Shankar Krishnan

Structure of supercooled liquid silicon

We report x-ray diffraction measurements of the structure factor S(Q) and the radial distribution function g(r) of levitated liquid silicon in the stable and supercooled states. Supercooling results in a sharpening of the first peak in S(Q) and shift to an 8% higher Q value, the appearance of a double shoulder on the high-r side of the first peak in g(r), a sharpening of the first peak in g(r) and a decrease in coordination number. These changes are consistent with a significantly enhanced degree of covalent bonding.

Structural changes on supercooling liquid silicon

X-ray diffraction experiments and ab initiomolecular dynamics (AIMD) simulations have been performed to investigate the atomic structure of liquidsilicon into the deeply supercooled region. The levitation technique used has made it possible to extend the measurements down to a temperature of 1458 K, 230 K below the equilibrium melting point. The x-ray and AIMD results, which are in reasonable agreement, show conclusively that the tetrahedral order is reinforced and that the coordination number decreases as the liquid is supercooled, with implications for the liquid–liquidphase transition.

Levitation apparatus for structural studies of high temperature liquids using synchrotron radiation

A new levitation apparatus coupled to a synchrotron-derived x-ray source has been developed to study the structure of liquids at temperatures up to 3000 K. The levitation apparatus employs conical nozzle levitation using aerodynamic forces to stably position solid and liquid specimens at high temperatures. A 270 W CO2 laser was used to heat the specimens to desired temperatures. Two optical pyrometers were used to record the specimen temperature, heating curves, and cooling curves. Three video cameras and a video recorder were employed to obtain and record specimen views in all three dimensions. The levitation assembly was supported on a three-axis translation stage to facilitate precise positioning of the specimen in the synchrotron radiation beam. The levitation system was enclosed in a vacuum chamber with Be windows, connections for vacuum and gas flow, ports for pyrometry, video, and pressure measurements. The vacuum system included automatic pressure control and multi-channel gas flow control. A phos...

X-ray diffraction from levitated liquids

The ability of containerless techniques to access very high liquid temperatures, maintain specimen purity, enable control of specimen chemistry and access the supercooled state has created new opportunities to study high-temperature liquids with a very high degree of control. Recently, these have been combined with x-ray diffraction at synchrotron sources to provide structural information not previously available. This article reviews and summarizes recent results obtained from x-ray diffraction on levitated liquid materials in both the normal and supercooled states. In particular, it addresses the effects of supercooling on the short-range structure, including interatomic distances and coordinations, of several high-temperature molten oxides, semiconductors and metallic materials. The properties of these liquids are discussed in the context of available molecular dynamics simulations and thermophysical property data.

X-ray and electrostatic levitation undercooling studies in Ti–Zr–Ni quasicrystal forming alloys

The first undercooling measurements on electrostatic-levitated droplets of TiZrNi alloys that form the icosahedral quasicrystal phase are presented. The reduced undercooling for crystallization decreases with an increasing polytetrahedral order of the primary solidifying phase, suggesting the development of icosahedral short-range order in the undercooled liquid. X-ray diffraction measurements made at the advanced photon source on liquid droplets of these alloys, aerodynamically levitated and heated to near their liquidus temperature, however, show only weak evidence for increased icosahedral order. This suggests that significant ordering occurs below the melting temperature.

Mueller-matrix ellipsometry using the division-of-amplitude photopolarimeter: a study of depolarization effects

A fully automated Mueller-matrix ellipsometer with a division-of-amplitude photopolarimeter as the polarization-state detector is described. This device achieves Mueller-matrix ellipsometry by measuring the Stokes parameters of reflected light as a function of the fast axis C of a quarter-wave retarder, which, in combination with a fixed linear polarizer, determines the polarization state of incident light. The reflected Stokes parameters were Fourier analyzed to give the 16 elements of the Mueller matrix. We investigated depolarization of polarized light on reflection from rough, heterogeneous, and anisotropic surfaces by obtaining measurements on rolled aluminum and plant leaves. The results demonstrate (1) a variation of degree of polarization of reflected light with the input polarization state, (2) the precision with which the measured matrices describe the depolarization results, (3) effects of surface anisotropy (rolling direction) on depolarization and cross polarization by reflection from aluminum surfaces, and (4) large values and differences in the depolarization effects from conifer and deciduous leaves. Depolarization of light reflected by the aluminum surfaces was most sensitive to the angle between the plane of incidence and the rolling direction when the incident Stokes parameters S(1), S(2), and S(3) were equal.

The electrical conductivity of levitated liquids

The electrical conductivities of aerodynamically levitated liquid spheres have been determined by an electrodeless method. It is shown that this technique is reliable over a wide range of temperatures; results are presented for a variety of systems including metals, semiconductors at room temperature and at their melting points, and solid and liquid Al2O3.

Spectral emissivities in the visible and infrared of liquid Zr, Ni, and nickel‐based binary alloys

This article reports: (i) values of the spectral emissivities at λ=0.633 μm for liquid Zr, Ni, Ni–25% Sn, Ni–32.5% Sn, Ni–40% Nb, and Ni–75% Zr over a wide temperature range including undercooled and superheated conditions, (ii) experimentally determined infrared radiance temperatures (effective center wavelength about 2.0 μm) as functions of true temperature, and (iii) derived values of the spectral emissivity of these liquid materials in the infrared. Based on the thermal arrest, which occurs upon heating the solid at the melting point, we also report melting points for some of the alloys. For zirconium, temperature dependence of emissivity is reported at other wavelengths in the visible. For Ni–75% Zr, the emissivity dependence on wavelength was determined and compared with previously reported results for liquid zirconium and nickel. The spectral emissivity measurements were obtained using pulsed‐dye laser ellipsometry on electromagnetically levitated liquid metals and the radiance temperatures of the ...

Spectralemissivity and optical properties at λ = 632.8 nm for liquid uranium and zirconium at high temperatures

Abstract The spectral emissivities, refractive indices, and extinction coefficients of pure liquid uranium and zirconium were measured versus temperature by He-Ne laser polarimetry at a wavelength of 632.8 nm. The experiments were conducted under containerless conditions using electromagnetic levitation and heating supplemented by CO2 laser beam heating. Clean liquid metal surfaces were achieved by heating the specimens to high temperatures at which oxides evaporated and nitrides decomposed. Results were obtained for liquid uranium and zirconium in the temperature ranges 2000–2800 K and 2000–2600 K, respectively and included data for liquid zirconium undercooled by 125 K. The spectral emissivity of zirconium was equal to 0.345 and was independent of temperature. The spectral emissivity of uranium increased with temperature from 0.272 at 2000 K to 0.294 at 2800 K. The melting temperature of zirconium was determined from its emissivity and apparent melting temperature to be 2125±11 K , in good agreement with values in the literature.

Short- and intermediate-range order in levitated liquid aluminates

We have used the aerodynamic levitation technique combined with CO 2 laser heating to study the structures of liquid CaAl 2 O 4 and MgAl 2 O 4 with x-ray and neutron diffraction. We determined the structure factors and corresponding pair correlation functions describing the short- and intermediate-range order in the liquids. The combination of the two scattering techniques makes it possible to derive information not accessible with a single measurement. In the case of the glass-forming liquid CaAl 2 O 4 we have made sequential measurements during free cooling to study the structural evolution during supercooling from the stable liquid phase to the cold glass below Tg.