Physical properties of lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanochemically synthesized powders

Modern electronics expect functional materials that are eco-friendly and are obtained with lower energy consumption technological processes. The multiferroic lead-free BaFe_1/2Nb_1/2O₃ (BFN) ceramic powder has been prepared by mechanochemical synthesis from simple oxides at room temperature. The development of the synthesis has been monitored by XRD and SEM investigations, after different milling periods. The obtained powders contain large agglomerates built by crystals with an estimated size about 12–20 nm depending on the period of milling. From this powder, the multiferroic BFN ceramic samples have been prepared by uniaxial pressing and subsequent sintering pressureless method. The morphology of the BFN ceramic samples strongly depends on high-energy milling duration. The properties of the ceramic samples have been investigated by dielectric spectroscopy, in broad temperature and frequency ranges. The high-energy milling of the powders has strongly affected the dielectric permittivity and dielectric loss of the BaFe_1/2Nb_1/2O₃ ceramic samples. The usage of the mechanochemical synthesis to obtain the multiferroic lead-free BFN materials reduces the required thermal treatment and simultaneously improves the parameters of the BFN ceramics.     

Rapid formation of lead magnesium niobate-based ferroelectric ceramics via a high-energy ball milling process

Pyrochlore-free nano-sized 0.90Pb(Mg_1/3Nb_2/3)O₃(PMN)-0.10PbTiO₃(PT) and 0.65PMN-0.35PT powders were synthesized from oxides via a high-energy ball milling process. Single perovskite phase PMN-PT were readily formed from the oxide mixture after milling for only 2 h. The grain size calculated from X-ray diffraction (XRD) patterns of all samples is about 20 nm, which is in agreement with the observation from scanning electron microscopy (SEM) (20-50 nm). PMN-PT ceramics were obtained by sintering the milled powders at temperature from 1000 to 1100°C for 2 h. The dielectric, ferroelectric properties of the PMN-PT ceramics derived from the synthesized powders were comparable with the reported results in the literature.     

Controlled mechanochemical synthesis and properties of a selected perovskite-type electroceramics

This paper presents mechanochemical synthesis as an alternative to the traditional high-temperature synthesis of advanced electrotechnical ceramic materials with a perovskite-type structure. The reaction conditions for high-energy ball milling, e.g. reaction environment, energy of milling and additives to BaTiO₃ such as metallic iron or zirconia from the exfoliation of the milling vessel and grinding media are discussed.     

Solid-state synthesis of nano-sized BaTiO<Subscript>3</Subscript> powder with high tetragonality

A solid-state reaction method was used to synthesize nano-sized, Ca-doped BaTiO₃ powder with high tetragonality (=c/a) in order to increase the volumetric efficiency of multilayer ceramic capacitors (MLCCs). The reaction temperatures for three different starting material combinations were examined by thermogravimetric/differential thermal analysis (TG/DTA). Nano-sized starting materials and the mechanochemical activation of the needle-shaped BaCO₃ via high-energy milling were effective in decreasing the reaction temperature. In addition, the results showed that the tetragonality of fine Ca-doped BaTiO₃ could be enhanced by 2-step heat treatment, consisting of holding at 800 °C for 1 h followed by consecutive heating to the target temperature, without any significant grain growth than that of the conventional 1-step calcination. The synthesized particles heat-treated at 950 and 1,000 °C by 2-step heat treatment were confirmed by characterization to have an average size of 128 and 212 nm, and a tetragonality of 1.0097 and 1.0105, respectively, which are higher tetragonality values than those previously reported for similar sized particles.     

Preparation and dielectric properties of Pb(Zn1/3Nb2/3)O3-based ferroelectric ceramics

The sintering behavior of mechanochemically prepared 0.9Pb(Zn_1/3Nb_2/3)O₃-0.1ABO₃ (ABO₃ = BaBiO₃, BaMnO₃, BaTiO₃) powders is studied. PbO and Bi₂O₃ are shown to volatilize at relatively low temperatures owing to partial reduction of these oxides during the mechanochemical synthesis. Dense (97% of theoretical density) ceramics are obtained under mild sintering conditions, and their dielectric properties are studied at different frequencies. The observed variations of their dielectric permittivity and loss tangent with frequency are typical of relaxor ferroelectrics, but the ceramics have a reduced dielectric permittivity, which is attributable to nanostructuring.     

Synthesis of visible-light responsive nitrogen/carbon doped titania photocatalyst by mechanochemical doping

Nitrogen and/or carbon doped titania photocatalysts TiO_2−x A _y (A = N, C) were prepared by a novel mechanochemical method. The samples were prepared by a high-energy ball milling of P25 titania with different nitrogen/carbon sources such as hexamethylenetetramine, admantane or ammonium carbonate, followed by calcination in air at 400 °C. The high mechanical energy accelerated the phase transformation of anatase to rutile, while the existence of the chemical reagents tended to block the transformation. The prepared powders possessed two absorption edges around 400 and 540 nm and showed excellent photocatalytic ability for nitrogen monoxide oxidation under visible light irradiation. Under the irradiation of visible light of wavelength >510 nm, 37% of nitrogen monoxide could be continuously removed by the carbon and nitrogen co-doped titania prepared by planetary ball milling of P25 titania–10% hexamethylenetetramine mixture followed by calcination in air at 400 °C. This mechanochemical technique might be widely useful for doping oxides with nonmetallic elements.     

Preparation of antiferroelectric lead zirconate titanate stannate ceramics by high-energy ball milling process

Lead zirconate titanate stannate powders doped with lanthanum and niobium, namely [Pb_0.99Nb_0.02(Zr_0.85Sn_0.13Ti_0.02)_0.98O₃, or PNZST) and (Pb_0.9La_0.02(Zr_0.65Sn_0.31Ti_0.04)O₃, or PLZST], were synthesized by a high-energy ball milling technique from their corresponding oxide mixtures. The milled powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and particle size analysis techniques. The sintering behaviors of the milled powders were investigated by a dilatometer from room temperature to 1100 °C. PZST ceramics formed from the high-energy ball milled powders were sintered at temperatures from 900 °C to 1200 °C. The measured electrical properties of the PZST ceramics were comparable to the results reported in the literature. These results have shown that the high-energy ball milling technique is a promising method to prepare PZST ceramics due to its simple procedure.     

铁电体中相变临界尺寸的研究现状

从铁电薄膜和铁电颗粒两大方面回顾了处理尺寸效应的理论研究方法(包括宏观的热力学唯像理论、微观的横场Ising模型和第一性原理计算)和铁电相变临界尺寸的理论预测. 总结了从50年代以来实验中观测到的各种铁电材料的临界尺寸, 简单介绍了铁电纳米陶瓷的一些尺寸效应, 并叙述了近几年才开始报道的铁电纳米线中的铁电相变. 最后指出了目前研究现状中存在的一些问题.     

On the feasibility of synthesizing complex perovskite ferroelectric ceramics via a B-site oxide mixing route

Several relaxor-based complex perovskite ferroelectric ceramics were synthesized by conventional solid-state reaction method via different routes, i.e., the B-site oxide mixing route, the straight calcination method and the columbite/wolframite precursor method. Structure analysis and physical properties measurements prove that the B-site oxide mixing route is efficient and feasible in the synthesizing of complex perovskite ferroelectrics since such a technique is superior in suppressing pyrochlore phases, stabilizing the perovskite structure, and improving the amount of perovskite phase as compared to the straight calcination method and the columbite/wolframite precursor method. The dielectric properties are improved correspondingly, namely exhibiting the largest value of dielectric maximum, the sharpest dielectric response peaks and the least frequency dispersion of dielectric properties in the ferroelectric ceramics prepared via the B-site oxide mixing route, which is considered as correlating with the amount of pyrochlore phases, ceramic density, grain size, and microstructure morphology of the ceramics synthesized.     

Processing and structure-property relation of fine-grained PLZT ceramics derived from mechanochemical synthesis

Lead lanthanum zirconate titanate with the nominal composition (Pb_0.93La_0.07)(Zr_0.60Ti_0.40)O₃ [designated as PLZT (7/60/40)] was synthesised from the constituent oxides by a mechanochemical processing route (MCP). Besides obtaining nano-sized powders, partial phase formation of the desired PLZT composition could be achieved at room temperature. DTA-TGA analyses were used to study the effect of reducing the particle size on the reaction kinetics of PLZT and thermomechanical studies were used to understand the sintering mechanism of the nano-scale powder compacts. Fully dense ceramics could be achieved at significantly reduced sintering temperatures compared to their conventionally synthesized counterparts. Transmission and scanning electron microscopy were employed to study the microstructure of the powders and CIP-sintered compacts, respectively. Resonance data were acquired in order to compute the electromechanical properties. Besides reduction in sintering temperatures, there are differences in electrical properties obtained on PLZT ceramics synthesized by MCP and conventionally processed samples.     

Microstructural evolution of Ni-NaCl mixtures during mechanochemical reaction and mechanical milling

The evolution of microstructure of Ni and NaCl mixtures formed by mechanochemical reaction and mechanical milling has been studied using X-ray diffraction, electron microscopy and magnetic measurements. Separate nano-sized Ni particles were formed by continuous solid-state reaction of NiCl₂ + 2Na during mechanical milling. Further milling resulted in the growth of clustered particles due to inter-particle welding during collision events. On the other hand, milling of micron-sized Ni and NaCl powders resulted in a layered particle morphology and continuous decrease in particle size with increasing milling time.     

Synthesis of La(Mg0.5Ti0.5)O3 ceramics for microwave applications

La(Mg_0.5Ti_0.5)O₃ ceramics were prepared by a non-conventional chemical route, which was based on the Pechini method. For the synthesis of La(Mg_0.5Ti_0.5)O₃ powders, special attention was paid to calcination and milling conditions. Powder morphology and composition were evaluated. Fine La(Mg_0.5Ti_0.5)O₃ powders were obtained at lower temperatures than by conventional methods. Sintering under different conditions was also tested. Dense La(Mg_0.5Ti_0.5)O₃ ceramics were obtained at lower temperatures showing a single phase composition and a homogeneous microstructure. Preliminary dielectric characterization at microwave frequencies was also performed.     

Mechanochemical synthesis, phase composition, and properties of Pb(Zn1/3Nb2/3)O3-based ferroelectric ceramics

Pb-containing relaxor ferroelectric ceramics are prepared by mechanochemical ceramic processing. Mechanochemical reactions in binary and ternary mixtures of the PbO-ZnO-Nb₂O₅ system are studied by x-ray diffraction. Disordered compounds with the columbite, changbaiite, and pyrochlore structures are prepared. The perovskite and pyrochlore phases in 0.9Pb(Zn_1/3Nb_2/3)O₃ + 0.1ABO₃ morphotropic phase boundary materials are shown to be in mechanochemical equilibrium. Among the ABO₃ additives studied, BaMnO₃ is the most effective for stabilizing the perovskite structure. The mechanochemical synthesis path has a strong effect on the phase composition of the resulting material. Conventional synthesis through a columbite phase leads to the predominant formation of a pyrochlore phase. Firing conditions also have a profound effect on the phase composition of the ceramics, but the disordered perovskite phase retains cubic symmetry.     

Calcining influence on the powder properties of hydroxyapatite

The effect of different calcination temperatures on the powder characteristics and the sintered density of synthetic hydroxyapatite (HA) powders, produced using two different processing routes, was examined. Powders were produced by either drying, milling and sieving an as-precipitated HA or by spray-drying a slurry of precipitated HA. Calcining the two powders at temperatures between 400 and 1000 °C did not significantly affect the powder particle size. The specific surface areas of the two powders, however, were reduced from 70–80 m²/g for a calcination temperature of 400 °C to approximately 5–7 m²/g for 1000 °C. Analysis of the surfaces of the HA powders using scanning electron microscopy (SEM) illustrated the coarsening and subsequent sintering of the sub-micron crystallites that constitute a powder particle as the calcination temperature increased, corresponding to the decrease in surface area of the powders. The sintered densities of the final ceramics were not significantly affected by calcining the powders. Microhardness measurements of ceramics prepared from powders calcined at different temperatures showed no significant variations with calcination temperature or powder processing method. The results of this study have illustrated that for applications where HA may be used in powder form, for example in plasma-spraying and for the production of HA-polymer composites, calcining the HA will significantly affect the powder properties, namely the surface area and morphology of the powders. For applications requiring HA in a dense ceramic form, for example as granules or blocks, calcining the powders does not significantly affect the properties of the final ceramic.     

Fine-grained multiferroic BaTiO3/(Ni0.5Zn0.5)Fe2O4 composite ceramics synthesized by novel powder-in-sol precursor hybrid processing route

Dense, homogeneous, and fine-grained multiferroic BaTiO₃/(Ni_0.5Zn_0.5)Fe₂O₄ composite ceramics are synthesized by a novel powder-in-sol precursor hybrid processing route. This route includes the dispersion of nanosized BaTiO₃ ferroelectric powders prepared via conventional sold-state ceramic process into (Ni_0.5Zn_0.5)Fe₂O₄ ferromagnetic sol-gel precursor prepared via sol-gel wet chemistry process. Uniformly distributed slurry is obtained after ball milling and used in the fabrication of the ceramics with low sintering temperatures. The ceramics show coexistence of ferromagnetic and ferroelectric phases with obvious ferromagnetic and ferroelectric hysteresis loops at room temperature, besides exhibiting excellent magnetic and dielectric properties in a wide range of frequency. The combination of high permeability and permittivity with low losses in the ceramics enables significant miniaturization of electronic devices based on the ceramics.     

Oxalate-precursor processing for high quality BaZrO<Subscript>3</Subscript>

BaZrO₃ is by far the most inert crucible material that has been used for melt processing of high quality single crystal YBCO superconductors. To overcome the processing difficulties of existing solid-state methods, solution processing methods are increasingly important in powder synthesis. This study investigates several methods of producing oxalate precursors for subsequent thermal decomposition to BaZrO₃ with a view to producing high quality BaZrO₃ ceramics. The most favourable system used barium acetate, ammonium oxalate and zirconium oxychloride, which unlike other previously reported oxalate processes allowed near stoichiometric precipitation without requiring a large excess of Ba reagents, elevated precipitation temperatures or slow addition of reagents. Precise control over precipitate stoichiometry was achieved by variation of the solution Ba:[Zr+Hf] mole ratio without requiring accurate control over oxalate addition. XRF, XRD, N₂ BET adsorption, DTA/TGA and TEM analysis showed this process to be capable of producing BaZrO₃ powders suitable for ceramics applications. The phase purity, particle size and surface areas of BaZrO₃ powders produced by calcination of these precursors can be adjusted by variation of stoichiometry and calcination temperature. Crucibles formed from oxalate precursors have been able to contain Y₂O₃-BaCuO₂-CuO melts for up to seven days.     

Preparation of Al2O3–TiB2 nanocomposite powder by mechanochemical reaction between Al,B2O3 and Ti

Mechanochemical processing is a novel technique for the synthesis of nano-sized materials. This research is based on the production of Al₂O₃–TiB₂ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of aluminum, titanium and boron oxide powders was subjected to high energy ball milling. The structural evaluation of powder particles after different milling times was conducted by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that during ball milling the Al/B₂O₃/Ti reacted with a combustion mode producing Al₂O₃–TiB₂ nanocomposite. In the final stage of milling, the crystallite sizes of Al₂O₃ and TiB₂ were estimated to be less than 50 nm.     

Mechanochemical activation assisted synthesis of bismuth layered-perovskite SrBi4Ti4O15

Strontium bismuth titanate oxide (SrBi₄Ti₄O₁₅) belongs to the Aurivillius structural type. The materials in this family are good candidates as lead-free ferroelectric materials for applications at high temperature due to their elevated Curie temperature. The mechanochemical activation method permits one to reduce the particle size of the initial products, so the specific surface is increased, improving in most cases its reactivity. In this paper we report on the synthesis of SrBi₄Ti₄O₁₅ Aurivillius-type material from activated precursors, obtained though mechanical treatment in vibrating and planetary mills. In some samples a new transitory Sr-Bi-Ti-O fluorite-type phase appears, depending on the milling time and the kind of mill used. The synthesis of SrBi₄Ti₄O₁₅ from activated precursors is obtained at a temperature about 450°C lower than it would have been required by a conventional solid-state reaction, with significantly reduced heating times.     

Effect of mechanical activation on the structure and ferroelectric property of Na0.5K0.5NbO3

Mechanical activation synthesis of Na_0.5K_0.5NbO₃ (NKN) was studied in order to explore the effect of mechanochemical interaction on the crystal structure and microstructure of NKN powder and ceramic. A single phase, nanocrystalline perovskite NKN powder has been derived from a mixture of oxide/carbonates via a mechanical activation route with heating at an elevated temperature. With the increase in milling time, the distortion of orthorhombic structure for NKN was weakened and the cell volume of NKN powder slightly decreased. The relative density and remnant polarization of NKN ceramics were improved, and the grain became uniform and smaller for prolonged milling NKN. The developed method is well suited for the production of NKN nanocrystallite powders and refined grain NKN ceramics.     

Mechanical and functional properties of ultrafine grained Al wires reinforced by nano-Al2O3 particles

A powder metallurgy route based on high-energy ball milling, powder consolidation by hot extrusion and cold rolling was used to produce Al composite wires reinforced with Al₂O₃ nanoparticles. The process was capable of preparing fully dense nanocomposites characterized by well dispersed nanoparticles in a ultra-fine grained matrix. Ball milling led to the fragmentation of the passivation oxide layer that covers the aluminum particles and of the alumina particle clusters added ex-situ in addition to embedding these nano-sized particles in the Al matrix and thus producing optimal precursors for subsequent consolidation. The nanocomposites showed improved mechanical performances in term of hardness and tensile strength. They also exhibited excellent damping behavior at high temperatures.