Production of the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> transparent nanostructured ceramics

The nanocrystalline Y₃Al₅O₁₂ ceramics with different phase compositions, microstructures, and optical characteristics have been prepared by low-temperature consolidation of the Y₃Al₅O₁₂ powder at pressures from 6 to 7.7 GPa in the temperature range from 250 to 550°C. The conditions have been defined for obtaining transparent nanostructured ceramics Y₃Al₅O₁₂ having grains of size 20–40 nm and a transmission coefficient in the visible region of 40–45%. The criteria for the transparency of the nanostructured ceramics have been formulated and the ways of an improvement of its optical characteristics are discussed.     

Nucleation of the Plasticity at Nanodeformation of the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> Yttrium-Aluminum Garnet

The nanoindentation in the continuous stiffness measurement (CSM) mode was used to study the nucleation of the plasticity at the nanodeformation of the yttrium-aluminium garnet (YAG) having a low ((111) YAG single crystal after the annealing at the temperature of 1300°C) and a high (polycrystalline yttrium-aluminium garnet with a grain size of ~ 20 μm after the mechanical polishing) density of dislocations. For a sample having a high dislocations density a smooth elastoplastic transition was observed in the nanocontact as a result of the motion and multiplication of dislocations that are already present in the sample. For (111) YAG single crystal after annealing at the temperature of 1300°C an abrupt elastoplastic transition (pop-in) caused by a homogeneous nucleation of dislocations in the region under the contact was observed.     

Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Recombination processes in the Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce<Superscript>3+</Superscript> scintillator

The roentgenoluminescence spectra, temperature-dependent activator luminescence, optically stimulated luminescence, and the effect of IR irradiation on the yield and spectral composition of the low-temperature roentgenoluminescence and thermoluminescence curves of the Y₃Al₅O₁₂:Ce³⁺ scintillator have been studied in the temperature range 85–295 K. The results, coupled with earlier data, suggest that the Ce³⁺ ions in the garnet crystal studied form Ce³⁺ p hole centers and increase the concentration of electronic F ^?-centers responsible for the IR stimulation band at 940 nm. The reduction in roentgenoluminescence yield on cooling Y₃Al₅O₁₂:Ce³⁺ to below 230 K is due to the significant localization of excited carriers at defects, which show up in thermoluminescence peaks and optical stimulation spectra. The low-temperature Ce³⁺ luminescence in Y₃Al₅O₁₂:Ce₃₊ seems to result from the recombination of activator-bound excitons.     

Crystallization and microstructures of Y-Si-Al-O-N glass-ceramics containing main crystal phase Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

A glass with the nominal composition of 28Y48Si24Al83O17N (in equal percentage) was chosen as parent glass in this paper to prepare Y₃Al₅O₁₂-based glass-ceramics. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to assess the crystallization process of the parent glass. YAG as the only crystalline phase appears in all glass-ceramics produced under 1250°C. A small amount of O’-Sialon secondary phase starts to precipitate from parent glass samples as heat treatment temperature increases to 1250°C. Grain size of the dendrite crystal which corresponds to YAG phase increases and the dendrite branches get thickened as heat treatment temperature increases. Moreover, grain size of YAG phase resulting from two-stage heat treatment is much smaller than that of YAG phase obtained by one-stage heat treatment. The results are relevant to developing improved crystallization treatments for glasses with potential for crystallization to YAG-based glass-ceramics and for heat treatments of YAG/β-SiAlON materials.     

Synthesis,luminescence, and effect of heat treatment on the properties of Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce phosphor

YAG:Ce nanosized spherical precursors were synthesized by coprecipitation using ammonium hydrogen carbonate as precipitant. Composition, phase transition, morphologies, and luminescence properties of the powders during calcination were studied. The precursors directly convert to pure YAG structure at about 900°C. The mean size of precursors is about 100 nm, and the grain size increases with increase of calcination temperature. When calcined above 1400°C, the grain size comes into a micrometer. With increase of calcination temperature, the luminescent intensity increases, and the emission band shows a redshift, while the lifetime increases.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> on the properties of nanocrystalline ZrO<Subscript>2</Subscript> + 3 mol % Y<Subscript>2</Subscript>O<Subscript>3</Subscript> powder

We have studied the properties of nanocrystalline ZrO₂〈3 mol % Y₂O₃〉 and 90 wt % ZrO₂〈3 mol % Y₂O₃〉-10 wt % Al₂O₃ powders prepared via hydrothermal treatment of coprecipitated hydroxides at 210°C. The results demonstrate that Al₂O₃ doping raises the phase transition temperatures of the metastable low-temperature ZrO₂ polymorphs and that the structural transformations of the ZrO₂ and Al₂O₃ in the doped material inhibit each other.     

Microwave-hydrothermal synthesis of Y<Subscript>3</Subscript>Fe<Subscript>3.35</Subscript>Al<Subscript>1.65</Subscript>O<Subscript>12</Subscript> nanoparticles for magneto-hyperthermia application

Crystalline aluminum substituted yttrium iron garnet nanoparticles Y₃Fe_3.35Al_1.65O₁₂ (YIG) was synthesized by hydrothermal microwave synthesis at 140 °C with soaking times ranging from 15 to 60 min. X-ray diffraction confirmed the single-phase YIG nanoparticles excluding the presence of any other phases in the reaction products. The Raman spectra revealed that the largest soaking time provides greater energy crystallization causing changes of lattice vibration and a certain degree of disorder in the crystal lattice. Field emission gun-scanning electron microscopy and high resolution transmission electronic microscopic revealed a homogeneous size distribution of nanometric YIG powders with agglomerated particles. Magnetic measurements were achieved by using a vibrating-sample magnetometer unit. YIG nanoparticles have great potential in magneto-hyperthermia application once in vivo applications magnetic induction heating ferromagnetic compounds generate heat in AC magnetic fields.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Directional solidification and growth characteristics of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary eutectic ceramic by laser floating zone melting

Directionally solidified Al₂O₃/Er₃Al₅O₁₂/ZrO₂ ternary eutectic ceramic in situ composite rods with length of 110 mm have been fabricated by laser floating zone melting. The microstructural characteristics of steady growth zone, initial growth zone and solid/liquid interface are investigated under high temperature gradient. In the steady growth zone, the eutectic spacing (λ) is rapidly decreased as increasing the growth rate (V), and the corresponding relationship between growth rate and eutectic spacing is determined to be λ = 11.14 × V ^?1/2. The temperature gradient has been measured to be about 5.3 × 10³ K/cm. In the initial growth zone, the melting process and temperature distribution are recorded by infrared thermal imager, and several unstable complex microstructures are observed. In the quenched zone, the regular eutectics with minimum eutectic spacing of 200 nm are obtained. Moreover, the solid/liquid interface during solidification shows convex interface morphology and the interface height is gradually decreased as increasing the growth rate. The eutectic growth behaviors at the center and edge of the as-grown rod are compared and discussed.     

Study of wear resistance and nanostructure of tertiary Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Y<Subscript>2</Subscript>O<Subscript>3</Subscript>/CNT pulsed electrodeposited ni-based nanocomposite

We study the electrodeposition of tertiary Alumina/Yitria/carbon nanotube (Al₂O₃/Y2O₃/CNT) nanocomposite by using pulsed currents. The process of coating is performed in a nickel-sulfate bath and the nanostructure of the obtained compound layer is examined with the help of high-precision figure analysis of SEM nanographs. The effects of process variables, i.e., the Y₂O₃ concentration, treatment time, current density, and the temperature of electrolyte are experimentally investigated. Statistical methods are used to achieve the minimum wear rate and average size of nanoparticles. Finally, the percentage contributions of different effective factors are revealed, and the confirmation run showed the validity of the obtained results. It is also revealed that the wear properties of the coatings undergo significant changes if the sizes of nanoparticles change. The atomic-force microscopy (AFM) and transmissionelectron microscopy (TEM) analyses confirm the smooth surfaces and average sizes of nanoparticles in the optimal coating.     

Strain localization in compressed ZrO<Subscript>2</Subscript>(Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) ceramics

Spatiotemporal distributions of local components of the distortion tensor of a nonplastic material—yttria partially stabilized tetragonal zirconia (YTZ) ceramics—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity in the elastic state of YTZ ceramics are considered.     

Effect of Pb ions on the optical absorption in Gd<Subscript>3</Subscript>Ga<Subscript>5</Subscript>O<Subscript>12</Subscript> epitaxial films

We have studied the effect of Pb²⁺ impurities and Pb²⁺-Pb⁴⁺ pairs on the optical absorption between 200 and 860 nm in single-crystal gadolinium gallium garnet films grown by liquid-phase epitaxy from supercooled PbO-B₂O₃ fluxed melts containing 0.1–0.5 mol% gadolinia.     

Influence of Mg<Superscript>2+</Superscript>/Ga<Superscript>3+</Superscript> doping on luminescence of Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce<Superscript>3+</Superscript> phosphors

Mg²⁺/Ga³⁺ doped Y₃Al₅O₁₂:Ce³⁺ phosphors were synthesized through a solid state reaction. The phase and luminescent of the synthesized phosphors were investigated. For Ga³⁺ codoped Y_2.96Ce_0.04Al_(5?x)Ga_xO₁₂ phosphors, the emission intensity increases with the increase of Ga³⁺ concentration up to Y_2.96Ce_0.04Al_4.80Ga_0.20O₁₂ and then decreases with a further increase of Ga³⁺ concentration, but the emission peak shifts to shorter wavelength continuously in the Ga³⁺ doping concentration range of 0.05–0.25. For Mg²⁺/Ga³⁺ codoped Y_2.96Ce_0.04Al_(4.8?y)Ga_0.20Mg_yO₁₂ phosphors, the emission intensity decreases and the emission peak shifts to longer wavelength continuously in the Mg²⁺ doping concentration range of 0.02–0.12. The emission spectra of Y_2.96Ce_0.04Al_(4.8?y)Ga_0.20Mg_yO₁₂ phosphors demonstrate that the codoped Mg²⁺/Ga³⁺ ions not only induce the enhancement of Y_2.96Ce_0.04Al₅O₁₂ emission intensity but also lead to the red shift of Y_2.96Ce_0.04Al₅O₁₂ emission peak. The decay lifetimes decrease in Mg²⁺/Ga³⁺ codoped Y_2.96Ce_0.04Al₅O₁₂ phosphors due to defects formed by substitutions of Y³⁺ by Mg²⁺/Ga³⁺.     

Role of Ti diffusion on the formation of phases in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> brazed interface

Diffusivities of Ti, Cu, Al and Ag in the interface of Al₂O₃–Al₂O₃ braze joints using Ag–Cu–Ti active filler alloy, have been calculated by Matano–Boltzman method. The Matano plane has been identified for each elemental diffusion at various brazing temperatures. The diffusivities of Ag, Cu and Al are almost insignificant on formation of interface during brazing, whereas the diffusivity of Ti changes significantly with the brazing temperature and controls the formation of different reaction product in the interface. Presence of TiO and Ti₃Cu₃O phases in the interface has been confirmed by transmission electron microscopy (TEM).     

Kinetics of mechanically activated P<Subscript>2</Subscript>O<Subscript>5</Subscript> incorporation into Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The incorporation of phosphoric anhydride into - and -aluminas during mechanical activation is investigated. An empirical equation is proposed for the kinetics of P₂O₅ incorporation. The effect of the specific power of the mill on the amount of bound P₂O₅ is analyzed.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 321–323.Original Russian Text Copyright © 2005 by Kosenko, Smirnova, Denisova.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Nucleation behavior and microstructure of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-poor LAS glass–ceramics

Influence of MgO and K₂O on the nucleation behavior of Al₂O₃-poor LAS (Li₂O–Al₂O₃–SiO₂) base glasses was investigated by thermal analysis and, the effect on microstructure and surface topography of glass–ceramics was also examined by SEM, AFM and TEM. According to results of thermal analysis, the exothermic peak temperature of the glass showed a decrease with increase of nucleation temperature to nucleation time of 6 h. But some glasses nucleated for 9 h showed nucleation rate-like curve with maximum point. The dependence of reciprocal value of the exothermic peak temperature on the nucleation temperature indicated that an introduction of MgO might accelerate the nucleation of the base glass and thus result in rough surface topography of glass–ceramics. On the other hand, in the case of glass–ceramics containing K₂O the main crystalline phase was lithium metasilicate and they showed fine microstructure resulting in smooth surface topography. TEM micrographs of as-quenched and nucleated glasses showed no trace of phase separation affecting nucleation or final microstructure.     

Relaxation of optical excitations in crystals of the Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system with radiation defects

The photoluminescence and short-time persistence (afterglow) kinetics in pure and doped Y₂O₃-Al₂O₃ crystals excited with UV laser pulses (12 ns, 337 nm) were studied using samples irradiated with gamma quanta from a ⁶⁰Co source to a dose from 10⁴ to 10⁷ Gy. The relaxation time of the samples studied increases, with decreasing symmetry of the crystal lattice, in the following order: garnet—orthoaluminate—ruby—yttria. The afterglow duration and intensity significantly decrease in gamma-irradiated crystals, which is explained by the predominant recombination of close electron-hole pairs. Garnet-neodymium crystals are characterized by high radiation stability and fast relaxation kinetics.     

Temperature-Dependent Thermal Boundary Conductance at Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interfaces

With the ever-decreasing size of microelectronic devices, growing applications of superlattices, and development of nanotechnology, thermal resistances of interfaces are becoming increasingly central to thermal management. Although there has been much success in understanding thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes in Al and Pt films on Al₂O₃ substrates are examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating potential inelastic phonon processes.