Ceramic Crystals Research Articles

Self-Irradiation of Ceramics and Single Crystals Doped With Pu-238: Summary of 5 Years of Research of the V. G. Khlopin Radium Institute

AbstractTo investigate the resistance of actinide host phases to accelerated radiation damage, which simulates radiation induced effects of long term storage, the following samples doped with plutonium-238 (from 2 to 10 wt. %) have been repeatedly studied using XRD and other methods: cubic zirconia, Zr0.79Gd0.14Pu0.07O1.99; monazite, (La,Pu)PO4; ceramic based on Pu-phosphate of monazite structure, PuPO4; ceramic based on zircon, (Zr,Pu)SiO4, and minor phase tetragonal zirconia, (Zr,Pu)O2; single crystal zircon, (Zr,Pu)SiO4; single crystal monazite, (Eu,Pu)PO4; ceramic based on Ti-pyrochlore, (Ca,Gd,Hf,Pu,U)2Ti2O7. No change of phase composition, matrix swelling, or cracking in cubic zirconia were observed after cumulative dose 2.77×1025 alpha decay/m3. The La-monazite remained crystalline at cumulative dose 1.19×1025 alpha decay/m3, although Pu-phosphate of monazite structure became nearly amorphous at relatively low dose 4.2×1024 alpha decay/m3. Zircon has lost crystalline structures under self-irradiation at dose (1.3-1.5)×1025 alpha decay/m3, however, amorphous zircon characterized with high chemical durability. The Ti-pyrochlore after cumulative dose (1.1-1.3)×1025 alpha decay/m3 became amorphous and lost chemical durability. Radiation damage caused crack formation in zircon single crystals but not in the matrix of polycrystalline zircon. Essential swelling and crack formation as a result of radiation damage were observed in ceramics based on Ti-pyrochlore and Pu-phosphate of monazite structure, but not so far in La-monazite doped with 238Pu.

Wide band gaps and low-loss waveguiding in ceramic high dielectric constant photonic crystals

Photonic crystals based on dielectric material with very high dielectric constant ε are investigated by transmission experiments in the microwave range and by calculations of the band structure and wave propagation. The material used is a tungsten-bronze-type ceramic (BaO–Nd2O3–4TiO2) with ε≈90. Our results demonstrate the superior properties of such a material, including omnidirectional and complete gaps, and improved waveguiding. This type of photonic crystals opens an additional road for basic research and applications regarding electronic devices in the radio frequency and microwave ranges as well as for spectroscopic techniques in the terahertz range.

Crystallization of spodumene-diopside in the las glass ceramics with CaO and MgO addition

Crystallization, morphology and mechanical properties of a spodumene-diopside glass ceramics with adding different amount of CaO and MgO in Li2O-Al2O3-2SiO2 were investigated. With CaO and MgO addition, the crystallization temperature (Tp) decreased, the value of Avrami constant (n) decreased from 3.2±0.3 to 1.4±0.2, the activation energy (E) increased from 299±3 kJ mol−1 to 537±5 kJ mol−1. The crystalline phases precipitated were h-quartz solid solution, β-spodumene and diopside. The mechanism of crystallization of the glass ceramics changed from bulk crystallization to surface crystallization. The grain sizes and thermal expansion coefficients increased while flexural strength and fracture toughness of the glass-ceramics increased first, and then decreased. The mechanical properties were correlated with crystallization and morphology of glass ceramics.

Ab Initio Calculation of Elastic Constants of Ceramic Crystals

An effective computational scheme to calculate the complete set of independent elastic constants as well as other structural parameters including bulk modulus, shear modulus, Young's modulus, and Poisson's ratio for crystals is reported. The scheme is based on the stress–strain analysis approach with the appropriate selection of strain governed by symmetry consideration. The first principles Vienna ab initio simulation package (VASP) is used in stress calculations. Comprehensive tests were performed for α‐SiO2 and spinel MgAl2O4 with different exchange‐correlation potentials, and different sets of computational parameters to investigate the relative accuracies of the calculations. A wide range of oxides, nitrides, and carbonate crystals with different crystal symmetries were chosen to test the scheme under both LDA and GGA approximations at zero temperature and pressure. Some of these calculations for large complex crystals are believed to be attempted for the first time. The calculated elastic constants show quite good agreement with the existing experimental data for almost all the examined systems with the exception of the relatively soft material such as α‐SiO2 and the C14 parameter of some trigonal crystals expressed in the hexagonal form such as in α‐Al2O3. Other structural properties derived from the elastic constants also show good agreements with the measured values.

Influence of superimposed biaxial stress on the tensile strength of perfect crystals from first principles

Influence of biaxial stresses applied perpendicularly to the [100] loading axis on the theoretical tensile strength is studied from first principles. Ten crystals of cubic metals and three crystals of diamond ceramics were selected as particular case studies. Obtained results show that, within a limited range of biaxial stresses, the tensile strength monotonously increases with increasing biaxial tensile stress for most of the studied metals. Within the range, the dependence can be approximated by a linear function. Beyond the range, the dependence shows a maximum that usually appears in the tensile range of biaxial stresses. On the other hand, some of the materials (Si, Ni, Cu, and Ge) exhibit a maximum tensile strength at nearly uniaxial stress state, and the superposition of both tensile and compressive biaxial stresses reduces the tensile strength. Unlike the other crystals, diamond revealed a maximum under compressive biaxial stress.

Fabrication and Measurement of Micro Three‐Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three‐dimensional (3D) photonic crystals with a diamond structure made of a dense SiO2 ceramic were successfully fabricated using a CAD/CAM micro‐stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO2 paste (before sintering) was around 15 μm. After the SiO2‐resin photonic crystals were formed via micro‐stereolithography, they were converted to pure SiO2 ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO2 photonic crystals was measured by terahertz‐time‐domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ–X〈100〉 direction, between 490 and 630 GHz in the Γ–K〈110〉 direction, and between 400 and 510 GHz in the Γ–L〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.

Effect of heat treatment on crystallization of Nd:YAG ceramics

(Nd0.01Y0.99)3Al5O12 nano-sized powders were synthesized by low temperature combustion (LCS), using Nd2O3, Y2O3, Al(NO3)3·9H2O, ammonia water and citric acid as starting materials. The powders were characterized by TG-DTA, XRD, FT-IR, ICP and TEM, respectively. The grain sizes were calculated by the Scherrer’s formula using the full width at half maximum (FWHM) of YAG (420) crystal plane diffraction lines. The study focused on crystallization of ceramics at different heat treatment temperatures. The experimental results show that crystallizing temperature of YAG is 850 °C, and the intermediate crystal phase YAP, appearing during heat treatment, transforms to YAG cubic crystal phase at the temperature of 1 050 °C. The particle size of the powders synthesized by LCS is nano-sized. With the temperature increasing, the mean grain sizes raise, the stand deviations keep almost at the value of 2.0 and the lattice parameters decrease. The grains mainly grow by grain boundary diffusion. The lattice parameter expansion is caused by an increase of the repulsive dipolar interactions on surfaces of crystallites.

Growth behavior, morphology and properties of lithium aluminosilicate glass ceramics with different amount of CaO, MgO and TiO 2 additive

Li 2O–Al 2O 3–SiO 2 glass with CaO, MgO and TiO 2 additive were investigated. With more CaO + MgO addition, the crystallization temperature ( T p) and the value of Avrami constant ( n) decreased, the activation energy ( E) increased. The mechanism of crystallization of the glass ceramics changed from bulk crystallization to surface crystallization. With more TiO 2 addition, the crystallization temperature decreased, E and n had a little change. The crystallization of the glass ceramics changed from surface crystallization to two-dimensional crystallization. Plate-like, high mechanical properties spodumene-diopside glass ceramics were obtained. The mechanical properties related with crystallization and morphology of glass ceramics.

Development of Functionally Structured Materials: Ceramic Photonic Crystals and Fractals

Functionally structured material is a tailored material to have unique geometric structures and create new functions or high performances. Design and fabrication of 3D ceramic photonic crystals and fractals using CAD/CAM stereolithography are demonstrated as well as their unique functions of reflection and localization of electromagnetic waves as typical examples of functionally structured materials. The outlook of functionally structured materials is briefly discussed.

Ion ordering and ferroelectricity in Pb(B′0.5 B″0.5)O3 perovskites

The Pb(B′0.5 B″0.5)O3 ferroelectric with a perovskite structure and the antiferroelectric of the same composition are usually considered to be the same compound regardless of whether the B′ and B″ ions in the octahedral positions are ordered even though they differ substantially in behavior. It is established that ion ordering can also occur in PZT-type solid solutions of the composition Pb(B′x B″1 − x )O3 with x ≈ 0.5. The mechanism of the influence of ion ordering on the behavior of crystals of both complex perovskites and PZT-type ceramics is discussed.

Crystallization and Mechanical Properties of Spodumene-Diopside Glass Ceramics

The mechanical properties of spodumene-diopside glasses ceramics with 4-12% TiO2 were investigated. The main crystalline phases precipitated were eucryptite, β-spodumene and diopside. As TiO2 content increasing, the morphology of glass ceramics transformed from coarse to fine microstructure, then to reverse. The flexural strength, elastic moduli, Vickers hardness and fracture toughness of the glass-ceramics were measured. The flexural strength of glass–ceramics containing 9%TiO2 was 198MPa, the Young’s modulus and fracture toughness were 91.3GPa and 2.3 MPa·m1/2 respectively. It was indicated that the mechanical properties were correlated with crystallization and morphology of glass ceramics

Effect of TiO2 addition on crystallization and machinability of potassium mica and fluorapatite glass ceramics

In this study, the effect of TiO2 addition as a nucleating agent on the crystallization and machinability of potassium mica and fluorapatite base glass ceramics were investigated. Glass compositions were prepared and casted at predetermined temperatures. Differential thermal analysis and XRD methods were applied to characterize phase precipitation sequence and identification of phases. Disc and cylindrical shaped samples were prepared to determine microstructural and mechanical properties in terms of microhardness and machinability. FEG-SEM was used to characterize variation of microstructural constituents depending on the amount of nucleating agent. The results indicate that optimum microstructure and machinability can be obtained in machinable glass-ceramic by the addition of 1 wt% TiO2 for the composition having 3:7 weight ratio of fluorapatite to potassium mica.

Giant strain in PbZr0.2Ti0.8O3 nanowires

The authors report a giant reversible piezoelectric strain of ∼4.2% achieved with single-crystalline PbZr0.2Ti0.8O3 nanowires of ∼70nm in diameter, which is nearly 300% greater than the nominal values reported for ceramic perovskite single crystals. The giant strain arises from the reversible switching of ferroelastic domains. In the nanowires, there is a competition between surface tension, which forces in-plane polarization, and applied electric field, which stabilizes out-of-plane polarization, through the entire voltage cycle, the result of which is the reversible movement of ferroelastic domains.

Cryogenic Yb$^{3+}$-Doped Solid-State Lasers

Cryogenically cooled solid-state lasers promise a revolution in power scalability while maintaining a good beam quality because of significant improvements in efficiency and thermo-optic properties. This is particularly true for Yb lasers because of their relatively low quantum defect and relatively broadband absorption even at cryogenic temperatures. Thermo-optic properties of host materials, including thermal conductivity, thermal expansion, and refractive index at low temperature, are reviewed and data presented for YAG (ceramic and single crystal), GGG, GdVO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , and Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . Spectroscopic properties of Yb:YAG and Yb:LiYF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (YLF) including absorption cross sections, emission cross sections, and fluorescence lifetimes at cryogenic temperatures are characterized. Recent experiments have pushed the power from an end-pumped cryogenically cooled Yb:YAG laser to 455-W continuous-wave output power from 640-W incident pump power at an of M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> 1.4.

Effect of Uniaxial Stress Upon the Electromechanical Properties of Various Piezoelectric Ceramics and Single Crystals

A systematic investigation of the stress‐dependent (σ) electromechanical properties of various ferroelectric ceramics and single crystals has been performed. Studies have been carried out on “hard” and “soft” piezoelectrics, electrostrictive ceramics, and various orientations of (1−x)Pb(Mg1/3Nb2/3)O3–(x) PbTiO3 PMN–x%PT single crystals. The large signal piezoelectric constant, acoustic power density, and coupling coefficient have been determined by calculation. The results are compared, in order to develop an understanding of the relative merits of the different types of active acoustic materials.

Fabrication of three-dimensional ceramic photonic crystals and their electromagnetic properties

Fabrication process for three-dimensional ceramic photonic crystals with a diamond structure was investigated. A diamond structure composed of epoxy lattice including SiO 2–TiO 2 ceramic particles at 10 vol.% was fabricated as a precursor by stereolithography. After burning off the epoxy resin in air, the diamond structure of SiO 2–TiO 2 was successfully sintered at 1400 °C for 2.5 h. The linear shrinkage ratio was 50%. Cracks were not found in the sintered diamond structure. Photonic bandgap was observed at around 19 GHz.

Synthesis and Greatly Enhanced Fluorescence Emission of Transparent Nd-doped Y3ScxAl5−xO12 Ceramics

Nd-doped Y3ScxAl5−xO12(Nd:YSAG) powder were prepared with a chemical combustion method. The powders were nano-sized and had a pure cubic phase when calcined at 900 °C. Transparent Nd:YSAG ceramics with up to 40% scandium substitution for aluminum were successfully fabricated by sintering the powder compact at 1800 °C under H2 atmosphere. The synthesis process and optical properties were investigated in detail. It was found that the light emission intensity at 1064 nm of the Nd:YSAG with 40% scandium substitution for aluminum can be enhanced by 2–3 times over that of Nd:YAG single crystal when pumped with the same 808-nm diode laser. In addition, the material was found to have prolonged fluorescence lifetime. This highly enhanced light emission intensity is fundamentally important for obtaining higher light output together with suppressed self-heating than Nd:YAG ceramic and single crystals.

Crystallization and properties of a spodumene-willemite glass ceramic

Spodumene-willemite glass ceramics were produced by replacement of Al 2O 3 in lithium aluminium silicate by ZnO. With replacement of Al 2O 3 by ZnO, the batch melting temperature, glass transition temperature ( T g) and crystallization temperature ( T p) all decreased. The main crystalline phases precipitated were eucriptite, β-spodumene and willemite (Zn 2SiO 4). All compositions of glass ceramics showed bulk crystallization. As ZnO content increased, the grain sizes and thermal expansion coefficients increased, while the flexural strength and fracture toughness of the glass-ceramics increased first, and then decreased. The mechanical properties were correlated with crystallization and morphology of glass ceramics.

Fabrication of TiO2–SiO2 Photonic Crystals with Diamond Structure

Ceramic photonic crystals with diamond structure were fabricated using stereolithography and successive sintering. The green body of an epoxy resin incorporating 10 vol% TiO2–SiO2 was formed by stereolithography and then heated in air at 1100°–1400°C for 2 h. The sintered products maintained the diamond structure with a linear shrinkage ratio of about 57% and a porosity of 38%. The ceramic photonic crystal with eight unit cells showed a photonic band gap at the center frequency of 23.5 GHz. This fabrication method of three‐dimensional (3D) ceramic photonic crystals is applicable to other 3D structural ceramics and does not require any molding techniques.

Fabrication and optical reflection behavior of a two-dimensional barium titanate ceramic photonic crystal

We demonstrate a method for fabricating a two-dimensional hexagonal array of nanoscale barium titanate ceramic pillars. A precursor array was prepared by introducing a sol solution into an electron-beam resist mold with air holes and subsequently dry etching the unnecessary gel layer formed on the mold. A ceramic array, consisting of barium titanate ceramic pillars with a radius of 85 nm, a lattice constant of 450 nm, and a height of 500 nm, was fabricated by sintering the precursor array at 1073 K. The reflection spectra for the ceramic array in the visible range revealed the underlying photonic band gaps, which were in good agreement with results calculated using a plane-wave method.