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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Giant Magnetocaloric effect and Magnetic Properties of selected Rare-Earth compounds

Mbulunge, Masevhe Hamisi January 2021 (has links)
Masters of Science / Rare-earth (RE) compounds have been an attractive subject, based on the unique electronic structures of the rare-earth elements. In particular, the RETX (RE = rare-earth, T = 3d/4d/5d, transition metals, and X = p – block elements) series is a large family of intermetallic compounds which crystallizes in different crystal structure depending on the constituents. Most of these compounds crystalize in the hexagonal, orthorhombic, and tetragonal crystal structure. On the other hand, the family of compounds RET2X2 adopted the tetragonal crystal structure of the ThCr2Si2 or the CaBe2Be2 with different space groups. Owing to the different crystal structure, these compounds show versatile magnetic and electrical properties such as Kondo effect, complex magnetic behaviour, valence fluctuation, unconventional and conventional superconductivity, heavy fermion behaviour, Fermi and non – Fermi liquid behaviour, metamagnetism, spin – glass, memory effect, crystal electric field (CEF), magnetoresistance and magnetocaloric effect. The history of magnetism reveals that it is closely related to practical applications and magnetic materials from the most vital components in many applications. These are memory devices, permanent magnets, transformer cores, magneto-mechanical devices and magneto-electronic devices. Recent additions to this list include magnetic refrigeration through the studies of magnetocaloric effect as well as spintronics. Magnetic refrigeration (MR) is an emerging technology and shows real potential to enter conventional markets and the principles of MR obeys the magnetocaloric effect (MCE), which is based on the effect caused by a magnetic field on the materials that accept the property of varying the magnetic entropy, as well as its temperature when varying the magnetic field. In this thesis, we report giant magnetocaloric effect and magnetic properties of NdPd2Al2 and RECuGa (RE = Nd, Dy, and Ho) compounds. These investigations were done through measurements of X – ray diffraction (XRD), magnetic susceptibility, ((T)), magnetization, (M(H)), isothermal magnetization, (M(H, T)), heat capacity, (Cp(T)) and electrical resistivity, ((T)). MCE has been studied from the isothermal magnetization and heat capacity measurements.The first chapter of the thesis describes the theoretical background from which the experimental results have been analyzed and interpreted. This is followed by the chapter which presents experimental details and methodology carried out in this thesis. Chapter three presents the results and discussion of the transport, magnetic and magnetocaloric properties of NdPd2Al2 compounds. XRD studies confirm the tetragonal CaBe2Ge2 – type structure with space group P4/nmm (No. 129). The results of (T), (T) and Cp(T) indicate a putative antiferromagnetic (AFM) phase transition at low temperature at, TN = 3 K. On the other hand, (T) data at high temperatures follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent Nd3+ ion. The magnetization results indicate metamagnetic – like transition at a low field that bears a first-order character which corroborates with the Below – Arrott plots. Giant MCE was obtained for the NdPd2Al2 compound similar to those reported for potential magnetic refrigerant materials. Chapter four discusses the magnetic and thermodynamic properties of the series of compounds RECuGa where RE = Nd, Dy, and Ho. XRD studies indicate the orthorhombic CeCu2 – type crystal structure with space group Imma (No. 74) for all three compounds. Magnetic measurements indicate a putative AFM phase transition below 𝑇𝑁 = 7.1, 8.5, and 3.7 K for Nd, Dy, and Ho compounds, respectively. The high-temperature (T) data for all three compounds follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent rare-earth ion. Again, large MCE were obtained for all three compounds similar to those reported for materials that can be used as magnetic refrigerant materials.
32

Dy2ScNbO7: a study of the effect of a disordered B-site on the spin ice magnetism typically seen in dysprosium pyrochlores / Dy2ScNbO7: the magnetism of a mixed B-site pyrochlore

Rutherford, Megan R. January 2021 (has links)
The thermodynamics of disorder have been studied for hundreds of years, with physicists using entropy to quantitatively connect the macroscopic properties of a system to its microscopic multiplicity (disorder). Here, we consider the effect of disorder in magnetic materials. The pyrochlore oxides (A2B2O7), comprised of a bipartite lattice of corner-sharing tetrahedra, have been central to the study of geometric frustration for the past several decades. Pyrochlores, in which the A-site is occupied by the magnetic cation dysprosium, tend to exhibit spin ice ordering down to low temperatures, in spite of chemical perturbations to the B-site lattice. With the motivation of this study being the investigation of how adding B-site disorder to the traditional Dy2ScNbO7 form of Dy-pyrochlores, a stoichiometric mixture of Sc-3+ and Nb-5+ was used to synthesize Dy2ScNbO7, the pyrochlore material that is central to this thesis work. We show using magnetometry, heat capacity, muon spin relaxation, and inelastic neutron scattering that the mixed B-site pyrochlore Dy2ScNbO7, does not adopt the spin ice ground state. The low temperature spin dynamics are much faster than other analogous dysprosium pyrochlores, the residual entropy is significantly smaller than that predicted for a spin ice and there are low-lying crystal field excitations. These results all indicate that the B-site disorder appears to destroy the predicted Ising anisotropy of dysprosium. / Thesis / Master of Science (MSc)
33

EVALUATING COSMO-RS FOR VAPOR LIQUID EQUILIBRIUM AND TURBOMOLE FOR IDEAL GAS PROPERTIES

Gazawi, Ayman January 2007 (has links)
No description available.
34

Specific Heat and Thermodynamic Properties of Metallic Systems: Instrumentation and Analysis

Lang, Brian E. 12 October 2005 (has links) (PDF)
A small-scale adiabatic calorimeter has been constructed as part of a larger project to study nano-particles and to facilitate specific heat measurements on samples where it is difficult to obtain enough material to run on the current large-scale adiabatic apparatus. This calorimeter is designed to measure sample sizes of less than 0.8 cc over a temperature range from 13 K to 350 K. Specific heat results on copper, sapphire, and benzoic acid show the accuracy of the measurements to be better than ±0.4% for temperatures higher than 50 K. The reproducibility of these measurements is generally better than ±0.25%. Experimental specific heat data was collected on this new apparatus for synthetic akaganeite, β-FeOOH, for samples with varying degrees of hydration. Our results yield values for Δ_0^298.15S°m of 79.94 ±0.20 J•K^-1•mol^-1 and 85.33 ±0.021 J•K^-1•mol^-1 for samples of β-FeOOH0.551H2O and β-FeOOH0.652H2O, respectively. From this data, we were able to determine the standard molar entropy for bare β-FeOOH, as Δ_0^298.15S°m = 53.8 ±3.3 J•K^-1•mol^-1, based on subtractions of the estimated contribution of water from the hydrated species. Additionally, the specific heats of α-uranium, titanium diboride, and lithium flouride have been measured on a low-temperature, semi-adiabatic calorimeter down to 0.5 K. For the α-uranium, the specific heat of a polycrystalline sample was compared to that of a single crystal, and it was found that there was a significant difference in the specific heats, which has been attributed to microstrain in the polycrystal. The third law entropy for the polycrystal at 298.15 K, Δ_0^298.15S°m, calculated from these heat capacities is 50.21 ±0.1 J•K^-1•mol^-1, which is good in agreement with previously published values of polycrystal samples. For the single crystal Δ_0^298.15S°m, calculated using the thermodynamic microstrain model, is 49.02 ±0.2 J•K^-1•mol^-1. The low-temperature specific heats of titanium diboride and lithium fluoride have been measured from 0.5 K to 30 K as part of a larger project in the construction of a neutron spectrometer. For this application, the measured specific heats were used to extrapolate the specific heats down to 0.1 K with lattice, electronic, and Schottky equations for the respective samples. The resultant specific heat values at 0.1 K for TiB2 and 6LiF are 4.08E-4 ±0.27E-4 J•K^-1•mol^-1 and 9.19E-9 ±0.15E-9 J•K^-1•mol^-1, respectively.
35

I. Thermodynamics and Magnetism of Cu2OCl2 II. Repairs to Microcalorimeter the "2"s are subscripts, and the second 2 is preceded by a lower case L, not a one

Parry, Thomas J. 13 August 2008 (has links) (PDF)
Adiabatic calorimetry provides accurate and precise specific heat (Cp) data. From this data, thermodynamic functions may be calculated. Cu2OCl2, melanothallite, became of interest as part of a study of a particular thermochemical cycle. The experimental specific heat data and the calculated thermodynamic functions are reported here. Free energies of formation, calculated from the thermodynamic functions, suggest the particular cycle of interest with this compound as an intermediate is not feasible; uncertainty as to the accuracy of CuO and CuCl2 data used in the calculations indicate further study may be necessary. Upon collection of the specific heat data, an antiferromagnetic transition was observed at 70 K; this led to examination of the magnetic heat capacity and entropy of the transition in melanothallite. The entropy of the transition was estimated to be 18.1 % and 7.5 % of 2Rln2 by two methods. A theoretical calculation using an Ising model produced a result of 39 %. This is consistently low when compared to the entropies of the antiferromagnetic transitions of CuO and CuCl2. This suggests geometric frustration. This thesis reports the thermodynamic functions calculated from the specific heat; the examination of the magnetic entropy; and repairs to an adiabatic apparatus involved in the collection of this data.
36

A comparative study between conventional fixed and advanced adaptive control system for resistance spot

Bohlin, Caroline January 2018 (has links)
Resistance spot welding is the main welding method used in the automotive industry to weld thin sheet metal. Today adaptive control systems have been developed for RSW, which means it can adjust the parameters in the weld process automatically during welding. The control systems can register the parameters and properties of the weld in real-time and from that calculate with algorithms how to adjust to give optimal weld conditions. This project is performed at Scania CV AB, Oskarshamn. Conducted in the part of body in white, where an adaptive control system called HCC is used in all weld processes. In this project, HCC was compared to the fixed control system CCR and another adaptive control system named Master mode. First step in the comparison was to create a weld schedule for each control system and test them on two different material combinations. The aim was to quantify gains and benefits that adaptive resistance spot welding systems have on the welding process. Benefits are quantified by examining the parameters and factors such as: weld time, expulsion, robustness, electrode wear and parameters in the control system. The tests were performed by welding as many approved spot welds as possible without tip-dressing the electrode. The experiment followed the requirements from international standards and the Scania standard for resistance spot welding.  The results from the experiment showed that HCC was the most robust process and the spot welds never decreased in size, which CCR and Master mode did. It is possible to weld several different material combinations with HCC, it increases flexibility in production and reduces the time needed to develop new weld schedules. The same schedule can handle many combinations with the same thickness. HCC allows the process to use several pulses and each pulse adds in time. Therefore, the weld schedule should be well developed and optimized to avoid waste in terms of long weld times. The results will give Scania knowledge about the processes and how to further optimize the welding processes in production. The result can also be used as foundation for selection of products or future investments. / Motståndspunktsvetsning är den huvudsakliga svetsmetoden som används inom fordonsindustrin för att svetsa tunn plåt. Idag har adaptiva styrsystem utvecklats för RSW vilket innebär att de automatiskt kan justera parametrarna i svetsprocessen under svetsning. Styrsystemen kan registrera parametrarna och egenskaperna hos svetsen i realtid och därmed beräkna med algoritmer hur de bör justeras för att ge optimala svetsförhållanden. Detta projekt är resultatet av ett examensarbete på Scania CV AB, Oskarshamn. Det utfördes i den nya karossfabriken, där ett adaptivt styrsystem som heter HCC används i alla svetsprocesser. I projektet jämfördes HCC med ett konstantströms styrsystem CCR samt ett annat adaptivt styrsystem kallat Master mode. Den primära metoden var att skapa ett svetsschema för varje styrsystem och testa dem på två olika materialkombinationer. Syftet var att kvantifiera vinster och fördelar som adaptiva punktsvetssystem har på svetsprocessen. Testerna utfördes genom att svetsa så många godkända punkter som möjligt utan att formera elektroden. Fördelarna kvantifieras genom att man undersökte parametrarna och faktorerna svetstid, sprut, robusthet, elektrodslitage och parametrar i styrsystemen. Experimentet följde kraven i enighet med internationella standarder och Scania-standarden för punktsvetsning. Resultaten från experimentet visade att HCC var den mest robusta processen och punkterna minskade aldrig i storlek, vilket CCR och Master mode gjorde. Det är möjligt att svetsa flera olika materialkombinationer med HCC, det ökar flexibiliteten i produktionen och minskar den tid som krävs för att utveckla nya svetsscheman eftersom samma schema kan hantera många kombinationer med samma tjocklek. HCC tillåter processen att använda flera pulser, och varje puls adderar tid och svetsschemat bör därför vara välutvecklat och optimerat för att undvika slöseri med avseende på långa svetstider. Resultaten kommer att ge Scania mer kunskap om processerna och hur man kan optimera processerna ytterligare i produktionen. Resultatet kan också användas som grund för val av produkter eller framtida investeringar.
37

Heat Capacity and Oxidation Kinetic Studies of Fe-Ti Composite Metal Oxide (ITCMO) using Simultaneous Differential Scanning Calorimetry and Thermogravimetric Analysis

Kumar, Prateek January 2017 (has links)
No description available.
38

Prediction of Specific Heat Capacity of Food Lipids and Foods

Zhu, Xiaoyi 20 October 2015 (has links)
No description available.
39

Thermal Properties of Candidate Coolant Salts

Ridder, Cathleen Elise 23 July 2024 (has links)
With the increasing research on advanced reactors, molten salt reactors have been recognized for their potential. As with any advanced reactor concept, each component and material must be thoroughly investigated before any reactors of that type are created. One of the most pressing issues in MSR research is that of the salts themselves. Though there are a multitude of salts to choose from when designing such a reactor, many of these salts lack the extensive research required to fully understand them. Across the decades there have been many studies that have investigated select molten salts, but there are a few problems with many of those studies. Those problems are the following: prior papers use obsolete and less reliable methods for their measurements, the papers don't investigate the salts across a wide enough range of temperatures nor at varying compositions, and finally many of the salts that are seen as candidates today were not given as much attention when molten salt reactors were first conceptualized which has resulted in a lack of research on them. Indeed, the research into these salts is lacking in many ways. This study seeks to investigate a collection of promising coolant salts in depth with acknowledgment to those past studies. LiF-NaF-KF (46.5-11.5-42.0 mol%) will be used as a calibration standard and for the purpose of verifying our methodology. Specifically, FLiNaK was used in the development of volume-height curves as calibration for density measurements. NaOH-KOH of four different compositions ( 0.5-0.5mol%, 0.55-0.45mol%, 0.6-0.4mol%, and 0.65-0.35 mol%) will be evaluated for their densities and heat capacities. And finally, BeF2-NaF(43-57mol%) will be evaluated within the question of if the properties are desirable enough that the dangers posed by beryllium are an acceptable risk. BeF2-NaF will have melting point, heat capacity, density, and vapor pressure measurements performed. Additionally, extensive impurity analysis and removal (via an HF gas system) was done to our BeF2-NaF samples. The melting point and heat capacity were evaluated using dynamic scanning calorimetry (DSC), the vapor pressure was evaluated using thermogravimetric analysis (TGA), and the density was measured using a system similar to the Arrhenius method that measures height. / Master of Science / Decades have passed since the discussion of nuclear energy began. Although great progress has been made in the field, the nuclear reactors in use today consist mainly of boiling water reactors (BWRs) or pressurized water reactors (PWRs). As reliable as these reactors have become, one can no longer ignore the fact that there is a multitude of other options for how a reactor can be built and operated. Options that provide greater safety and more energy output. Many reactor concepts of the past were discounted for the extensive research that would be required to make use of them. However, as time has passed and technology has improved, that research has become more and more possible. Many advanced reactors are the result of that attention to the reactor concepts and materials of the past that couldn't be given the attention that they deserve until now. Molten salt reactors (MSRs) are one of those promising concepts. However, before they can be built every part of the reactor, from the structure to the materials, must be entirely understood. One of the most pressing issues in MSR research is the properties of the salts in consideration for use. Though there are a multitude of salts to choose from when designing such a reactor, many of these salts lack the extensive research required to fully understand them. Across the decades there have been many studies that have investigated select molten salts, but there are a few problems with many of those studies. Those problems are the following: the papers are so old that the methods that were used are now obsolete, the papers don't investigate the salts across a wide enough range of temperatures nor at varying compositions, and finally many of the salts that are seen as candidates today were not given as much attention when molten salt reactors were first conceptualized which has resulted in a lack of research on them. Indeed, the research into these salts is lacking in many ways. This study seeks to investigate a collection of promising coolant salts in depth with acknowledgment to those past studies. LiF-NaF-KF will be used as a calibration standard and for the purpose of verifying our methodology. A multitude of different compositions of NaOH-KOH will be evaluated for their densities and heat capacities. And finally, BeF2-NaF will be evaluated within the question of if the properties are desirable enough that the dangers posed by beryllium are an acceptable risk. BeF2-NaF will have melting point, heat capacity, density, and vapor pressure measurements performed. Additionally, extensive impurity analysis and removal was done to our BeF2-NaF samples.
40

Heat Transfer Enhancement using Iron Oxide Nanoparticles

Stuart, Dale 07 September 2012 (has links)
Two different iron oxide nanofluids were tested for heat transfer properties in industrial cooling systems. The nanofluids either had 30 nm particles with a wide size distribution to include particles greater than 1 micrometer or 15 nm particles with greater than 95% of the particles less than 33 nm. Calorimetry and thermal circuit modeling indicate that the 15 nm particle ferrofluid enhanced heat capacity. The smaller particle ferrofluid also demonstrated up to a 39% improvement in heat transfer, while the larger particle ferrofluid degraded the heat transfer performance. Particles from the larger particle ferrofluid were noted as settling out of a circulating system and therefore not participating in the bulk fluid properties. Application of 0.32% 15nm particles in an open cooling system improved cooling tower efficiency by 7.7% at a flow rate of 11.4 liter per minute and improved cooling tower efficiency by 3.3% at a flow rate of 22.7 liter per minute, while applying 0.53% 15 nm particles also improved cooling tower efficiency but was less effective than the lower concentration.

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