The system LiHo_<em>x</em>Y_1-<em>x</em>F₄ is a nearly perfect example of a dilute, dipolar-coupled Ising magnet and, as such, it is an ideal testing ground for many theories in statistical mechanics. At low holmium concentration (<em>x</em> = 0. 045) an unusual spin liquid or "anti-glass" state was discovered in previous work [1]. This state does not exhibit a spin glass freezing transition as is expected for a long-range interaction. Instead, it shows dynamics which are consistent with a collection of low-frequency oscillators [2]. It was also seen to have sharp features in its specific heat [3]. <br /><br /> We present heat capacity measurements on three samples at and around the concentration of the spin liquid state in zero magnetic field and in a temperature range from around 50 mK to 1 K. In contrast to previous measurements, we find no sharp features in the specific heat. The specific heat is a broad feature which is qualitatively consistent with that of a spin glass. The residual entropy as a function of <em>x</em>, obtained through a numerical integral of the data, however, is consistent with numerical simulations which predict a disappearance of spin glass ordering below a critical concentration of dipoles [4]. <br /><br /> Also presented here, is ac susceptibility data on an <em>x</em> = 0. 45 sample which exhibits a paramagnetic to ferromagnetic transition and is found to be consistent with previous work.

This thesis will present research that studies the role of disorder, geometric frustration and the long range dipolar interaction on the collective behaviour of several insulating, rare earth magnets. Experiments were performed at low temperatures to measure the specific heat and magnetic susceptibility of several materials. Susceptibility was measured with a SQUID magnetometer that has been designed and constructed primarily for the study of slow dynamics in glassy systems. Specifically, this thesis will discuss three distinct topics. The first is the series of materials LiHo(x)Y(1-x)F(4), which are manifestations of the dilute, dipolar coupled Ising model. The low-x portion of the phase diagram has become a rather contentious issue in recent years with both theoretical and experimental groups disagreeing on the existence of a spin glass freezing transition and one experimental group arguing for the existence of an exotic "antiglass'' or spin liquid state resulting from quantum entanglement at x=0.045. We present specific heat and dynamical susceptibility measurements on four stoichiometries in this series: x = 0.018, 0.045, 0.080 and 0.012. No evidence of an unusual antiglass state is observed. Instead, our results show evidence, at all dilution levels studied, of a spin glass freezing transition. Interpretation of experimental data is found to be complicated by the anomalously slow dynamics in these materials. The relaxation time scales are found to increase as the concentration of Ho(3+) ions is reduced, an effect which can be attributed to single-ion physics and the importance of the nuclear hyperfine coupling in this system. A second set of materials studied here is a series of several Gd garnet materials, the most famous of which is Gd(3)Ga(5)O(12) (GGG), a material previously argued to be a disorder-free spin glass. Our specific heat experiments reproduce previous experiments on GGG and show that the homologous Gd garnets Gd(3)Te(2)Li(3)O(12) and Ga(3)Al(5)O(12) do not share the same glassy physics but exhibit sharp ordering features. By experimenting with the introduction of random site dilution, it is concluded that a 1-2% off-stoichiometry inherent in GGG is likely a special kind of disorder that is particularly effective in inducing random frustration and the formation of a spin glass. Finally, specific heat measurements on the pyrochlore antiferromagnet Gd(2)Sn(2)O(7) (GSO) are presented. While GSO has generally been found to be a well behaved and well understood model magnet, with long range order developing at around 1 K, like many other geometrically frustrated magnets, it has been discovered to possess persistent spin dynamics down to very low temperatures as measured by μSR and Mössbauer spectroscopy. Measurement of the low temperature limit of the specific heat when compared with linear spin-wave theory, however, presents a consistent picture of gapped magnon excitations that freeze out at low temperatures and make the existence of the proposed dynamic ground state unlikely.

The Department of Energy funded a project to determine if the specific heat of thermal energy storage materials could be improved by adding nanoparticles. The standard thermal energy storage materials are molten salts. The chosen molten salt was a sodium nitrate and potassium nitrate eutectic, commercially called Hitec Solar Salt. Two nanoparticle types were chosen, alumina and silica. The nanoparticle composite materials were fabricated by mixing the components in an aqueous solution, mixing that solution for a set amount of time using a sonic mixer, then removing the water from the aqueous solution, leaving the composite molten salt behind as a fine white powder. The thermal properties of the composite and plain material were measured using two techniques: American Society for Testing and Materials (ASTM) 1269E and Modulating Differential Scanning Calorimetry (MDSC). These two techniques measured the specific heat and the heat of fusion of the plain and composite materials. The results of all the ASTM and MDSC measurements suggest that the addition of the nanoparticles using the given manufacturing technique increased the specific heat of the molten salt by approximately 20 percent, with both measurement techniques showing approximately the same level of increase. The silica and the alumina improved the specific heat by nearly the same amount over the base material. The heat of fusion did not seem to be significantly altered compared to the observed heat of fusion value of the unmodified material. It was also observed that the nitrate and silica composite material's specific heat decreased if the material was raised to a temperature above 400C. The specific heat was observed to decrease over time, even when the temperature was well below 400C. It is unknown why this occurred. The nitrate plus alumina composite and the plain nitrate were stable to a temperature of 450C for the test duration.

The molecular magnet [FeII(£G)FeII(£N)(ox)2(Phen)2]n, whose chemical formula is C28H16Fe2N4O8 for unity, has been studied by magnetization measurements, neutron diffraction, and field-dependent specific heat. From the magnetization measurements, the quasi-ferrimagnetic behavior at T>Tm region can be well described with alternating Land&#x00E9; factors within 1D Ising chain model. However, in T<Tm region, the construction of long-range magnetic ordering due to the increase of interchain interaction was investigated, which is consistent with the anomaly shown in the low temperature specific heat measurement. Furthermore, an intrinsic antiferromagnetic configuration is deduced from analyzing Bragg pattern of neutron scattering. In specific heat measurement, a £f-type anomaly indicating the long-range magnetic ordering was observed. In addition, the magnetic entropy due to this anomaly is much smaller than expected value indicating the spin fluctuated as short-range ordering at T>Tm.

The heat capacity of 3He adsorbed on Grafoil (exfoilated graphite) preplated with four layers of 4He was measured between 1 and 50mK. The heat capacity was found to be linear up to 4OmK. At 3He surface densities below 4nm-2 two dimensional Fermi liquid behaviour was found and values of the hydrodynamic effective mass and Landau parameter Ff inferred. Subsequent steps in the heat capacity as a function of coverage were taken as evidence of independent 2D Fermi fluids. A low field DC SQUID pulsed NMR spectrometer was developed for future studies of magnetic order in 2D solid 3He films at ultralow temperatures

Dissociation and specific heats of synthetic methane and ethane hydrates were measured under high-pressure condition by using a heat-flow type calorimeter to understand thermodynamic properties of gas hydrates under submarine/sublacustrine environments. Ice powder was put into the sample cell and pressurized by methane and ethane up to 5MPa and 2MPa, respectively. After the completion of gas hydrate formation, samples were heated from 263K to 288K at the rate of 0.01 K min-1. Large negative peaks of heat flow corresponded to the dissociation of gas hydrates were detected in a temperature range 279-282K at a pressure of 5MPa for methane hydrate and 283-286K at 2MPa for ethane hydrate, respectively. We also obtained the specific heats of gas hydrates in the range 264-276K for methane and 264-282K for ethane under pressure.

This thesis will present research that studies the role of disorder, geometric frustration and the long range dipolar interaction on the collective behaviour of several insulating, rare earth magnets. Experiments were performed at low temperatures to measure the specific heat and magnetic susceptibility of several materials. Susceptibility was measured with a SQUID magnetometer that has been designed and constructed primarily for the study of slow dynamics in glassy systems. Specifically, this thesis will discuss three distinct topics. The first is the series of materials LiHo(x)Y(1-x)F(4), which are manifestations of the dilute, dipolar coupled Ising model. The low-x portion of the phase diagram has become a rather contentious issue in recent years with both theoretical and experimental groups disagreeing on the existence of a spin glass freezing transition and one experimental group arguing for the existence of an exotic "antiglass'' or spin liquid state resulting from quantum entanglement at x=0.045. We present specific heat and dynamical susceptibility measurements on four stoichiometries in this series: x = 0.018, 0.045, 0.080 and 0.012. No evidence of an unusual antiglass state is observed. Instead, our results show evidence, at all dilution levels studied, of a spin glass freezing transition. Interpretation of experimental data is found to be complicated by the anomalously slow dynamics in these materials. The relaxation time scales are found to increase as the concentration of Ho(3+) ions is reduced, an effect which can be attributed to single-ion physics and the importance of the nuclear hyperfine coupling in this system. A second set of materials studied here is a series of several Gd garnet materials, the most famous of which is Gd(3)Ga(5)O(12) (GGG), a material previously argued to be a disorder-free spin glass. Our specific heat experiments reproduce previous experiments on GGG and show that the homologous Gd garnets Gd(3)Te(2)Li(3)O(12) and Ga(3)Al(5)O(12) do not share the same glassy physics but exhibit sharp ordering features. By experimenting with the introduction of random site dilution, it is concluded that a 1-2% off-stoichiometry inherent in GGG is likely a special kind of disorder that is particularly effective in inducing random frustration and the formation of a spin glass. Finally, specific heat measurements on the pyrochlore antiferromagnet Gd(2)Sn(2)O(7) (GSO) are presented. While GSO has generally been found to be a well behaved and well understood model magnet, with long range order developing at around 1 K, like many other geometrically frustrated magnets, it has been discovered to possess persistent spin dynamics down to very low temperatures as measured by μSR and Mössbauer spectroscopy. Measurement of the low temperature limit of the specific heat when compared with linear spin-wave theory, however, presents a consistent picture of gapped magnon excitations that freeze out at low temperatures and make the existence of the proposed dynamic ground state unlikely.

The results of photoluminescence (PL) spectra of TlGaS2 single crystal were reported in the 500-1400 nm wavelength and in the 15-115 K temperature range. Three broad PL bands with an asymmetric Gaussian lineshapes were observed to be centered at 568 nm (A-band), 718 nm (B-band) and 1102 nm (C-band). The shift of the emission band peak energy as well as the change of the half-width of the emission band with temperature and excitation laser intensity were also studied. We analyzed the observed results using the configurational coordinate (CC) model. The powder diffraction patterns of TlInS2 and TlGaS2 crystals were obtained and the diffraction data were indexed using CRYSFIRE computer program packet. TlInS2 has hexagonal system with parameters a = 3.83 and c = 14.88 Ao. TlGaS2 has monoclinic system with parameters a = 9.62, b = 4.01 and c = 7.52 Ao with &amp / #946 / = 96.30o. Our diffraction studies at low temperatures did not reveal any phase transition for TlInS2 as reported in the literature. The specific heat capacities of both TlInS2 and TlGaS2 crystals calculated from Differential Scanning Calorimetry (DSC) measurements at low temperatures are reported in the thesis.

[pt] Propomos um hamiltoniano de spin para descrever as propriedades térmicas e magnéticas de uma série de borocarbetos. O efeito do campo elétrico cristalino é aproximado por um campo magnético efetivo.Aplicamos a teoria de onda de spin e encontramos a relação de dispersão e a densidade apenas se desloca rigidamente em energia. A partir destes resultados calculamos a contribuição dos magnons ao calor específico molar com e sem campo magnético externo. O efeito do campo externo, porém é pequeno.A seguir calculamos a magnetização de subrede a T=0K, a qual sofre uma pequena redução devido às flutuações quânticas, em relação ao valor iônico obtido pela regra de Hund. Encontrmos também a magnetização em função da temperatura, com e sem aplicação de campo magnético. Com esse modelo conseguimos descrever bastante bem os dados experimentais de calor específicoda série. / [en] We propose a spin hamiltonian to describe the magnetic and thermal properties of a series of borocarbides. The effect of the crystal electric field is simulated by an effective magnetic field. By applying spin wave theory we have found the dispersion relation and the density of state for the magnos as well. In presence of an external magnetic field this density os state shifts rigidly in energy. From these results we have calculated the magnon contribuition to the molar specific heat without applied magnetic field. the effect of the field, however, is very small. We also have found the sublattice magnetization at T = 0K which is slightly reduced with respect to the ionic value given by the Hund's rule. Then we have calculated the sublattice magnetization as a function of T, both in presence and in absence of the magnetic field. With this model we succed in fitting fairly well the experimental data for the specific heat of the borocarbides series.

[pt] A ocorrência de fluxo crítico de calor – FCC – é o principal fator termo-hidráulico limitante à produção de energia em reatores nucleares do tipo PWR (Reator a Água Pressurizada). O método usual de determinação de FCC é baseado em simulação numérica, utilizando programas como os COBRA, desenvolvidos a partir da análise dos subcanais do núcleo do reator. Esses programas implementam uma correlação, ou função empírica, que interpola os resultados obtidos por simulação experimental, realizada nas Seções de testes – ST-, de forma a obter o FCC numa ampla faixa operacional do reator. Esta dissertação propõe e investiga um método alternativo de determinação de FCC empregando, como correlação, redes neuronais artificiais – RNA. Neste método, as RNA são obtidas a partir de treinamento, utilizando o paradigma de backpropapagation, realizado com o mesmo conjunto de dados experimentais oriundos das STs. / [en] Critical Heat Flux – CHF – occurence is the main thermo-hydraulical factor that restrains the energy produced in Pressurized Water Reactor – PWR – nuclear plants. The usual method of determining CFCH is based upon numerical simulation performed by computer programs such as COBRA, which were developed considering the reactor core sub-channel analysis. These programs implement a correlation, or empirical function, wich interpolates the results obtained through experimental simulation, acocomplished on test sections – TSs – for the sake of obtaining CHF in a wide core operational range. This work investigate and analyze an alternate method of detrmining CHF using, as a correlation, artificial neural networks – ANNs. In this method, the ANNs are obtained through trainning, making use of backpropagation paradigm, against the same experimental data set that came from the TSs.