H. R. Xia

Monte Carlo simulation of liquid water in a magnetic field

A Monte Carlo simulation of liquid water was carried out in order to study the influence of an external magnetic field on the internal energy and heat capacity of pure water. Analysis of the structure of water was made by calculating the radial distribution function of water molecules. A multipeak function of the magnetic effect versus magnetic flux density was found. A significant change in the internal energy and heat capacity occurred at a magnetic flux density of about 0.2 T. In particular, when the magnetic flux density increases, the second peak of the water–water radial distribution function increases markedly. This has been demonstrated by calculations for both 64- and 125-molecule systems.

Thermal and mechanical properties of BaWO4 crystal

A large single crystal of barium tungstate (BaWO4) with dimensions of 22-mmdiameter×80-mm length was grown by the Czochralski method using an iridium crucible. The melting point, molar enthalpy of fusion, and molar entropy of fusion of the crystal were determined to be 1775.10 K, 96913.80Jmol−1, and 54.60JK−1mol−1, respectively. The average linear thermal-expansion coefficients are αa=10.9526×10−6∕K, αb=10.8069×10−6∕K, and αc=35.1063×10−6∕K in the temperature range from 303.15 to 1423.15 K along the three respective crystallographic axes. The density of the crystal follows an almost linear decrease from 6.393×103 to 6.000×103kgm−3 when the temperature is increased from 303.15 to 1423.15 K. The measured specific heat are 115.56–130.96JK−1mol−1 in the temperature range of 323.15–1173.15 K. The thermal diffusion coefficient of the crystal was measured in the temperature range of 297.15–563.15 K. The calculated thermal conductivity is 2.256Wm−1K−1 along the [001] direction and 2.324Wm−1K−1 along the [100] dir...

Raman spectra and laser properties of Yb-doped yttrium orthovanadate crystals

Yb-doped yttrium orthovanadate YbxY1−xVO4 (YYV), with x=0.02, crystallizes with a zircon-type structure in the tetragonal system, conforming to the space group I41/amd. The lattice constants are a=0.7122(5) and c=0.6291(3) nm at room temperature. The primitive cell contains four formula units. The Raman spectra and crystallography show no distortion of the VO4 and Y/YbO8 groups. The crystal field is similar to that of the YVO4 crystal and is suitable for the dopant Yb3+ ions. A strong absorption at 975 nm arises from the 2F7/2 to 2F5/2 transition; with a bandwidth of 70 nm it may provide a useful pump band for InGaAs diode lasers. This band is about seven times broader than that of Nd-doped yttrium aluminum garnet and more than three times as broad as that of Nd-doped yttrium orthovanadate. The measured absorption of the YYV is compared to Judd–Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric-dipole transitions of rare earth ions on noncentrosymmetric sites demonstrates good agr...

Characterization of mixed Nd:LuxGd1−xVO4 laser crystals

A series of laser crystals Nd:LuxGd1−xVO4 (x=0.14,0.32,0.50,0.61,0.70,0.80) was grown by the Czochralski method. The thermal properties, including the average linear thermal expansion coefficients, thermal diffusion coefficients, specific heats, and thermal conductivities, of the mixed crystals were obtained. The material constants Ms for the thermal stress resistance figure were calculated and showed that the thermal fracture limits of the mixed crystals should be comparable with that of Nd:YVO4. The polarization absorption spectra from 240to1000nm were measured at room temperature and the absorption cross sections at 809nm were calculated. Using the Judd-Ofelt theory, the theoretical radiative lifetimes were calculated and compared with the experimental results. Continuous wave laser performances were achieved with the mixed crystals at the wavelength of 1.06μm when they were pumped by a laser diode. Thermal, optical, and laser properties have shown variation as a function of x and proved that the mixed...

Vibrational Spectra of a K0.30Na0.10Sr0.48Ba0.32Nb0.2O0.6 Single Crystal Studied by Raman and Infrared Reflectivity Spectroscopy

Potassium and sodium (K/Na)-modified strontium barium niobate (SBN) of the idealized formula (K x Na 1 x)0.4(SryBa1-y)0.8Nb2O6 (KNSBN). with x = 0.75. y = 0.60, a Curie temperature of (200 ± 2) C, and a laser damage threshold larger than 600 MW/cm 2 at 4 pps, crystallizes with a filled tungsten bronze (TB) type structure in the tetragonal system. The lattice constants are a = (1.2488 ± 0.0001) and c = (0.3955 ± 0.0001) nm at room temperature. There are five formulas per unit cell. The lattice vibrational spectrum of KNSBN, which arises mainly from internal vibrations of the Nb-O octahedra and studied by Raman and infrared reflectivity spectroscopy, typifies a tetragonal TB type structure. The results shown in Figs. 1 to 6 are typical for vibrational spectra, The NbO 6 clusters form the rigid Nb-O octahedra, and the rigidity of the lattice is mainl ascribed to five vibrations, as a non-degenerate symmetric Nb-O stretching vibration (v 1 ), a doubly degenerate symmetric O-Nb-O stretching vibration (v 2 ). >a >triply |>|>>degenerate >antisymmetric >Nb-O>stretching >vibration >(v 3 ). a triply degenerate antisymmetric O-Nb-O bending vibration (v 4 ), and a triply degenerate symmetric O-Nb-O bending vibration (v 5 ). The KNSBN crystals as well as the SRN crystals show good structural quality and mechanical properties, and large linear electro-optical and pyroelectric coefficients. Besides, the nonlinear optical property of KNSBN is better than that of SBN,

Growth and thermal properties of SrWO4 single crystal

A strontium tungstate (SrWO4) single crystal with dimensions of Φ 25×40mm2 has been grown by the Czochralski method using an iridium crucible. X-ray powder diffraction results show that as grown the SrWO4 crystal belongs to the tetragonal system in the scheelite structure. The effective segregation coefficients of elemental W and Sr in the crystal growth are measured by x-ray fluorescence, and the respective values are close to 1. The thermal properties of the SrWO4 crystal were systemically studied by measuring the thermal expansion, specific heat, and thermal diffusion coefficient. These results show that the crystal possesses a large anisotropic thermal expansion with thermal-expansion coefficients αa=8.61×10−6∕K, αb=8.74×10−6∕K, αc=18.78×10−6∕K over the temperature range of 303.15–773.15K. The measured value of the specific heat is 0.30–0.34Jg−1K−1 when the temperature is increased from 323.15to1073.15K. The thermal diffusion coefficient was measured over the temperature range of 303.15–543.15K. The t...

Thermal properties of monoclinic crystal Er3+:Yb3+:Ca4YO(BO3)3

A large Er3+–Yb3+ co-doped yttrium calcium oxoborate [Er3+:Yb3+:Ca4YO(BO3)3, abbreviated as Er:Yb:YCOB] single crystal, with dimensions of 2.5 cm in diameter and 9.0 cm in length, has been grown along the crystallographic b axis by the Czochralski method. X-ray powder diffraction results show that the as-grown Er:Yb:YCOB crystal belongs to the monoclinic system with space group Cm; the unit-cell constants are a = 8.085, b = 16.048, c = 3.528 A and β = 101.11°. The high crystalline quality of the as-grown single crystal was confirmed by high-resolution X-ray diffraction, which showed the full width at half-maximum of the rocking curves to be less than 20 arcseconds on the (060) and (\overline 201) diffraction planes. The measurement and calculation of the symmetrical second-rank tensor for the monoclinic crystal are described in detail in this paper. The principal coefficients of thermal expansion of the as-grown Er:Yb:YCOB crystal are αI = 11.95 × 10−6 K−1, αII = 9.20 × 10−6 K−1 and αIII = 18.93 × 10−6 K−1 over the temperature range 303.15–873.15 K. The specific heat of the crystal is 725.6 J kg−1 K−1 at 328.15 K. The principal thermal conductivity parameters are KI = 2.882 W m−1 K−1, KII = 2.687 W m−1 K−1 and KIII = 2.692 W m−1 K−1 at 328.15 K.

Spectral parameters of Nd-doped yttrium orthovanadate crystals

Nd-doped yttrium orthovanadate (Nd:YVO4, abbreviated as NYV) crystal has been grown using a modified Czochralski method. Its main structural parameters and optical and laser properties are all presented. The absorption spectrum of Nd3+ ions doped in NYV in a wavelength range from 190 to 900 nm and that of free Nd3+ ions in HCl solution of 0.2 mol in the range of 190–900 nm have both been measured. The width of the absorption band around 808 nm to match the laser diode pumping is about 28 nm, which is approximately three times broader than that of Nd-doped yttrium aluminum garnet crystal. The line intensity of the absorption spectrum of NYV has been calculated. The calculated luminescence parameters of NYV are also listed.

Optical parameters and luminescent properties of Nd:GdVO4 crystals

Based on the quantum electrodynamics and using Einstein’s coefficient and Planck’s energy density, the stimulated emission cross section was given. The measured absorption of 0.5 at. % Nd-doped Nd:GdVO4 crystal was compared to the Judd–Ofelt (JO) theory. When applied, the JO theory of parity-forbidden electric–dipole transitions of rare-earth ions on noncentrosymmetric site lacking inversion symmetry demonstrates good agreement. An absorption band around 808 nm is suitable to match laser diode pumping. The branching ratio of the fluorescence emission and stimulated emission cross section at the emissive wavelength of 1064 nm are of great advantage to produce a laser output.

Thermal characterization of lowly Nd 3+ doped disordered Nd:CNGG crystal

Thermal properties of a lowly Nd(3+)-doped disordered Nd:CNGG crystal have been symmetrically investigated. At room temperature, the specific heat, thermal diffusion coefficient and density were determined to be 0.595 J/g.K, 1.223 mm(2)/s and 4.718 g/cm(3), corresponding the thermal conductivity of 3.43 W/m.K. By measuring the thermal lens at different pump power, the thermal-optical coefficient was calculated to be 9.2x10(-6) K(-1) for the first time to our knowledge. All the thermal properties recovered that this material can be used in the moderate and even high pump power.