhanced performance can be attributed to improved interfacial conductivity between the cathode and electrolyte and the protective effects of coating.     

Sintering behavior,phase structure and microwave dielectric properties of CeO2 added CaTiO3-SmAlO3 ceramics prepared by reaction sintering method

Novel 0.695CaTiO₃-0.305SmAlO₃+xwt% CeO₂ (x = 0, 0.5, 1.0, 1.5) ceramics were fabricated using a reaction-sintering (RS) approach. The crystal structure, morphology, and microwave dielectric properties of ceramics were systematically studied. The addition of CeO₂ could effectively improve the sintering behavior of 0.695CaTiO₃-0.305SmAlO₃ (CTSA) ceramics. When x = 0.5 wt%, the ceramics exhibited optimal microwave dielectric properties, with ε_r = 43.9, Q×f = 48 779 GHz, and τ_? = ?0.24 ppm/°C, thereby indicating that the samples prepared via the RS route possess superior dielectric properties compared to those prepared by the conventional solid phase reaction. The results demonstrate that CaTiO₃-SmAlO₃ ceramics can be prepared simply and efficiently through a reaction-sintering process.     

Electrical and microwave dielectric properties of K2Nb8O21 microwires

K₂Nb₈O₂₁ microwires with diameters of several hundred nanometers and lengths up to tens of microns were prepared by molten salt synthesis, and characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The as-synthesized microwires have semiconductor characteristics with band gap E_g of about 3.1 eV, consistent with electrical transport measurement of single microwire, which gives a resistivity of about 0.46 Ω m at room temperature. Microwave dielectric property measurement at 2–18 GHz shows that K₂Nb₈O₂₁ microwires have a relative low complex permittivity. A weak dielectric relaxation occurs in the 12–16 GHz band due to the free charges polarization on the interfaces between K₂Nb₈O₂₁ microwires and the PVB matrix.     

Effect of processing parameters on the development of anisotropic α-Al2O3 platelets during molten salt synthesis

It is of utmost importance to control the size and morphology of anisotropic α-Al₂O₃ platelets owing to its extensive usage in various applications, including most recent synthetic biomimetic abalone nacre-like composites. In the present article, a wide range of well-developed and well-dispersed α-Al₂O₃ platelets were produced with diameter and thickness ranging from 4 - 8 μm and 0.6–1 μm respectively, by taking γ-Al₂O₃ precursor and sodium sulfate as low melting flux. The effects of various processing parameters like calcination temperatures, molar ratios of Al/Na and soaking time on the phase formation and morphology development of the final platelets were investigated in rigor. The results indicated that high calcination temperature of 1200 °C and moderate soaking time (60 min) helped to develop α-Al₂O₃ platelets with hexagonal morphology and uniform sizes. The sulfate salts created favorable conditions for growth of anisotropic platy α-Al₂O₃ and Al/Na molar ratio has significant effect in controlling the aspect ratio of the platelets. The impact of deagglomeration treatment by the ultrasound irradiation on the calcined aggregates of platelets was also examined. The breakage of platelets clusters by fracture/erosion due to the interaction with imploding cavitation bubbles under ultrasound energy, reached maximum after 90 min of deagglomeration treatment. The detail phase transformation sequences from γ-Al₂O₃ to α-Al₂O₃ was explained by correlating thermal (DT-TGA) and X-ray analysis. The growth mechanism of the anisotropic α-Al₂O₃ particles in the presence of molten Na₂SO₄ liquid was also unraveled.     

Fabrication and topchemical transformation mechanism of PbTiO3 microplatelets

Two-dimensional platelets of the perovskite-structured PbTiO₃ were synthesized from a layer-structured PbBi₄Ti₄O₁₅ precursor, using a topochemical microcrystal route. The PbBi₄Ti₄O₁₅ precursor showed a high aspect ratio with an average size of ～7.74 μm and thickness of ～0.29 nm. Later, PbTiO₃ platelets coexisting with Pb₄Bi₄Ti₇O₂₄ mesophase were obtained, which had a more stable Aurivillius structure and was difficult to completely convert to the perovskite structure during topochemical microcrystal conversion. The reason that the Pb₄Bi₄Ti₇O₂₄ mesophase coexisted in PbTiO₃ platelets was deeply discussed. Both the crystal structure difference between the unit cell parameters for (Pb₄Bi₂Ti₇O₂₂)^2- and the thickness for the (Bi₂O₂)²⁺ layer, and the stability of (Bi₂O₂)²⁺ layer were used to analyze the existence of Pb₄Bi₄Ti₇O₂₄ phase. However, the result showed that the stability of (Bi₂O₂)²⁺ layer was the dominant reason, because the [OBi₄]/[OPb₄] tetrahedron formed in the (Bi₂O₂)²⁺ layer due to the similar chemical properties of Pb²⁺ and Bi³⁺. The work provided deep theoretical guidance to fabricate anisotropic perovskite platform-based topchemical microcrystal conversion mechanisms.