Development of moderate temperature CVD Al2O3 coatings

Chemically vapor deposited Al₂O₃ coatings, due to their high hardness and chemical inertness, are currently the state of art in the cutting tool industry. The conventional high deposition temperature of about 1050 °C for Al₂O₃ coatings, based on the water–gas shift process, has to a great extend restricted the development of several hybrid coatings, such as TiC/TiN/TiCN/Al₂O₃. To overcome this limitation, alternate systems to deposit Al₂O₃ at moderate temperatures have been investigated. Systems using NO–H₂, H₂O₂, NO₂–H₂ and HCOOH were identified and thermodynamic calculations were performed to evaluate them as potential sources of oxygen donors to form Al₂O₃ in the moderate temperature range of 700–950 °C. Preliminary results have clearly demonstrated that it is possible to grow moderate temperature alumina (using such alternate sources) on the TiC/TiN coated cemented carbide substrates.     

Effect of Y203 on microstructure and oxidation of γ-Ni＋γ′-Ni3Al coatings transformed from electrodeposited Ni-Al films at 1 000℃

The electrodeposited Y2O3-dispersed γ-Ni＋γ-Ni3Al coatings on Ni substrates were developed by the conversion of electrodeposited Ni-Al-Y2O3 films with dispersed AI microparticles in Ni matrix into Ni3Al by vacuum annealing at 800 ℃ for 3 h. For comparison, Y2O3-free γ-Ni＋γ＇-Ni3Al coatings with a similar AI content were also prepared by vacuum annealing the electrodeposited microparticle-dispersed composite coatings of Ni-AI under the same condition. SEM and TEM characterizations show that the electrodeposited Y2O3-dispersed γ＋γ＇ coatings exhibit finer grains, a more homogeneous distribution of γ＇, and a narrowed γ＇ phase spacing compared with the electrodeposited Y2O3-free γ-γ＇ coatings. The oxidation at 1 000 ~C shows that the addition of Y2O3 significantly improves the oxidation resistance of the electrodeposited γ＋γ＇coatings. The effect of Y2O3 particles on the microstructure and oxidation behavior of the electrodeposited γ＋y＇ coatings was discussed in detail.     

Microstructure and properties of CVD γ-Al2O3 coatings

This work deals with the microstructures and wear properties of chemical vapour deposited γ-Al₂O₃. The γ-Al₂O₃ coatings were deposited at 800 °C on TiN and Ti(C,N) pre-coated cemented carbide substrates. The microstructures developed in the γ-Al₂O₃ coatings and the influence of the nucleation surface on the growth of γ-Al₂O₃ were characterised using transmission electron microscopy, electron energy-loss spectroscopy and X-ray diffraction. The γ-Al₂O₃ coatings were fine-grained with a high density of {1 1 1} growth twins and contained some residual sulphur. γ-Al₂O₃ was found to grow epitaxially on the investigated substrates. The mechanical properties were evaluated in metal cutting and were compared with those of κ-Al₂O₃ coated tools. As compared with the κ-Al₂O₃ coatings, the γ-Al₂O₃ coatings exhibited slightly worse adhesion and tendency for edge chipping. However, the γ-Al₂O₃ coatings showed better crater wear resistance on the rake face than κ-Al₂O₃ coatings.     

Thermochemistry of GaBO3 and phase equilibria in the Ga2O3–B2O3 system

Employing a Tian-Calvet-type calorimeter operating in the scanning mode at temperatures from 1120 to 1220 K, the enthalpy change, Δ_dH, associated with the decomposition of GaBO₃ (=1/2β-Ga₂O₃+1/2B₂O₃(liq.)) and the corresponding decomposition temperature, T_d, were determined: Δ_dH=30.34±0.6 kJ/mol, T_d=1190±5 K. Using the transposed-temperature-drop method the thermal enthalpy, H(T)−H(295 K), of GaBO₃ was measured as a function of temperature, T, in the region from 760 to 1610 K; the results obtained are

[H(T)−H(295 K)]/(J/mol)=104.8·(T/K)−31 300 (760 K<T<1190 K),

[H(T)−H(295 K)]/(J/mol)=138.8·(T/K)−41 480 (1190 K<T<1590 K).

On the basis of the experimental results, the enthalpy and entropy of formation, Δ_fH and Δ_fS, respectively, of GaBO₃ from the component oxides were derived:

Δ_fH=−30.34 kJ/mol,Δ_fS=−25.50 J/(K·mol) at 1190 K,

Δ_fH=−10.55 kJ/mol,Δ_fS=−5.48 J/(K·mol) at 298 K.

The enthalpy versus temperature curve shows, apart from a step associated with the decomposition of GaBO₃, a further step at 1593 K which is attributed to a monotectic equilibrium.     

Study on phase formation and sintering kinetics of BaTi0.6Zr0.4O3 powder synthesized through modified chemical route

BaTi_0.6Zr_0.4O₃ powder was prepared from barium oxalate hydrate, zirconium oxy-hydroxide and titanium dioxide precursors. Barium oxalate hydrate and zirconium oxy-hydroxide were precipitated from nitrate solution onto the surface of suspended TiO₂. Phase formation behaviour of the materials was extensively studied using XRD. BaTiO₃ (BT) and BaZrO₃ (BZ) start forming separately in the system upon calcinations in the temperature range 600–700 °C. BT–BZ solid solution then forms by diffusion of BT into BZ from 1050 °C onwards. The precursor completely transforms into BaTi_0.6Zr_0.4O₃ (BTZ) at 1200 °C for 2 h calcination. The activation energy (AE) of BT (134 kJ mol⁻¹) formation was found to be less than that of BZ (167.5 kJ mol⁻¹) formation. BTZ formation requires 503.6 kJ mol⁻¹ of energy. The sintering kinetics of the powder was studied using thermal analyzer. The mean activation energy for sintering was found to be 550 kJ mol⁻¹.     

Preparation of neodymium sulphides by the reaction of Nd2(So4)3 with carbon disulphide

The reaction to synthetize neodymium sulphides from neodymium sulphate octahydrate in a stream of carbon disulphide gas was studied. The dehydration of the octahydrate in vacuum was finished at 300 °C. At 1050–1100 °C in air neodymium oxysulphide, Nd₂O₂SO₄, was formed. Neodymium oxysulphide, Nd₂O₂S, was formed upon heating with a reducing agent such as annealed carbon. The reaction of neodymium sulphate with carbon disulphide commenced at 500–600 °C, resulting in formation of the disulphide, NdS₂. The crystal structure of NdS₂ heated at 500 °C was, however, different from that of the sample heated at 600 °C. In the temperature range 800–900 °C -Nd₂S₃ was obtained as a single phase after heating for at least 3 h in high flow rates of gas mixtures of nitrogen and high concentrations of carbon disulphide. The sesquisulphide, γ-Nd₂S₃ (or Nd₃S₄), was formed at temperatures as high as 1100 °C. The reaction conditions for the compounds mentioned above are discussed together with the analysis of their crystal structures by X-ray powder diffractometry.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Sm0.5Sr0.5CoO3 cathode material from glycine-nitrate process: Formation, characterization, and application in LaGaO3-based solid oxide fuel cells

Cathode material Sm_0.5Sr_0.5CoO₃ (SSC) with perovskite structure for intermediate temperature solid oxide fuel cell was synthesized using glycine-nitrate process (GNP). The phase evolution and the properties of Sm_0.5Sr_0.5CoO₃ were investigated. The single cell performance was also tested using La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) as electrolyte and SSC as cathode. The results show that the formation of perovskite phase from synthesized precursor obtained by GNP begins at a calcining temperature of 600 °C. The single perovskite phase is formed completely after sintering at a temperature of 1000 °C. The phase formation temperature for SSC with complete single perovskite phase is from 1000 to 1100 °C. The SrSm₂O₄ phase appeared in the sample sintered at 1200 °C. It is also found that the sample sintered at 1200 °C has a higher conductivity. The electrical conductivity of sample is higher than 1000 S/cm at all temperature examined from 250 to 850 °C, and the highest conductivity reaches 2514 S/cm at 250 °C. The thermal expansion coefficient of sample SSC is 22.8 × 10⁻⁶ K⁻¹ from 30 to 1000 °C in air. The maximum output power density of LSGM electrolyte single cell attains 222 and 293 mW/cm² at 800 and 850 °C, respectively.     

Subsolidus phase relation and crystal structure of the PrBa2−xSrxCu3O7±δ system

The PrBa_2−xSr_xCu₃O_7±δ solid solution was investigated by means of X-ray powder diffraction in combination with Rietveld analysis. The Sr-doped Pr123 single phase could be synthesized at 950 °C in air. The solubility of PrBa_2−xSr_xCu₃O_7±δ solid solution is 0.2≤x≤0.6. The structure of PrBa_2−xSr_xCu₃O_7±δ is orthorhombic for x=0.2. The structure transforms into tetragonal for 0.3≤x≤0.6. In the PrBa_2−xSr_xCu₃O_7±δ structure, Sr ions can replace Ba ions, the highest value is x=0.6 under our experimental condition. But Sr ions could not replace Pr ions. Furthermore Pr ions could not occupy the sites of Ba ions in the PrBa_2−xSr_xCu₃O_7±δ system. Both ionic radii and chemical properties play an important role in the mutual substitution of Pr, Ba and Sr ions in the Pr123 structure of the PrBa_2−xSr_xCu₃O_7±δ system.     

Microstructural evolution of some MgO–TiO2 and MgO–Al2O3 powder mixtures during high-energy ball milling and post-annealing studied by X-ray diffraction

High-energy dry ball-mill and post-anneal processing were applied to synthesize MgTiO₃ and Mg₂TiO₄ single crystalline phases from the predetermined compositions of MgO–TiO₂ powder mixtures. Also, the experiments were performed to show that it is possible to prepare MgAl₂O₄ single crystalline phase from the predetermined composition of MgO–Al₂O₃ powder mixture only by employing high-energy dry ball milling, i.e. without post-annealing the milled samples. In contrast, fully developed single crystalline powders of MgTiO₃ and Mg₂TiO₄ were obtained after post-annealing the milled samples for 1 h at 900 and 1200 °C, respectively.     

Properties of ZrO2–Al2O3 composite as a function of isothermal holding time

Zirconia and alumina based ceramics present interesting properties for their application as implants, such as biocompatibility, good fracture resistance, as well as high fracture toughness and hardness. In this work the influence of sintering time on the properties of a ZrO₂–Al₂O₃ composite material, containing 20 wt% of Al₂O₃, has been investigated. The ceramic composites were obtained by sintering, in air, at 1600 °C for sintering times between 0 and 1440 min. Sintered samples were characterized by microstructure and crystalline phases, as well as by mechanical properties. The grain growth exponents, n, for the ZrO₂ and Al₂O₃ were 2.8 and 4.1, respectively, indicating that different mechanisms are responsible for grain growth of each phase. After sintering at 1600 °C, the material exhibited a dependency of hardness as function of sintering time, with hardness values between 1500 HV (120 min) and 1310 HV (1440 min) and a fracture toughness of 8 MPa m^1/2, which makes it suitable for bioapplications, such as dental implants.     

High temperature oxidation behavior of Ti3SiC2-based material in air

The oxidation behavior of Ti₃SiC₂-based material in air has been studied from 900°C to 1200°C. The present work showed that the growth of the oxide scale on Ti₃SiC₂-based material obeyed a parabolic law from 900°C to 1100°C, while at 1200°C it followed a linear rule. The oxide scale was generally composed of an outer layer of coarse-grained TiO₂ (rutile) and an inner layer of fine-grained TiO₂ and SiO₂ (tridymite) above 1000°C. A discontinuous coarse-grained SiO₂ layer was observed within the outer coarse-grained TiO₂ layer on the samples oxidized at 1100°C and 1200°C. Marker experiments showed that the oxidation process was controlled by the inward diffusion of oxygen, outward diffusion of titanium and CO or SiO, and that internal oxidation predominated. The TiC content in Ti₃SiC₂ was deleterious to the oxidation resistance of Ti₃SiC₂.     

Effect of annealing on the morphology evolution and densification of LiMn2O4 film from chitosan-containing solution

Dense LiMn₂O₄ films deposited on a Pt-coated silicon substrate were obtained by annealing the deposited Li–Mn–O-chitosan films under a two-stage heat-treatment procedure. It was demonstrated that the heat-treatment at 300 °C plays an important role in the subsequent densification of LiMn₂O₄ films. This is attributed to the formation and rearrangement of the nano-sized LiMn₂O₄ crystallites. The surface morphology of the calcined Li–Mn–O-chitosan films was highly related to the annealing temperature. Ridge-like bumps formed on the surface of the films after being heated at 200 °C for 1 h. With calcination at 400 °C or higher, the surface morphology turned into a wrinkle-like microstructure. This morphology transformation is ascribed to the flowing characteristics of the Li–Mn–O-chitosan films during heat-treatment and subsequent thermal decomposition of the precursor at higher temperatures. Moreover, the electrochemical tests showed that the 700 °C-annealed LiMn₂O₄ film possesses the highest discharge capacity of 56.3 μA h/(cm² μm) and best capacity retention of 90.7% after 50 charge/discharge cycles of all annealed films.     

Ba3YB3O9: phase transition and crystal structure

A new modification of the compound Ba₃YB₃O₉, β phase, has been attained through solid phase transition from phase at 1125–1134 °C. β-Ba₃YB₃O₉ crystallizes in the hexagonal space group with cell parameters a=13.0529(8) Å, c=9.5359(9) Å. The crystal structure of -Ba₃YB₃O₉ has been determined from powder X-ray diffraction (XRD) data. The refinement was carried out using the Rietveld methods and the final refinement converged with R_p=8.8%, and R_wp=11.8% with R_exp=5.65%. In its structure, the isolated [BO₃]³⁻ anionic groups are parallel to each other and distributed layer upon layer along the c-axis. The Y atoms are six-coordinated by the O atoms to form octahedra. The result of IR spectrum confirmed the existence of [BO₃]³⁻ triangular groups.     

Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

Subsolidus phase relations in the ZnO–MoO3–B2O3, ZnO–MoO3–WO3 and ZnO–WO3–B2O3 ternary systems

The subsolidus phase relations in the ZnO–MoO₃–B₂O₃, ZnO–MoO₃–WO₃ and ZnO–WO₃–B₂O₃ ternary systems have been investigated by the means of X-ray powder diffraction (XRD). There is no ternary compound in all the systems. There are five binary compounds and five tie lines in the ZnO–MoO₃–B₂O₃ system. This system can be divided into six 3-phase regions. There are three binary compounds and three tie lines in the ZnO–MoO₃–WO₃ system. This system can be divided into four 3-phase regions. There are four binary compounds and four tie lines in the ZnO–WO₃–B₂O₃ system. This system can be divided into five 3-phase regions. The possible component regions for ZnO single crystal flux growth were discussed. The phase diagram of Zn₃B₂O₆–ZnWO₄ pseudo-binary system has been constructed, and the result reveals this system is eutectic system. The eutectic temperature is 1007 °C and eutectic point component is 70 mol% Zn₃B₂O₆.     

Structural and thermal studies on Na2U(MoO4)3 and Na4U(MoO4)4

In Na–U(IV)–Mo–O system, two quaternary compounds Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄ were prepared by solid state reactions of Na₂MoO₄, UMoO₅ and MoO₃ in the required stoichiometric ratio at 500 °C in evacuated sealed quartz ampoules. The crystal structure of both the compounds were derived from X-ray powder diffraction data in the tetragonal system by Rietveld profile method. Na₂U(MoO₄)₃ has scheelite structure, whereas Na₄U(MoO₄)₄ has scheelite superlattice structure.

TG curves of Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄ did not show any significant weight change up to 750 °C in an inert atmosphere. During the heating cycle in an inert atmosphere, DTA curves of Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄ showed endothermic peaks due to the melting of the compounds at 740 °C and 730 °C, respectively. Na₂U(MoO₄)₃ and Na₄U(MoO₄)₄, when heated in air atmosphere at 1200 °C, decomposed to form Na₂U₂O₇ which was confirmed by weight loss calculation and XRD.     

Pore morphology in sintered ZrO2–8 mol% Y2O3 ceramic: a small-angle neutron scattering investigation

Pore morphology and pore size distribution in yttria-stabilized zirconia (ZrO₂–8 mol% Y₂O₃) have been investigated, for two sintering temperatures, namely 1200 and 1270 °C, using small-angle neutron scattering. The results show that the reduction in the porosity, at 1270 °C compared to that at 1200 °C, occurs by the elimination of the pores at the lower end of the pore size distribution. In addition, the polydispersity is also lower at 1270 °C and the nature of the distribution is altered significantly near the smaller radius range. The average pore size shifts towards the higher radius range. The specific surface area of the pores is also diminished at 1270 °C because of the elimination of the finer pores.     

Preparation of nanocrystalline Lu2O3:Eu phosphor via a molten salts route

The paper reports on the course of decomposition of hydrated lutetium nitrate and lutetium chloride to Lu₂O₃ in the eutectic mixture of NaNO₂ and KNO₂. It was shown that a crystallographically pure phase of the cubic Lu₂O₃ is formed at temperature as low as 250 °C. IR spectra revealed that the recovered powder contains some OH-contamination, however. The powders are characterized by crystallites sizes in the range of 18–30 nm in average. Emission and excitation spectra of Eu-doped powders show characteristic features for Eu³⁺ ion in an oxide host, which indicates that the procedure is appropriate for making activated nanoparticulate oxide phosphors. Most profound emission appears around 611 nm and the luminescence from the powder made starting with Lu(NO₃)₃ was noticeably higher compared to the product obtained from LuCl₃. The excitation spectrum of Eu³⁺ emission at 611 nm contains a band related to the fundamental absorption of the lutetia host lattice, which indicates an existence of the host-to-activator energy transfer.     

Synthesis and characterization of LiGaxMn2−xO4 (0 ≤ x ≤ 0.05) by triethanolamine-assisted sol–gel method

Spinel LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) cathode materials with phase-pure particles and nano-sized distribution were synthesized by sol–gel method using triethanolamine as the chelating agent. The effects of heat treatment on the physicochemical properties of the spinel LiGa_xMn_2−xO₄ powders were examined with thermogravimetric and differential thermal analysis (TG/DTA), powder X-ray diffraction (XRD) and scanning electron micrograph (SEM). The LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) electrodes were characterized electrochemically by charge/discharge experiments under a current rate of 0.5C at 55 °C. Although the Ga-doped spinel electrode showed smaller initial discharge capacity, it exhibited better cycling performance than the undoped-LiMn₂O₄ electrode. The dQ/dV versus potential plots at 55 °C revealed that the improvement in cycling performance of the Ga-doped spinel electrode is attributed to stabilization of the spinel structure by the presence of gallium ion.