Freeze casting of aqueous coal fly ash/alumina slurries for preparation of porous ceramics

A porous mullite-matrix composite with a bimodal pore structure has been prepared by a freeze casting route using water/coal fly slurry system. The top and bottom parts of the sintered freeze cast body consisted of solid particles and micropores, which were irregularly distributed. However, the middle section was made up of small lamellar pores and porous ceramic walls, aligned along the solidification direction. The porosity of mullite composites was in the range 67-55% after sintering at 1300-1500 °C. The addition of 3Y-ZrO₂ reduced the porosity, especially material in sintered at 1500 °C due to relatively high densification. The compressive strength of the porous composite with 10 wt% 3Y-ZrO₂ addition, sintered at 1500 °C exhibited a maximum value of ∼41 MPa.     

The microstructure and specific properties of La/HAP composite powder and its coating

La/HAP composite powder, a novel bioactive material, was prepared using co-precipitation method. The La/HAP coating was obtained for the first time through the dip-coating method, starting from the sols of La/HAP and TiO₂ particles. The compositions and coating of as-produced La/HAP composite powder sintered at temperatures from 300 to 750 °C were analyzed by means of X-ray diffraction (XRD). The changes of the ion groups in as-prepared La/HAP composite powder were characterized by using Fourier transform infrared (FTIR) spectroscopy. Their surface morphologies were observed by means of scanning electron microscope (SEM). The results show that the La/HAP composite powder has higher thermostability than pure HAP powder and La can refine HAP particle and restrain the decomposition of HAP. Consequently, in coating process the heat-treatment temperature is lower (750 °C) using the synthesized La/HAP powder than that using pure HAP (900 °C). The La/HAP coating mainly contains HAP, TiP or Ti₃P₅ and TiO₂ phases as well as a little CaTiO₃ crystal, a very ideal composition to enhance bioactivity of biomaterials. These unique properties of the La/HAP composite powder are beneficial to enhance the strength and bioactivity of coating when it is used as a starting material in coating process.     

Magnetic properties in Fe-doped NiO synthesized by co-precipitation

The magnetic properties of 1.5 at% Fe-doped NiO bulk samples were investigated. The samples were prepared by sintering the corresponding precursor in air at temperatures between 400 and 800 °C for 6 h. The synthesis was by a chemical co-precipitation and post-thermal decomposition method. In order to allow a comparison, a NiO/0.76 at% NiFe₂O₄ mixture was also prepared. The X-ray diffraction pattern shows that the samples that were sintered at 400 and 600 °C remain single phase. As the sintering temperature increased to 800 °C, however, the sample becomes a mixture of NiO and NiFe₂O₄ ferrite phases. The samples were investigated by measuring their magnetization as a function of magnetic field. The samples sintered between 400 and 800 °C and the one mixed directly with NiFe₂O₄ nanoparticles show a coercivity value of H_c≈200, 325, 350 and 110 Oe, respectively. The magnetic properties of the samples depend strongly on the sintering temperature. Simultaneously, the field-cooling hysteresis loop shift also observed after cooling the sample sintered at 600 °C to low temperature suggests the possibility of the existence of a ferromagnetic/antiferromagnetic exchange coupling.     

Effects of sintering temperature on grain growth and the complex permeability of Co0.2Ni0.3Zn0.5Fe2O4 material prepared using mechanically alloyed nanoparticles

Nanoparticle-sized Co_0.2Ni_0.3Zn_0.5Fe₂O₄ was prepared using mechanical alloying and sintering. The starting raw materials were milled in air and subsequently sintered at various temperatures from 600 to 1300 °C. The effects of sintering temperature on physical, magnetic and electrical characteristics were studied. The complex permittivity and permeability were investigated in the frequency range 10 MHz to 1.0 GHz. The results show that single phase Co_0.2Ni_0.3Zn_0.5Fe₂O₄ could not be formed during milling alone and therefore requires sintering. The crystallization of the ferrite sample increases with increasing sintering temperature; which decrease the porosity and increase the density, crystallite size and the shrinkage of the material. The maximum magnetization value of 83.1 emu/g was obtained for a sample sintered at 1200 °C, while both the retentivity and the coercivity decrease with increasing the sintering temperature. The permeability values vary with both the sintering temperature and the frequency and the absolute value of the permeability decreased after the natural resonance frequency. The real part of the permittivity was constant within the measured frequency, while the loss tangent values decreased gradually with increasing frequency.     

Wear behavior of sintered hexagonal boron nitride under atmosphere and water vapor ambiences

Hexagonal boron nitride was pressed and sintered at 2000 °C with CaB₂O₄ as an additive to promote its crystallization, which was used as an abradable sealing coating for aircraft turbo engines. Microstructures, phase compositions and tribological properties of the sintered hBN were tested, and the results show that CaB₂O₄ can effectively promote crystal growth of hBN at 2000 °C for 5 h in N₂ ambience. The friction coefficients of the sintered hBN under atmosphere ambience increase as the temperature increasing from room temperature to 400 °C, and then decrease with further increasing of temperature up to 800 °C. Under water vapor ambience, friction coefficients of the sintered hBN are much lower than those under atmosphere ambience, which are attributed to a lamella-slip of hBN and the solid lubrication effect of H₃BO₃.     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

Milling time and BPR dependence on permeability and losses of Ni0.5Zn0.5Fe2O4 synthesized via mechanical alloying process

Ni_0.5Zn_0.5Fe₂O₄ has been synthesized using mechanical alloying method with two variables (milling time and ball-to-powder weight ratio (BPR)) were varied in order to study its effect on the magnetic properties of the material. The effects of these two variables were studied using XRD, SEM, TEM and later by impedance analyzer with the frequency range from 1 MHz to 1.8 GHz. The results obtained however show that there are no significant trends to relate the milling time and BPR with the permeability and losses of the material studied. After being sintered at 1150 °C, all the effects of alloying process seem to diminish.     

Effects of excessive grain growth on the magnetic and mechanical properties of hot-deformed NdFeB magnets

The magnetic and mechanical properties of rare-earth magnets hot-deformed at temperature range 750-950 °C have been investigated. The grains tended to grow excessively from dozens of nanometers to several microns at the temperatures above 850 °C. The alignment of grains was disrupted by the hot deformation at the high temperatures. The Nd-rich phase was extruded at the temperatures which are higher than 850 °C. The Nd-rich phase extrusion resulted in the reduction of density by 1% and the reduction of remanence from 1.42 to 0.72 T. The reduction of grain boundaries caused by flat platelet-shaped grains changing to spherical grains and the weak binding strength among large grains of Nd₂Fe₁₄B phase may be the main reasons for the low mechanical strength of hot-deformed magnets.     

Fabrication of solid oxide fuel cell based on doped ceria electrolyte by one-step sintering at 800 °C

Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) electrolyte was prepared by a carbonate co-precipitation method. Lithium nitrate at 1, 1.5, 2 and 3 mol% was added to GYDC as sintering additive. 96% relative density was achieved for GYDC at sintering temperature of 800 °C with addition of 1.5 mol% LiNO₃. The conductivities of GYDC with sintering aids LiNO₃ were measured by a.c. impedance spectroscopy and showed comparable values to that of pure GYDC sample sintered at 1400 °C. A single cell with 1.5 mol% LiNO₃ infiltrated GYDC electrolyte was fabricated by sintering at 800 °C for only 2 h. Lithiated NiO was synthesized by the glycine-nitrate combustion method and employed as cathode material. The cell was tested at temperatures from 500 to 575 °C and a maximum power density of 73 mW cm^− 2 was obtained at 575 °C. These preliminary results indicate that LiNO₃ is a very effective sintering additive for intermediate temperature solid oxide fuel cell fabrication.     

Magnetic and bioactivity evaluation of ferrimagnetic ZnFe2O4 containing glass ceramics for the hyperthermia treatment of cancer

Glass ceramics of the composition xZnO·25Fe₂O₃·(40−x)SiO₂·25CaO·7P₂O₅·3Na₂O were prepared by the melt-quench method using oxy-acetylene flame. Glass-powder compacts were sintered at 1100 °C for 3 h and then rapidly cooled at −10 °C. X-ray diffraction (XRD) revealed 3 prominent crystalline phases: ZnFe₂O₄, CaSiO₃ and Ca₁₀(PO₄)₆(OH)₂. Vibrating sample magnetometer (VSM) data at 10 KOe and 500 Oe showed that saturation magnetization, coercivity and hence hysteresis area increased with the increase in ZnO content. Nano-sized ZnFe₂O₄ crystallites were of pseudo-single domain structure and thus coercivity increased with the increase in crystallite size. ZnFe₂O₄ exhibited ferrimagnetism due to the random distribution of Zn²⁺ and Fe³⁺ cations at tetrahedral A sites and octahedral B sites. This inversion/random distribution of cations was probably due to the surface effects of nano-ZnFe₂O₄ and rapid cooling of the material from 1100 °C (thus preserving the high temperature state of the random distribution of cations). Calorimetric measurements were carried out using magnetic induction furnace at 500 Oe magnetic field and 400 KHz frequency. The data showed that maximum specific power loss and temperature increase after 2 min were 26 W/g and 37 °C, respectively for the sample containing 10% ZnO. The samples were immersed in simulated body fluid (SBF) for 3 weeks. Scanning electron microscope (SEM), energy dispersive spectroscopy (EDX) and XRD results confirmed the growth of precipitated hydroxyapatite phase after immersion in SBF, suggesting that the ferrimagnetic glass ceramics were bioactive and could bond to the living tissues in physiological environment.     

In vitro reactivity of Na2O-MgO-SiO2 glasses

A variety of bioactive glasses have been investigated over the last two decades as substitute material for diseased or damaged tissues in a human body. In this investigation, three different melt derived bioactive glasses, each having 55% by mole SiO₂ and ratio of MgO to Na₂O varying from 1:8 to 8:1, were prepared by melting various oxides at temperature >1250 °C. After microstructure evolution, vitro reactivity of these glasses was examined by keeping them in simulated body fluid (trans buffered pH 7.25 at 25 cc). The surface reactivity of these glasses gradually increased with increasing Na₂O/MgO ratio.     

Effect of sintering temperature on structure and nonlinear dielectric properties of Ba0.6Sr0.4TiO3 ceramics prepared by the citrate method

Ba_0.6Sr_0.4TiO₃ ceramics were prepared by a citrate precursor method. The structure and nonlinear dielectric properties of the resulting ceramics were investigated within the sintering temperature range 1200-1300 °C. Adopting fine Ba_0.6Sr_0.4TiO₃ powder derived from the citrate method was confirmed to be effective in reducing the sintering temperatures required for densification. The ceramic specimens sintered at 1230-1280 °C presented relative densities of around 95%. A significant influence of sintering temperature on the microstructure and nonlinear dielectric properties was detected. The discrepancy in nonlinear dielectric behavior among the specimens sintered at different temperatures was qualitatively interpreted in terms of the dielectric response of polar micro-regions under bias electric field. The specimens sintered at 1230 and 1250 °C attained superior nonlinear dielectric properties, showing relatively low dielectric losses (tan δ) of 0.24% and 0.22% at 10 kHz together with comparatively large figure of merits (FOM) of 121 and 142 at 10 kHz and 20 kV/cm, respectively.     

Effect of annealing temperature on structure, magnetic properties and optical characteristics in Zn0.97Cr0.03O nanoparticles

The Cr-doped zinc oxide (Zn_0.97Cr_0.03O) nanoparticles were successfully synthesized by sol-gel method. The relationship between the annealing temperature (400 °C, 450 °C, 500 °C and 600 °C) and the structure, magnetic properties and the optical characteristics of the produced samples was studied. The results indicate that Cr (Cr³⁺) ions at least partially substitute Zn (Zn²⁺) ions successfully. Energy dispersive spectroscopy (EDS) measurement showed the existence of Cr ion in the Cr-doped ZnO. The samples sintered in air under the temperature of 450 °C had single wurtzite ZnO structure with prominent ferromagnetism at room temperature, while in samples sintered in air at 500 °C, a second phase-ZnCr₂O₄ was observed and the samples were not saturated in the field of 10000 Oe. This indicated that they were mixtures of ferromagnetic materials and paramagnetic materials. Compared with the results of the photoluminescence (PL) spectra, it was reasonably concluded that the ferromagnetism observed in the studied samples was originated from the doping of Cr in the lattice of ZnO crystallites.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

Surface study of fine MgFe2O4 ferrite powder prepared by chemical methods

To study surface behaviors, MgFe₂O₄ ferrite materials having different grain sizes were synthesized by two different chemical methods, i.e., a polymerization method and a reverse coprecipitation method. The single phase of the cubic MgFe₂O₄ was confirmed by the X-ray diffraction method for both the precursors decomposed at 600-1000 °C except for a very small peak of Fe₂O₃ was detected for the samples calcined at 600 and 700 °C by the polymerization method. The crystal size and particle size increased with an increase in the sintering temperature using both methods. The conductance of the MgFe₂O₄ decreased when the atmosphere was changed from ambient air to air containing 10.0 ppm NO₂. The conductance change, C = G(air)/G(10 ppm NO₂), was reduced with an increase in the operating temperature. For the polymerization method, the maximum C-value was ca. 40 at 300 °C for the samples sintered at 900 °C. However, the samples sintered at 1000 °C showed a low conductance change in the 10 ppm NO₂ gas, because the ratio of the O₂ gas adsorption sites on the particle surface is smaller than those of the samples having a high C-value. The low Mg content on the surface affects the low ratio of the gas adsorption sites. For the reverse coprecipitation method, the particle size was smaller than that of the polymerization method. Although a stable conductance was obtained for the sample sintered at 900 and 1000 °C, its conductance change was less than that of the polymerization method.     

High-k Mg-doped ZST for microwave applications

The (Zr_0.8Sn_0.2)TiO₄ material (ZST), has been prepared by solid state reaction and characterized. The samples were sintered in the temperature range of 1260-1320 °C for 2 h. The effects of sintering parameters like sintering temperature (T_s) and MgO addition (0.2 wt.%) on structural and dielectric properties were investigated. Bulk density increases from 4900 to 5050 kg/m³ with the increase of sintering temperature. The effect of MgO addition is to lower the sintering temperature in order to obtain well sintered samples with high value of bulk density. The material exhibits a dielectric constant ?_r ∼ 37 and high values of the Q × f product, greater than 45,000, at microwave frequencies. The dielectric properties make the ZST material very attractive for microwave applications such as dielectric resonators, filters, dielectric antennas, substrates for hybrid microwave integrated circuits, etc.     

Microstructure and magnetic properties of low-temperature sintered CoTi-substituted barium ferrite for LTCC application

In this article, the influences of the BaCu(B₂O₅) (BCB) additive on sintering behavior, structure and magnetic properties of iron deficient M-type barium ferrite Ba(CoTi)_xFe_11.8−2xO₁₉ (BaM) have been investigated. It is found that the maximum sintered densities of BaM change from 86% to 94% as the BCB content varies from 1 to 4 wt%. Single-phase BaM can be detected by the XRD analysis in the sample with 3 wt% BCB sintered at 900 °C, and the microstructure is hexagonal platelets with few intragranular pores. This is attributed to the formation of the BCB liquid phase. Meanwhile, the experimental results illuminate that the CoTi ions prefer to occupy the 4f2 and 2b sites and the magnetic properties depend on the amount of CoTi-substitution. In addition, the chemical compatibility between BaM and silver paste is also investigated; it can be seen that BaM is co-fired well with the silver paste and no other second phase is observed. Especially, the 3 wt% BCB-added Ba(CoTi)_0.9Fe₁₁O₁₉ sintered at 900 °C has good properties with the sintered density of 4.9 g/cm³, saturation magnetization of 49.7 emu/g and coercivity of 656.6 Oe. These results indicate that it is cost effective in the production of Low Temperature Co-fired Ceramics (LTCC) multilayer devices.     

Effect of sintering temperature on the structural, dielectric and ferroelectric properties of tungsten substituted SBT ceramics

Structural, dielectric and ferroelectric properties of tungsten (W) substituted SrBi₂(Ta_1−xW_x)₂O₉ (SBTW) [x=0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] have been studied as a function of sintering temperature (1100-1250 °C). X-ray diffraction patterns confirm the single-phase layered perovskite structure formation up to x=0.05 at all sintering temperatures. The present study reveals an optimum sintering temperature of 1200 °C for the best properties of SBTW samples. Maximum T_c of ∼390 °C is observed for x=0.20 sample sintered at 1200 °C. Peak-dielectric constant (ε_r) increases from ∼270 to ∼700 on increasing x from 0.0 to 0.20 at 1200 °C sintering temperature. DC conductivity of the SBTW samples is nearly two to three orders lower than that of the pristine sample. Remnant polarization (P_r) increases with the W content up to x≤0.075. A maximum 2P_r (∼25 μC/cm²) is obtained with x=0.075 sample sintered at 1200 °C. The observed behavior is explained in terms of improved microstructural features, contribution from the oxygen and cationic vacancies in SBTW. Such tungsten substituted samples sintered at 1200 °C exhibiting enhanced dielectric and ferroelectric properties should be useful for memory applications.     

On the thermal stability of Co2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

Co₂Z hexaferrite Ba₃Co₂Fe₂₄O₄₁ was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T<1300 °C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 °C and the Y-type ferrite at 1230 °C. The Z-type material has its stability interval between 1300 and 1350 °C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 °C, intermediate grinding and sintering at 1330 °C. The permeability of Co₂Z-type ferrite of about μ=20 is stable up to several 100 MHz, with maximum losses μ′′ around 700 MHz. Addition of 3 wt% Bi₂O₃ as sintering aid shifts the temperature of maximum shrinkage down to 950 °C and enables sintering of Z-type ferrite powders at 950 °C. However, the permeability is reduced to μ=3. It is shown here for the first time that Co₂Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co₂Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 °C.     

Preparation and properties of ferromagnetic Z-type hexaferrite from wet milled mixtures of intermediates

Almost pure Z-type phases of hexaferrites were synthesized by firing preliminarily milled M- and Y-type phases intermediates at 1250 °C. These phases were obtained by calcining the stoichiometric powder mixture precursors at 1080 °C, followed by wet milling in a planetary mill for 1 h and subsequent heating at 1250 °C that increased the fractional crystallization of the Z-phase up to 96%. Addition of 0.2 wt% SiO₂ to the intermediates reduced the milling time necessary for the sintered density required for practical permeability measurement. Z-phase hexaferrite sintered at 1250 °C for 2 h exhibited fairly good high-frequency properties, i.e. an initial real permeability of up to 19.3 below 100 MHz and 8–10 at around 1 GHz.