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First Principles Theory of the Lattice Thermal Conductivity of SemiconductorsWard, Alistair Norman January 2009 (has links)
Thesis advisor: David A. Broido / Using density functional perturbation theory and a full solution of the linearized phonon Boltzmann transport equation (BTE), a parameter-free theory of semiconductor thermal properties is developed. The approximations and shortcomings of previous approaches to thermal conductivity calculations are investigated. The use of empirical interatomic potentials in the BTE approach is shown to give poor agreement with measured values of thermal conductivity. By using the adiabatic bond charge model, the importance of accurate descriptions of phonon dispersions is highlighted. The extremely limited capacity of previous theoretical techniques in the realm of thermal conductivity prediction is highlighted; this is due to a dependence on adjustable parameters. Density functional perturbation theory is coupled with an iterative solution to the full Boltzmann transport equation creating a theoretical construct where thermal conductivity prediction becomes possible. Validation of the approach is demonstrated through the calculation of a range of thermal properties for a set of polar and non-polar semiconductors which are compared with measured values. The agreement between theory and measurement is very good, confirming the promise of the theoretical approach. Due to the significant computational effort required by the parameter-free calculations, new forms for room temperature relaxation time approximations are derived. The resulting forms produce thermal conductivity values in very good agreement with the ab initio data across a wide temperature range. It is therefore shown that accurate relaxation time approximations can be developed, fixing the adjustable parameters to the ab initio theory avoiding any comparison with measured data. This approach improves the accuracy of phonon relaxation times compared with previous models. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.
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The partial equivalent conductances of salts in seawaterConnors, Donald Nason 04 May 1967 (has links)
Graduation date: 1967
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Development and application of a methodological model that allows evaluate and compare the behaviour of external walls exposed to moisture phenomenonsVeas, Leonardo 20 April 2006 (has links)
The thesis has the objective of design a methodological model for evaluate and
compare the behaviour of external walls exposed to moisture phenomena. The
comparison is related to different variables us for example: thermal
conductivity, thermal transmitance, moisture content in the element along the
time, P.O. Fanger theory of comfort, risk grouwth of mould among the others
parameters.
The model is developed in function of two softwares that permit sensibilyze
the performance of building elements in relation of the presence of different
quantities of moisture inside of them along the year. In this case, the model
is probe with the use of TRNSYS 15 and WUFI 3.2 Pro softwares.
The results show that there are many differences in the analysis of the
different parameters in the cases with the materials in dry and wet state.
Also, is possible to realize that the improve of any constructive solutions
they are amortized in periods of time that no exceed more than three years in
relation to the save of energy for the improvement in the themal conductivity
of the materials.
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The effects of acid contact time and rock surfaces on acid fracture conductivityMelendez Castillo, Maria Georgina 02 June 2009 (has links)
The conductivity created in acid fracturing is a competition between two phenomena: etching of the rock surface and weakening of the rock. This study presents experimental results of acid fracturing conductivity experiments with polymer gelled acid, while varying contact time and rock type. The experiments were conducted in a laboratory facility properly scaled from field to laboratory conditions to account for the hydrodynamic effects that take place in the field. The rocks of study were Indiana limestone, San Andres dolomite and Texas Cream chalk. Our results illustrate that acid fracturing conductivity is governed by the etching pattern of the rock surface and influenced by the hardness of the rock. If channels are created, the fracture is more likely to retain conductivity after closure. The hardness of the rock is the dominating factor to determine the conductivity response when no channeling is present. Among the rocks tested, Texas Cream chalk had the lowest hardness measurement before and after acidizing and the fracture closed at a much lower stress compared with limestone and dolomite. Dolomite had the highest conductivity under all closure stresses even without a channeling pattern. Additionally, it was observed that a higher reduction in rock strength at the contact points for dolomite yielded lower conductivity after closure. The effects of hardness variation on conductivity are higher in dolomite than in limestone and chalk. It is apparent that longer contact times do not always provide higher conductivity after closure.
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Experimental Investigation of Propped Fracture Conductivity in Tight Gas Reservoirs Using The Dynamic Conductivity TestRomero Lugo, Jose 1985- 14 March 2013 (has links)
Hydraulic Fracturing stimulation technology is used to increase the amount of oil and gas produced from low permeability reservoirs. The primary objective of the process is to increase the conductivity of the reservoir by the creation of fractures deep into the formation, changing the flow pattern from radial to linear flow. The dynamic conductivity test was used for this research to evaluate the effect of closure stress, temperature, proppant concentration, and flow back rates on fracture conductivity. The objective of performing a dynamic conductivity test is to be able to mimic actual field conditions by pumping fracturing fluid/proppant slurry fluid into a conductivity cell, and applying closure stress afterwards. In addition, a factorial design was implemented in order to determine the main effect of each of the investigated factors and to minimize the number of experimental runs. Due to the stochastic nature of the dynamic conductivity test, each experiment was repeated several times to evaluate the consistency of the results.
Experimental results indicate that the increase in closure stress has a detrimental effect on fracture conductivity. This effect can be attributed to the reduction in fracture width as closure stress was increased. Moreover, the formation of channels at low proppant concentration plays a significant role in determining the final conductivity of a fracture. The presence of these channels created an additional flow path for nitrogen, resulting in a significant increase in the conductivity of the fracture. In addition, experiments performed at high temperatures and stresses exhibited a reduction in fracture conductivity. The formation of a polymer cake due to unbroken gel dried up at high temperatures further impeded the propped conductivity.
The effect of nitrogen rate was observed to be inversely proportional to fracture conductivity. The significant reduction in fracture conductivity could possibly be due to the effect of polymer dehydration at higher flow rates and temperatures. However, there is no certainty from experimental results that this conductivity reduction is an effect that occurs in real fractures or whether it is an effect that is only significant in laboratory conditions.
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Thermal and charge conductivities of superconducting skutterudite compounds, PrRu4Sb12 and PrOs4Sb12Rahimi, Somayyeh Jay January 2007 (has links)
The measurement of thermal conductivity is a powerful probe that can be used for identifying the nature of heat and charge carriers and structure of the gap in the superconducting compounds. At low temperature when the effect of phonons in transporting heat becomes smaller, one can obtain information about the quasiparticle distribution and the superconducting gap structure.
In order to do a sensitive thermal conductivity measurement, we designed and built a thermal conductivity mount. The charge conductivity was measured through the same leads that we used for making the thermal conductivity measurements. To test the mount, we measured the heat and charge conductivity of a silver wire and determined the accuracy with which we could satisfy the Wiedemann--Franz law within 5 \%.
We will report the measurements of thermal and electrical conductivities of two filled skutterudite superconducting compounds, PrRu4Sb12 and PrOs4Sb12 at 1.1--35 K temperature range. The differences and similarities between the transport properties of these compounds in the superconducting and normal states along with the results of investigation of the Wiedemann--Franz law will be discussed in the following chapters.
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Thermal and charge conductivities of superconducting skutterudite compounds, PrRu4Sb12 and PrOs4Sb12Rahimi, Somayyeh Jay January 2007 (has links)
The measurement of thermal conductivity is a powerful probe that can be used for identifying the nature of heat and charge carriers and structure of the gap in the superconducting compounds. At low temperature when the effect of phonons in transporting heat becomes smaller, one can obtain information about the quasiparticle distribution and the superconducting gap structure.
In order to do a sensitive thermal conductivity measurement, we designed and built a thermal conductivity mount. The charge conductivity was measured through the same leads that we used for making the thermal conductivity measurements. To test the mount, we measured the heat and charge conductivity of a silver wire and determined the accuracy with which we could satisfy the Wiedemann--Franz law within 5 \%.
We will report the measurements of thermal and electrical conductivities of two filled skutterudite superconducting compounds, PrRu4Sb12 and PrOs4Sb12 at 1.1--35 K temperature range. The differences and similarities between the transport properties of these compounds in the superconducting and normal states along with the results of investigation of the Wiedemann--Franz law will be discussed in the following chapters.
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Investigation of the effect of gel residue on hydraulic fracture conductivity using dynamic fracture conductivity testMarpaung, Fivman 15 May 2009 (has links)
The key to producing gas from tight gas reservoirs is to create a long, highly
conductive flow path, via the placement of a hydraulic fracture, to stimulate flow from the
reservoir to the wellbore. Viscous fluid is used to transport proppant into the fracture.
However, these same viscous fluids need to break to a thin fluid after the treatment is over
so that the fracture fluid can be cleaned up. In shallower, lower temperature (less than
250oF) reservoirs, the choice of a fracture fluid is very critical to the success of the
treatment. Current hydraulic fracturing methods in unconventional tight gas reservoirs
have been developed largely through ad-hoc application of low-cost water fracs, with little
optimization of the process. It seems clear that some of the standard tests and models are
missing some of the physics of the fracturing process in low-permeability environments.
A series of the extensive laboratory “dynamic fracture conductivity” tests have
been conducted. Dynamic fracture conductivity is created when proppant slurry is
pumped into a hydraulic fracture in low permeability rock. Unlike conventional fracture
conductivity tests in which proppant is loaded into the fracture artificially, we pump
proppant/ fracturing fluid slurries into a fracture cell, dynamically placing the proppant
just as it occurs in the field.
Test results indicate that increasing gel concentration decreases retained fracture
conductivity for a constant gas flow rate and decreasing gas flow rate decreases retained
fracture conductivity. Without breaker, the damaging effect of viscous hydraulic
fracturing fluids on the conductivity of proppant packs is significant at temperature of
150oF. Static conductivity testing results in higher retained fracture conductivity when
compared to dynamic conductivity testing.
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Laboratory-scale fracture conductivity created by acid etchingPournik, Maysam 15 May 2009 (has links)
Success of acid fracturing treatment depends greatly on the created conductivity
under closure stress. In order to have sufficient conductivity, the fracture face must be
non-uniformly etched while the fracture strength maintained to withstand the closure
stress. While there have been several experimental studies conducted on acid fracturing,
most of these have not scaled experiments to field conditions and did not account for the
effect of rock weakening and etching pattern. Hence, acid fracture conductivity
predictions based on the above works have not been able to match actual results.
In order to develop a more appropriate and accurate prediction of acid fracturing
treatment outcome, a laboratory facility was developed that is properly scaled to field
conditions and enables analysis of etching pattern and rock strength. A systematic
experimental study that covered a variety of formations, acid types, and acid contact
times was conducted. An acid fracture conductivity correlation was developed based on
etched volume, etched pattern, and fracture strength under closure stress.
Results suggested that there is an optimal time of acid exposure resulting in
maximum fracture conductivity. There were large differences in the conductivity created with the different acid systems tested due to different etching patterns and degree of rock
strength weakening. There was an optimal acid system depending on formation type,
contact time and overburden stress. The acid fracture conductivities measured did not
agree with the predictions of the Nierode-Kruk correlation. The newly developed
correlation predicts conductivity much closer as it includes the effect of rock strength
and surface etching pattern on resulting conductivity.
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The effects of acid contact time and rock surfaces on acid fracture conductivityMelendez Castillo, Maria Georgina 02 June 2009 (has links)
The conductivity created in acid fracturing is a competition between two phenomena: etching of the rock surface and weakening of the rock. This study presents experimental results of acid fracturing conductivity experiments with polymer gelled acid, while varying contact time and rock type. The experiments were conducted in a laboratory facility properly scaled from field to laboratory conditions to account for the hydrodynamic effects that take place in the field. The rocks of study were Indiana limestone, San Andres dolomite and Texas Cream chalk. Our results illustrate that acid fracturing conductivity is governed by the etching pattern of the rock surface and influenced by the hardness of the rock. If channels are created, the fracture is more likely to retain conductivity after closure. The hardness of the rock is the dominating factor to determine the conductivity response when no channeling is present. Among the rocks tested, Texas Cream chalk had the lowest hardness measurement before and after acidizing and the fracture closed at a much lower stress compared with limestone and dolomite. Dolomite had the highest conductivity under all closure stresses even without a channeling pattern. Additionally, it was observed that a higher reduction in rock strength at the contact points for dolomite yielded lower conductivity after closure. The effects of hardness variation on conductivity are higher in dolomite than in limestone and chalk. It is apparent that longer contact times do not always provide higher conductivity after closure.
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