Preparation and characterization of Ni0.6Mn2.4O4 NTC ceramics by solid-state coordination reaction

The impact of synthesis parameters, such as calcination temperature and reaction time, on the microstructure, phase compositions and electrical properties of Ni_0.6Mn_2.4O₄ negative temperature coefficient (NTC) ceramics, which were synthesized via solid-state coordination reaction method, was systematically explained. With enhancing calcination temperature and time, the relative densities were raised, while the porosities diminished. A compact single-phase cubic spinel ceramic could be obtained after annealing at 1,100 and 1,150 °C, while a secondary phase Mn₃O₄ was detected when the annealing temperature raised to 1,200 °C. For the ceramics calcinated at 1,100 or 1,150 °C for 3 h, their resistivities were 2,133 and 2,178 Ω cm, the thermal constant B, which reflects the temperature sensitivity of the NTC ceramics, were 3,820 and 3,857 K. While the ceramic prepared at 1,200 °C for 3 h, the resistivity and the B value reached 2,273 Ω cm and 3,810 K, respectively. This phenomenon was attributed to the increase of lattice parameters and the reduction of Mn⁴⁺–Mn³⁺ at high temperatures.     

Preparation and properties of low temperature sintered CaO-B2O3-SiO2 microwave dielectric ceramics using the solid-state reaction

The LTCC CaO-B₂O₃-SiO₂ (CBS) ceramics were synthesized via solid-state reaction process without any sintering aid. The effects of different sintering temperatures and B₂O₃ content on the microwave and mechanical properties were investigated. The results show that the best sintering temperature is around 950 °C and increasing amount of B₂O₃ promotes the crystallization of CaB₂O₄ enhancing the flexure strength of the CBS ceramics. However, the dielectric and mechanical properties deteriorated rapidly while the amount of B₂O₃ exceeded 25 wt.%. The sample with 20.5 wt.% B₂O₃ sintered at 950 °C had the best properties with ? _r = 6.06; tanδ = 0.0015 (1 MHz) and a high flexure strength σ _f > 180 MPa.     

低温镁铝尖晶石纳米粉末的制备与表征

以Mg(NO3)2.6H2O、Al(NO3)3.9H2O为原料,Na2CO3、NaOH为沉淀剂,采用化学共沉淀法制得MgAl2O4尖晶石前驱体。用TG、XRD、TEM及物理吸附仪研究了试样的热稳定性、物相组成、显微形貌和表面性能。结果表明:前驱体经800℃处理后,几乎全部转变成粒径为10nm左右的MgAl2O4尖晶石超细粉末,并且颗粒分散性好,比表面积大,为136.35m2/g,比传统制备镁铝尖晶石粉末的温度低600～700℃。     

Preparation and characterization of ZrB 2 -SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Preparation and characterization of ZrB2-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Preparation of nanostructured NiFe2O4 catalysts by combustion reaction

Nanosize nickel ferrite powders (NiFe₂O₄) have been prepared by combustion reaction using nitrates and urea as fuel. The resulting powders were characterized by X-ray diffraction (XRD), nitrogen physical adsorption (BET), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and catalytic activity. The results showed nanosize nickel ferrite powders with high specific surface area (55.21 m²/g). The powders showed extensive XRD line broadening and the crystallite size calculated from the XRD line broadening was 18.0 nm. The nickel ferrite powder presented significant activity as catalyst for the water gas shift reaction, over the temperature range of 250–450 °C.     

Preparation and characterization of bulk ZnGa2O4

High-quality bulk ZnGa₂O₄ has been synthesized from equimolar mixtures of ZnO and Ga₂O₃ by the conventional solid-state method. For the first time, the sample has been characterized in detail to confirm the formation of pure single phase of spinel ZnGa₂O₄. The formation of ZnGa₂O₄ has been confirmed by sintering the mixtures of ZnO and Ga₂O₃ at different temperatures, ranging from 900–1200 °C. It is observed that the single phase of ZnGa₂O₄ has been formed at and above 1000 °C sintering temperature for 24 h. The crystallinity and phase formation of this single phase has been confirmed by X-ray diffraction. X-ray photoelectron spectroscopic studies have been carried out for bulk ZnGa₂O₄ sintered at 1000 °C for 24 h which showed 14% Zn, 28% Ga and 58% O, indicating stoichiometric ZnGa₂O₄. A new parameter, the energetic separation between the Zn 2p_3/2 and Ga 2p_3/2 peaks, has been used as a sensitive tool to distinguish between a complete formation of ZnGa₂O₄ compound and a mixture of ZnO and Ga₂O₃ powders. Surface morphology studies by scanning electron microscopy reveal that the formation of ZnGa₂O₄ takes place in mosaic rod-like structure. The purity of the compound has also been checked by the energy dispersive X-ray method, indicating the absence of foreign ions and the ratio of zinc to gallium has been calculated and found to be 1 : 2, indicating stoichiometric ZnGa₂O₄.     

纳米铝酸锌的制备及其表征

用醇-水乳浊液法制备了立方系尖晶石结构纳米ZnAl2O4,用XRD和SEM对其进行了表征.结果表明,该ZnAl2O4纳米颗粒粒径在30～50nm之间,分散均匀,大部分粒子都已独立成核,粒径随热处理温度的升高而变大.     

Characterization of nanoparticles of LiMn2O4 synthesized by a one-step intermediate-temperature solid-state reaction

Nanoparticles of lithium manganese oxide (LiMn2O4) with a spinel structure have been synthesized by a one-step intermediate temperature solid-state reaction. The influence of the molar ratio of citric acid to the metal ions on the physicochemical properties of LiMn2O4 powders in air has been analyzed by means of X-ray diffraction and electron microscope techniques. The electrochemical behavior of the material has been examined by charge/discharge tests and cyclic voltammetry. Test results reveal that LiMn2O4 particles with lower molar ratios of citric acid to metal ions (1:2) are highly crystalline and highly electrochemically reversible, with better cycle capabilities when compared with a sample with a higher molar ratio (2:1). The LiMn2O4 powders obtained by this method have a uniform morphology with a narrow size distribution.     

Preparation and characterization of K-substituted NaCa4Nb5O17 microwave dielectric ceramics

The effect of K substitution for Na on the phase, microstructure and microwave dielectric properties of the Na_1?x K _x Ca₄Nb₅O₁₇ (x = 0–1) composition series was investigated. The compositions with x = 0, 0.5 and 1 formed single-phase Na_1?x K _x Ca₄Nb₅O₁₇ ceramics within the detection limit of the in-house XRD facility when sintered at 1,200–1,300 °C. At x = 0.25 and 0.75, the major Na_1?x K _x Ca₄Nb₅O₁₇ phase formed but along with a secondary CaNb₂O₆ phase. Relative permittivity (ε _r ) and temperature coefficient of resonant frequency (τ_f) increased from 45 to 51 and ?120 to +473 ppm/ °C respectively while the quality factor (Q × f _o ) decreased from 13,838 to 2,374 GHz with an increase in x from 0 to 1. Optimum microwave dielectric properties (i.e. ε _r  = 47, Q × f _o  = 5,047 GHz and τ_f = ? 23 ppm/ °C) were achieved for the x = 0.5 (i.e. Na_0.5K_0.5Ca₄Nb₅O₁₇) composition. Further investigations are required to improve the density and hence microwave dielectric properties of Na_1?x K _x Ca₄Nb₅O₁₇ ceramics.     

NiMn2O4热敏材料纳米粉体烧结过程研究

本文采用溶胶－凝胶法（ｓｏｌ－ｇｅｌ）制备ＮｉＭｎ２Ｏ４纳米粉体。将ＮｉＭｎ２Ｏ４纳米粉体压制成型,进行烧结。对ＮｉＭｎ２Ｏ４的米粉体烧结过程进行研究。实验结果表明：ＮｉＭｎ２Ｏ４纳米粉体的烧结温度比传统烧结方法低２００℃左右。在１１００℃以下的温度范围,烧结试样的相结构均为尖晶石结构。ＮｉＭｎ２Ｏ４纳米粉体烧结过程分为三个过程：（１）烧结前期（≤８００℃）;（２）烧结初中期（８００℃￣１０００℃     

Preparation and characterization of two new dielectric ceramics Ba4NdTiNb3O15 and Ba3Nd2Ti2Nb2O15

Two new cation-deficient hexagonal perovskites Ba₄NdTiNb₃O₁₅ and Ba₃Nd₂Ti₂Nb₂O₁₅ were prepared in the BaO–Nd₂O₃–TiO₂–Nb₂O₅ system by high temperature solid-state reaction route. The phase and structure of the ceramics were characterized by X-ray diffraction and scanning electron microscope (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. Ba₄NdTiNb₃O₁₅ has a dielectric constant of 38.15, a high-quality factors (Q _u ×f >18 700 at 5.4422 GHz), and a small temperature coefficient of resonant frequency ( _f )+12 ppm °C^–1 at room temperature; Ba₃Nd₂Ti₂Nb₂O₁₅ has a higher dielectric constant of 46.83 with high-quality factors Q _u ×f >19 500 at 5.0980 GHz, and _f +28 ppm °C^–1.     

Preparation and characterization of LiMn2O4 and LiCo0.16Mn1.84O4 as cathode materials for lithium-ion batteries by low heating solid state coordination method

Spinel lithium manganese oxide LiMn₂O₄ and its substituted forms LiCo_0.16Mn_1.84O₄ were prepared by annealing of the mixed precursors which were synthesized by low heating solid state coordination method using lithium acetate, cobalt acetate, manganese acetate and oxalic acid as starting materials. The structures and morphologies of the LiMn₂O₄ and LiCo_0.16Mn_1.84O₄ were investigated as a function of annealing temperature. The results show that all samples in different annealing temperature have the same spinel structure. The higher the annealing temperature, the more complete the crystal structure, and the larger the particle size. In addition, the LiMn₂O₄ and LiCo_0.16Mn_1.84O₄ were used as cathode active materials for lithium secondary batteries and their charge/discharge properties have been investigated. As a result, it could be seen that low heating solid state coordination method is an effective method to prepare lithium manganese oxide (LiMn₂O₄) and its substituted forms.     

球形尖晶石LiMn2O4的制备及性能

通过氧化还原法在室温下制备出球形MnO2前驱体,以LiOH·H2O为锂源,按照一定锂锰摩尔比混合,在750℃下焙烧8h,得到球形尖晶石LiMn2O4.采用X射线衍射和扫描电镜对MnO2和LiMn2O4进行了表征,并对LiMn2O4样品做了充放电性能及循环性能测试.结果表明:合成的样品以球形颗粒存在,粒度大小均匀,分散性和流动性好;首次充放电比容量分别为130.5和128.2 mAh·g-1,充放电效率为98.2%,50次循环后容量保持率为90%,球形LiMn2O4具有较高的比容量和优良的循环性能.     

Preparation and microwave dielectric properties of Bi2Ti4O11 ceramics

Single phase of Bi₂Ti₄O₁₁ ceramics, which belong to meta-stable phase compounds, were synthesized by controlling the reaction time through conventional solid-state method. The effects of annealing time on phase composition of Bi₂Ti₄O₁₁ ceramic powders and sintered ceramics were studied by XRD analysis. Second phase Bi₂Ti₂O₇ appeared when the annealing time shorter than 4 h. However, pure phase of Bi₂Ti₄O₁₁ powders can be formed by prolonging the annealing time to 6 h at 1,000 °C. The sintering temperatures on microstructure and microwave dielectric properties of Bi₂Ti₄O₁₁ ceramics were investigated. The results show that ceramics sintered at 1,075–1,175 °C are single phase of Bi₂Ti₄O₁₁ and present two different sizes of prismatic shape grains. Smaller size crystals grow into larger ones with increasing sintering temperature. The ceramics sintered at 1,125 °C reach a maximum density and have a microwave dielectric properties of ε_r = 51.2, Q × f = 3,050 GHz and τ_f = ?297 ppm/°C.     

Preparation of Sr2MgSi2O7:Eu2+, Dy3+ nanofiber by electrospinning assisted solid-state reaction

Novel one-dimensional polyvinylpyrrolidone (PVP)/[Sr(NO₃)₂ + Mg(NO₃)₂ + TEOS + Eu(NO₃)₂ + Dy(NO₃)₂] composite fibers were prepared via electrospinning, and then it was calcined at high temperature to fabricate the long-persistent luminescence Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ nanofiber by solid-state reaction under a reducing atmosphere. X-ray diffraction, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy and fluorescence spectrophotometer as well as afterglow brightness tester were used to characterize the resulting samples. The results revealed that diameter of the composite nanofibers decreased firstly and then increased with the increasing of the calcination temperature. FTIR analysis manifested that there was two new absorption peaks around 660 and 834 cm^?1, which corresponded to Mg–O stretching and Sr–O stretching, when PVP/[Sr(NO₃)₂ + Mg(NO₃)₂ + TEOS + Eu(NO₃)₂ + Dy(NO₃)₂] composite nanofibers was calcined at the temperature of 1,100 °C, indicating that the crystalline Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ had formed. Furthermore, its luminescence properties were tested. Upon excitation in the UV region, the emission spectra consisted of broad emission band, and the emission peak was located at the wavelength of 468 nm. Decay process contained both rapid-decaying and the slow-decaying processes, which due to the short survival time of the electro in Eu²⁺ and the deep trap energy center of Dy³⁺.     

Preparation and characterization of MnFe2O4 by a microwave-assisted oxidative roasting process

An innovative and effective technological process is proposed to synthetize spinel manganese ferrite (MnFe₂O₄) by a microwave-assisted oxidization roasting process. In the current paper, the effect of operating variables on the formation mechanism of Fe-MnO systems for synthesizing spinel MnFe₂O₄ is initially investigated to reveal the oxidation mechanism and phase transformation during the microwave–assisted oxidative roasting process. The synthetic MnFe₂O₄ microparticles with a saturation magnetization (Ms) of 70.5 emu/g, a coercivity (Hc) of 4.65 Oe and a ratio of the saturation magnetization to remanent magnetization (Ms/Mr) of 0.055 were successfully prepared by heating for 30 min at 1000 °C in an air atmosphere. The MnFe₂O₄ samples exhibited normal and inverse spinel crystal structures, inevitably coexisting and co-transforming simultaneously at high temperature (≥950 °C). Furthermore, this innovative and effective technological process can be extended to fabricate other spinel ferrite particles of interest.     

Control of temperature coefficient of resonant frequency in Ba4Sm9.33Ti18O54 ceramics by templated grain growth

A novel tuning method of temperature dependence of dielectric properties in Ba₄Sm_9.33Ti₁₈O₅₄ (BSmT) microwave material was demonstrated by texture engineering. Two kinds of grain-oriented BSmT ceramics were obtained using a templated grain growth technique. Highly fiber-textured ceramics were fabricated by tape casting of slurry with a mixture of 〈001〉-elongated BSmT template particles and fine-grained BSmT powder. On the other hand, partially grain-oriented ceramics were also obtained from the uniaxially pressed mixture of the template particles and the fine-grained powder. Orientation degree of sintered BSmT ceramics increased with a content of template particles. As compared with the specimen prepared by normal sintering technique, the textured ceramics displayed a large anisotropy in the dielectric properties, especially in the temperature coefficient of resonant frequency (τ_f). Consequently, near-zero τ_f was obtained in the textured ceramics with (hk0)-orientation degree of approximately 0.10 in the surface of the disk specimen. In addition, the τ_f could be controlled to a desired value from ranging −90 to +25 ppm/°C by adjusting the grain orientation in the specimen. Measurement of the temperature dependence of dielectric constant, which correlates with τ_f, revealed positive and negative behaviors for (hk0) and (001) textured BSmT ceramics, respectively. The tuning of τ_f was explained as a rule of mixture of dielectric anisotropy in BSmT.