Interaction of lithium-titanium ceramics with air during sintering

Interaction of lithium-titanium polycrystalline ferrites with ambient air during their sintering is investigated using the methods of electroconductivity and thermal gravimetry within the temperature interval T = 600–1300 K. It is found out that the temperatures exceeding T = 1100 K correspond to effective course of reducing reactions in this material, which results in the formation of an oxygen-deficient ceramic structure in the stage of isothermal tempering (T = 1280 K). It is shown that in the final cooling stage the process of exchange of the ferrite ceramics with the ambient atmosphere exhibits a pronounced oxidative character. The role of these redox processes in the formation of the oxygen non-stoichiometry of the sintered specimen and the depth of its distribution are analyzed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 35–40, February, 2007.     

Defect driven multiferroicity in Gd doped BiFeO3 at room temperature

This paper reports that defect driven magnetism can be obtained at room temperature by optimizing metal ion concentration in bismuth ferrite (BFO) following our novel slow step solid state sintering route. We observed a clean signature of enhanced multiferroic behavior in Gd doped bismuth ferrite (Gd-BFO) bulk ceramics at room temperature (RT). Bismuth rich iron deficient Gd-BFO ceramics were prepared by solid state route through slow step sintering schedule at 850 °C. At particular composition, (Bi_1.2Gd_0.1Fe_0.8O₃), this materials completely transform from rhombohedral R3c to orthorhombic Pn2₁a space group. We emphasized that excess bismuth is expected to act as point defects and occupy interstitials positions, which in turn interact by oxygen vacancies. These defects are likely to promote defect driven ferromagnetism in BFO system. Incorporation of Gd in presence of excess bismuth in BFO enhanced both spin and electric polarization at room temperature. We also infer that Gd substitution in BFO is likely to suppress spiral spin modulation, which also favors ferromagnetism in Gd-BFO.     

On the Computation of Local-Demagnetization Tensor for Nonellipsoidal Ferrites

The distribution of the magnetization in ferrite devices is nonuniform due to their nonellipsoidal shapes. In this paper, the local-demagnetization tensor of magnetized nonellipsoidal ferrite is proposed and calculated. By analysing the distribution of the tensor elements, the distribution of the demagnetizing field can be obtained. The method is simple yet accurate. It can be used to calculate the nonuniform magnetizing distribution in ferrite devices as well as optimize the ferrite devices in the CAD of MMIC.     

The effects of oxygen vacancies on the electrical properties of W,Ti doped CaBi2Nb2O9 piezoceramics

Different type doped CaBi₂Nb₂O₉ (CBN) ceramics were prepared by a conventional solid state sintering method. The number of oxygen vacancies were decreased or increased by the introduction of W⁶⁺ and Ti⁴⁺ doping in CBN ceramics. The influence of W⁶⁺, Ti⁴⁺ and W⁶⁺/Ti⁴⁺ dopants on the microstructures and electrical properties of CBN-based ceramics was investigated. The voids and spherical morphology in the SEM image of W, Ti co-doped ceramics indicate that W, Ti co-doping could change the microstructure of CBN-based ceramics. Impedance analysis results show that the electrical properties of CBN-based ceramics have a close relationship with the number of oxygen vacancies. W doping and W, Ti co-doping decrease the oxygen vacancies, as a result, the resistance and piezoelectric properties were increased and the frequency dispersion of dielectric properties were restrained.     

Scattering characteristics of conducting cylinder coated with nonuniform magnetized ferrite

An analytical technique, referred to as the scattering matrix method （SMM）, is developed to analyse the scattering of a planar wave from a conductolution for the nonuniform fering cylinder coated with nonuniform magnetized ferrite. The SMM srite coating can be reduced to the expressions for the scattering and penetrated coefficients in four particular cases： nonuniform magnetized ferrite cylinder, uniform magnetized ferrite-coated conducting cylinder, uniform ferrite cylinder as well as homogeneous dielectric-coated conducting cylinder. The resonant condition for the nonuniform ferrite coating is obtained. The distinctive differences in scattering between the nonuniform ferrite coating and the nonuniform dielectric coating are demonstrated. The effects of applied magnetic fields and wave frequencies on the scattering characteristics for two types of the linear profiles are revealed.     

Laser-quality oxide Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> ceramics. Comparative studies of its basic characteristics and laser ceramics of a known manufacturer

The effect of sintering aids of SiO₂, ZrO₂, B₂O₃, and MgO oxides on the optimum sintering temperature, ceramics grain growth, total volume of residual pores, and optical quality of obtained ceramics is studied. The best combinations of sintering aids are found; as a result, YAG:Nd (1 at%) samples of ceramics of high optical quality are obtained. An original method for characterizing laser properties of ceramics is developed. Comparative measurements of main laser characteristics of the obtained ceramics and ceramics of the Konoshima Chemical Corp. Ltd wellknown in the world practice, are performed.     

On synthesis of BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>, SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>, and PbFe<Subscript>12</Subscript>O<Subscript>19</Subscript> hexagonal ferrite ceramics with multiferroid properties

We analyze the possibility of obtaining M-type hexagonal ferrites of barium, strontium, and lead with multiferroid properties with the help of ceramic technology. Using the modified ceramic technology (especially pure initial raw materials, admixture of B₂O₃, and sintering in the oxygen atmosphere), we obtained for the first time the BaFe₁₂O₁₉ and SrFe₁₂O₁₉ samples with intense multiferroid properties at room temperature. At the same time, the employed technology does not make it possible to obtain PbFe₁₂O₁₉ samples exhibiting ferroelectricity. The multiferroid characteristics of experimental samples are compared with the characteristics of classical high-temperature multiferroic BiFeO₃ and with the characteristics of BaFe₁₂O₁₉, SrFe₁₂O₁₉, and PbFe₁₂O₁₉ ferrite ceramics obtained in accordance with polymer precursor technology. We propose a mechanism explaining multiferroid properties of the hexagonal ferrite ceramic samples and note the importance of our results for applications.     

Effect of Annealing on Ferroelectric Properties of Nanometre BaTiO3 Ceramics Prepared by High Pressure Sintering Method

Dense nanocrystalline BaTiO3 ceramics with a grain size of 5Onto are prepared under 6 GPa at 1273K using a high pressure sintering method. The sintered bulk is uniform and the relative density is above 97%. We anneal the ceramic samples in oxygen with various temperatures and for the annealing, several broadened peaks can be observed at different times without apparent grain growth. After about 378K（ by dielectric measurements. However, these peaks are very different from those of coarser-grained ceramics. It is indicated that both the elimination of oxygen vacancies and the release of residual stresses caused by high pressure greatly improve the overall ferroelectric properties of BaTiO3 ceramics. The observation of nearly linear polarization hysteresis loop after anneal provides the solid evidence of ferroelectricity in these nano-sized BaTiO3 ceramics. It is believed that the absence of 90° domains and the existence of poor-permittivity nonferroelectric grain boundaries contribute to the slim loop.     

烧结压力对碳化钽陶瓷维氏硬度的影响

为了探究烧结压力对不同晶粒尺寸碳化钽(TaC)力学性能的影响,通过高温高压技术对纳米、微米尺寸TaC粉末进行高温高压烧结,制备不同烧结条件下的块状TaC陶瓷。利用X射线衍射等表征方法对烧结样品的物相、元素分布、压痕形态进行表征,结果表明:TaC在烧结过程中物相稳定,且无杂质渗入。利用维氏硬度计对不同烧结压力(3.0、4.0和5.5 GPa)条件下的3种陶瓷样品进行维氏硬度测试,并进行微观结构分析,结果表明:随着烧结压力由3.0 GPa提升到5.5 GPa,微米尺寸TaC的维氏硬度(21.0 GPa)优于3.0、4.0 GPa下的纳米尺寸TaC维氏硬度(17.5、19.2 GPa)。此外,研究发现,测试维氏硬度时,3.0 kg应用载荷对测试TaC维氏硬度更加精确。研究结果对结构陶瓷烧结和超高温陶瓷硬度研究具有指导意义。     

Effects of sintering condition on phase formation,microstructure and dielectric properties of lead titanate ceramics

Lead titanate ceramics have been prepared by two different processing methods: conventional (or single-stage) and two-stage sintering. Effects of designed sintering conditions on phase formation, densification, microstructure and dielectric properties of the ceramics were characterized via X-ray diffraction, Archimedes density measurement, scanning electron microscopy and dielectric measurement, respectively. The potentiality of a two-stage sintering technique as a simple ceramic fabrication method to obtain highly dense and pure lead titanate ceramics was demonstrated. It has been found that, under suitable two-stage sintering conditions, dense perovskite lead titanate ceramics can be successfully achieved with better dielectric properties than those of ceramics from a single-stage sintering technique. PACS 77.22.-d; 77.84.-s; 77.84.Dy     

Grain-boundary diffusion of oxygen in polycrystalline ferrites

It is shown that layer-by-layer measurements of the electron-transfer activation energy in polycrystalline ferrites allow the profiles of depth distribution of oxygen introduced from the atmosphere into a ferrite at the stage of sintering or thermal treatment to be studied using a simple experimental technique. The analytical equations proposed here make it possible to determine the volume and grain-boundary diffusion coefficients of oxygen using a minimum number of adjustable parameters. Tomsk Polytechnic University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 64–69, May, 1999.     

Low temperature fired Ni-Cu-Zn ferrite with Bi4Ti3O12

The Ni-Cu-Zn ferrites with different contents of Bi₄Ti₃O₁₂ ceramics (1-8 wt%) as sintering additives were prepared by the usual ceramic technology and sintered at 900 °C to adapt to the low temperature co-fired ceramic (LTCC) technology. The magnetic and dielectric properties of the ferrite can be effectively improved with the effect of an appropriate amount of Bi₄Ti₃O₁₂. For all samples, the ferrite sintered with 2 wt% Bi₄Ti₃O₁₂ has relatively high density (98.8%) and permeability, while the ferrite with 8 wt% Bi₄Ti₃O₁₂ has relatively good dielectric properties in a wide frequency range. The influences of Bi₄Ti₃O₁₂ addition on microstructure, magnetic and dielectric properties of the ferrite have been discussed.     

Electrical properties of heterovalently doped apatite-like oxosilicates

Oxygen-ion-conductive apatite-like oxosilicate and silicophosphate ceramics has been prepared by sintering sol-gel synthesis products at temperatures 200°C below those of solid-phase synthesis. The electrical properties of the ceramics obtained have been studied by impedance spectroscopy in the range 400–1000°C. A positive effect of heterovalent substitutions in the silicon sublattice on the bulk and intergrain conductivities is established. The increase in the oxygen-ion conductivity in La_9.6Si_5.7Mg_0.3O_26.1 in comparison with the other studied oxosilicates by more than two orders of magnitude is related to the formation of additional interstitial positions for oxygen ions as a result of partial substitution of the silicon ion, tetrahedrally coordinated by oxygen, with the Mg ion having a characteristic octahedral coordinatio     

Mössbauer study of MnZn ferrite formation under low oxygen pressure conditions

The solid state reaction of ferrite formation was studied under low oxygen partial pressure (10^-3% O₂) by Mössbauer spectroscopy, X-ray diffraction and density measurements. At temperatures below 500°C, a Mn ferrite was formed, whereas the Zn ferrite formation starts later at the same temperature than in air sintering. The appearance of an intermediate Mn ferrite is caused by the low valency state of Mn ions under such strong reducing conditions. The MnZn ferrite formation is completed by 800°C.No direct correlation was found between ferrite formation and densification.     

Dependence of the magnetic and magnetoelastic properties of cobalt ferrite on processing parameters

The dependence of the magnetic and magnetoelastic properties of highly magnetostrictive cobalt ferrite on processing parameters has been investigated. The cobalt ferrite samples used in this study were prepared via conventional ceramic processing methods. The processing parameters of interest were sintering temperature, holding time at the sintering temperature and powder compaction pressure. It was observed that the crystal structure, composition and saturation magnetization of the samples studied did not vary with changes in processing parameters but coercive field decreased with increasing sintering temperature. The amplitude of peak to peak magnetostriction was dependent on the holding time and powder compaction pressure. The strain derivative on the other hand was found to depend on powder compaction pressure at any given sintering temperature or holding time. The results show how the magnetoelastic properties of cobalt ferrite can be varied by changing the processing parameters.     

基于电子顺磁共振的锶铁氧体磁特性研究

为了研究配料、温度、氧环境和掺杂等条件对锶铁氧体的磁性能的影响问题, 利用溶胶-凝胶法制备了锶铁氧体粉末, 建立了一种基于电子顺磁共振技术研究锶铁氧体粉末的磁特性的方法. 用电子顺磁共振波谱仪对烧结后的产物进行测试发现: 400 ℃预烧下, 锶铁摩尔比为1:9时, 中间产物顺磁相α-Fe₂O₃含量最多, 高于400 ℃时其含量减少, 亚铁磁相增加, 并确定最佳煅烧温度介于800-900 ℃. 这是由于外磁场和其他磁场综合作用产生亚铁磁相, 进而产生较强的磁矩相互作用所致. 结合工业实际应用, 发现缺氧退火环境下, 顺磁相α-Fe₂O₃含量较大, 不利于亚铁磁相生成; X-射线衍射(XRD)表征结果表明: 除了少量杂相, 其余均为顺磁相和亚铁磁相; 电子顺磁共振谱和XRD 谱检测结果综合表明, 锶铁摩尔比为1:9时, 最终产物的顺磁相含量最少, 亚铁磁相含量最多, 磁性最强; 毫特斯拉计的剩磁检测结果也证实了上述结果. 掺杂实验发现镧离子占锶镧总摩尔数的20% 至30% 时, 能够有效降低顺磁相的产生, 增强最终产物的亚铁磁性.     

Microstructure characterization and cation distribution of nanocrystalline cobalt ferrite

Nanocrystalline cobalt ferrite has been synthesized using two different methods: ceramic and co-precipitation techniques. The nanocrystalline ferrite phase has been formed after 3 h of sintering at 1000 °C. The structural and microstructural evolutions of the nanophase have been studied using X-ray powder diffraction and the Rietveld method. The refinement result showed that the type of the cationic distribution over the tetrahedral and octahedral sites in the nanocrystalline lattice is partially an inverse spinel. The transmission electronic microscope analysis confirmed the X-ray results. The magnetic properties of the samples were characterized using a vibrating sample magnetometer.     

Magnetic ceramics based on hydroxyapatite modified by particles of M-type hexagonal ferrite for medical applications

Magnetic bioceramics based on hydroxyapatite Ca₅(PO₄)₃OH and particles of the M-type hexagonal (barium or calcium) ferrite with a high bioactivity and magnetic characteristics providing hyperthermal treatment of oncology diseases has been developed and studied. The phase composition, microstructure, and magnetic properties of the synthesized bioceramics have been determined. It has been shown that the synthesized biomaterial consists of the biocompatible matrix with the apatite structure into which particles of hexagonal ferrite are incorporated. The magnetic parameters of the synthesized ceramics are substantially higher than those of the bioglass ceramics modified by iron oxides that have been used in medicine, which suggests good potential and effectiveness of application of the created ceramics for medical purposes. Thus, a new class of magnetic bioceramics combining hydroxyapatite Ca₅(PO₄)₃OH, which exhibits good properties in biocompatibility and bioactivity, with particles of the M-type hexagonal ferrite, which possess high magnetic characteristics, has been created.     

Contribution of oxygen vacancies to the giant dielectric response in Sm1.5Sr0.5NiO4−δ ceramics

The dielectric properties of Sm_1.5Sr_0.5NiO_4?δ ceramics with different concentrations of oxygen vacancies were characterized. The ceramics with lower concentration of oxygen vacancies were prepared by directly sintering the sol–gel derived powders in air, while the higher one could be obtained by annealing the as-sintered ceramics in the flow of nitrogen. The post-densification annealing in the flow of nitrogen decreased the dielectric constant at low temperature and increased it at high temperature, while the dielectric loss increased in overall temperature range. The activation energy of low-temperature dielectric relaxation decreased with increasing the concentration of oxygen vacancies, and so did that of bulk electrical resistances although the values of resistances increased, while the activation energy of electrical resistances for grain boundary increased though the values of resistances decreased. The giant dielectric response in the as-sintered Sm_1.5Sr_0.5NiO_4?δ ceramics should be mainly attributed to the small polaronic hopping process, while that of annealed ceramics should be directly linked to the oxygen vacancies.     

Microstructure and dielectric properties of PMN–PT ceramics prepared by the molten salts method

The sintering characteristic and dielectric properties of 0.67PMN–0.33PT ceramics prepared by the molten salt synthesis (MSS) method were investigated. PMN–PT particles synthesized by MSS with smaller grain size and good dispersion could lower the sintering temperature of ceramics; PMN–PT ceramics with relative density above 96% could be obtained in the range 1150–1180 °C. The molten salts species could significantly affect the microstructure and properties of MPN-PT ceramics. In the range 1100–1200 °C, PMN–PT ceramics from the sulfate flux MSS powders showed intergranular fracture, but that from the chloride flux MSS powder showed transgranular fracture. At the same sintering condition, the properties of PMN–PT ceramics from the powders prepared in the chloride flux are better than that from the powders prepared in the sulfate flux, their maximum dielectric constant ε_max≈29,385 and piezoelectric constant d₃₃≈660 pC/N. The above results demonstrated that PMN–PT ceramics prepared by the molten salts method possessed excellent piezoelectric and dielectric properties.