Transparent Polycrystalline Research Articles

Densification of transparent yttrium aluminum garnet (YAG) by SPS processing

Nano-size YAG powder co-mixed with 0.25 wt.% LiF was used to fabricate transparent polycrystalline YAG specimens by means of the Spark Plasma Sintering (SPS) technique. The presence of the LiF additive in the initial nano-powder allows obtaining fully dense disc shaped (up to 4 mm thick) transparent specimens at the outcome of a 2 h treatment at 1300 °C. The presence of LiF plays a key role in the mass transport related effects during the densification of YAG to full density and also in the elimination of the residual carbon contamination, allowing reaching a level of optical transmittance close to the theoretical value.

Fabrication, microstructure and optical properties of polycrystalline Er 3+:Y 3Al 5O 12 ceramics

Highly transparent polycrystalline Er 3+:Y 3Al 5O 12 (Er:YAG) ceramics with different Er 3+ ions content from 1% to 90% were prepared by the solid-state reaction and the vacuum-sintering technique. The grain boundary is clean and narrow with a width of about 1 nm. The best sintering temperature of the ceramics is about 1800 °C. The relationships between fabrication, microstructure and transparency of the ceramics were discussed. Grain size distributions in axial direction of cylinder samples were characterized by electron probe micro-analyzer (EPMA). The luminescence spectra were measured and discussed.

Sintering of Transparent Yttria Ceramics in Oxygen Atmosphere

A novel method is reported for the preparation of transparent polycrystalline yttria ceramics in oxygen atmosphere. Zirconia and other additives were added to control the grain growth. Pores can be eliminated clearly at a temperature <1650°C with the grain size around 1 μm. The grain growth kinetics and the mechanisms controlling grain growth were studied. Sintering in oxygen atmosphere is beneficial for making samples with a big size at low cost and avoiding the posttreatment of samples sintered in vacuum or hydrogen atmosphere.

Characterizations of a Hot‐Pressed Polycrystalline Spinel:Ce Scintillator

Here, we report a new polycrystalline ceramic scintillator for possible use in γ‐ray detection and medical imaging applications. The goal was to develop a cerium‐doped spinel (MgAl2O4:Ce), which can be processed utilizing ceramic forming techniques. High‐purity MgAl2O4 powders were used as the starting materials. Lithium fluoride (LiF) was used as a sintering aid and CeO2 powder was used as the dopant. The mixed and dried powders were hot pressed in a vacuum environment to achieve high‐density MgAl2O4:Ce. The hot‐pressed sample shows a transparent polycrystalline appearance. In‐line transmission was measured to determine the transparency of the structure. Microstructures were characterized using X‐ray diffraction and scanning electron microscopy. Fluorescence absorption and emission peaks were also measured in addition to the decay time measurement.

Elastic and Mechanical Properties of Polycrystalline Transparent Yttria as Determined by Indentation Techniques

The Young's modulus (E) and nanoindentation (NI) stress–strain curves of fully dense, transparent yttria were determined using a nano‐indenter capable of continually measuring the stiffness (S). Two hemispherical indenters with radii of 1.4 and 5 μm were used, in addition to a Berkovich tip. The E values—calculated from the S vs. a (contact radius) curves obtained with both spherical indenters (158–171 GPa) were ≈5% lower than the 176 GPa dynamic modulus measured on the same sample using ultrasound. Depending on range of penetration depths, the Berkovich modulus was ≈1%–5% higher than the dynamic modulus. When the NI load‐displacement curves were converted to NI stress–strain curves, the yield point obtained was 7 ± 1 GPa. This value was ≈20% and ≈33% lower than the Vickers and Berkovich hardness values measured on the same sample, respectively. The Vickers microhardness values (8.8 ± 0.2 GPa) and the fracture toughness extracted from the latter (1.5 ± 0.3 MPa·m1/2) are in good agreement with published results on samples with comparable microstructures. The strain hardening rates were almost identical for both tips; no tip size effect was observed. Based on this work, we conclude that S vs. a plots are a powerful, and relatively simple, technique to measure the Young's moduli of polycrystalline ceramics and other hard solids. The fact that one also obtains NI stress–‐strain curves is a distinct advantage over the more commonly used Berkovich tip.

Transparent polycrystalline ruby ceramic by spark plasma sintering

Abstract Fine-grained and transparent polycrystalline ruby ceramics (Cr2O3-doped Al2O3) were successfully prepared by spark plasma sintering (SPS). The effect of Cr2O3 concentration on the grain size, hardness, fracture toughness and thermal conductivity of ruby ceramics was investigated systematically. For 0.05 wt.% Cr2O3, high in-line transmittance of 85% at 2000 nm can be reached, further increase of Cr2O3 concentration leads to the decrease in transmittance. High hardness of 23.95–25.05 GPa can be achieved due to the fine grain size in all ruby ceramics. The fracture toughness of 1.9–2.29 MPa m1/2 indicates that no improvement in fracture toughness over pure Al2O3 can be obtained by Cr2O3 doping in these submicron grained ruby ceramics. High thermal conductivity of 28–29.8 W/(m K) at room temperature, close to that of single crystal sapphire, can be achieved. The change in grain size for different Cr2O3 concentrations is the major reason for the change in mechanical and thermal properties, but not for the change in optical properties.

Role of Nitrogen Additive and Temperature on Growth of Diamond Films from Nanocrystalline to Polycrystalline

In this work, the coupled effect of nitrogen addition into CH4/H2 mixtures and surface temperature on diamond growth ranging from large grained polycrystalline to fine-grained nanocrystalline were investigated. Moreover a new growth parameter window for simultaneous growth of nanocrystalline diamond (NCD) and {100} textured large-grained diamond films was developed by using a high power high pressure 5 kW microwave plasma assisted chemical vapor deposition (MPCVD) reactor. Scanning electron microscope (SEM), Raman spectroscopy, and X-ray diffraction (XRD) are employed to characterize the morphology, crystalline quality and texture of the diamond samples. Our results can be grouped by two catalogs: First, deposition run without and with 0.24% N2 addition, while keeping all the other parameters constant, resulted in a high quality transparent large-grained polycrystalline diamond film and a NCD film, respectively. This result clearly evidences nitrogen induced nanocrystallinity. Then, two different substrate surface temperatures were obtained by overlapping a small silicon slice on the top centre of a large silicon wafer of 5.08 cm in diameter in only one deposition run using 0.24% N2 addition and the same set of parameters as the previous runs. From this growth run, a NCD film of growth rate around 2.3 microm/h was obtained at low temperature, while a {100} textured large-grained diamond film of much higher growth rate about 10.4 microm/h was grown at high temperature. These results not only confirm the reproducibility of NCD by N2 addition, but also indicate that distinct growth modes were involved at different substrate temperatures with 0.24% nitrogen addition, or coupled effect of nitrogen addition and temperature on the growth of CVD diamond films happened. Finite element method (FEM) analysis was employed to simulate the temperature gradient and distribution on these two samples, and based on this simulation and other simulation results in the literature, the growth mechanism is briefly discussed.

Development of Transparent MgAl<sub>2</sub>O<sub>4</sub> Spinel Ceramics

The present paper summarizes the technology and basic process for fabricating transparent ceramic MgAl2O4 spinel and introduces a scalable production technique. Fully transparent polycrystalline spinel has been prepared by combined vacuum sintering, hot-pressing, sintering in H2 atmosphere and hot isostatic pressing(HIP). The optical transmittance of the specimen sintered at 1500°C following HIP at 1800°C was nearly the same as that of single crystal spinel. When radiated by γ-ray and X- ray, colorless spinel became brown due to V-type color centers, which decreased its transmittance. On the other hand, the spinel ceramics’ resistance to radiation improved by doping CeO2.

Compositional and physico-optical characterization of 0–5% Al-doped zinc oxide films prepared by chemical spray pyrolysis

Highly transparent polycrystalline Al-doped ZnO thin films were deposited in open atmosphere by chemical spray pyrolysis on fused silica and glass substrates at 623 K. The influence of Al doping, 0 to 5%, was studied. XPS results revealed a linear relationship between Al content in the precursor solutions and Al content in the films. XPS depth profiling showed that any carbon contamination is restricted to the uppermost surface of the films. Optical transmission measurements revealed an increasing number of dispersion centres as well as a band gap shift to higher values with increasing Al content in the films. At fixed Al concentration, the comparison of the absorption coefficient for increasing film thickness showed that the films are very homogeneous, not changing their materials properties such as absorption coefficient and band gap.

Correlations between polycrystalline fabric and the polarization of transmitted light

Strong correlations have been found in the polarization of light transmitted through a polycrystalline material and the grain sizes and orientations of that material. Experiments and supporting simulations with irregularly shaped single quartz crystals show that linear polarization is lost more rapidly as grain sizes decrease and the angular spread of the crystal orientations increase. A quantitative method using Stokes matrices to predict such changes is described and experimentally verified using an apparatus to vary the orientation of irregular quartz crystals. Grain sizes are varied between 1mm and 4mm, and the angular spreads in the crystal orientation are varied between 9 degrees and 27 degrees . This technique has applications to identify changes in crystal structure of transparent uniaxial polycrystalline materials, especially in the nondestructive characterization of glacial ice.

Spectroscopic properties and Judd-Ofelt theory analysis of La, Nd codoped Y2O3 high refractivity transparent ceramics

Nd，La:Y2O3 nanocrystalline powder was prepared by co-precipitation method. The transparent ceramic was fabricated by vacuum sintering at 1700℃ for 4 h in vacuum. Transparent polycrystalline ceramics with uniform lanthanum distribution was obtained. The grain size was around 22 μm. No pores were found in the grain and grain boundary. The absorption spectrum and the fluorescence spectrum of the sample were measured. The highest absorption peak is centered at 821 nm and the absorption cross section is 43×10-24 m2 The main emission peak is at 1078 nm, and the fluorescence lifetime is 0287 ms. The Judd-Ofelt intensity parameter Ωλ（λ=2，4，6）, spontaneous transition probability, radiative lifetime and fluorescence branching ratio of Nd3+ were calculated by Judd-Ofelt theory. The calculated emission cross section of 4F3/2→4I11/2 transition is 20×10-24 m2 It was found that the addition of La ion can change the crystalline field of yttria, which helps to obtain suitable solid laser materials.

The fluorescence and thermoluminescence characteristics of α-Al2O3 transparent ceramics

Transparent polycrystalline α-Al2O3 ceramics were fabricated by conventional solid-state processing with MgO as additive. The absorption, excitation, emission spectra and thermoluminescence of α-Al2O3 transparent ceramics were investigated. There are two characteristic absorptions, one is centered at 192 nm, and the other at from 230 to 250 nm, which are caused by F+ and F color centers, respectively. There is one obvious emission peak located at 410 nm. Based on the excitation spectra, it is shown that the emission peaks are caused by the absorption of F+ and F color centers. The thermoluminescence curves show three peaks centered at 368, 456 and 614 K, respectively. The peak centered at 456 K may be caused by F and F+ centers.

Transparent polycrystalline alumina using spark plasma sintering: Effect of Mg, Y and La doping

Abstract Transparent polycrystalline alumina (PCA) is a promising replacement for sapphire. Its optical properties however are highly dependent on the grain size and residual porosity which need to be controlled for real inline transmittances (RIT), that are high enough for possible applications. To achieve high RITs, doping as well as pressure assisted sintering is often used. In this study spark plasma sintering (SPS) and doping are investigated. A systematic experimental design is used to study the influence of Mg, Y and La single or co-doping (75–450 ppm) as well as the SPS sintering pressure and temperature on the RIT and grain size of PCA. Using optimized sintering parameters, RITs of >50% were attained in the visible wavelength (640 nm) for 0.8 mm thick samples for almost all doping strategies. The best RIT of 57% was for triple-doped samples at a total dopant level of 450 ppm. These results are significantly better than previously published SPS studies and illustrate that SPS sintered alumina can attain high and reproducible optical transmittances under various doping and sintering conditions.

Fabrications of Nb-doped TiO2 (TNO) transparent conductive oxide polycrystalline films on glass substrates by sol–gel method

Anatase Ti0.94Nb0.06O2 (TNO) films were fabricated on glass substrates by sol–gel method using a dip-coating technique. The annealing treatment was separated into two steps, first in air at 350–550 °C for 1 h and then in vacuum of 4.0 × 10−4 Pa at 550 °C for 1 h. The influence of vacuum annealing treatment to the electrical and optical properties was discussed. Especially, the role of air annealing treatment from 350 to 550 °C on the crystallization and the structure of the films was analyzed. It is proved that the films annealed at 550 °C in air and then 550 °C in vacuum exhibited the minimum resistivity of 19.3 Ω·cm and the average optical transmittance of about 75% in the visible range, indicating that the sol–gel method is a feasible and promising method to fabricate TNO films.

${\rm Lu}_{2}{\rm SiO}_{5}:{\rm Ce}$ Optical Ceramic Scintillator for PET

Phase-pure transparent LSO ceramic was obtained, with its scintillation properties rivaling those of single crystalline LSO. The transparent LSO ceramic was prepared by a nano-technology approach. The densities of the LSO ceramic increased with increasing sintering temperature. Further SEM examination confirmed the decrease of the porosity during densification of the ceramic. Pores were segregated at grain boundaries after sintering. Hot isostatic pressing (HIPing) was employed to further densify the ceramic and eliminate porosity after sintering. Transparent polycrystalline LSO ceramics were obtained after the final HIP. XRD examination confirms single monoclinic LSO phase. A light output as high as 30,100 ph/MeV was obtained using a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sup> Na excitation source. LSO ceramic showed an energy resolution of 15% (FWHM) at 662 keV ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs source) and a fast scintillation decay of 40 ns due to the 5d \ura 4f transition of Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> . The excellent scintillation and optical properties make LSO ceramic a promising candidate for future gamma-ray spectroscopy as well as medical imaging applications.

Fabrication of lanthanum doped yttria transparent ceramics

Yttria nanocrystalline powder doped with 9% lanthanum was synthesized by co-precipitation method using ammonia for pH adjusting. After calcinations, finer particles with narrow distribution and large surface area were obtained. After dry pressing, samples were sintered at 1500–1700° for 4 h in vacuum to produce transparent polycrystalline ceramics with uniform grains. Samples with 9 mol% lanthanum were transparent in visible light after being sintered at 1500° for 4 h. The grain sizes increased with lanthanum doping compared with those of pure yttria transparent ceramic sintered at the same conditions. Relative density of the transparent ceramic was 99.7%. The in-line transmittance was 73% at 580 nm wavelength after milling and polishing.

Spark‐Plasma‐Sintering Condition Optimization for Producing Transparent MgAl2O4 Spinel Polycrystal

By controlling the heating rate at &lt;10°C/min during spark‐plasma‐sintering (SPS) processing, transparent polycrystalline spinel with an in‐line transmission of 50% and 70% in the visible‐ and infrared‐wavelengths, respectively, can be successfully fabricated for only a 20‐min soak at 1300°C. The high transmission can be attained by reducing the residual porosity and pore size, which was achieved by the low‐heating rate. At high heating rates, many closed pores are formed due to the high densification rate during the heating process and remain as large pores around grain junctions. At temperatures &gt;1300°C, the coalescence of the residual pores and the precipitation of second phases, which are caused by rapid grain growth, degrade the transparency. The present study demonstrates that although the high heating rates have been regarded as a primary advantage for the SPS processing, the low heating rate is highly effective in attaining a high transparency in the spinel even at low temperatures and for short sintering times.

The effect of silica doping on neodymium diffusion in yttrium aluminum garnet ceramics: implications for sintering mechanisms

The Nd 3+ cation diffusion into transparent polycrystalline YAG (Y 3Al 5O 12) was investigated as a function of temperature and silica content. Thin neodymium oxide layers were deposited on sintered YAG substrates prior to annealing under air at temperatures from 1400 to 1600 °C. Bulk and grain boundary neodymium diffusion coefficients were measured by secondary ion mass spectrometry. The experimental results show that silica addition increases the diffusivity of Nd 3+ by a factor 10 whatever the diffusion path, probably as a result of extrinsic point defects formation, especially rare-earth vacancies. The experimental diffusion data were used to elucidate the sintering mechanism of Nd:YAG ceramics in the temperature range 1450–1550 °C. Firstly, it appeared that the intermediate stage of solid-state sintering should be controlled by the rare-earth diffusion along the grain boundary with an activation energy of about 600 kJ mol −1. Secondly, grain growth mechanism at the final stage of liquid-phase sintering was investigated for silica-doped Nd:YAG samples. Thus, the grain growth should be limited by the reaction at interfaces at a temperature lower than 1500 °C, with an activation energy of about 880 kJ mol −1. At higher temperature, it seems to be limited by the ionic diffusion through the intergranular liquid phase, with an activation energy of 250 kJ mol −1.

Precipitation synthesis and sintering of lanthanum doped yttria transparent ceramics

Co-precipitation method was used to prepare lanthanum doped yttria nanopowder. After calcinations at 1000 °C, lanthanum ion entered into the yttria lattice. Effect of lanthanum oxide as an equal valent additive on the sintering of yttria was investigated. The addition of lanthanum ions was found to be effective for promoting the densification process during sintering, and make it easier for the ceramic to be transparency at moderate 1700 °C in vacuum. After sintering at 1500 and 1700 °C for 4 h in vacuum, translucent and transparent poly-crystalline ceramics with uniform lanthanum distribution was obtained, respectively. The grain size (75 ± 13 μm) of 7% lanthanum doped yttria sintered at 1700 °C was about 10 times bigger than that sintered at 1500 °C (7.6 ± 1.9 μm). The refractive indices of lanthanum ion doped sample were higher than undoped sample in vis–UV light range. The in-line transmittance of 2 mm thick samples sintered at 1700 °C reached 74% at 800 nm (89.7% of theoretical transmittance) wavelength.

Synthesis and Densification of Transparent Magnesium Aluminate Spinel by SPS Processing

Mixtures of elementary oxides, MgO–Al2O3, were used to fabricate transparent polycrystalline magnesium aluminate spinel specimens by means of the spark plasma sintering technique. A sintering aid, 1 wt% of LiF, was added to the mixed powder. The presence of the additive promotes the synthesis of spinel that starts at 900°C and is completed at 1100°C. The LiF additive wets spinel on its melting and promotes densification, which is completed at 1600°C. LiF vapor plays a cardinal role in eliminating residual carbon contamination and in the fully dense state, allows attaining a 78% level of optical transmittance. The optimal conditions for achieving adequate transparency were determined and the role of the LiF addition in the various stages of the process is discussed.