Polycrystal Powders Research Articles

Comparative study of composite single crystal and polycrystalline YAG:Ce phosphors for laser-based lighting applications

YAG:Ce, a widely employed phosphor for LED wavelength conversion applications, comes in different forms, including polycrystal (powder), single crystal, and composite single crystal varieties. We investigated Epoch NeoTM, a composite single crystal incorporating Al2O3 as a heat distribution and optical guiding material embedded within its luminescent core. The properties of Epoch Neo were analyzed and compared with those of its polycrystal counterparts in this paper. Multiple characterization techniques like chromaticity points, color temperature, phosphor temperature, and speckle pattern were employed to study the effect of variable laser input power on both polycrystal powder and composite single crystal of YAG:Ce. It was found that under the influence of pumping blue laser high input optical power the composite single crystal phosphor’s performance was significantly better compared to that of the polycrystal.

The Effect of Y3+ Addition on Morphology, Structure, and Electrical Properties of Yttria-Stabilized Tetragonal Zirconia Dental Materials.

Zirconia (ZrO2), a widely known material with an extensive range of applications, is especially suitable for dental applications. This kind of dental material is produced in the form of blocks or discs (mostly discs—depending on CAD/CAM machines) by cold isostatic pressing (CIP). Such discs are subsequently milled by CAM/CAD technology into a desirable form. Due to the application of CIP, the resulting discs consist of different yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) powders, additives and pigments. The diverse composition of the discs (e.g., different Y3+ content) governs material properties, such as hardness, toughness and translucence. The aim of this work was to investigate the impact of Y3+ addition on the grains and grain boundaries, on the ZrO2 phases lattice parameter values and on the electrical equivalent circuit parameters of the prepared Y-TZP samples. The disc-shaped samples were prepared by using CAM/CAD technology. It was observed that the grain size and the grain density were increased by Y3+ addition. The sample with the lowest Y3+ content was characterized by the highest portion of the tetragonal phase, whilst the disc with the highest Y3+ addition consisted mainly of the cubic phase. It was also observed that at the higher Y3+ ion concentrations, these ions mainly incorporated the tetragonal phase. Furthermore, conductivity investigations showed that the resistivity of the grains in the samples with the higher Y3+ concentrations was decreased as these ions were mainly trapped in the grain boundary. On the other hand, the Y3+ trapping increased the capacitance of the grain boundary.

Effect of Powder Characteristics on Slip Casting Fabrication of Dental Zirconia Implants.

Dense zirconia compacts were fabricated by slip casting and sintering of nanoscale zirconia powders, and the effect of the powder characteristics (crystallite size, specific surface area, yttria content, and agglomeration) on the slurry and sintered properties was investigated. Three types of commercial 3 mol% yttria-stabilized tetragonal zirconia polycrystals powders were used as the starting powders after the powder characteristic analysis. A zirconia slurry for slip casting was prepared by mixing zirconia powder (solid loading of 60, 65, and 70 wt.%), distilled water, and a dispersant of Darvan C. The green compacts obtained from slip casting were cold isostatic pressed to enhance the close packing and densified by sintering at 1450 °C for 2 h. Highly dense zirconia compacts with a relative density of 99.5% and grain size of 350 nm were obtained based on the powder type and solid loading in the slurry. The microstructure and mechanical hardness of the sintered specimen after slip casting were dependent on the yttria content in the 3 mol% yttria-stabilized tetragonal zirconia polycrystal powder and the solid loading within the slurry.

Dielectric SrTiO3 mesocrystal derived from a H2Ti4O9·H2O single crystal

Strontium titanate (SrTiO3, ST) is a typical paraelectric material that has been widely used in many fields as a dielectric material and an energy-storage material, especially in the electronics industry, due to its excellent dielectric properties. Herein, we report the preparation of pure ST polycrystal powder with uniform granules by the solvothermal treatment of layered titanate H2Ti4O9·H2O (HT) and Sr(OH)2·8H2O as the raw materials. The effects of specimen morphology and electric field strength on the electrical properties of ST ceramics were investigated by studying their responses to heat treatment at various temperatures. The results show that pure ST mesocrystals, nanoparticles, and ceramics were successfully prepared. The ST ceramics show a rare bimodal phenomenon and strong dielectric response, which is due to the specimens containing two forms of ST crystal structure, namely the traditional ST polycrystal and a rare ST mesocrystal. When sintered at 900 °C, the ST ceramic exhibited good electrical-response performance due to the uniform particle morphology of the specimen and a large electric field strength at 2 kV. This study demonstrates the influence of particle morphology and electric field strength on the electrical properties of ceramics and lays the foundation for the development of multifunctional oxide ceramics with excellent electrical properties.

Green up-conversion of C12A7–Ho3+ prepared by co-precipitation method

7CaO⋅12Al2O3 (C12A7) and Ho3+-doped 7CaO⋅12Al2O3 (C12A7–Ho3+) polycrystal powders were synthesized via co-precipitation followed by sintering at 1173K. Crystal structures, thermodynamics characters and optical properties of crashed samples were examined by XRD, DSC/TG, FT-IR and Raman spectra analysis, respectively. Activation energy of crystallization for C12A7 polycrystals was calculated according to both Ozawa/Kissinger and modified Ozawa/Kissinger methods to be 813 or 791 (for the former, only considering crystallization) and 1084 or 1077kJmol−1 (for the latter, taking both crystallization and nucleation into consideration) respectively. Growth morphology parameter (m) and crystallization mechanism index (n) of C12A7 were determined to be m=3 and n=4. Fluorescence effects of Raman spectra around 2000–2300cm−1 and pure visible up-conversion emission centered at 540nm were observed from C12A7–Ho3+ (1, 2 and 5mol.%) powders under 488nm laser diode and 808nm Xe lamp excitation, respectively. Two mechanisms were proposed to explain up-conversion of pure green light process.

뜨거운 곁쌓기 법에 의해 성장된 MgGa2Se4단결정 박막의 열처리 효과

The evaporating materials for single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, compounded polycrystal powder was deposited on thoroughly etched semi-insulated GaAs(100) substrate by the hot wall epitaxy (HWE) method system. The source and substrate temperatures of optimized growth conditions, were and , respectively.The source and substrate temperatures were and , respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The temperature dependence of the energy band gap of the obtained from the absorption spectra was well described by the Varshni`s relation, $E_g(T)

低温燃烧法制备的Ho∶YbGG多晶粉体及其发光性能

Holmium doped ytterbium gallium garnet (Ho:YbGG) polycrystal powder was synthesized by low temperature combustion method using citric acid as combustion agent. The mole fraction of Ho3+ was 2%. The Ho:YbGG polycrystal powder was determined by X-ray diffraction analysis and SEM analysis. The results show that 900 is the optimum calcination temperature. The absorption spectrum of the sample shows that the absorption peaks of Ho3+ are centered at 536 nm and 645 nm, corresponding to 5I85F4 (5S2) and 5I85F5 level transitions, respectively. The strong absorption bands of Ho3+ are centered at 940 nm and 982 nm, corresponding to 2F5/22F7/2 level transition. The fluorescence spectra of the samples show that the strong emission band of Ho3+ is centered at 1 989 nm, corresponding to 5I75I8 level transition. The upconversion spectrum of the sample was tested and the corresponding conversion mechanism was analyzed. Ho:YbGG can emit strong green and red light under 980 nm LD excitation, which are corresponding to 5F4(5S2)5I8 and 5F55I8 level transitions of Ho3+.

SiAlON-based crystal–glass composite phosphor with tunable yellow-green-blue color emission induced by SiO2 melting corrosion

SiAlON-based crystal–glass composite phosphors with tunable yellow-green-blue emission are exploited. Substitution of Al–N by Si–O in a SiAlON lattice and the introduction of a Si–Al–O–N glass phase are induced by melting corrosion as SiAlON polycrystal powders were heat-treated with nano-SiO2 additive. The melting corrosion process involving liquid penetration, solid dissolution, liquid transportation is discussed and assigned to the origin of morphology transformation. The substitution extent and the glass content are identified to increase with the addition of nano-SiO2 additive. The resultant tunable photoluminescence of Eu2+ in the SiAlON lattice and Si–Al–O–N glass matrix and the microstructure evolution are detailed, respectively. The emission color of the SiAlON-based crystal–glass composite phosphors is traced continuously from yellow to green then to blue with the combination of changeable emissions of Eu2+ originated from the SiAlON phase and glass phase. This artful SiAlON-based crystal–glass composite phosphor shows flexible color-adjustable capability with the control of SiO2 melting corrosion, indicating the potential applications in lighting and display fields.

Thermodynamic analysis of the sublimation process in cadmium selenide

Abstract In the present work, the thermodynamic properties of the sublimation process in cadmium selenide were discussed. Thermodynamic equation was established on the basis of the classical Maxwell–Boltzmann distribution law, state equation of ideal gas and Clapeyrons equation. Thermogravimetic analysis (TGA) results of CdSe polycrystal powder were obtained at different temperature. It was found that the CdSe evaporation temperature was about 840 °C and the rapid sublimation of this material took place almost at 1100 °C. Furthermore, by using the TGA curves, the parameters such as the activation energy and index value pre-factor in thermodynamic equation were calculated as 21.899 kJ mol−1 and 8.401 × 103 mg mm2 min Pa, respectively. The results were discussed.

Preparation and phase development of yttria-doped ceria coated TZP powder

This paper presents a simple technique for preparation of yttria-doped ceria (YDC) coated tetragonal zirconia polycrystal (3Y-TZP) powder and its phase development upon firing. The coating solution was prepared using yttrium nitrate hexahydrate and cerium nitrate hexahydrate as starting reagents. Thermochemical reactions of the coated powder were studied using TGA and FTIR while phase development upon firing was examined using XRD. Inward diffusion of the coating YDC into the TZP particles was monitored by observing the change of crystal structure and lattice parameter as a function of sintering temperature and time. At sintering temperature of 1300 °C for 1 h, crystal structure of the sample was still tetragonal (t-ZrO2). Increasing sintering time to 5 h at 1300 °C, diffusion of YDC into TZP particles occurred drastically and the structure was changed to cubic (c-ZrO2) as indicated by the disappearance of (002)/(200) peak splitting. Increasing sintering temperature to 1400 and 1500 °C, however, resulted in the co-existence of tetragonal and cubic phases as indicated by the appearance of triples around 72.5–74° 2θ and also the decrease of cubic lattice parameter. When the sintering temperature was further increased to 1600 °C, lattice parameter was only slightly changed, suggesting that inward diffusion of YDC reached saturation point around this temperature.

Synthesis of Single-Crystalline Barium Dititanate Nanobelts

In this paper, we report a novel and simple molten-salt method, for the first time, to synthesize long single-crystal ferroelectric and photocatalyst material barium dititanate (BaTi2O5) nanobelts (width of 30−500 nm and lengths of up to several hundred micrometers) using BaTiO3 polycrystal powders as the precursor by a molten-salt medium process. This method is one-step and uses no organic dispersant or capping agent, which is a very green chemistry route. The product was characterized using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS).

Spectroscopic Studies on Charge Transfer from Ce3+ to Yb3+ in KMgF3: Ce, Yb Nanocrystals

Nanocrystals of KMgF3 single-doped and codoped with Ce3+ or/and Yb3+ were synthesized separately by the microemulsion method. The X-ray diffraction(XRD) patterns were indexed to show that the KMgF, crystal system was unchanged. The fluorescent spectra of KMgF3:Ce, Yb polycrystal powders were studied and compared with those of the Ce, Yb doped KMgF3 crystals produced using the high-temperature solid phase method. The diffuse reflection spectra and infrared. emission of KMgF3:Ce, Yb were investigated. From the results, the authors could confirm that there were charge transfer processes from Ce3+ to Yb3+ in both KMgF3: Ce,Yb nanocrystals and polycrystal powders.

Mechanical properties and microstructure of TiN/TZP nanocomposites

TiN/TZP nanocomposite powders were obtained by ball milling with titanium nitride (TiN) nanopowders and tetragonal zirconia polycrystal (TZP) powders as starting materials. TiN/TZP nanocomposites were prepared by hot pressing sintering. The effects of TiN nanoparticles on the relative density, mechanical properties and microstructures of TiN/TZP nanocomposites were studied. Experimental results show that the addition of TiN nanoparticles did not decrease the sintering ability of the composites; the Vickers hardness of the composites reached to 15 GPa when the content of TiN is 20 vol%; the bending strength of the composite with 5 vol% TiN fillings can be enhanced with 200 MPa higher than pure TZP ceramic. The dispersivity of TiN particles has a distinct effect on the strength of the composites.

Sinterability of Alumina-Dispersed 3Y-TZP

The effect of Al2O3 addition on sinterability of Tetragonal Zirconia Polycrystal powders including 3mol% Y2O3 (3Y-TZP) was investigated. Each 3Y-TZP powder dispersed with 0.3, 0.6, 0.9 and 1.2wt.% of Al2O3 was prepared by the spray-drying method. The prepared powders were pressed into a disk type and sintered at 1350, 1400, 1450 and 1500 for 2 hours in the air. Resultant microstructures and mechanical properties of specimens were investigated by using Vickers/Micro hardness Tester, FE-SEM and XRD. Most of the specimens showed high relative density over 99% and a higher fracture toughness than pure 3Y-TZP. Al2O3 particles dispersed in 3Y-TZP microstructure depressed grain growth of 3Y-TZP by the pinning effect. Increase in fracture toughness of 3Y-TZP was explained by the crack deflection due to dispersed Al2O3 particles.

Uniaxial plastic deformation in the zirconia-based nanocrystalline ceramics containing a silicate glass

Abstract Nanocrystalline yttria-stabilized tetragonal zirconia polycrystal (nc-Y-TZP) powders coated with silicate based glasses were cold isostatically pressed and sintered near to the full density (98–99%). Two glasses with different compositions were used: 93 SiO 2 –1 Na 2 O–6 SrO (mol%) (designated as SNS glass) and 58 SiO 2 –29 Al 2 O 3 –13 SrO (designated as SAS glass). Uniaxial compression tests of the pure (glass-free) nc-Y-TZP samples yielded strain rates as high as 2·10 −4 s −1 under 60 MPa at 1300 °C. Comparable strain rates were measured in the SNS glass-containing samples, with the maximum of 3·10 −4 s −1 at 1300 °C under a stress of 80 MPa (5 vol.% SNS glass content). Compression tests under 100 MPa exhibited relatively high strain rates of 5·10 −4 and 10 −4 at 1300 °C and 1200 °C, respectively, in the 15 vol.% SAS glass samples. The strain rates measured in the SAS glass-containing samples were achieved at temperatures lower by 100 °C compared to the similar strain rates in the glass-free and SNS glass-containing samples. The microstructure of the deformed samples was similar to that of samples before deformation, within which the ultrafine and equiaxed character of the grains was preserved. Clear evidence for cooperative grain boundary sliding was observed in the SAS glass-containing samples.

Microstructural evolution in nanocrystalline Y–TZP containing a silicate glass

Abstract Nanocrystalline yttria stabilized tetragonal zirconia polycrystal (NC–Y–TZP) powders were coated with a sodium strontium silicate glass using the sol-gel technique. The compact powders were sintered at 1400°C for 1 h resulting in dense nanocrystalline pellets with 5, 10 and 15 vol.% glass. Short pre-sintering heat treatment at 800°C caused the full stabilization of the tetragonal phase while long treatments led to the devitrification of the glassy phase. The mean grain size decreased from 196 nm in the glass-free specimen to 140, 110 and 100 nm in the specimens containing 5, 10 and 15 vol.% glass, respectively. The grain size distribution width also decreased with increase in the glass content. TEM revealed the morphological changes of the highly facetted polyhedral grains in the glass-free specimens into the round-shape grains with increase in the glass content. The effect of the glass on the phase formation, densification and microstructure have been discussed.

Preparation and characterisation of lanthanum doped BaTiO3 nanosize polycrystals by sol–gel processing

Sol–gel processing has been refined to prepare lanthanum doped barium titanate nanosize polycrystal powders, LaxBa1-xTiO3 (x=0.05, 0.15, 0.20, 0.25), in the presence of Ba(OAc)2, La(OAc)3, and Ti(OnBu)4 as starting materials with 0.7 mol L-1 total concentration of precursor. The sol was obtained by cosolubilised hydrolysis of starting materials using HOAc as medium and catalysis with pH∼3.8. The wet gel was acquired by concentrating and aging at room temperature. The wet gel to dry gel conversion was carried out at 50°C for ∼24 h. First the dry gel was characterised by differential thermal analysis–thermal gravimetry (DTA–TG) and then it was calcined at various temperatures (600, 700, 800, 900°C) to prepare the nanocrystals. The final synthesised polycrystals were characterised by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction, and scanning electron microscopy.

INFLUENCE OF Gd3+ AND Y3+-DOPING ON LOW TEMPERATURE THERMOLUMINESCENCE OF Pb WO_4

The influence of Gd3+ and Y3+ doping on the low temperature thermoluminescence (TL) of PbWO4(PWO) is investigated. In the case of polycrystal powder, the TL above 180K is eliminated by doping Y3+ or Gd3+ ions. Meanwhile, a new TL peak at lower temperature appeare. For the PWO single crystal there are two TL peaks at about 130 and 253K, but only one single peak at about 130K for PWO:Y crystals. The common effect of doping trivalent ions is to decrease or eliminate the TL above 180K and create a new TL peak at lower temperature. It indicates that the traps responsible for TL above 180K are due to the intrinsic defects which probably are Pb3+ and O-. During the preparation of PWO, the evaporation of the PbO component is easier than that of WO3, which will lead to the deficiency of PbO. The deficiency of O ions can be compensated by annealing in air while the deficiency of Pb is permanent. Therefore in the volume surrounding the Pb vacancy, there is a lack of positive charge. So the charge deficiency in the Pb-sublattic can be most easily balanced by an increase of concentration of Pb3+ and O- Doping Gd3+ or Y3+ can decrease the concentration of Pb3+ and O-. Gd3+ or Y3+ can also play a role of electron traps, which create new TL peaks at 150 and 125K, respectively. The trap depth of Gd3+ and Y3+ are 0.32 and 0.29eV, respectively. The difference between doping Gd3+ and Y3+ can be explained by the difference between their coulomb potentials.

ChemInform Abstract: In situ Synthesis of Yttria‐Stabilized Tetragonal Zirconia Polycrystal Powder Containing Dispersed Titanium Carbide by Selective Carbonization.

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In Situ Synthesis of Yttria‐Stabilized Tetragonal Zirconia Polycrystal Powder Containing Dispersed Titanium Carbide by Selective Carbonization

Reaction thermodynamics was utilized to analyze the selective carbonization conditions of ZrO2‐TiO2‐Y2O3 ultrafine powder and to define the temperature range of the selective carbonization. The ZrO2‐TiO2‐Y2O3 powder was prepared by coprecipitation from a solution containing Zr4+, Ti4+, and Y3+. The powder was selectively carbonized at 1350°, 1450°, 1550°, and 1650°C, respectively, for 2 h under argon atmosphere with sucrose as the carbon source. The resulting product was analyzed by X‐ray diffraction. The experimental results indicated that the ZrO2‐TiO2‐Y2O3 powder could be selectively carbonized in situ at 1450°C to form a carbonized powder which was composed of TiC and t‐ZrO2. It was also found that zirconium carbide could be formed at 1550°C, which is much lower than the carbonization temperature of ZrO2 defined by thermodynamics.