Synthesis and characterization of bulk and coatings of hydroxyapatite using methanol precursor

Hydroxyapatite, an important bioceramic was synthesized in the bulk form and developed as a coating by a sol-gel route using alcoholic precursor. The bioactive coating was developed on bio-inert α-alumina and yttria stabilized zirconia substrates. The apatite phase began to form after the heat treatment of the precursor at 500 °C for 10 min. The complete crystallization of the apatite was obtained at 800 °C heat treatment for 10 min. The phase composition of the bulk and the coatings was identified by FT-IR spectroscopic and powder X-ray diffraction (XRD) techniques. Surface morphology was determined by scanning electron microscopy. The study indicates different surface textures for the powder and for the coatings on α-alumina and yttria stabilized zirconia substrates.     

A novel process for the synthesis of lithium aluminum silicate powders from rice husk ash and other water-based precursor materials

Lithium aluminum silicate (LAS) powders were synthesized through sol-gel technique by utilizing agro-based ‘waste’ material—rice husk ash and other water-based precursor materials. The gel and oxide powders were characterized by thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). DTA, XRD and FTIR results confirmed that crystallization of lithium aluminum silicate in the form of β-eucryptite and β-spodumene took place at about 800 °C. Furthermore, XRD and FTIR results showed that a substantial crystallization of β-eucryptite for the composition Li₂O.Al₂O₃.2SiO₂ (LA2S) and that of β-spodumene for the composition Li₂O.Al₂O₃.4SiO₂ (LA4S) occurred at 1000 °C. A cobble-stone-like shaped particles of β-eucryptite and β-spodumene were examined by SEM.     

Effect of post vacuum heating on the microstructural feature and bonding strength of plasma-sprayed hydroxyapatite coatings

Plasma-sprayed HA coatings (HACs) on Ti−6Al−4V substrate with post vacuum heating treatment were employed to improve the properties of HACs. The heating temperatures are 400, 500, 600, 700 and 800 °C. According to the experimental results, post vacuum heating treatment could improve the average bonding strength and the minimum strength of HACs by using Weibull's reliability analysis. The HACs with 600 °C vacuum heating shows the highest strength value of all. On the other hand, the index of crystallinity (IOC) of heat-treated HACs increases and impurity phase content decreases with increasing heating temperature. The crystallization during vacuum heating also leads the variation in thermal expansion coefficient and the volume contraction of heat-treated HACs, especially at higher than that of 600 °C vacuum heating. Therefore, the volume contraction induces apparent and concentrated large cracks and reduces the bonding strength of heat-treated HACs.     

Effect of calcination condition on phase formation and particle size of lead titanate powders synthesized by the solid-state reaction

A perovskite-like phase of lead titanate, PbTiO₃, has been synthesized by a solid-state reaction via a rapid vibro-milling technique. Phase formation of the calcined powders has been investigated as a function of calcination temperature, soaking time and heating/cooling rates by DTA and X-ray diffraction (XRD) techniques. Moreover, morphology and particle size evolution have been determined via SEM technique, respectively. It has been found that single-phase PbTiO₃ powders were successfully obtained for calcination conditions of 550 °C for 4 h or 600 °C for 1 h with heating/cooling rates of 20 °C/min. Higher temperatures clearly favoured particle growth and the formation of large and hard agglomerates.     

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

Increase of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La₂Zr₂O₇) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.     

Silver diffusion and high-temperature lubrication mechanisms of YSZ–Ag–Mo based nanocomposite coatings

Yttria-stabilized zirconia (YSZ) nanocomposite coatings consisting of silver and molybdenum were produced by a hybrid process of filtered vacuum arc, magnetron sputtering and pulsed laser depositions for tribological investigations at different temperatures. The coatings with 24 at.% Ag and 10 at.% Mo contents showed a friction coefficient of 0.4 or less for all temperatures from 25 to 700 °C. The wear scar surfaces and coating cross-sections were studied using scanning electron, transmission electron, scanning transmission electron and focused ion beam microscopes, which also provided the information on chemical composition distributions of silver and molybdenum along with microstructure features. It was demonstrated that silver diffusion and coalescence on surfaces played an important part in the high-temperature lubrication mechanism of the YSZ–Ag–Mo coatings. Silver was found to be an effective lubricant at temperatures below 500 °C and its coalescence on the surface isolated molybdenum inside coatings from ambient oxygen. Lubricious oxides of molybdenum were formed and lubricated at temperatures above 500 °C when the silver was worn off the contact surface. For silver containment inside the coating at high temperatures, a multilayer architecture was built by inserting a TiN diffusion barrier layer in the composite coatings. Microscopic observations showed that this barrier layer prevented silver exit to the coating surface. At the same time, this enabled a subsequent lateral lubricant supply toward a wear scar location where the diffusion barrier layer was worn through and/or for a next thermal cycle. The multilayer coating maintained a friction coefficient of 0.4 or less for more than 25,000 cycles, while the monolithic coating lasted less than 5000 cycles. In addition, a TiN surface barrier layer with pinholes was deposited on the YSZ–Ag–Mo composite surface to control vertical silver diffusion. With this coating design, the coating wear lifetime was significantly increased beyond 50,000 cycles.     

Effect of ageing on martensitic transformation in NiTi shape memory alloy

The shape memory effect in Ni-49 Ti alloy after solution treatment at 800° C for 0.5 h and ageing at 400, 500, 600and 700° C for 1 h has been investigated by means of transmission electron microscopy and differential thermal analysis (DTA) measurement. Ageing at 400 and 500° C for 1 h causes the precipitation of characteristic temperatures towards higher values. In sample aged at 500° C three distinct DTA peaks arise giving evidence of intermediate stages of the martensite transformation.     

Hydrothermal-electrochemical codeposited hydoxyapatite/yttria-stabilized zirconia composite coating

Hydroxyapatite(HA)/yttria-stabilized zirconia(YSZ) composite coatings were deposited on titanium substrates using a hydrothermal electrochemical method in an electrolyte containing calcium, phosphate ions and YSZ particles. HA/YSZ composite coatings were prepared in different conditions with different electrolyte temperatures(100 ∼ 200°C), current densities(0.1 ∼ 10.0 mA/cm²), and particles content in bath(0 ∼ 100 g/L). The effect of YSZ additions on the phase composition, microstructure, thermal stability, corrosion behavior and the bonding strength of HA/YSZ composite coatings were studied. The results show that crystallinity of HA in HA/YSZ composite coatings increase continuously with the electrolyte temperature and close to stoichiometric HA. The n(Ca)/n(P) ratio at 200°C is about 1.67 according with stoichiometric HA. YSZ particles are imbedded uniformly between the HA crystals. The average HA crystal size are reduced owing to the additions of YSZ particles. After annealing at 1200°C, tetragonal phase YSZ tend to react with the released CaO to form cubic phase YSZ and CaZrO₃, which cause destabilization of HA to decompose into more α-TCP phase. The bonding strength between HA/YSZ composite coatings and titanium substrates increase with increasing volume content of YSZ in the composite coatings (V %). HA/YSZ composite coatings exhibit a better electrochemical behavior than pure HA coatings and uncoated Ti metals.     

Synthesis and characterization of lanthanum phosphate sol for fibre coating

Various routes to synthesize lanthanum phosphate sols (LaPO₄) were explored. Prepared lanthanum phosphate sols were characterized using differential thermal analysis/ thermogravimetric analysis (DTA/TGA) and powder X-ray diffraction (XRD) after heat treatments. Appreciable crystallization of lanthanum phosphate was observed above 400 °C and the average crystallite sizes were calculated to be 15 nm and 46 nm after heat treatment at 700 and 1100 °C, respectively. Sapphire fibres were dip coated in LaPO₄ sol, and subsequently heated to 1100 °C. The thickness of the coating ranged from 60 to 150 nm depends upon the sol concentration and the number of times of dip coatings. XRD of coated fibres revealed the formation of LaPO₄ after heat treatment above 700 °C. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis and characterization of nanocrystalline barium strontium titanate powders prepared by citrate precursor method

Nanocrystalline and well dispersed barium strontium titanate (BST) powders were prepared by a novel and simple citrate precursor method. This method involved direct crystallization of a white precursor from a stable solution in the citric acid (CA)–ethylene glycol (EG)–tetrabutyl titanate–M²⁺ (M = Ba, Sr) system under a specific pH value range. Subsequent heat treatment of the precursor at 850 °C led to a pure phase BST powder. TG/DTA was used to examine the decomposition behaviour of the precursor. The crystalline phase and morphology of the BST powders were investigated by XRD and TEM. It was found that the BST powders synthesized by citrate precursor process were more homogeneous and uniform than that obtained by the citrate gel method.     

Effects of calcination conditions on phase formation and particle size of indium niobate nanopowders synthesized by the solid-state reaction

A wolframite-type phase of indium niobate, InNbO₄, has been synthesized by a solid-state reaction via a rapid vibro-milling technique. The formation of the InNbO₄ phase in the calcined powders has been investigated as a function of calcination conditions by TG–DTA and XRD techniques. Morphology, particle size and chemical composition have been determined via a combination of SEM and EDX techniques. It has been found that single-phase InNbO₄ powders have been obtained successfully at the calcination condition of 950 °C for 2 h with heating/cooling rates of 30 °C/min. Higher temperatures and longer dwell times clearly favoured particle growth and the formation of large and hard agglomerates.     

工艺参数对EB-PVD制备YSZ涂层的影响

为探讨电子束物理气相沉积(EB-PVD)制备8 mol.%氧化钇稳定氧化锆(8YSZ)涂层过程中工艺参数对涂层致密性、表面粗糙度和晶粒择优取向生长的影响,利用扫描电镜、原子力显微镜和X射线衍射技术对涂层的上述性能进行了分析.分析结果表明,随沉积速率由750 nm/min下降至20 nm/min,YSZ涂层的晶粒逐渐聚合长大,晶粒之间的孔隙减少,涂层的气体扩散系数相应地由2.41×10-4cm4/(N·s)下降至6.56×10-5cm4/(N·s).YSZ涂层的表面粗糙度随靶基距的提高逐渐降低,涂层的晶体学取向随蒸汽粒子入射角的改变而改变,入射角为30°时(111)晶面具有平行于涂层表面排列的趋势,入射角为45°时(311)和(420)晶面具有平行于表面排列的趋势,而入射角为60°时(220)和(331)晶面具有平行于表面排列的趋势.     

Preparation of aluminosilicate precursor by mechanochemical method from gibbsite-fumed silica mixtures

Aluminosilicate (mullite) precursor was prepared by mechanochemical treatment of gibbsite and fumed silica mixtures. The effect of grinding on its structure and thermal behaviour was examined by²⁷Al and²⁹Si MAS NMR, XRD, DTA-TG and FTIR. Thermal treatment of this precursor led to the crystallization of mullite at about 1200°C via a spinel-phase.²⁷Al and²⁹Si MAS NMR spectroscopies show lowering of mullitization temperature, which is associated with increased homogeneity of the precursor. Mechanochemical treatment of gibbsite and fumed silica mixtures resulted in the formation of a more homogeneous aluminosilicate precursor.     

Thermal treatment of a Y-123 precursor prepared using the hydroxide co-precipitation method

A Y-123 precursor has been obtained using the hydroxide co-precipitation method. The influences of the calcination time and temperature (with and without sintering and annealing) on the formation of the superconducting phase, YBa₂Cu₃O_7−δ, are examined using XRD and T_c measurements. A comparison between the co-precipitated hydroxide precursor and the conventional dry precursor, obtained by mixing oxides and carbonates, is made. It shows that thermal treatment of the hydroxide precursor at 750°C already gives a superconductor, while the oxide–carbonate precursor is only superconducting after thermal treatment at 850°C.     

Synthesis and characterization of sol-gel derived TiO<Subscript>2</Subscript> thin films: Effect of different pretreatment process

Titanium dioxide (TiO₂) thin films have been successfully synthesized deposited on glass substrates by the sol-gel dip-coating method through different pretreating processes, including heated at 100, 500°C, via freeze drying, microwave heating for 10 min and subsequently annealed at 500°C for 2 h. The as-synthesized TiO₂ films were characterized using X-ray diffraction (XRD), atomic force microscopy (AFM) and ultravioletvisible (UV-vis) absorption spectra analysis technology. The preparation of the precursor sols and TiO₂ films were described in detail. Effects of 100, 500°C, freeze drying and microwave heating pretreatment on crystalline structure, surface morphology, roughness, particle size, optical property and electronic transition of TiO₂ thin films have been primarily investigated. The XRD results demonstrate that the TiO₂ films were well-crystallized and consisted of anatase TiO₂ phase only with (101) plane. The average crystalline size is only about 15 nm at 100°C pretreatment and the absorption edge shifts to shorter wavelength comparing with that at 500°C, freeze drying and microwave heating pretreatment. Pretreatment process is important during the preparation of thin films and has obviously effect on the structure and optical property of TiO₂ films due to the different heating mechanisms.     

Effect of high heating rate on thermal decomposition behaviour of titanium hydride (TiH2) powder in air

DTA and TGA curves of titanium hydride powder were determined in air at different heating rates. Also the thermal decomposition behaviour of the aforementioned powder at high heating rates was taken into consideration. A great breakthrough of the practical interest in the research was the depiction of the P _H2-time curves of TiH ₂ powder at various temperatures in air. In accordance with the results, an increase in heating rate to higher degrees does not change the process of releasing hydrogen from titanium hydride powder, while switching it from internal diffusion to chemical reaction. At temperatures lower than 600 °C, following the diffusion of hydrogen and oxygen atoms in titanium lattice, thin layers TiH _x phase and oxides form on the powder surface, controlling the process. On the contrary, from 700 °C later on, the process is controlled by oxidation of titanium hydride powder. In fact, the powder oxidation starts around 650 °C and may escalate following an increase in the heating rate too.     

Synthesis of spodumene and spodumene — zirconia composite powders using aqueous sol-gel method

Pure spodumene and spodumene-zirconia (5, 10, 15 mol%) composite powders were prepared using aqueous sol-gel method employing lithium formate, aluminium formate, zirconium formate and tetraethoxy silane (TEOS) as starting materials in aqueous medium. The gels prepared by this method were dried at 100°C for 24h and then calcined for 2h at different temperatures ranging from 500°C to 800°C. X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis (TGA) and infra-red spectroscopy analysis (IR) were utilized to characterize the gel powders and calcined powders. Transmission electron microscope (TEM) was used to measure the average particle size of the calcined powders.     

Phase transformation and thermal stability of ZnSe QDs due to annealing: emergence of ZnO

In the present study ZnSe quantum dots (QDs) were synthesized by chemical co-precipitation method using mercaptoethanol as the capping agent. These nanostructures were characterized for structure and surface morphology by using X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrometry respectively. The average size of ZnSe quantum dots from XRD and HRTEM is found to be 4–5 nm having cubic crystalline phase. Effect of annealing temperature, which were determined as phase transformation temperatures (299 and 426 °C) using differential scanning calorimetry has been investigated for structural and thermal stability of QDs. The XRD of annealed samples at temperatures 325 and 442 °C (slightly higher than the temperatures corresponding to two crystallization peaks in the DSC scan) have been carried out to find structural changes corresponding to these annealing temperatures. Sample annealed at 325 °C showed no change in the phase except improvement in intensity of peaks (crystallinity) whereas sample annealed at 442 °C showed transitions from cubic phase of ZnSe to ZnO and orthorhombic phases of ZnSeO₄, ZnSeO₃. Emergence of ZnO peaks in the XRD pattern of annealed samples have been further verified by Raman spectroscopy of the annealed samples. Besides this crystallization kinetics of ZnSe quantum dots has been employed to determine activation energies of these transitions due to oxidation by employing Kissinger, Augis Bennett and Ozawa’s models. Higher activation energy of crystallization corresponding to first crystallization peak shows that the cubic phase is more thermally stable.     

Structural and electrical properties of SiO<Subscript>2</Subscript>–Li<Subscript>2</Subscript>O–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> glass and glass-ceramics obtained by thermoelectric treatments

Glass and glass-ceramics with the molar composition of 60SiO₂–30Li₂O–10Nb₂O₅ (mole %) were studied. Ferroelectric lithium niobate (LiNbO₃) nanocrystals were precipitated in the glass matrix trough a thermal treatment, with and without the simultaneous application of an external electric field. The as-prepared sample, yellow and transparent, was heat-treated (HT) at 600 and 650 °C and thermoelectric treated (TET) at 600 °C. The applied electric fields were the following ones: (i) 5 × 10⁴ V/m; (ii) 1 × 10⁵ V/m. Differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and dielectric spectroscopies were used to investigate the glass samples properties. The LiNbO₃ crystalline phase was detected in the 650 °C HT sample and in the 600 °C TET samples. The presence of an external electric field, during the heating process, promotes the glass crystallization at lower temperatures. In the TET samples, the surface crystallization of the cathode and the anode are different. The number and size of the crystallites, in the glass network, dominate the electrical dc behavior while the ac conductivity process is more dependent of the glass matrix structure. The obtained results reflect the important role carried out by the temperature and the applied electric field in the glass-ceramic structures.     

Entwicklung von Oxid – Keramik zur Anwendung als Wärmedämmschichten

Development of Oxide Ceramics for an Application as TBC The standard thermal barrier coating material yttria stabilised zirconia (YSZ) is limited in long term operation to a maximum temperature of about 1200°C. As a result further increase of the gas inlet temperature and hence the efficiency of gas turbines are hardly to achieve with YSZ coatings. In a screening procedure especially perowskite (ABO₃, A = Sr,Ba, B = Zr) and pyrochlore (A₂B₂O₇, e.g. A = La and other rare earth elements, B = e.g. Zr) materials have been identified as possible candidates for thermal barrier coatings. Basic physical properties (e.g. thermal expansion coefficient, thermal diffusivity and conductivity) of several candidates have been determined using sintered, dense samples. The possibility of optimization of the properties by using specific compositions will be discussed. From promising materials powders which are suitable for plasma‐spraying have been produced by spray‐drying. New TBC systems consisting of new materials (BaZrO₃, La₂Zr₂O₇) deposited by atmospheric plasma spraying and vacuum plasma sprayed MCrAlY bondcoats were tested in a gas burner facility. Especially La₂Zr₂O₇ coatings gave promising results. A further improvements could be achieved by the use of layered or graded coatings with a YSZ coating at the bondcoat interface and on top a layer of the new TBC material. First results of thermal cycling tests with 1250 and 1350°C surface temperature will be presented.