MOCVD of YSZ coatings using β-diketonate precursors

Metallorganic chemical vapor deposition (MOCVD) was investigated as a more efficient means to fabricate yttria-stabilized zirconia (YSZ) for thermal barrier coating. The MOCVD precursors were Y(tmhd)₃ and Zr(tmhd)₄ (tmhd, 2,2,6,6-tetramethyl-3,5-heptanedianato) and delivered via aerosol-assisted liquid delivery (AALD). The maximum YSZ coating rate was 14.2 ± 1.3 μm h⁻¹ at 827 °C yielding a layered coating microstructure. The growth was first-order with temperature below 827 °C with an apparent activation energy of 50.9 ± 4.3 kJ mol⁻¹. Coating efficiency was a maximum of approximately 10% at the highest growth rate. While homogeneous nucleation remained a problem, the deposition of YSZ with only minor carbon content was achieved.     

Structural characteristics and resistive switching properties of thermally prepared TiO2 thin films

Polycrystalline TiO₂ thin films were formed on Pt(1 1 1)/Ti/SiO₂/Si by thermal oxidation of Ti films with temperatures ranging from 600 °C to 800 °C. Results of Raman spectra testing indicate that the structure of the oxidized TiO₂ films is rutile phase. The resistance switching behaviors (RSB) have been confirmed in Pt/TiO₂/Pt structures. A stable RSB with a narrow dispersion of the resistance states and switching voltages was observed in the sample fabricated with the oxidation temperature of 600 °C. The resistance ratios of high resistance states to low resistance states are larger than 10³ with the set and reset voltage as low as 2.5 V and 0.6 V, respectively.     

Oxidation behaviour of PACVD TiBN coating at elevated temperatures

The elevated-temperature oxidation behaviour of a TiBN coating on a plasma-nitrided hot-work tool steel (DIN 1.2367) by means of plasma-assisted chemical vapour deposition (PACVD) was investigated under the condition where a coated die would be preheated prior to being mounted on the press for aluminium extrusion. The TiBN coating was found to possess good resistance to oxidation up to 400 °C. Rapid oxidation started to occur at 450 °C. Radio frequency glow discharge optical emission spectroscopy (rf-GDOES) indicated that the oxidised layer was thickened from 100 nm to 1.0 μm, as the soaking time at 500 °C was prolonged from 2 to 16 h, which was attributed to the high temperature that promoted the penetration of oxygen into the coating. rf-GDOES also showed that boron initially in the coating vanished from the oxidised layer when the temperature was 450 °C or higher. X-ray diffractometry confirmed that the oxidised layer was composed mainly of TiO₂. SEM revealed that the TiO₂ layer was pulverised, leaving many microcracks and cavities, as a result of the losses of boron oxide and nitrogen. The rapid oxidation at above 450 °C was attributed to the pulverised TiO₂ layer that was unable to hinder the diffusion of oxygen into the coating. It is therefore recommended to apply a protective gas during the preheating of the TiBN-coated die for aluminium extrusion. Alternatively, an advanced TiBN coating with enhanced resistance to oxidation must be developed, which will be conducive to its application for aluminium extrusion dies.     

Effect of CrB2 addition on densification, properties and oxidation resistance of TiB2

This paper presents the results of detailed studies carried out on the densification of TiB₂ with CrB₂ as sinter additive by hot pressing. The dense compacts were characterized by measurement of hardness, indentation fracture toughness, flexural strength, coefficient of thermal expansion and electrical resistivity. Oxidation characteristics were investigated between 600 °C and 1000 °C and isothermal oxidation kinetics at 850 °C. Phase identification and surface morphology analysis of hot pressed and oxidized samples were done using XRD and SEM. A high density of 96.61% Τ.D was obtained with the addition of 2.5% CrB₂ by hot pressing at 1750 °C under 35 MPa pressure. Hardness values of composites with 2.5–10% CrB₂ were close to 24 GPa and fracture toughness in the range of 3–5 MPa m^1/2. Coefficient of thermal expansion of the composite with 10% CrB₂ was measured in the range of 6.21–7.43 × 10⁻⁶/K from room temperature to 1000 °C. Electrical resistivity of TiB₂ + 10%CrB₂ was measured as 32.83, 75.97 and 120 μΩ cm at 25 °C, 500 °C and 900 °C, respectively. Observed nature of oxidation was parabolic for all composites. Formation of continuous and thick glassy film was observed with increased CrB₂ content in the composite. TiO₂ and CrBO₃ phases were identified on the oxidized surface which are responsible for the improved oxidation resistance of this composite.     

Synthesis of ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders by low-energy milling and subsequent carbothermal reduction–nitridation reaction

Ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders with globular-like particles of 50–300 nm were synthesized at static nitrogen pressure from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation (CRN) techniques. Reaction path of the (Ti, W, Mo, V)(C, N)–Ni system was discussed by X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG–DSC), and microstructure of the milled powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that CRN reaction has been enhanced by nano-TiO₂ and nano-carbon powders. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N)–20Ni is at only 1300 °C for 1 h. During synthesizing reaction, Ni solid solution phase forms at about 700 °C and reduction–carbonization of WO₂ and MoO₂ occurs below 900 °C. The reactions of TiO₂ → Ti₃O₅, Ti₃O₅ → Ti(C, O) and Ti(C, O) → Ti(C, N) take place at about 930 °C, 1203 °C and 1244 °C, respectively.     

Low temperature sintering and properties of CaO–B2O3–SiO2 system glass ceramics for LTCC applications

The P₂O₅ + ZnO, ZrO₂ + TiO₂, B₂O₃ and a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) are selected as the sintering additives, and the effect of their additions on the microwave dielectric properties, mechanical properties and microstructures of CaO–B₂O₃–SiO₂ system glass ceramics is investigated. It is found that the sintering temperature of pure CBS glass is higher than 950 °C and the sintering range is about 10 °C. With the above additions, the glass ceramics can be sintered between 820 °C and 900 °C. The dielectric properties of the samples are dependent on the additions, densification and microstructures of sintered bodies. The major phases of this material are CaSiO₃, CaB₂O₄ and SiO₂. With 10 wt% B₂O₃ and LG glass additions, the CBS glass ceramics have better mechanical properties, but worse dielectric properties. The _r values of 6.51 and 7.07, the tan δ values of 0.0029 and 0.0019 at 10 GHz, are obtained for the CBS glass ceramics sintered at 860 °C with 2 wt% P₂O₅ + 2 wt% ZnO and 2 wt% ZrO₂ + 2 wt% TiO₂ additions, respectively. This material is suitable to be used as the LTCC material for the application in wireless communications.     

A sol-gel route to nanocrystalline TiN coated cubic boron nitride particles

For the purpose of increasing microstructural homogeneity and enhancing the reinforcement-matrix interfacial area, cubic Boron Nitride, cBN particles were coated by nanocrystalline TiN by a sol-gel route that required neither the need for pH adjustment nor the use of surfactants or additives. Uniform shells of amorphous titania having thicknesses in the nanometers scale were formed on the surface of the cBN particles by hydrolysis and condensation reactions of titanium (IV) isopropoxide. The amorphous coated cBN powder was nitrided to crystalline TiN coated cBN by treating in NH₃ gas at 900 °C. After nitridation the amorphous layer was completely converted to nanocrystalline TiN particles that uniformly covered the surface of cBN. Changes in the TiO_x coated layer thickness and the size of the TiN particles were investigated as a function of alkoxide content. TiO₂ nanoparticles were synthesized using the same reaction conditions, but without the presence of cBN. These nanoparticles were calcined in air at different temperatures (250-700 °C) and then nitrided at 900 °C. The nitridation behavior of TiO₂ nanoparticles was studied as a function of calcination temperature.     

Thermo-physical and thermal cycling properties of plasma-sprayed BaLa2Ti3O10 coating as potential thermal barrier materials

Increased turbine inlet temperature in advanced turbines has promoted the development of thermal barrier coating (TBC) materials with high-temperature capability. In this paper, BaLa₂Ti₃O₁₀ (BLT) was produced by solid-state reaction of BaCO₃, TiO₂ and La₂O₃ at 1500 °C for 48 h. BLT showed phase stability between room temperature and 1400 °C. BLT revealed a linearly increasing thermal expansion coefficient with increasing temperature up to 1200 °C and the coefficients of thermal expansion (CTEs) are in the range of 1 × 10^− 5–12.5 × 10^− 6 K^− 1, which are comparable to those of 7YSZ. BLT coatings with stoichiometric composition were produced by atmospheric plasma spraying. The coating contained segmentation cracks and had a porosity of around 13%. The microhardness for the BLT coating is 3.9–4.5 GPa. The thermo-physical properties of the sprayed coating were investigated. The thermal conductivity at 1200 °C is about 0.7 W/mK, exhibiting a very promising potential in improving the thermal insulation property of TBC. Thermal cycling result showed that the BLT TBC had a lifetime of more than 1100 cycles of about 200 h at 1100 °C. The failure of the coating occurred by cracking at the thermally grown oxide (TGO) layer due to severe oxidation of bond coat. Based on the above merits, BLT could be considered as a promising material for TBC applications.     

Production of nanoscale Ba(Zn1/3Nb2/3)O3 microwave dielectric ceramics by polymerised complex method

Ba(Zn_1/3Nb_2/3)O₃ nanoparticles have been synthesized by a polymerised complex method by using precursor materials of barium nitrate, zinc acetate, niobium oxide, hydrofluoric acid and citric acid. Thermal decomposition characteristics and crystallization behavior of the powders were investigated by the thermogravimetric and differential thermal analysis, X-ray diffractometer and Fourier transform infrared spectroscopy. Ba(Zn_1/3Nb_2/3)O₃ phase started to form at low temperature of 400 °C and, single phase Ba(Zn_1/3Nb_2/3)O₃ perovskite structure was obtained at 1000 °C. Microstructural investigation revealed that the major particle size of Ba(Zn_1/3Nb_2/3)O₃ nanoparticles were in the range of 80–110 nm with spherical morphology and homogeneous size distribution. But the powders also contained some agglomeration.     

Self-assembled polyaniline nanotubes and nanoribbons/titanium dioxide nanocomposites

Self-assembled polyaniline (PANI) nanotubes, accompanied with nanoribbons, were synthesized by the oxidative polymerization of aniline with ammonium peroxydisulfate in an aqueous medium, in the presence of colloidal titanium dioxide (TiO₂) nanoparticles of 4.5 nm size, without added acid. The morphology, structure, and physicochemical properties of the PANI/TiO₂ nanocomposites, prepared at various initial aniline/TiO₂ mole ratios, were studied by scanning (SEM) and transmission (TEM) electron microscopies, FTIR, Raman and inductively coupled plasma optical emission (ICP-OES) spectroscopies, elemental analysis, X-ray powder diffraction (XRPD), conductivity measurements, and thermogravimetric analysis (TGA). The electrical conductivity of PANI/TiO₂ nanocomposites increases in the range 3.8 × 10^?4 to 1.1 × 10^?3 S cm^?1 by increasing aniline/TiO₂ mole ratio from 1 to 10. The morphology of PANI/TiO₂ nanocomposites significantly depends on the initial aniline/TiO₂ mole ratio. In the morphology of the nanocomposite synthesized using aniline/TiO₂ mole ratio 10, nanotubes accompanied with nanosheets prevail. The nanocomposite synthesized at aniline/TiO₂ mole ratio 5 consists of the network of nanotubes (an outer diameter 30–40 nm, an inner diameter 4–7 nm) and nanorods (diameter 50–90 nm), accompanied with nanoribbons (a thickness, width, and length in the range of 50–70 nm, 160–350 nm, and ～1–3 μm, respectively). The PANI/TiO₂ nanocomposite synthesized at aniline/TiO₂ mole ratio 2 contains polyhedral submicrometre particles accompanied with nanotubes, while the nanocomposite prepared at aniline/TiO₂ mole ratio 1 consists of agglomerated nanofibers, submicrometre and nanoparticles. The presence of emeraldine salt form of PANI, linear and branched PANI chains, and phenazine units in PANI/TiO₂ nanocomposites was proved by FTIR and Raman spectroscopies. The improved thermal stability of PANI matrix in all PANI/TiO₂ nanocomposites was observed.     

Enhanced hydrogen sorption properties in the LiBH4-MgH2 system catalysed by Ru nanoparticles supported on multiwalled carbon nanotubes

The LiBH₄-MgH₂ system has a high reversible hydrogen storage capacity. However, the hydrogen de/absorption kinetics has to be further enhanced for its practical application. Motivated by the possibility that the metal catalysts facilitating the dissociation and combination of hydrogen molecules and activating Mg-H and B-H bonds, a novel catalyst, ruthenium nanoparticles supported on multiwalled carbon nanotubes (Ru/C) is prepared and its effect on the hydrogen sorption properties of LiBH₄-MgH₂ systems is investigated. The experimental results show that the Ru/C catalyst is active in reducing the dehydrogenation temperature and enhancing the dehydrogenation kinetics. Furthermore, the reversible capacity is also markedly enhanced under moderate conditions, and the catalytically enhanced hydrogen absorption capacity persists well during three de/rehydrogenation cycles.     

Oxidation mechanism and resistance of ZrB2–SiC composites

The oxidation mechanism of ZrB₂–SiC composites was investigated based on a combination of theory and experiments. The oxidation reactions, microstructure evolution, scale stability and temperature limit were examined in our research and a good correspondence was obtained between theoretical predictions and experimental results. Microstructure evolution and stability are significantly dependent on both temperature and composition. SiO₂ is thermochemically stable below 1800 °C and will lose its protective properties at temperatures above 2300 °C. The temperature limit for ZrB₂–SiC composites is strongly dependent on the vapor pressure of the gaseous products and volume content of ZrB₂.     

Polymer-induced generation and characterization of electrophoretic properties of hollow TiOX nanospheres for electronic paper

Hollow TiO_X nanospheres have been successfully prepared using hollow core–double shell latex particles (poly(styrene-co-methyl methacrylate-co-butyl acrylate-co-methacrylic acid) (abbreviated in poly(St-co-MMA-co-BA-co-MAA)) as template, which involves the deposition of inorganic coating on the surface of hollow core–shell latex particles and subsequent removal of the latex by calcinations in air or ammonia gas. Ti(OBu)₄ was used as precursor for the preparation of hollow TiO_X nanospheres. TEM of white hollow core–double shell polymers particles with an aperture of approximately 225 nm displays the perfect characteristic hollow nanospheres structure of primary core–double shell particles. The formation of TiO_X was confirmed by XRD analysis and hollow structure of the particles was revealed by transmission electron microscopy (TEM). When the calcined temperature was at 800 °C, hollow TiO₂ nanospheres were arranged regularly with the diameter range of 130–170 nm. The electrophoretic properties were characterized by JS94J micro-electrophoresis apparatus. The electrophoretic mobility of white TiO₂ and black TiO hollow spheres in tetrachloroethylene were 1.09 × 10⁻⁵ and 3.12 × 10⁻⁵ cm²/V s, and the zeta potentials were 7.10 and 20.24 mV, respectively. The results show that white TiO₂ particles and black TiO hollow nanoparticles are suitable as electrophoretic particles and possess the application potential in the future electrophoretic display.     

Charge transport and recombination in dye-sensitized solar cells based on hybrid films of TiO2 particles/TiO2 nanotubes

In this paper, anodic TiO₂ nanotubes are blended into the TiO₂ mesoporous films based on P25 nanoparticles to assemble a list of dye-sensitized solar cells (DSSCs) with different nanotube concentrations. The electron properties of transport and recombination in the fabricated DSSCs are studied by using electrochemical impedance spectroscopy and the open-circuit voltage decay technique under AM 1.5 illumination. Results indicate that the electron lifetime increases with increasing the concentration of the anodic TiO₂ nanotubes, the electron transport time at a blending level of 10 wt% TiO₂ nanotubes is short as compared to that at 0 wt%, and above 10 wt%, the electron transport time has a trend of becoming large. Due to the combining effects of the electron transport and recombination, the electron collecting efficiency and the electron diffusion length obtain maxima at a blending level of 10 wt% nanotubes, which results in a highest short circuit current and a maximum energy conversion efficiency at this point in the DSSCs. This study gives a clear explanation for the performance enhancement of TiO₂ particle-based DSSCs at a blending level of 10 wt% anodic TiO₂ nanotubes and for the performance decrease at a blending level over 10 wt% anodic TiO₂ nanotubes from the angle of the electron transport and recombination. This study also supplies a feasible and easy way to improve the performance of particle-based DSSCs by restraining electron recombination and accelerating electron transportation.     

New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

On the four-phase reactions in the Ti–Ni–Al system

Two four-phase reactions of transition type in the Ti–Ni–Al system were studied on several alloys, which were annealed at carefully set temperatures and quenched. The phase constitution was established by XRD and EPMA analyses. Due to sluggish reaction kinetics, the transition temperatures were defined by annealing and quenching techniques as no DTA signals could be received. For the reaction NiAl + TiNiAl  TiNiAl₂ + TiNi₂Al, the transition temperature was found to be 925 °C ± 15 °C and for the reaction TiNiAl + Ti₃NiAl₈  TiAl₂ + TiNiAl₂, the transition temperature was found to be 990 °C ± 15 °C. Furthermore we confirmed the three-phase field TiNi₂Al + Ti₃Al + Laves phase (TiNiAl), as reported at 900 °C by Huneau et al. in 1999.     

Cyclic oxidation of Cr2AlC between 1000 and 1300 °C in air

Fully dense, monolithic ternary Cr₂AlC compounds were synthesized via a powder metallurgical route, and their cyclic oxidation behavior was investigated between 1000 and 1300 °C in air for up to 100 h. At 1000 and 1100 °C, Cr₂AlC displayed excellent cyclic oxidation resistance by forming a less than 5 μm-thick Al₂O₃ oxide layer and a narrow Cr₇C₃ underlayer. At 1200 and 1300 °C, an outer (Al₂O₃, Cr₂O₃)-mixed oxide layer, an intermediate Cr₂O₃ oxide layer, an inner Al₂O₃ oxide layer, and a Cr₇C₃ underlayer formed on the surface. From 1200 °C, scale cracking and spalling began to occur locally to a small extent. At 1300 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids and the spallation of the scales.     

Investigation of physicochemical transformation at mechanochemical, hydrothermal and microwave treatment of barium titanyloxalate

The effect of mechanochemical (in air and water) as well as hydrothermal and microwave treatment on physicochemical transformations of barium titanyloxalate was studied. The samples were examined using XRD and thermal analysis, FTIR spectroscopy, argon thermodesorption and granulometry. Barium metatitanate is already formed during mechanochemical treatment of barium titanyloxalate in air atmosphere and the following thermal treatment at 550 °C improves its crystal structure. At the same time barium metatitanate is only formed at 800 °C in the case of usual thermal decomposition of barium titanyloxalate.Hydrothermal and microwave treatment of barium titanyloxalate promotes formation of the barium orthotitanate (Ba₂TiO₄) and barium carbonate (BaCO₃) mixture.     

Effects of temperature on the protective property, structure and composition of the oxide film on Alloy 625

The paper mainly investigated the protective property, structure and composition of the oxide film on Alloy 625 in a lithium borate buffer solution (pH_300°C = 6.93) in the temperature range of 25–300 °C. The methods used were electrochemical measurements and XPS analysis. As temperature increased, the protective property of the oxide film degraded, and the structure varied from a singe-layer to double-layer. The oxide film consisted of Cr₂O₃ and Cr(OH)₃ at 25 and 150 °C, while it contained Ni(OH)₂, in addition to Cr₂O₃ and Cr(OH)₃ at 250 and 300 °C. This was mainly attributed to the temperature-induced variation of composition and protective property of the barrier layer.     

Thermal conductivity of titanium dioxide films grown by metal-organic chemical vapor deposition

We studied the effect of the microstructures on the thermal conductivity of the titanium dioxide (TiO₂) films. TiO₂ films were grown by MOCVD, their morphologies were observed using a scanning electron microscope (SEM). The chemical composition was determined through Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) measurements. The thermal conductivity of the in-plane direction was measured using an alternating current calorimetric method (laser-heating Angstrom method) in the temperature range of 300 to 470 K. The authors fabricated a TiO₂ film with extremely low thermal conductivity (~ 0.5 Wm^− 1 K^− 1), in which a feather-like texture is regularly arranged in the direction perpendicular to the heat flow. The origins of the extremely low thermal conductivity were studied from a microstructural viewpoint.