Low-temperature sintering of 12Pb(Ni1/3Sb2/3)O3–40PbZrO3–48PbTiO3 with V2O5 and excess PbO additives

Low-temperature sintering of 12Pb(Ni_1/3Sb_2/3)O₃–40PbZrO₃–48PbTiO₃ (12PNS–40PZ–48PT) calcined powders with V₂O₅ and excess PbO additives has been investigated. Adding 0.20 to 0.40 wt.% V₂O₅ and 1.0 wt.% excess PbO to 12PNS–40PZ–48PT calcined powders and sintering at 950 °C for 4 h, the sintered samples only contain the perovskite structure. The calcined powders are doped with 3.0 wt.% excess PbO and 0.20 to 1.0 wt.% V₂O₅ and sintered at 950 °C for 4 h, the coexistence of both tetragonal and rhombohedral phases with the minor phase of pyrochlore is observed. During the calcined powders contain 1.0 wt.% excess PbO and are sintered at 950 to 975 °C for 2 h, the bulk density decreases with V₂O₅ addition greater than 0.6 wt.%. When the calcined powders with 3.0 wt.% excess PbO are sintered at 900 to 975 °C for 2 h, the bulk density decreases with added V₂O₅ content increased. The values of the planar coupling coefficient (K_p) approach the maxima, namely, 0.51 obtained for the compacts containing 0.40 wt.% V₂O₅ and 1.0 wt.% excess PbO and sintered at 950 °C. As the calcined powders are added with 3.0 wt.% excess PbO and 0.80 wt.% V₂O₅ and sintered at 975 °C for 2 h, the maximum Q_m value 1100 is obtained.     

Effect of calcination temperature on the microstructure,composition and properties of nanometer agglomerated 8YSZ powders for plasma spray-physical vapor deposition (PS-PVD) and coatings thereof

Homemade nano-agglomerated powders 8YSZ powders for PS-PVD were prepared by the spray drying, then calcination processes at four different temperatures (500 °C, 700 °C, 900 °C and 1100 °C) were carried out on the spray-dried powders. Checked by laser particle sizer, scanning electron microscope (SEM) and X-ray diffraction (XRD), the physical properties, microstructure and phase constitutions of the calcined powders were investigated. The results show that the size of powders calcined at 500 °C is increased relative to the spray-dried powder, whereas the powders calcined at 700 °C, 900 °C and 1100 °C possess smaller size. The binding force of the primary particles tend to rise with the increase of calcination temperature. When the temperature was up to 900 °C and above, it was found that the sintering neck indicating with strong binding was formed between the primary particles. In parallel, the powders underwent an m-ZrO₂ to t-ZrO₂ transition as the calcination temperature rose. It is also found that the PS-PVD prepared coatings which were obtained by using the above powders undergo a transformation from a feather-like to a dense laminate structure as the calcination temperature rises. It is noteworthy that the coating obtained by the powders calcined at 700 °C have a special three-layer composite structure of near dense surface layer, columnar intermediate layer and dense sub-layer. The composite structural coating has excellent adhesion and thermal shock resistance, with a bonding strength of 81MPa and no major spalling when water quenched 100 cycles at 1100 °C.     

Effects of calcination temperature on structure and magnetic properties of pure FeCo2O4 powders

A series of FeCo₂O₄ powders was initially synthesized using a hydrothermal method and subsequently calcined at various temperatures to produce the final product. Pure phase FeCo₂O₄ powders can only be formed in the temperature range of 950–1050 °C. In this work, we study the cation occupation, cation valence, bond length and bond angle changes of the pure phase FeCo₂O₄ powders formed in such a narrow temperature range. Octahedral lattice distortion in the pure phase FeCo₂O₄ samples has been observed. More tetrahedral Fe³⁺ and octahedral Co²⁺ are excited and exchanged their sites as the calcination temperature increases from 950 °C to 1000 °C, and part of Co³⁺ ions are reduced to Co²⁺ in the sample calcined at 1050 °C. The structure of the sample calcined at 1000 °C is close to that of the ideal FeCo₂O₄ spinel. Magnetic measurements show that ferrimagnetism and anti-ferromagnetism coexist in the pure phase FeCo₂O₄ samples. Interaction changes between ferrimagnetism and antiferromagnetism caused by the structural changes of the samples have been studied. Due to the pinning of the local anti-ferromagnetism to ferrimagnetism in the sample, the sample shows a Barkhausen jump below 150 K. As the measurement temperature increases further, the system enters into a reentrant spin glass state.     

Synthesis of Fe:ZnSe nanopowders via the co-precipitation method for processing transparent ceramics

Fe:ZnSe nanopowders were synthesized via the co-precipitation method for fabricating transparent ceramics. Fe_xZn_1−xSe (0.00 ≤ x ≤ 0.06) powders that were calcined at 400°C yielded a single-phased cubic ZnSe, but when the calcination temperature was raised to 500-600°C, ZnO phase was created. Introduction of pressure could avoid appearance of ZnO. XRD Scherrer analysis revealed a monotonic increase in lattice parameter with increasing Fe²⁺ content. The average powder particle size increased with calcination temperature from several nanometers at 80°C to hundreds of nanometers at 600°C. Attempts to pressurelessly sinter ZnSe powders resulted in the partial decomposition of ZnSe, thus spark plasma sintering was employed to sinter Fe_0.01Zn_0.99Se transparent ceramics with pure ZnSe phase composition, which could be well sintered at 950°C for 30 minutes under an applied pressure of 60 MPa. SEM observations of the polished and thermally etched microstructure of the ceramic revealed a dense microstructure with average grain size of approximately 35 μm, and a few micropores were observed at the grain boundaries. The transparent ceramic exhibited good transmittance in the mid-far infrared range, with the highest transmittance 57% at 12 μm. This paper confirmed the scheme of synthesis of Fe:ZnSe nanopowders by liquid-phase co-precipitation method for sintering transparent ceramics.     

Synthesis of NdAlO3 with good microwave dielectric properties by stearic acid method

In this study, NdAlO₃ with perovskite structure was synthesized by the stearic acid method at relatively low temperature. The structural characteristics of the as-synthesized product were identified by TG–DSC, XRD, FT–IR, SEM, and TEM techniques. Using the powders as starting materials, NdAlO₃ bulk microwave ceramics were prepared, and the corresponding densification process, microstructural and dielectric properties were studied. The XRD and FI–IR results confirmed that single phase NdAlO₃ could be prepared at low temperature by the stearic acid method. A unique two-dimensional platelike morphology with an unevenly dispersed bubble shape structure was observed in the calcined powder. However, the TEM result revealed that the powder calcined at 800 °C had a good dispersity accompanied with narrow particle size distribution within a range of 20–35 nm. The average particle size of 27.3 nm was in accordance with that calculated from the XRD data. Using the powder calcined at 800 °C as raw materials, the as-obtained NdAlO₃ ceramics sintered at 1500 °C for 4 h possessed the highest density and favorable combined microwave dielectric properties (i.e., ε_r = 23.02, Q × f = 65320 GHz, and τ_f = −32.4 ppm/^°C). The present work developed a fast, energy-efficient approach to synthesize NdAlO₃ powder used as promising raw materials of microwave dielectric ceramics.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Effects of powder stoichiometry on the sintering of β-tricalcium phosphate

The impact of weak stoichiometry variations on β-TCP sintering behaviour was studied. β-Tricalcium phosphate (β-TCP) powders were synthesised by chemical precipitation through aqueous solution of diammonium phosphate and calcium nitrate. Excess or deficiency of nitrate salt leads to compositions with Ca/P ratios below or over 1.5. These powders, calcined at various temperatures (800–950 °C), were shaped by slip casting process and sintered at 1100 °C. The microstructure, phase composition, specific surface area and density of powders and sintered compacts were analysed by SEM, XRD, FTIR, BET, Archimedes methods and dilatometry.This study shows that the presence of calcium pyrophosphate or the hydroxyapatite phases affects considerably the physical characteristics of the β-TCP powders and in particular specific surface area and consequently their sinterability.A precise determination of the β-TCP chemical composition after synthesis allows to adapt the calcination temperature of the raw powder in order to obtain a maximum densification of the compact. The beneficial role of small quantity of HA phase inside β-TCP powder on their sinterability was also shown in this work.     

Pb(Zr0.95Ti0.05)O3 powders and porous ceramics prepared by one-step pyrolysis process using non-aqueous Pechini method

Using non-aqueous Pechini method, Pb(Zr_0.95Ti_0.05)O₃ powders were prepared at low temperature by one-step pyrolysis process. The polymeric gels and powders were characterized using a range of techniques, such as DTG, XRD, SEM, Raman spectroscopy, and laser particle size distribution. The perovskite phase was formed at about 350–400 °C and some oxocarbonate impurities can be detected in all samples after calcining at 400–850 °C by one-step pyrolysis process. Phase pure and porous Pb(Zr_0.95Ti_0.05)O₃ ceramics were obtained without pore formers from the powders by one-step pyrolysis process at 500 °C for 4 h. The relative densities were 87%, 91% and 94% for the ceramics sintered at 1100, 1150 and 1200 °C for 2 h, respectively. The porous ceramics sintered at 1200 °C for 2 h have homogeneously dispersed pores and fine-grain structures with an individual grain size of 0.7–2 μm.     

Flame Spray Synthesis of BaZr0.8Y0.2O3–δ Electrolyte Nanopowders for Intermediate Temperature Proton Conducting Fuel Cells

Flame Spray Synthesis (FSS) technique has been used for the preparation of BaZr_0.8Y_0.2O_3–δ (BZY20) nanoprecursors. The nanoprecursors were composed of a perovskite phase mixed with doped Zirconia and barium nitrate. Pure phase powder could be obtained after calcining the precursors at 1,200 °C. Both nanoprecursors and pure phase powder were then sintered at 1,600 °C to obtain dense specimen. AC impedance spectroscopy performed on the sintered samples allowed correlation of the electrical properties of the samples to their microstructures. The sintered nanoprecursors compared with the sintered pure phase powders showed enhanced grain growth associated with higher grain boundary conductivity. The influence of the reactive sintering on the enhanced grain growth and electrical properties in the nanoprecursors is discussed. The high total proton conductivity measured (7.7·10⁻³ S cm⁻¹ at 450 °C) promotes FSS as an effective powder synthesis method for the preparation of BZY20 electrolyte material for proton conducting fuel cells operating in the intermediate temperature range.     

Transparent mullite ceramic from single-phase gel by Spark Plasma Sintering

Monophasic mullite precursors with composition of 3Al₂O₃·2SiO₂ (3:2) were synthesized and then were sintered by Spark Plasma Sintering (SPS) to form transparent mullite ceramics. The precursor powders were calcined at 1100 °C for 2 h. The sintering was carried out by heating the sample to 1450 °C, holding for 10 min. The sintered body obtained a relative bulk density of above 97.5% and an infrared transmittance of 75–82% in wavelength of 2.5–4.3 μm without any additive. When the precursor powders were calcined at below 1100 °C, it was unfavorable for completely eliminating the residual OH, H₂O and organic compound. However, when calcined temperature was too high, it was unfavorable either for full densification due to the absence of viscous flow of amorphous phase. At the same calcined temperature, the transmittance of sintered body was decreased with the increase of the sintering temperature above 1450 °C owing to the elongated grain growth.     

A modified two-Stage mixed oxide synthetic route to lead magnesium niobate and lead iron niobate

A modified mixed oxide synthetic route has been developed for the synthesis of lead magnesium niobate [Pb(Mg_1/3Nb_2/3)O₃; PMN] and lead iron niobate [Pb(Fe_1/2Nb_1/2)O₃; PFN] powders. The formation of perovskite and pyrochlore phases in the calcined PMN and PFN powders has been investigated as a function of calcination temperature and time by XRD and DTA techniques. The particle size distribution of calcined powders was determined by laser diffraction, with the morphology, phase composition and crystal structure determined via SEM, TEM and EDX techniques. In both cases it has been found that cubic pyrochlore phases in the PbO–Nb₂O₅ system tend to form, as well as the perovskite phase. However, pyrochlore-free PMN and PFN powders were successfully obtained for a calcination temperature of 800°C for 4 and 3 h, respectively, without the introduction of excess PbO and/or MgO.     

Transparent polycrystalline Gd2Hf2O7 ceramics

We have successfully developed transparent polycrystalline Gd₂Hf₂O₇ ceramics with high in‐line transparency. A sol–gel process was used to synthesize the Gd₂Hf₂O₇ powder. Simultaneous thermal gravimetric analysis and differential thermal analysis (TGA/DTA) was used to identify the decomposition sequence as a function of temperature for the as‐synthesized sol–gel powders. The calcined powder is single phase and was formed with an estimated average particle size of 120 nm. Crystallization was confirmed by x‐ray diffraction (XRD) and a single phase was achieved by calcining at 1000°C. The calcined powders were hot‐pressed at 1500°C to achieve >95% theoretical density with closed pore structure followed by a hot isostatic pressing at 1500°C at 207 MPa to achieve a fully dense structure. Microstructural characterization shows a uniform grain size distribution with an average grain size of about 11 μm. In‐line transmission measurements revealed high transparency in the red and infrared. Dielectric properties remain stable with relative permittivity values around 180 and loss tangents less than 0.005 up to 350°C. Thermal conductivity was measured to be ~1.8 W/m°K at room temperature, decreasing to ~1.5 W/m°K by 500°C.     

Effect of calcination temperature on use of high-boron-content waste for low-temperature wall tile production

The effects of the calcination temperature on raw-colemanite-waste properties and calcined waste content on wall tile production were investigated. Waste containing 11.24% B₂O₃ calcined between 500 and 800°C was added to wall tile granules in various ratios (0–100 wt.%) to produce a low-temperature-sintered wall tile by adding the maximum content of boron waste, as determined through optimal calcination. The low-temperature (850–1000°C) sinterability of the samples and the effect of the calcined colemanite-waste content on the wall tile properties were investigated. The samples were characterised using X-ray fluorescence, X-ray powder diffraction, differential thermal analysis, thermogravimetric analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy, and colourimetry. The waste calcined at 800°C exhibited a substantially different phase distribution, bond structure, morphology, and colour. The wall tile produced using 40 wt.% colemanite waste calcined at 800°C and subsequently sintered at 950°C exhibited the optimal properties. The linear firing shrinkage, water absorption, and flexural strength of the optimised wall tile were 0.88%, 16.04%, and 36.07 MPa, respectively. The optimised wall tile exhibited major albite, quartz, and diopside phases and 64% higher strength. The sample calcined at 800°C showed that high colemanite-waste content could be incorporated into ceramic bodies.     

A benzoate coprecipitation route for synthesizing nanocrystalline GDC powder with lowered sintering temperature

Electrolyte powders with low sintering temperature and high-ionic conductivity can considerably facilitate the fabrication and performance of solid oxide fuel cells (SOFCs). Gadolinia-doped ceria (GDC) is a promising electrolyte for developing intermediate- and low-temperature (IT and LT) SOFCs. However, the conventional sintering temperature for GDC is usually above 1200 °C unless additives are used. In this work, a nanocrystalline powder of GDC, (10 mol% Gd dopant, Gd_0.1Ce_0.9O_1.95) with low-sintering temperature has been synthesized using ammonium benzoate as a novel, environmentally friendly and cost-effective precursor/precipitant. The synthesized benzoate powders (termed washed- and non-washed samples) were calcined at a relatively low temperature of 500 °C for 6 h. Physicochemical characteristics were determined using thermal analysis (TG/DTA), Raman spectroscopy, FT-IR, SEM/EDX, XRD, nitrogen absorptiometry, and dilatometry. Dilatometry showed that the newly synthesized GDC samples (washed and non-washed routes) start to shrink at temperatures of 500 and 600 °C (respectively), reaching their maximum sintering rate at 650 and 750 °C. Sintering of pelletized electrolyte substrates at the sintering onset temperature for commercial GDC powder (950 °C) for 6 h, showed densification of washed- and non-washed samples, obtaining 97.48 and 98.43% respectively, relative to theoretical density. The electrochemical impedance spectroscopy (EIS) analysis for the electrolyte pellets sintered at 950 °C showed a total electrical conductivity of 3.83 × 10^?2 and 5.90 × 10^?2 S cm^?1 (under air atmosphere at 750 °C) for washed- and non-washed samples, respectively. This is the first report of a GDC synthesis, where a considerable improvement in sinterability and electrical conductivity of the product GDC is observed at 950 °C without additives addition.     

Preparation of spherical nanoparticles of LaAlO3 via the reverse microemulsion process

Spherical LaAlO₃ nanoparticles in a reverse microemulsion consisting of solution (water phase), Tween-80 and Span-80 (surfactant), n-butanol (cosurfactant, and cyclohexane (oil phase) were prepared. Precursor powders and calcined powders were characterized by differential thermal analysis (DTA), thermogravimetry analysis (TG), X-ray diffraction (XRD) and transmission electron microscopy (TEM). A pure perovskite LaAlO₃ formed when the precursor hydroxides calcined at 800 °C for 2 h. The particle size was about 50 nm and the shape of the monodisperse particles is spherical. The reverse microemulsion process can dramatically lower the crystallization temperature of LaAlO₃ about 700 °C than the classical solid-state reaction method.     

Synthesis of La0.9Sr0.1Al0.85Mg0.1Co0.05O2.875 using a polymeric method

Nanocrystalline La_0.9Sr_0.1Al_0.85Mg_0.1Co_0.05O_2.875 (LSAMC) powders were synthesized via a polymeric method using poly(vinyl alcohol) (PVA). The effect of PVA content on the synthesized powders was studied. When the ratio of positively charged valences (Mⁿ⁺) to hydroxyl groups (OH) is 1.5:1, crystalline LaAlO₃ could be obtained at such a low calcination temperature as 700 °C. While at 900 °C the ratio is of less importance, since pure LaAlO₃ perovskite could be formed for all powders after calcination at 900 °C. Thermal analysis (TG/DTA) was utilized to characterize the thermal decomposition behaviour of precursor powders. The chemical structure of the calcined powder was studied by Fourier transform infrared (FTIR) spectroscopy. The powder morphology and microstructure were examined by SEM. Dense pellets with well-developed submicron microstructures could be formed after sintering at 1450 °C for 5 h. Compared with the solid-state reaction method, the sintering temperature is substantially lower for powder prepared by the PVA method. This is due to the ultrafine and highly reactive powder produced.     

Lanthanum-doped PZT synthesized by the oxidant peroxide method and sintered by conventional and microwave routes

Lanthanum-doped lead zirconate titanate (Pb_0.95La_0.05Zr_1−xTi_xO₃) with two different compositions, x=0.20 (PLZT20) and 0.80 (PLZT80), were successfully obtained by the oxidant peroxide method (OPM). This environmental friendly synthetic route showed to be very attractive to prepare lead based ferroelectric powders since no carbon or halides compounds were used. Tetragonal perovskite structure was observed for PLZT80 and rhombohedral perovskite for PLZT20, with single phase identified in the powders calcined at temperatures higher than 700 °C. Powders crystallized at 700 °C were used to produce ceramic samples that were sintered at 1000 °C for 2 h by conventional and microwaves techniques. The composition of the fractured surface of ceramics were quite close to the nominal molar composition, indicating that the OPM promotes the formation of materials with high compositional homogeneity and similar microstructures regardless the sintering method used.     

New Method of Synthesizing Aluminum Oxynitride Spinel Powders

Aluminum oxynitride spinel (AlON) powders were synthesized by aluminothermic reaction in a reducing N₂‐CO atmosphere. Low cost and easily available aluminum and γ‐Al₂O₃ alumina micrometer‐sized powders were employed as starting materials. Mixed powders consisting of 75 wt% Al and 25 wt% Al₂O₃ were milled together and pressed into billets with diameter of 20 mm and height of 15 mm. Green‐body billets were then calcined in charcoal‐protected condition (namely in a N₂‐CO atmosphere) at 1600°C. Phase composition and microstructure of final sintered products were analyzed. The results showed that AlON phase with AlN as a minor phase was formed at 1600°C for 3 h. At the same time, grains of AlON were tabular in shape and whiskers can be found in samples after being sintered at 1600°C.     

Processing and properties of 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 thick films

We have investigated the processing of 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ (PMN–PT) thick films on platinised alumina substrates. Nanosized PMN–PT powder with 2 mol% of excess PbO was prepared by high-energy milling and deposited on the substrate using screen-printing technology. The films were then sintered at 950 °C in a PbO-rich atmosphere. The influence of the sintering time and the amount of PbO-containing packing powder was studied and related to the structural, microstructural, dielectric and piezoelectric properties of the film. In order to obtain a homogeneous and dense thick film without any secondary phase, the PMN–PT films had to be sintered in the presence of a PbO-based liquid phase that had to be completely removed from the thick film during the final stage of the sintering. Under optimal sintering conditions we obtained a room temperature relative dielectric permittivity of 3600, dielectric losses of 0.036, a T_m of 174 °C, a permittivity at the T_m of 21,000 and a d₃₃ of 140 pC/N.     

Effect of Pb excess in sol–gel process on phase composition in PFN ceramics

Lead iron niobate Pb(Fe_0.5Nb_0.5)O₃ (PFN) precursors were prepared using sol–gel synthesis by mixing acetates Pb and Fe with Nb-ethylene glycol–tartarate (Pechini) complex at 80 °C and calcination of gels at 600 °C. Single pyrochlore phase with structure close to Pb₃Nb₄O₁₃ was formed in stoichiometric precursor and Pb₃Nb₄O₁₃ with small amount of perovskite phase Pb(Fe_0.5Nb_0.5)O₃ in nonstoichiometric precursor prepared with the excess of Pb in molar ratio (Pb:Fe:Nb = 1.2:0.5:0.5). Average particle sizes of PFN calcined powders were ～120 nm. The metastable pyrochlore phase was partially decomposed to perovskite phase at sintering temperature of 1150 °C for 2, 4 and 6 h. Excess of Pb caused increasing of the density (7.4 g/cm³) and content of the perovskite phase (～53 vol.%) in ceramics sintered for 4 h. In microstructures of PFN ceramics sintered at 1150 °C for different times, the bimodal grain size distribution was observed with small spherical grains of perovskite phase and larger octahedral grains, which represent the pyrochlore phase. Results of EDX analysis confirm that complex types of pyrochlore phases that differ in iron content were present in ceramics.