PrOs₄Sb₁₂ is the first known heavy fermion superconductor containing Pr. Many experiments show results indicating unconventional superconductivity in this system. Most notably, PrOs₄Sb₁₂ exhibits two superconducting transitions. The upper transition occurs at Tc₁ ≈ 1.89 K and the lower transition occurs at Tc₂ ≈ 1.72 K. There are many features of this system that are not understood and do not have a unifi ed consensus among researchers. Many issues remain unanswered, such as the origin of superconductivity, symmetry of the superconducting energy gap, and the nature of the two superconducting transitions. We present specific heat data for a single PrOs₄Sb₁₂ crystal. The data show two superconducting transitions. Variance in the lower superconducting transition (among various samples) suggests multiple superconducting phases. Power law dependence of the low temperature specific heat is indicative of asymmetric energy gap. Cuprates are unconventional high temperature superconductors. The theory on origin and characteristics of superconductivity in these systems is still a debated issue. Experiments indicate presence of a phase in these systems that occurs above the superconducting transition temperature which exhibits some common characteristics with the superconducting state. An important issue is the nature of this so-called "pseudogap" phase and its relationship to the superconducting state. We have developed an experimental apparatus and procedure for measuring the specific heat of a high temperature superconductor and demonstrated it by measuring the heat capacity of a YBCO high-Tc superconductor sample.

The focus of this research was to create and characterize high temperature alumina and nitrate salt eutectic nanofluids for use in thermal energy storage (TES) systems. The nitrate eutectic was originally used in the TES system demonstrated as part of the Solar Two power tower and is currently employed as the TES material at Andasol 1 in Spain. Concentrations of alumina nanoparticles between 0.1% and 10% by weight were introduced into the base material in an effort to create nanofluids which would exhibit improved specific heat capacity to reduce the $/kWht thermal energy storage system costs. The composite materials were created using an aqueous mixing method in which both the nanoparticles and nitrate eutectic were placed into solution using acidic water. This solution was then sonicated in an ultrasonic bath in an effort to reduce nanoparticle agglomeration and to improve homogeneity. After boiling off the excess water, the nanoparticle-nitrate eutectic composite was recovered for characterization. The thermal properties of both the composite and base materials were characterized using the differential scanning calorimetry techniques outlined in ASTM E 1269. The created nanofluids were not stable and did not offer a cost-effective alternative to the current nitrate eutectic TES material. Despite these setbacks, a positive correlation between alumina concentration and nanofluid specific heat was demonstrated. Additionally, the specific heat capacities of the created nanofluids exceeded that predicted by the current theoretical models. These findings suggest that further work in the field of high temperature nanofluids for use in TES systems is warranted.

Order-disorder phenomena in geometrical frustrated systems are the attractive topics because of the intrinsic fluctuation. Among the geometrical frustrated systems, the material with spinel structure (AB2X4) is one of the appropriate candidates to investigate the long range ordering behavior. Corner sharing of tetrahedron and edge sharing of octahedron in the unique structural network of spinel structure are the characteristics for geometrical frustration. Hence, to study the 3d transition metal substituted in spinel system which leads to fruitful physical behavior becomes rapidly attractive. In this dissertation, long range ordering behavior in spin frustrated systems including three interesting materials LiTi2O4, NaxCoO2¡DyH2O, and CdCr2S4 were investigated. LiTi2O4 was found to show the highest superconducting transition temperature (Tc ~ 11 K) while first hydrated superconductor NaxCoO2¡DyH2O (Tc~ 4.5 K) was discovered in 2003. Superconductivity of LiTi2O4 and NaxCoO2¡DyH2O had been measured and analyzed by low temperature specific heat under magnetic field. According to the analyses of specific-heat results, isotropic (s-wave) and nodal (d-wave) gaps of superconducting pairing symmetry were proposed for LiTi2O4 and NaxCoO2¡DyH2O, respectively. Finally, LiTi2O4 was confirmed to be a typical BCS-like, fully gapped, and electron-phonon moderate-coupling type-II superconductor. Not like the case of LiTi2O4, the superconducting parameters of NaxCoO2¡DyH2O, such as Tc, HC2 and pairing symmetry, were strongly dependent on synthesized conditions. However, the evidence of nodal gap was found to be an intrinsic feature in this peculiar material NaxCoO2¡DyH2O. In the ferromagnetic insulator CdCr2S4, we first found several interesting features induced by external electric field in dielectric and magnetization measurements. Exchangestriction was proposed to be associated with the colossal change of dielectric constant value and suppression of magnetization under external electric and magnetic field in CdCr2S4. Therefore, our results supported that CdCr2S4 was a typical multiferroic material. In a conclusion, the intrinsic fluctuation of spin frustrated systems wasnecessary to pay more attention in the near future due to its fruitful physical properties and behind theoretical description.

PrOs₄Sb₁₂ is the first known heavy fermion superconductor containing Pr. Many experiments show results indicating unconventional superconductivity in this system. Most notably, PrOs₄Sb₁₂ exhibits two superconducting transitions. The upper transition occurs at Tc₁ ≈ 1.89 K and the lower transition occurs at Tc₂ ≈ 1.72 K. There are many features of this system that are not understood and do not have a unifi ed consensus among researchers. Many issues remain unanswered, such as the origin of superconductivity, symmetry of the superconducting energy gap, and the nature of the two superconducting transitions. We present specific heat data for a single PrOs₄Sb₁₂ crystal. The data show two superconducting transitions. Variance in the lower superconducting transition (among various samples) suggests multiple superconducting phases. Power law dependence of the low temperature specific heat is indicative of asymmetric energy gap. Cuprates are unconventional high temperature superconductors. The theory on origin and characteristics of superconductivity in these systems is still a debated issue. Experiments indicate presence of a phase in these systems that occurs above the superconducting transition temperature which exhibits some common characteristics with the superconducting state. An important issue is the nature of this so-called "pseudogap" phase and its relationship to the superconducting state. We have developed an experimental apparatus and procedure for measuring the specific heat of a high temperature superconductor and demonstrated it by measuring the heat capacity of a YBCO high-Tc superconductor sample.

Orientador: Flavio Cesar Guimarães Gandra / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-10T12:45:58Z (GMT). No. of bitstreams: 1 Loula_GeorgeDourado_M.pdf: 2173914 bytes, checksum: ff1208aeb34d1619646a01ea6f3f1ec8 (MD5) Previous issue date: 2005 / Resumo: Neste trabalho realizamos medidas de difração de raios-X, magnetização, calor específico e resistividade elétrica nos compostos da série U(Cu,Au)Sn, além do composto UPtSn, que pode cristalizar-se tanto na forma hexagonal (sem tratamento térmico) quanto cúbica, com tratamento térmico de 850ºC por uma semana. Na série UCu(1-x)A u(x)Sn verificamos que todos os compostos apresentam ordenamento antiferromagnético, sendo que a temperatura de ordenamento magnético ( TN) decresce com o aumento da concentração do substituinte. Além disso, percebe-se um pequeno aumento no calor especifico eletrônico ( d ) e no volume da célula unitária ao se inserir Au. O momento efetivo do urânio diminui de 3.44 m B para x=0 até 3 m B para x=1, devido a uma maior hibridização 5f-s,d decorrente da substituição do Cu por Au. Essa hibridização propicia diversas alterações nas propriedades físicas dos compostos. Quanto ao sistema UPtSn, verificamos que o tratamento térmico ao qual a amostra é submetida influencia na formação estrutural do composto e as propriedades físicas de cada estrutura são muito distintas. A fase cúbica ordena antiferromagneticamente em 35K e o urânio apresenta momento magnético efetivo igual a 3.24 m B, enquanto que a fase hexagonal ordena ferromagneticamente em 27K, e apresenta momento magnético efetivo de 2.82 m B. Esta redução no momento efetivo pode ser devida ao efeito Kondo, mas os resultados experimentais não são conclusivos quanto a isto / Abstract: In this work we report on measurements of x-ray diffraction, magnetization, specific heat and electrical resistivity for the series U(Cu,Au)Sn and for UPtSn, which crystallize either with hexagonal (as-cast) or with cubic simmetry (annealed). In the series UCu(1-x)Ausub(x)Sn we verify that all the compounds order antiferromagnetically and the magnetic ordering temperature (TN) decreases when the substitute's concentration is increased. We also noticed a small increase of the electronic specific heat coefficient and unit cell volume by replacing Au for Cu. The effective moment is reduced from 3.44 m B at x=0 to 3 m B at x=1 due to a more effective 5f-s,d hybridization. This hybridization is responsible for the phisical properties of these compounds. For UPtSn we found that the thermal treatment can define the structure of the compound. The phisical properties of both compound are quite different. The cubic phase is AF with TN = 35K and present m eff = 3.24 m B while the hexagonal phase orders at 27K with m eff = 3B. Such reduction of the effective moment may be due to the Kondo effect but the experiments where not conclusive about this / Mestrado / Física da Matéria Condensada / Mestre em Física

[pt] O presente trabalho é o estudo de um aquecedor de água para fins domésticos que utiliza energia radiante oriunda do sol, captada por um coletor de forma hemisférica. Na primeira parte do trabalho é feita uma tentativa de avaliação da radiação solar direta e difusa, a partir dos dados de incidência global em superfícies horizontais. A segunda parte trata do projeto do aquecedor, cálculo das perdas de calor no decorrer do processo de aquecimento e a simulação do sistema num computador digital. A terceira e última consta da comparação dos resultados fornecidos pela simulação no computador, com os obtidos durante a experimentação do aparelho e a discussão desses resultados. / [en] This work is a study of a domestic water heater which uses radiant energy from the sun, captured by a hemispherical collector. In the first part of the study, the direc and diffuse solar radiation is evalueted from environmental data. The second part deals with the collecter desing the heat losses involved, and the computer simulation of the overall system. In the third part, experimental results of a prototype system constructed in João Pessoa are compared with the theoretical simulation.

[pt] Analisa-se a resistência térmica da parede de trocadores de calor tipo tubo-placa. Quatro modelos do trocador, imersos em um fluido são considerados; uma placa com tubos paralelos e igualmente espaçados sobre uma das superfícies; dois cilindros ocos, um com tubos uniformemente distribuídos sobre a superfície interna e segundo a direção circunferencial e finalmente uma esfera oca com distribuição paralela e uniforme de tubos sobre a superfície interna. Em todos os casos, a analise baseia-se na existência de fluxo uniforme de calor entre tubo e placa, extendendo-se porém, para a placa plana, a condição de fluxo conectivo. Os resultados mostraram um aumento considerável na resistência térmica da placa quando as razões(largura do tubo/ distancia entre tubos) e/ou (espessura da placa/ distancia entre tubos) diminuem ocasionando um estrangulamento das linhas de fluxo do calor através da placa. Essa resistência mostrouse-se também ser uma função crescente da resistência térmica do fluido externo e da condutividade térmica da placa, e crescer também com o aumento da resistência, obteve-se para os modelos cilíndricos, desvios de até 10 por cento, ao passo que valores até 3 (três) vezes maiores que aqueles foram obtidos para o modelo esférico. Um exemplo típico de uma aplicação sub-ocêanica é feito com o modelo cilíndrico de tubos axiais, usando-se titânio com material da parede do vaso. / [en] The thermeal costriction resisteance in the wall of a tube-plate heat excchanger in analyzed in this study. Four heat exchanger models, are considered; a flat plate with parallel tubes aqual spaced on one of the surfaces; two hollow cylindrs, one with tube uniformaly distributed axially over the inside surface and the other with tubes uniformly distributed circumferencially: and finally a hollow shere with a parallel distribution of tubes uniformly spaced on the inside surface. In each of the geometries it is assumed that the heat flux from the tubes is uniform. In addition, the condition of covective heat transfer from the tubes is considered for the case of the flat plate. The outer surface of the plate is cooled convectively. The results show a considerable increase in the therminal resitance of teh when the ratios (tube midth/tube spacing) and/or ( wall thickness/tube spacing) decreases, giving rise to the pincirg of the lines of heat flux through the plate. This resistance is shown also to be an increasing fuction of the therminal resistance of the fluid that flows in the tubes. Using the results of the rectangular model (flat plate) as a reference, it is found that the resistance for the cylindrical models differs model differs by up to 3 times greater. As an example of the application of the data obtained in this study, a desing of a deep seen cylindrical vessel made of titanium with axially distributed tubes is considered.

The surface properties of coal greatly affect its use in industrial processes such as gasification, liquefaction and the use of coal slurries. In addition, such processes also involve reagent penetration into coal pores and the oxidative state of the coal surface. Reagent penetration into coal was examined by determining the heat and kinetics of immersion of Pocahontas No. 3 coal in water, methanol, a series of n-alkanes, and several cyclic and heterocyclic hydrocarbons. Results indicate that the heat of immersion is sensitive to the carbon chain length of the wetting liquid and its ability to hydrogen bond to coal. The time of immersion is insensitive to carbon chain length but is increased by the presence of nitrogen in a cyclic hydrocarbon. The oxidation of both unextracted and extracted coal at 3200C was followed by immersional measurements in water. The heat of immersion increased rapidly with oxidation time, leveling off after 3 hours of oxidation. Extraction with methanol or pyridine prior to oxidation and immersion lowered the time of immersion in water compared to the unextracted case. The oxidation of coal was also followed by x-ray photoelectron spectroscopy (XPS). The XPS oxygen/carbon ratios correlated linearly with the heats of immersion for the extracted, oxidized coals. Thus, XPS was shown to be a useful ancillary technique to heat of immersion for the study of coal surfaces. / Master of Science

Studies on strongly correlated electron systems over decades have allowed physicists to discover unusual properties such as spin density waves, ferromagnetic and antiferromagnetic states with unusual ordering of spins and orbitals, and Mott insulating states, to name a few. In this thesis, the focus will be on the specific heat property of these materials exhibiting novel electronic ground states in the presence and absence of a field. The purpose of these measurements is to characterize the phase transitions into these states and the low energy excitations in these states. From measurements at the phase transitions, one can learn about the amount of order involved [i.e. entropy: ΔS = ∫Δc p/T dT], while measurements at low temperatures illuminate the excitation spectrum. In order to study the thermodynamic properties of the materials at their phase transitions, a high sensitive technique, ac-calorimetry was used. The ac-calorimeter, workhorse of our low dimensional materials lab, is based on modulating the power that heats the sample and measuring the temperature oscillations of the sample around its mean value. The in-house ac-calorimetry set up in our lab has the capability to produce a quasi-continuous readout of heat capacity as a function of temperature. A variety of single crystals were investigated using this technique and a few among them are discussed in my thesis. Since many of the crystals that are studied by our group are magnetically active, it becomes useful for us to also study them in the presence of a moderate to high magnetic field. This motivated me to design, develop, and build a heat capacity probe that would enable us to study the crystals in the presence of non-zero magnetic fields and at low temperatures. The probe helped us not only to revisit some of the studied materials and to draw firm conclusions on the previous results but also is vital in exploring the untouched territory of novel materials at high magnetic fields (~ 14 T).

No description available.