White, Mary Anne

Permanent URI for this collectionhttps://hdl.handle.net/10222/27750

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Open Access
High-resolution, high-sensitive ac calorimeter.
(1992) Bednarz, Grzegorz; Millier, Brian; White, Mary Anne
A high-resoln., high-sensitivity, automated ac calorimeter capable of heat capacity measurements of very small samples (m < 20 mg) with a temp. resoln. of a few mK in the ac mode is described. This calorimeter also can be operated in the relaxation mode to provide abs. heat capacity values with precision and accuracy of around 2%. This calorimeter was tested by measuring the heat capacity of gadolinium over its ferromagnetic phase transition. The results agree well with the literature data giving, however, substantially better resoln. of the heat capacity in the crit. region.
Open Access
Automated, small sample-size adiabatic calorimeter.
(1987) Van Oort, Michiel J. M.; White, Mary Anne
An automated adiabatic calorimeter is described with an internal vol. of 5 cm3, operable at 30-380 K. One of the main advantages of this calorimeter over others in use is the shorter down time during sample changes, due to interchangeable sample vessels that fit into the heater/thermometer assembly. This calorimeter was tested by measuring the heat capacity of benzoic acid, and the results agreed with the literature values to within 0.5%.
Open Access
Spin waves and quantum criticality in the frustrated XY pyrochlore antiferromagnet Er2Ti2O7
(2008-10) Ruff, J. P. C.; Clancy, J. P.; Bourque, A.; White, M. A.; Ramazanoglu, M.; Gardner, J. S.; Qiu, Y.; Copley, J. R. D.; Johnson, M. B.; Dabkowska, H. A.; Gaulin, B. D.
We report detailed measurements of the low temperature magnetic phase diagram of Er2Ti2O7. Heat capacity and time- of- flight neutron scattering studies of single crystals reveal unconventional low- energy states. Er3+ magnetic ions reside on a pyrochlore lattice in Er2Ti2O7, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long- range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of noncollinear phases, divided by a zero temperature phase transition at mu(0)Hc similar to 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.
Open Access
Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates (vol 85, pg 114, 2000)
(2001-05) Tse, JS; Uehara, K.; Rousseau, R.; Ker, A.; Ratcliffe, CI; White, MA; MacKay, G.
No abstract available.
Open Access
X-ray absorption spectroscopy studies of local structure and electronic properties of NaxSi136 (0 < x < 24) clathrates
(2010-10) Ritchie, Andrew D.; MacDonald, Mark A.; Zhang, Peng; White, Mary Anne; Beekman, Matt; Gryko, Jan; Nolas, George S.
We have probed both the local structure and the electronic properties of Na-containing type II Si clathrates by x-ray absorption spectroscopy. The near-edge region of the spectrum is particularly sensitive to the local structure surrounding the Na atom. Through experimental investigations of a series of eight samples with general formula NaxSi136, with x ranging from similar to 0 to 21.5, and simulation of extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure spectra (XANES), we find that Na is primarily in the larger Si-28 cages at low loadings, and loss of the Na relative to full cage occupancy occurs preferentially from the smaller Si-20 cages. Our EXAFS results also show that Na is dynamically disordered in the Si-28 cages but less disordered at higher Na loadings. Local-orbital density of states calculations indicate that the Na in the Si-20 cage has a charge of 0.7e(-) while Na in the Si-28 cages has a slightly higher (loading-dependent) charge (0.72e(-) to 0.8e(-)).
Open Access
Structural principles and amorphouslike thermal conductivity of Na-doped Si clathrates
(2000-07) Tse, JS; Uehara, K.; Rousseau, R.; Ker, A.; Ratcliffe, CI; White, MA; MacKay, G.
The postulated low thermal conductivity and the possibility of altering the electronic conductivity of metal-doped clathrates with semiconducting host elements have stimulated great interest in exploring these compounds as promising thermoelectric materials. The electronic and thermal properties of the prototypical NaxSi46 system are studied in detail here. It is shown that, despite the fact that the Na/Si clathrate is metallic, its thermal conductivity resembles that of an amorphous solid. A theoretical model is developed to rationalize the structural stability of the peculiar structural topology, and a general scheme for rational design of high efficiency thermoelectric materials is presented.
Open Access
Thermal and lattice dynamical properties of Na8Si46 clathrate
(2001-07) Qiu, LY; White, MA; Li, ZQ; Tse, JS; Ratcliffe, CI; Tulk, CA; Dong, JJ; Sankey, OF
The experimental heat capacity of Na8Si46 has been determined from 35-300 K, and its lattice parameters have been measured over the range 100-330 K. The experimental heat capacity and the thermal-expansion coefficient are compared with theoretical lattice-dynamical calculations for Na8Si46. The latter accurately reproduce the experimental thermal expansion and also give the first reliable assignment of the vibrational spectrum of this material, as judged by comparison of the calculated and experimental heat capacities. In addition, the theoretical results allow a calculation of the Gruneisen parameter of Na8Si46, which shows enhanced anharmonicity at low temperatures.
Open Access
Heat capacity of gadolinium near the Curie temperature.
(1993) Bednarz, Grzegorz; Geldart, D. J. W.; White, Mary Anne
High-resoln. ac calorimetric data near the Curie point are reported for several single crystals of gadolinium. The crit. temp. and the shape of the heat-capacity curve near Tc both depend on the sample-prepn. procedure, including heat treatments. The heat-capacity data are analyzed in terms of predictions of renormalization-group theory. This anal. shows that the crit. behavior of Gd is consistent with the picture of a complex crit. behavior consisting of a series of crossovers dictated by the interplay between short-range and magnetic-dipolar interactions.
Open Access
Structure, thermal, and transport properties of the clathrates Sr8Zn8Ge38, Sr8Ga16Ge30, and Ba8Ga16Si30
(2004-07) Qiu, LY; Swainson, IP; Nolas, GS; White, MA
The structural parameters, thermal properties, and transport properties of three type I clathrates, namely Sr8Zn8Ge38, Sr8Ga16Ge30, and Ba8Ga16Si30, have been determined at or below room temperature. The structural parameters of these clathrates were determined by powder neutron diffraction. Their lattice thermal expansion is two to four times greater than that of the diamond phases of silicon and germanium, consistent with more anharmonic lattice vibrations. From the temperature dependence of the isotropic atomic displacement parameters, the estimated rattling frequencies of guests in the large cages of these clathrates are in the range 50-60 cm(-1). The heat capacities of these three clathrate materials increase smoothly with increasing temperatures and approach the Dulong-Petit value around room temperature. The Gruneisen parameter of these materials is constant between 100 and 300 K but increases below 100 K, due to the dominance of the low-frequency guest-rattling modes. The room-temperature electrical resistivity and the Seebeck coefficient show that these materials are metallic. The temperature profile of the thermal conductivities and calculated phonon mean free paths of these materials show glasslike behavior, although they are crystalline materials, indicating strong resonant scattering of heat-carrying acoustic phonons via the rattling of the guests in the clathrate cages.
Open Access
Characterization of the Incommensurate Phase of [P(CH3)4]2CuCl4
(1989-02) PRESSPRICH, MR; BOND, MR; WILLETT, RD; WHITE, MA
No abstract available.
Open Access
Technique for determination of accurate heat capacities of volatile, powdered, or air-sensitive samples using relaxation calorimetry
(2006-09) Marriott, Robert A.; Stancescu, Maria; Kennedy, Catherine A.; White, Mary Anne
We introduce a four-step technique for the accurate determination of the heat capacity of volatile or air-sensitive samples using relaxation calorimetry. The samples are encapsulated in a hermetically sealed differential scanning calorimetry pan, in which there is an internal layer of Apiezon N grease to assist thermal relaxation. Using the Quantum Design physical property measurement system to investigate benzoic acid and copper standards, we find that this method can lead to heat capacity determinations accurate to +/- 2% over the temperature range of 1-300 K, even for very small samples (e.g., < 10 mg and contributing ca. 20% to the total heat capacity). (c) 2006 American Institute of Physics.
Open Access
Apparatus for dynamical thermal measurements of low-thermal diffusivity particulate materials at subambient temperatures
(1998-12) Murashov, VV; White, MA
Apparatus for measurement of thermal diffusivity of particulate matter in the temperature range of 10-300 K is described. The basis of the measurement is modulated radial heat flow into a cylindrical sample, with a relatively small temperature gradient. Data is analyzed by Fourier transformation, and thermal diffusivity is determined from both the phase lag and amplitude ratio of the temperature at various locations along the cross section of the cylinder. This apparatus was tested with solid benzene, and found to give measurements within +/- 30%, in accord with the error analysis. It is especially useful for the determination of low thermal diffusivity materials, in the range of 1 x 10(-8) to 5 x 10(-6) m(2) s(-1).
Open Access
A thermodynamic investigation of dynamical disorder in phase II of CBr4.
(1995) Michalski, Darek; White, Mary Anne
CBr4 is an archetypal example of a pseudospherical mol. that can form an orientationally disordered phase (phase I, 320 K < T < 365 K). At lower temps., phase II is the stable phase; although it must be more ordered than phase I, different techniques give conflicting results concerning order in phase II. In order to investigate dynamic disorder in phase II, the heat capacity of CBr4 was measured in the temp. range 30-305 K. No phase transitions were found in this region. The calcd. heat capacity, with contributions due to low-frequency translation and rotational-vibrations of the rigid CBr4 mols., as well as internal motions, fits the exptl. derived C? data to within 3%. Virtually full excitation of rigid mol. rotation-vibrations at very low temps. (?45 K) is consistent with the observation of a low-temp. elevation in the calcd. Grueneisen parameter.
Open Access
Transport properties of Na8Si46
(2001-10) Nolas, GS; Ward, JM; Gryko, J.; Qiu, L.; White, MA
We report on transport properties of polycrystalline Na8Si46 in the temperature range 10 to 300 K. The compound is a type I clathrate with Na incorporated into polyhedra formed by framework Si atoms. Seebeck coefficient and resistivity measurements indicate metallic behavior, The thermal conductivity is low in comparison with crystalline Si, but high in comparison with other type I clathrates with a large contribution due to electronic conduction. The potential for thermoelectric applications also is discussed.
Open Access
Thermal and Diffraction Studies of Phase-Transitions in the Incommensurate Compound [P(Ch3)(4)]2CuCl4
(1994-01) BEDNARZ, G.; WHITE, MA; PRESSPRICH, MR; WILLETT, RD
Calorimetric and x-ray-diffraction studies of the commensurate
Open Access
Thermal conductivity of an organic clathrate: Possible generality of glasslike thermal conductivity in crystalline molecular
(1997-04) Michalski, D.; White, MA
No abstract available.
Open Access
Heat capacity, lattice dynamics, and thermodynamic stability of the negative thermal expansion material HfMo2O8
(2007-06) Kennedy, Catherine A.; White, Mary Anne; Wilkinson, Angus P.; Varga, Tamas
We explore the lattice dynamics of the negative thermal expansion material, cubic HfMo2O8, through analysis of its heat capacity (measured from 0.5 to 300 K) and its room-temperature Raman spectrum. Its heat capacity is quantitatively very similar to that of ZrW2O8, as is its Raman spectrum. The heat capacity of HfMo2O8 can be well represented by the present lattice dynamical assignment and by C-P(HfW2O8)-C-P(ZrW2O8)+C-P(ZrMo2O8), but not by C-P(HfO2)+2C(P)(MoO3), likely because the AB(2)O(8) compounds have low-frequency optic modes, not present in HfO2 and MoO3. The present thermodynamic data also allow an analysis of the thermodynamic stability of cubic HfMo2O8, and it is shown to be unstable with respect to MoO3 and HfO2 at room temperature.
Open Access
Low thermal conductivity of the negative thermal expansion material, HfMo2O8
(2007-04) Kennedy, Catherine A.; White, Mary Anne; Wilkinson, Angus P.; Varga, Tamas
The thermal conductivity of the cubic polymorph of hafnium molybdate, HfMo2O8, was determined over the temperature range of 2-400 K. The values of thermal conductivity were low (0.64 +/- 0.15 W m(-1) K-1 at T=300 K for a fully dense sample), with a positive temperature coefficient throughout the temperature range examined. Calculations of the theoretical minimum thermal conductivity and the effective phonon mean free path show that the heat-carrying phonons are nearly fully coupled in HfMo2O8, consistent with the rather large magnitude of its Gruneisen parameter. The low-frequency optic modes which lead to negative thermal expansion in HfMo2O8 are likely responsible for the low thermal conductivity. (c) 2007 American Institute of Physics.
Open Access
Temperature dependence of thermal conductivity enhancement in single-walled carbon nanotube/polystyrene composites
(2010-02) Jakubinek, Michael B.; White, Mary Anne; Mu, Minfang; Winey, Karen I.
The thermal conductivity of single-walled carbon nanotube (SWCNT)/polystyrene composites, prepared by a method known to produce a uniform distribution of SWCNT bundles on the micrometer length scale, was measured in the temperature range from approximately 140 to 360 K. The thermal conductivity enhancement (50% for 1 mass % at 300 K) is reasonably constant above room temperature but is reduced at the lower temperatures. This result is consistent with the expected, large contribution of interfacial thermal resistance in SWCNT/polymer composites. Enhancements in electrical conductivity show that 1 mass % loading is in the region of the electrical percolation threshold.
Open Access
Neutron-Scattering Spectra from the Hindered Rotation of Methane in Methane + Krypton Mixtures Adsorbed on Graphite
(1985) ALLEN, MP; HOVENDEN, CB; HUMES, RP; METHA, SK; THOMAS, RK; WHITE, MA
No abstract available.

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