Elaboration and dielectric study of ferroelectric or relaxor ceramics in the ternary system BaTiO3-NaNbO3-BaSnO3

The present work reports the elaboration and physical investigation of new compounds of the following composition Ba_1−xNa_x(Ti_1−ySn_y)_1−xNb_xO₃ (BTSnNxy). The studied ternary system presents some continuous solid solutions between the next 3 phases: the NaNbO₃ antiferroelectric phase that becomes easily ferroelectric at low rate substitutions, the BaTiO₃ ferroelectric phase and the paraelectric stannate phase BaSnO₃. Two different dielectric behaviors can be observed once some substitutions are made either in A or B sites of an ABO₃ perovskite. These substitutions modify the dielectric properties of the material. The introduction of Sn⁴⁺ and Ti⁴⁺ cations in the B site favors, respectively, a decrease of the transition temperature and an increase in the value of the real dielectric permittivity. The transition temperature should be modulated by varying the rate of cationic substitution. Some relaxor materials can be obtained at a temperature around room temperature.     

Formation of Na3AlH6 from a NaH/Al mixture and Ti-containing catalyst

To investigate the effects of Ti-containing catalyst on the hydride formation in the Na–Al–H system, Ti(OBuⁿ)₄- and TiF₃-doped NaH/Al powders were hydrogenated into Na₃AlH₆ by controlling the hydrogen pressure at 5 MPa and the temperature at 100–140 °C. X-ray diffraction and differential scanning calorimetry showed that TiF₃ was more catalytically favorable than Ti(OBuⁿ)₄ to enhance the formation of Na₃AlH₆. X-ray absorption spectroscopy revealed that the initial Ti⁴⁺ or Ti³⁺ ions were reduced to Ti¹⁺ or Ti⁰ and form Ti–Al pairs with two coordination shells during the doping and hydrogenation processes. The difference in catalytic activity between the two Ti-containing catalysts can be attributed to their ability to form Ti–Al pairs with the NaH/Al mixture.     

Stabilization of supported platinum nanoparticles on γ-alumina catalysts by addition of tungsten

The thermal stabilization of Al₂O₃ using W⁶⁺ ions has been found useful to the synthesis of Pt/Al₂O₃ catalysts. The simultaneous and sequential methods were used to study the effect of W⁶⁺ upon Pt/γ-Al₂O₃ reducibility, Pt dispersion, and benzene hydrogenation. The W/Pt atomic ratios were from 0.49 to 12.4. In the first method we found that the W⁶⁺ ions delayed reduction of a fraction of Pt⁴⁺ atoms beyond 773 K. At the same time, W⁶⁺inhibited sintering of the metallic crystallites once they were formed on the surface. For the sequential sample with a W/Pt atomic ratio of 3.28 W⁶⁺ did not inhibit the H₂ reduction of Pt oxides even below of 773 K, the Pt oxides were reduced completely. After reduction at 1073 K, sequential samples impregnating Pt on WO_x–γ-Al₂O₃ were more active and stable during benzene hydrogenation. TOF of the reaction did not change when the W/Pt atomic ratio, preparation technique and reduction temperature changed and its value was of 1.1 s⁻¹. W⁶⁺ ions promoted high thermal stability of Pt crystallites when sequential catalysts were reduced at 1073 K and decreased their Lewis acidity_.     

Coatings based on niobium oxides and phosphates formed on niobium alloy

Coatings were formed by plasma electrolytic oxidation in electrolytes containing Ni(II) hexametaphosphate complexes. Depending on the n = [P₆O ₁₈ ^6? ]]/[Ni²⁺] molar ratio in the electrolyte, layers with specific niobium, phosphorus, and nickel contents and of a particular thickness were produced. Upon annealing in air at a temperature of 500 or 800°C, either Nb₂O₅, or Nb₂O₅ +NbO(PO₄) + P₂O₅ · 9Nb₂O₅, or Nb₂O₅ + NbO(PO₄), or Nb₂O₅ + Ni₂P₂O₇ crystal phases are formed in the coatings depending on the electrolyte composition and temperature of annealing. These surface systems formed on niobium for 10 min at room temperature as a result of a one-stage process in ecologically safe electrolytes can be catalytically active.     

Microwave-induced heating and sintering of metallic glasses

Metallic glasses exhibit low viscosity in a temperature range between the glass-transition and crystallization temperature which allows successful sintering of glassy powders. Microwave heating being a volumetric heating has significant advantages over conventional heating in materials processing, such as substantial energy savings, rapid heating rates and process cleanness. In the present study, we investigate the stability of the Fe₇₃Si₇B₁₇Nb₃, Fe₆₅Co₁₀Ga₅P₁₂C₄B₄, Ni_52.5Zr₁₅Nb₁₀Ti₁₅Pt_7.5, Ni₆₀Nb₂₀Ti₁₅Zr₅, Zr₅₅Cu₃₀Al₁₀Ni₅ and Ti_47.5Zr₁₀Cu₃₀Pd_7.5Sn₅ glassy powders and formation of the porous bulk metallic glassy samples by microwave heating in a single-mode cavity with separated electric (E) and magnetic (H) field maxima in an inert atmosphere. The Fe₇₃Si₇B₁₇Nb₃ and Fe₆₅Co₁₀Ga₅P₁₂C₄B₄ alloys crystallized upon MW heating in both E- and H-field maxima forming a nanostructure. The Ni_52.5Zr₁₅Nb₁₀Ti₁₅Pt_7.5 and Zr₅₅Cu₃₀Al₁₀Ni₅ alloys were heated well in H-field and were not heated well enough in E-field. In case of Ni_52.5Zr₁₅Nb₁₀Ti₁₅Pt_7.5 and Ni₆₀Nb₂₀Ti₁₅Zr₅ alloys sintered samples were obtained.     

Anodic oxidation and dielectric behaviour of aluminium-niobium alloys

The anodizing behaviour of sputtering-deposited Al-Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb₂O₅ and Al₂O₃ in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb⁵⁺, Al³⁺ and O²⁻ species, with cation transport numbers, in the range 0.31-0.35, and formation ratios, in the range 1.35-1.64 nm V⁻¹, intermediate between those of anodic alumina and anodic niobia. Al³⁺ ions migrate slightly faster than Nb⁵⁺ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb₂O₅ into anodic alumina increases the dielectric constant from about 9 to the range 11-22 for the investigated alloys.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.     

Fabrication and characteristics of strontium barium niobate/barium strontium titanate ceramics by powder–sol method

Barium strontium titanate (Ba_1-xSr_xTiO₃, BST) and strontium barium niobate (Sr_xBa_1-xNb₂O₆, SBN) are important ferroelectric materials with excellent pyroelectric, dielectric properties and faster response time of infrared radiation. SBN/BST composite ceramics with different mole ratios of Nb and Ti were fabricated using a powder-sol (P-S) method with Nb₂O₅ fine powders suspended in the barium strontium titanate (BST in short) sol solution. The X-ray diffraction results indicate that three intermediate phases, i.e. TiO₂, BaNb₂O₆ and SrNb₂O₆, are developed during the formation of SBN-BST. Powders obtained from dried gels are calcined at 800 ? for 3 h. The Ti_2p spectra of only one spin-orbit doublet are observed, which indicates one 4+ chemical state in the composite ceramics. The binding energies of Nb element depend strongly on composition.     

Thermodynamic modeling of the Nb–B system

In the present work, the Nb–B binary system was thermodynamically optimized. The stable phases in this system are BCC (niobium), Nb₃B₂, NbB, Nb₃B₄, Nb₅B₆, NbB₂, B (boron) and liquid L. The borides Nb₃B₂, NbB, Nb₃B₄ and Nb₅B₆ and the B (boron) were modeled as stoichiometric phases and the liquid L, BCC (niobium) and NbB₂ as solutions, using the sublattices model, with their excess terms described by the Redlich–Kister polynomials. The Gibbs energy coefficients were optimized based on the experimental values of enthalpy of formation, low temperature specific heat, liquidus temperatures and temperatures of invariant transformations. The calculated Nb–B diagram reproduces well the experimental values from the literature.     

New data on phase equilibria in the Nb-rich region of the Nb-B system

The currently accepted Nb-B phase diagram shows Nb_ss (solid solution), Nb₃B₂, NbB, Nb₅B₆, Nb₃B₄, NbB₂, B, and liquid L as the stable phases in this system. There is a general agreement in the literature about the stability of the NbB, Nb₃B₄, and NbB₂ phases. However, the stability of Nb₃B₂, Nb₅B₆, and Nb₂B₃ phases is arguable. The aim of this work was to reevaluate the phase equilibria in the Nb-rich region (0-50at.% B) of the Nb-B system. The alloys were arc melted from high purity materials and heat-treated at 1700 °C under high vacuum. The samples were characterized by scanning electron microscopy/back-scattered electron image (SEM/BSE) and x-ray diffraction (XRD). The most important findings were: (1) no liquid formation was observed during heat-treatments of the alloys at 1700 °C; (2) the eutectic reaction in the Nb-rich region is L ↔ Nb_ss+ NbB with liquid eutectic composition close to 16 at.%B; and (3) the Nb₃B₂-phase is formed through the peritectoid reaction Nb_ss+ NbB ↔ Nb₃B₂. These results support the phase diagram proposed by Rudy [1969 Rud] for the Nb-rich region, which is not in agreement with the currently accepted Nb-B phase diagram.     

Effect of lattice defects on thermal conductivity of Ti-doped,Y2O3-stabilized ZrO2

The evolution of lattice structure and thermal conductivity has been studied systematically for a range of Ti-doped, Y₂O₃-stabilized ZrO₂ (YSZ) solid solutions. The mechanism of reducing the thermal conductivity by Ti doping has been determined. Ti⁴⁺ mainly substitutes for Zr⁴⁺ below a critical composition factor (x ? 0.08), above which the interstitial Ti⁴⁺ need to be considered separately. The effect of lattice defects caused by mass and radius differences between Ti⁴⁺ and Zr⁴⁺ ions on the phonon scattering coefficient was discussed quantitatively. And the reduction of oxygen vacancy by interstitial Ti⁴⁺ ions which increases the thermal conductivity at high Ti doping content was also determined. Concerning the integrated phase stability and thermo-mechanical properties, Ti-doped YSZ is believed to be a promising candidate for thermal barrier coatings at higher temperature.     

New data on phase equilibria in the Nb-rich region of the Nb-B system

The currently accepted Nb-B phase diagram shows Nb_ss (solid solution), Nb₃B₂, NbB, Nb₅B₆, Nb₃B₄, NbB₂, B, and liquid L as the stable phases in this system. There is a general agreement in the literature about the stability of the NbB, Nb₃B₄, and NbB₂ phases. However, the stability of Nb₃B₂, Nb₅B₆, and Nb₂B₃ phases is arguable. The aim of this work was to reevaluate the phase equilibria in the Nb-rich region (0-50at.% B) of the Nb-B system. The alloys were arc melted from high purity materials and heat-treated at 1700 °C under high vacuum. The samples were characterized by scanning electron microscopy/back-scattered electron image (SEM/BSE) and x-ray diffraction (XRD). The most important findings were: (1) no liquid formation was observed during heat-treatments of the alloys at 1700 °C; (2) the eutectic reaction in the Nb-rich region is L ↔ Nb_ss+ NbB with liquid eutectic composition close to 16 at.%B; and (3) the Nb₃B₂-phase is formed through the peritectoid reaction Nb_ss+ NbB ↔ Nb₃B₂. These results support the phase diagram proposed by Rudy [1969 Rud] for the Nb-rich region, which is not in agreement with the currently accepted Nb-B phase diagram.     

Thermal properties of zirconia co-doped with trivalent and pentavalent oxides

Zirconia doped with 6–8 wt% (3.2–4.2 mol%) yttria (6–8YSZ), the most common thermal barrier coating material, relies mostly on oxygen vacancies to provide the phonon scattering necessary for low thermal conductivity. The present study examines whether specific substitutional defects—in addition to, or instead of, oxygen vacancies—can provide similar or greater reductions in conductivity. To this end a series of zirconia samples co-doped with varying levels of yttrium (trivalent) and tantalum/niobium (pentavalent) oxides were synthesized, thereby allowing oxygen vacancy and substitutional atom concentration to be varied independently. The results show that Nb–Y and Ta–Y co-doped zirconia samples containing only substitutional defects produce stable single-phase tetragonal materials with thermal conductivities very close to that of the conventional 6–8YSZ. In these samples, Nb⁵⁺ and Td⁵⁺ are similarly effective in lowering thermal conductivity, in contradiction to phonon scattering theories that consider primarily mass effects and thereby predict significantly greater conductivity reduction due to Ta⁵⁺ doping than Nb⁵⁺ doping. Finally, Nb⁵⁺/Ta⁵⁺–Y³⁺ doped samples, which contain both oxygen vacancies and substitutional defects, are found not to be stable in single-phase form; however, the thermal conductivities of the two-phase tetragonal+cubic mixtures are again as low as that of the conventional 6–8YSZ.     

Study of the role of Hf,Mo and W additions in the microstructure of Nb–20Si silicide based alloys

The effects of Hf, Mo and W on the microstructure and hardness of as cast and heat treated (1500 °C/100 h) Nb–20Si–5Hf–5W (YG5), Nb-20Si–5Mo–3W (YG6) and Nb–20Si–5Hf–5Mo–3W (YG8) alloys were studied. The macrosegregation of Si was strong in the as cast alloy YG6 and decreased in the order of the synergy of alloying elements as follows (Mo + W), (Hf + W) and (Hf + Mo + W). All three alloys were contaminated by oxygen. The phases present in the as cast alloys YG5, YG6 and YG8 were the Nb_ss, Nb₅Si₃ and HfO₂, Nb_ss, Nb₃Si, and Nb₅Si₃ and Nb_ss, Nb₅Si₃ and HfO₂ respectively, with a lamellar Nb_ss + Nb₅Si₃ microstructure formed in all three alloys. In the as cast alloys YG5 and YG6 there were Nb_ss grains with no Si content. There was microsegregation of Hf and W in the Nb_ss in the as cast alloy YG8 and microsegregation of Hf in Nb₅Si₃ in the as cast and heat treated alloys YG5 and YG8. It is concluded that the concentrations of Si and refractory metal (RM) in the Nb_ss depend on the synergy of RMs with/without Hf. The Nb_ss with no Si and Nb₅Si₃ were present in all three heat treated alloys and HfO₂ was formed in the heat treated alloys YG5 and YG8 via the consumption of the Hf in the Nb_ss. The stability of the Nb_ss + Nb₅Si₃ lamellar microstructure decreased in the order of the heat treated alloys YG5, YG8 and YG6. In the presence of Hf the transformation of βNb₅Si₃ to αNb₅Si₃ was enhanced. It is concluded that the equilibrium microstructures of the alloys consist of the Nb_ss with no Si and αNb₅Si₃ phases. Alloying with Hf caused reduction of the hardness of the alloys YG5 and YG8 after heat treatment.     

Influence of Nb on microstructure and mechanical properties of Ti-Sn ultrafine eutectic alloy

In this study, A series of the high strength (T₈₂Sn₁₈)_100-xNb_x (x=0, 1, 3, 5, and 9 at%) ultrafine eutectic alloys with large plasticity are developed by suction casting method. The Ti₈₂Sn₁₈ binary eutectic alloy consists of a mixture of a hcp Ti₃Sn and a α-Ti phases having the plate-like lamellar type duplex structure with micro scaled eutectic colony. From the (T₈₂Sn₁₈)₉₇Nb₃, the alloy display structural heterogeneous distribution of ultrafine-scaled phases composed of β-Ti(Nb) solid solution surrounded by alternating plate-like shaped Ti₃Sn and α-Ti phases. With increasing Nb content, the volume fraction of β-Ti is continuously increased, which induced improving mechanical properties both strength and plasticity. Especially, (Ti₈₂Sn₁₈)₉₁Nb₉ alloy has the outstanding combination of the high strength (σ _y ≈1.1 GPa) and large plasticity (ε _p ≈36%) at room temperature.     

Powder metallurgical production of TiNiNb and TiNiCu shape memory alloys by combination of pre-alloyed and elemental powders

The common Ti₄₄Ni₄₇Nb₉ and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys were produced by sintering techniques and the microstructure, phase structure and phase transformation behaviour were investigated. A combination of pre-alloyed binary TiNi powder and elemental Nb, Ni and Cu, Ti powders, respectively, were used. In contrast to the use of pre-alloyed ternary powders, which have to be produced in each new composition, a higher flexibility in the alloy composition becomes possible. In case of the Ti₄₄Ni₄₇Nb₉ alloy, liquid phase sintering was done to obtain the eutectic phase structure known from cast material. In case of the Ti₅₀Ni₄₀Cu₁₀ alloy, the pore size and porosity can be improved by choosing a two-step sintering process, as a eutectic melt between Ti and Cu is formed at low temperatures which influences the sintering behaviour. Controlling the impurity contents and the resulting secondary phases is necessary for both alloys in the same way as for binary TiNi alloys.     

激光热喷涂Co30Cr8W1.6C3Ni1.4Si涂层在PAO+2.5%MoDTC油中摩擦学特性

为了提高TC4合金的耐磨性能,采用激光热喷涂技术在其表面制备了Co₃₀Cr₈W_1.6C₃Ni_1.4Si涂层。通过扫描电子显微镜（SEM）和X射线衍射（XRD）分析了涂层的形貌和物相,并通过摩擦磨损实验研究了涂层在PAO+2.5% MoDTC（质量分数）油中的磨损行为。结果表明,激光热喷涂的Co₃₀Cr₈W_1.6C₃Ni_1.4Si涂层主要由Ti、WC_1-x、CoO、Co₂Ti₄O和CoAl相组成,在涂层界面形成冶金结合。在激光功率为1000、1200和1400 W时所制备的涂层平均摩擦因数分别为0.151、0.120和0.171,其对应的磨损率分别为1.17×10^-6、1.33×10^-6和2.80×10^-6 mm³?N^-1?m^-1,磨损机理为磨粒磨损,其枝晶尺寸对降磨起主要作用。     

Study of synthesis and electrochemical properties of spinel LiNi0.5Mn1.2Ti0.3O4 by different methods

Two spinel LiNi_0.5Mn_1.2Ti_0.3O₄ samples were successfully synthesized by the sol-gel method using chemicals LiAc·2H₂O, Mn(Ac)₂·2H₂O, Ni(Ac)₂·4H₂O and Ti(OCH₃)₄ as reactants. When reactants are calcined in air, a sample of LiNi_0.5Mn_1.2Ti_0.3O₄ (1), which contains Mn³⁺ and Mn⁴⁺ ions, is obtained. The sample of LiNi_0.5 Mn_1.2Ti_0.3O₄ (2), which contains only Mn⁴⁺ ions, is obtained when reactants are calcined in an oxygen atmosphere. X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge-discharge test and cyclic voltammogram test were employed to investigate the two samples. XRD results show that there is a small shift towards a larger diffraction angle for peaks of the LiNi_0.5Mn_1.2Ti_0.3O₄ (2) sample. SEM indicates that the two samples exhibit polyhedral shapes. The cyclic voltammogram test demonstrates that reduction-oxidation reactions take place at different voltages for the two samples. The prepared sample of LiNi_0.5Mn_1.2Ti_0.3O₄ with Mn³⁺ ions exhibits excellent cycle performance at different current rates. Its discharge capacity is 133.9 mAh/g at 0.1C.     

Interstitial and substitutional diffusion of metallic solutes in Ti3Al

Solute diffusion of Ni, Fe and Nb in the intermetallic compound Ti₃Al was studied. The diffusion measurements of Ni and Fe were performed with radiotracers applying the standard precision grinding technique for sectioning, while the concentration profiles of Nb in Ti₃Al were analysed through in-depth profiling by secondary ion mass spectrometry (SIMS). Ni and Fe exhibit fast diffusion behaviour of about 4 and 2 orders of magnitude, respectively, larger than titanium self-diffusion. The temperature dependencies of the diffusion coefficients yield the Arrhenius parameters D₀^Ni=1.76 × 10⁻⁵ m²/s, Q^Ni=195 kJ/mol and D₀^Fe=1.96 × 10⁻³ m²/s, Q^Fe=277.2 kJ/mol, respectively. The high mobility of these solutes with comparatively small atomic radii suggests in the D0₁₉-structure of Ti₃Al some type of interstitial diffusion that is discussed in the context of results of analogous diffusion investigations in α-Ti. Nb-diffusion coefficients in Ti₃Al were found to be lower by about one order of magnitude than self-diffusion. The Arrhenius relation is characterized by D₀^Nb=3.15 × 10⁻⁴ m²/s and Q^Nb=338.7 kJ/mol. It is concluded that Nb-diffusion occurs substitutionally via thermal vacancies in the Ti-sublattice.