Synthesis and characterisation of chemical bath deposited TiO2 thin-films

A series of titania thin films was prepared by chemical bath deposition (CBD) of TiCl₃ on indium tin oxside (ITO) glass at room temperature, followed by calcinations at 500 °C for 4 h. The effect of cyclic deposition on phase composition, microstructure and electrical resistivity of TiO₂ thin films was characterised using X-ray diffraction, scanning electron microscopy and four-point probe respectively. Results showed that TiO₂ films produced by single deposition cycle were amorphous. In contrast, those produced by 5 and 6 deposition cycles were partly amorphous and partly crystalline with the formation of rutile. Both the film thickness and electrical resistivity increased with an increase in the number of deposition cycles.     

Synthesis of BaCeO3 and BaCe0.9Y0.1O3−δ from mixed oxalate precursors

An oxalate precipitation route is proposed for the synthesis of BaCe_1−xY_xO₃ (x = 0 and 0.1) after calcination at 1100 °C. The precipitation temperature (70 °C) was a determinant parameter for producing a pure perovskite phase after calcination at 1100 °C for 1 h. TG/DTA measurements showed that the co-precipitated (Ba, Ce and Y) oxalate had a different thermal behaviour from single oxalates. Despite a simple grinding procedure, sintered BaCe_0.9Y_0.1O_3−δ pellets (1400 °C, 48 h) presented 90.7% of relative density and preliminary impedance measurements showed an overall conductivity of around 2 × 10⁻⁴ S cm⁻¹ at 320 °C.     

Fabrication and properties of Y2O3 transparent ceramic by sintering aid combinations

Transparent Y₂O₃ ceramics were fabricated by the solid-state reaction and vacuum sintering method using La₂O₃, ZrO₂ and Al₂O₃ as sintering aids. The microstructure of the Y₂O₃ ceramics sintered from 1550 °C to 1800 °C for 8 h were analyzed by SEM. The sintering process of the Y₂O₃ transparent ceramics was optimized. The results showed that when the samples were sintered at 1800 °C for 8 h under vacuum, the average grain sizes of the ceramics were about 3.5 µm. Furthermore, the transmittance of Y₂O₃ ceramic sintered at 1800 °C for 8 h was 82.1% at the wavelength around the 1100 nm (1 mm thickness), which was close to its theoretical value. Moreover, the refractive index of the Y₂O₃ transparent ceramic in the temperature range from 30 °C to 400 °C were measured by the spectroscopic ellipsometry method.     

Mechanical and optical properties of spark plasma sintered transparent Y2O3 ceramics

Commercial Y₂O₃ nanopowder was used to fabricate transparent Y₂O₃ ceramics by spark plasma sintering under the pressure of 100 MPa for 20 min with the heating rate of 100 °C/min. The microstructures, mechanical and optical properties of the Y₂O₃ ceramics sintered at different temperatures were investigated in detail. Densification occurred up to a sintering temperature of 1500 °C, and above 1500 °C, rapid grain growth and pore growth occurred. The highest relative density of 99.58% and the minimum average grain size of 0.58±0.11 µm were obtained at 1500 °C. The flexural strength, hardness and fracture toughness of the optimal spark plasma sintered Y₂O₃ ceramic were 122 MPa, 7.60 GPa and 2.06 MPa.m^1/2, respectively. The Y₂O₃ ceramic sintered at 1500 °C had the in-line transmission of about 11–54% and 80% in the wavelength range of 400–800 nm and 3–5 µm, respectively.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.     

Spark plasma sintering of transparent Y2O3 ceramic using hydrothermal synthesized nanopowders

Y₂O₃ nanopowders were synthesized by the hydrothermal treatment of Y(NO₃)₃·6H₂O and citric acid (CA) as Y⁺³ and the capping agent, respectively. The effect of different CA:Y⁺³ mol ratios, heat treatment time, and calcination temperature was investigated in order to determine their influence on the morphology, particle size and phase of Y₂O₃ nanopowders. The narrow size distribution of particles was obtained with CA:Y⁺³ mol ratio=1.6, heat treatment time of 6 h, and a calcination temperature at 900 °C for 90 min. Then, the synthesized Y₂O₃ nanopowder was consolidated by the spark plasma sintering technique at 1500 °C with a heating rate of 100 °C/min and held for 8 min before turning off the power. As a result, the ceramic prepared with 3 mm thickness got the highest transmission of 80% at 2.5–6 µm wavelength. The highest density and the grain size of yttria ceramic were 99.58% and 1–1.2 µm at 1500 °C, respectively.     

Effect of sintering temperature on the microstructure and transparency of Nd,Y:CaF2 ceramics

1 at% Nd, 3 at% Y doped CaF₂ transparent ceramics were obtained by hot pressing at the sintering temperature varing from 500 to 800 °C under vacuum environment with co-precipitated CaF₂ nanopowders. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grain size around 26 nm. Scanning electron microscopy (SEM) observations of the Nd, Y: CaF₂ ceramics indicated that the mean grain size of the ceramic sintered at 800 °C was about 748 nm. The influence of the temperature on the grain size, microstructure and optical transmittance was investigated. For the ceramic sintered at 800 °C, the transmittance was 85.49% at the wavelength of 1200 nm. The room temperature emission spectra of Nd: CaF₂ and Nd, Y: CaF₂ ceramics were measured and discussed.     

Synthesis and pyrolysis behavior of a soluble polymer precursor for ultra-fine zirconium carbide powders

A soluble polymer precursor for ultra-fine zirconium carbide (ZrC) was successfully synthesized using phenol and zirconium tetrachloride as carbon and zirconium sources, respectively. The pyrolysis behavior and structural evolution of the precursor were studied by Fourier transform infrared spectra (FTIR), differential scanning calorimetry, and thermal gravimetric analysis (DSC–TG). The microstructure and composition of the pyrolysis products were characterized by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscope (SEM) and element analysis. The results indicate that the obtained precursor for the ultra-fine ZrC could be a Zr–O–C chain polymer with phenol and acetylacetone as ligands. The pyrolysis products of the precursor mainly consist of intimately mixed amorphous carbon and tetragonal ZrO₂ (t-ZrO₂) in the temperature range of 300–1200 °C. When the pyrolysis temperature rises up to 1300 °C, the precursor starts to transform gradually into ZrC, accompanied by the formation of monoclinic ZrO₂ (m-ZrO₂). The carbothermal reduction reaction between ZrO₂ and carbon has been substantially completed at a relatively low temperature (1500 °C). The obtained ultra-fine ZrC powders exhibit as well-distributed near-spherical grains with sizes ranging from 50 to 100 nm. The amount of oxygen in the ZrC powders could be further reduced by increasing the pyrolysis temperature from 1500 to 1600 °C but unfortunately the obvious agglomeration of the ZrC grains will be induced.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends

Samarium borate (SmBO₃) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm₂O₃) and boron oxide (B₂O₃) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B₂O₃ phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700–1250 °C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV–vis) techniques. Fine-grained and pure SmBO₃ powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO₃ powders were synthesized after mechanical activation for 1 h and annealing at 700 °C for 2 h. The polymorphic transformation temperature of SmBO₃ powders from triclinic to hexagonal is about 1080 °C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO₃ phase and its transformation to hexagonal form were found to take place at ∼50–100 °C lower temperatures than those reported in other methods. Mainly hexagonal SmBO₃ powders were obtained after annealing at 1150 °C in the presence of a very small amount of triclinic SmBO₃. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO₃ phase) and 0.68% (hexagonal SmBO₃ phase) depending on their crystal structures.     

High optical quality Y2O3 transparent ceramics with fine grain size fabricated by low temperature air pre-sintering and post-HIP treatment

High optical quality Y₂O₃ transparent ceramics with fine grain size were successfully fabricated by air pre-sintering at various temperature ranging from 1500 to 1600 °C combined with a post-hot-isostatic pressing (HIP) treatment using co-precipitated powders as the starting material. The fully dense Y₂O₃ transparent ceramic with highest transparency was obtained by pre-sintered at 1550 °C for 4 h in air and post-HIPed at 1600 °C for 3 h (the pressure of HIP 200 MPa), and it had fine microstructure and the average grain size was 0.96 μm. In addition, the in-line transmittance of the ceramic reached 81.7% at 1064 nm (1 mm thickness). By this approach, the transparent Y₂O₃ ceramics with fine grain size (<1.6 μm) were elaborated without any sintering aid.     

Microwave sintering of CeO2 and Y2O3 co-stabilised ZrO2 from stabiliser-coated nanopowders

Tetragonal ZrO₂ polycrystalline (TZP) composites with 2 wt.% Al₂O₃ and co-stabilised with 1 mol% Y₂O₃ and (4, 6 or 8) mol% CeO₂ were sintered at 1450 °C for 20 min in a single mode 2.45 GHz microwave furnace. For comparison, conventional sintering was performed in air at 1450 °C for 20 min. The starting powder mixture was obtained by a suspension coating technique using yttrium nitrate, cerium nitrate and pure m-ZrO₂ nanopowder. Fully dense material grades were obtained by both sintering methods. The influence of the composition and the sintering methods on the final phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscopy. Finer and more uniform microstructures were observed in the microwave sintered ceramics when compared to the conventionally sintered samples. The fracture toughness increases with decreasing stabiliser content, whereas a reverse relation was found for the Vickers hardness. Comparable toughness and hardness values were obtained for the microwave and conventionally sintered samples.     

Effects of heat treatment conditions on the structural and magnetic properties of MgCuZn nano ferrite

Mg_0.5Cu_0.05Zn_0.45Fe₂O₄ nanoparticles were prepared through sol–gel method using polyvinyl alcohol as a chelating agent. The as prepared sample was annealed at three different temperatures (500 °C, 700 °C and 900 °C). The phase formation, morphology and magnetic properties with respect to annealing temperature were studied using the characterisation techniques like X-ray diffraction (XRD) as well as Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometer (VSM), respectively. The crystallite size and magnetisation showed increasing trend with annealing temperature. The coercivity increased up to a particular annealing temperature and decreased thereafter, indicating transition from single domain to multi domain state with increasing annealing temperature. Further, to know the suitability of the material, as a ferrite core, in multilayer chip inductors, the powder sample annealed at 500 °C was compacted in the form of torroids and sintered at three different temperatures (800 °C, 900 °C and 950 °C). The permeability showed increasing trend with the increase of sintering temperature since the permeability depends on microstructure. The frequency dispersion of permeability, for the sintered samples, demonstrated high frequency stability as well as high operating frequency. The cut-off frequency for the sintered samples 800 °C, 900 °C and 950 °C is 32 MHz, 30.8 MHz and 30.4 MHz, respectively.     

High temperature dielectrics in the ceramic system K0.5Bi0.5TiO3-Ba(Zr0.2Ti0.8)O3-Bi(Zn2/3Nb1/3)O3

Ceramics in the system (1-x)[0.5K_0.5Bi_0.5TiO₃-0.5Ba(Zr_0.2Ti_0.8)O₃]-xBi(Zn_2/3Nb_1/3)O₃ have been fabricated by a solid-state processing route for compositions x≤0.3. The materials are relaxor dielectrics. The temperature of maximum relative permittivity, T_m, decreased from 150 °C for composition x=0, to 70 °C for x=0.2. The x=0.2 sample displayed a wide temperature range of stable relative permittivity, ε_r, such that ε_r=805±15% from −20 to 600 °C (1 kHz). Dielectric loss tangent was ≤0.02 from 50 °C to 450 °C (1 kHz), but due to the tanδ dispersion peak, the value increased to 0.09 as temperatures fell from 50 °C to −20 °C. Values of dc resistivity were of the order of ~10⁹ Ω m at 300 °C. These properties are promising in the context of developing new high temperature capacitor materials.     

Preparation of dual-phase composite BaCe0.8Y0.2O3/Ce0.8Y0.2O2 and its application for hydrogen permeation

Dual-phase ceramic membranes composed of BaCe_0.8Y_0.2O₃ (BCY) and Ce_0.8Y_0.2O₂ (CYO) were successfully synthesized by solid state reaction method for hydrogen permeation. The influences of the BCY/CYO volume ratios on phase composition, microstructure, chemical stability and electrical property were investigated. The hydrogen permeation of the dual-phase composite was characterized as a function of temperature and feed side hydrogen partial pressure. The results showed that there was no reaction between the two constituent oxides observed under the preparation conditions. The dual-phase composite with different BCY/CYO volume ratios after sintering at 1550 °C exhibited dense structure, as well as good stability in 4% H₂/Ar, wet Ar and pure CO₂ atmosphere. The conductivity of the dual-phase composite increased with the content of CYO increasing and 30BCY–70CYO exhibited the highest total conductivity of 2.6×10⁻² S cm⁻¹ at 800 °C in 4% H₂/Ar. The hydrogen permeability of 30BCY–70CYO sample was improved as the temperature and the hydrogen partial pressure in feed gas increased. The hydrogen permeation flux of 1.7 μmol cm⁻² s⁻¹ was achieved at 850 °C.     

Tribological behavior of Ti3SiC2 and Ti3SiC2/Pb composites sliding against Ni-based alloys at elevated temperatures

The Ti₃SiC₂ and Ti₃SiC₂/Pb composites were tested under dry sliding conditions against Ni-based alloys (Inconel 718) at elevated temperatures up to 800 °C using a pin-on-disk tribometer. Detailed tribo-chemical changes of Pb on sliding surface were discussed. It was found that the tribological behavior were insensitive to the temperature from 25 °C (RT) to 600 °C (friction coefficient ≈0.61–0.72, wear rate ≈10⁻³ mm³ N m⁻¹). An amount of Pb in the composites played a key role in lubricating with the temperature below 800 °C. The friction coefficient (≈0.22) and wear rate (≈10⁻⁷ mm³ N m⁻¹) at elevated temperatures were both decreased by the added PbO. The wear mechanisms of Ti₃SiC₂/Pb-Inconel 718 tribo-pair at elevated temperatures were believed to be the combined effect of abrasive wear and tribo-oxidation wear. During the sliding, two oxidization reactions proceed, 2Pb+O₂=2PbO (below 600 °C) and 6PbO+O₂=2Pb₃O₄ (800 °C). The friction coefficient and wear rate of the composites were reduced due to the self-lubricating effect of the tribo-oxidation products.     

Microstructural evolution of multi-walled carbon nanotubes in the presence of mixture of silicon and silica powders at high temperatures

Microstructural evolution of multi-walled carbon nanotubes (MWCNTs) in the presence of mixture of silicon and silica powders in a coke bed is studied in the temperature range of 1000–1500 °C by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and thermogravimetry–differential scanning calorimetry (TG–DSC). The results showed that a thin amorphous SiO₂ coating was formed on the surface of MWCNTs at the temperature below 1300 °C. With the increase of the treated temperature, the coating became thicker, 3–7 nm in thickness at 1400 °C and a maximum of 10 nm at 1500 °C. Meanwhile, SiC nanowires and SiC nanocrystals around Ni catalyst at the tip of MWCNTs were formed at 1400 °C and 1500 °C, which were related to the vapor–vapor (V–V) and vapor–liquid–solid (V–L–S) reactions between SiO (g) and CO (g) or C (s), respectively. The oxidation resistance of all the treated MWCNTs was better than that of as-received ones. The oxidation peak temperature reached 804.2 °C for the treated MWCNTs, much higher than 652.2 °C for as-received ones.     

Linear thermal expansion coefficients of relaxor-ferroelectric 0.57Pb(Sc1/2Nb1/2)O3–0.43PbTiO3 ceramics in a wide temperature range

The objective of this work was to examine linear thermal expansion of virgin and poled 0.57Pb(Sc_1/2Nb_1/2)O₃–0.43PbTiO₃ ceramics between 30 °C and 600 °C by contact dilatometry. The thermal expansion dL/Lo of the virgin ceramic increases with increasing temperature until approximately 260 °C. The physical and technical thermal expansion coefficients were determined. At 260 °C the physical thermal coefficient is 2.08 × 10^?6 K^?1. Between 260.0 °C and 280.0 °C an anomaly in the thermal expansion vs. temperature and an endothermic peak in the differential scanning calorimetry curves correspond to the phase transition region from tetragonal to cubic phase. At temperatures from 280 °C to 600 °C the thermal expansion dL/Lo increases again.In the derivative of the dL/Lo heating curves of the poled ceramics, additionally to the anomaly at 270 °C, also the anomaly at 160 °C is observed, which is associated with the depolarization of the material during heating.     

Effects of Y2O3 addition on structure and properties of LZAS vitrified bond for CBN grinding tools application

The effects of Y₂O₃ addition on the structure and properties of Li₂O–ZnO–Al₂O₃–SiO₂ (LZAS) vitrified bonds were firstly investigated for CBN grinding tools application. Glasses and glass-ceramics were characterized using differential scanning calorimetry, X-ray diffractometry, scanning electron microscopy and infrared spectroscopy. The thermal expansion coefficient (TEC), microhardness, bending strength and chemical durability of the obtained products were also evaluated. Results showed that Y₂O₃ acted as the network former in the track of SiO₄ tetrahedrals. Introducing Y₂O₃ in the glasses increased the glass transition temperature and crystallization temperature. The crystallization of the main β-quartz_ss phase increased with increase of Y₂O₃ content. The morphology of the crystals was dependent on the Y₂O₃ content. The TEC (5.15×10⁻⁶/°C) of vitrified bond containing 1.0 mol% Y₂O₃ (Y1.0) was very close to the TEC (5.0×10⁻⁶/°C) of CBN grains. Moreover, Y1.0 vitrified bond exhibits a high microhardness (5.98 GPa), a high bending strength (202 MPa) and a good chemical durability (20 days, DR=2.8×10⁻⁹ g/cm² min), suggesting that it would be a promising material for CBN grinding tool.