The thermal conductivity of PMMA films with thicknesses from 5 to 50 nanometers and layered over a treated silicon substrate is explored numerically by the application of the reverse non-equilibrium molecular dynamics (NEMD) technique and the development of a coarse-grained model for PMMA, which allows for the simulation time of hundreds of nanoseconds required for the study of large polymer systems. The results showed a constant average thermal conductivity of 0.135 W/m_K for films thickness ranging from 15 to 50 nm, while films under 15 nm in thickness showed a reduction of 30% in their conductivity. It was also observed that polymer samples with a degree of polymerization equal to 25% of the entanglement length had 50% less thermal conductivity than films made of longer chains. The temperature profiles through the films thickness were as predicted by the Fourier equation of heat transfer. The relative agreement between the thermal conductivity from experiments (0.212 W/m_K for bulk PMMA) and the results from this investigation shows that with the proper interpretation of results, the coarse-grained NEMD is a useful technique to study transport coefficients in systems at larger nano scales.

<p>The last years a TC-sensing module called HSS-440 has been developed at AppliedSensor. The sensor is used in hydrogen powered cars to detect H2-leakages. TC-sensing is a technique that uses small changes in thermal conductivity when H2 is present to determine concentrations. Today these small changes are estimated with a prediction model that uses several hundreds of parameters.</p><p>A sensor substrate from a new manufacturer is now introduced. This means an opportunity to look over the current solution. The task for this thesis is to investigate system properties and new solutions regarding a prediction model with minimal need for calibration.</p><p>System properties are investigated and relations for heat flow and influence of H2 are established. In the process an earlier not known nonlinearity are proved to exist. From this, a new open loop nonlinear greybox model is estimated and the nonlinearity are concluded to improve the model. The model is then closed with an earlier implemented PI-regulator and concluded to be useful for H2-predictions. The new model also utilizes 11 parameters instead of hundreds which is a big improvement.</p> / <p>Sista åren har en sensor, med beteckningen HSS-440, mätandes Termisk konduktivitet utvecklats på AppliedSensor. Sensorn används för att upptäcka läckage av H2-gas i vätgasdrivna bilar. Vid Termisk Konduktivitets mätning används små förändringar av den termiska konduktiviteten, då H2 är närvarande i det omgivande mediumet, som ett mått på koncentrationen. Idag änvänder prediktionsmodellen flera hundra parametrar för att skatta denna koncentration.</p><p>Nu introduceras ett sensorsubstrat från en ny tillverkare, vilket innebär ett bra tillfälle att se över den gamla lösningen. Syftet med examensarbetet är därför att undersöka nya systemegenskaper i och med introduktionen av det nya sensorsubstratet samt nya lösningar på en prediktionsmodel med ett minimalt behov av kalibrering.</p><p>Systemegenskaperna undersöks och samband för värmeflöden och H2's påverkan på systemet fastställs. Vid denna undersökning upptäcks en tidigare okänd olinjäritet. Utifrån detta bestämms en ny olinjär greybox modell där den nyfunna olinjäriteten bevisas förbättra modellen. Modellen sluts med en tidigare implementerade PI-regulator och bevisas vara användbar vid H2-prediktion. Den nya modellen använder även bara 11 parametrar istället för flera hundra vilket är en stor förbättring.</p>

Two-dimensional graphene, a single layer of graphite, has emerged as an excellent candidate for future electronic material due to its unique electronic structure and remarkably high carrier mobility. Even higher carrier mobility has been demonstrated in graphene devices using hexagonal boron nitride as an underlying dielectric support instead of silicon oxide. Interestingly, both graphene and boron nitride exhibit superior thermal properties, therefore may potentially offer a solution to the increasingly severe heat dissipation problem in nanoelectronics caused by increased power density. In this thesis, we focus on the investigation of the thermal properties of graphene and hexagonal boron nitride. First, scanning thermal microscopy based on a sub-micrometer thermocouple at the apex of a microfabricated tip was employed to image the temperature profiles in electrically biased graphene devices with ~ 100 nm scale spatial resolution. Non-uniform temperature distribution in the devices was observed, and the "hot spot" locations were correlated with the charge concentrations in the channel, which could be controlled by both gate and drain-source biases. Hybrid contact and lift mode scanning has enabled us to obtain the quantitative temperature profiles, which were compared with the profiles obtained from Raman-based thermometry. The temperature rise in the channel provided an important insight into the heat dissipation mechanism in Joule-heated graphene devices. Next, thermal conductivity of suspended single and few-layer graphene was measured using a micro-bridge device with built-in resistance thermometers. Polymer-assisted transfer technique was developed to suspend graphene layers on the pre-fabricated device. The room temperature thermal conductivity values of 1-7 layer graphene were measured to be lower than that of bulk graphite, and the value appeared to increase with increasing sample thickness. These observations can be explained by the impact of the phonon scattering by polymer residue remaining on the sample surfaces. Lastly, thermal conductivity of few-layer hexagonal boron nitride sample was measured by using the same device and technique used for suspended graphene. Measurements on samples with different suspended lengths but similar thickness allowed us to extract the intrinsic thermal conductivity of the samples as well as the contribution of contact thermal resistance to the overall thermal measurement. The room temperature thermal conductivity of 11 layer sample approaches the basal-plane value reported in the bulk sample. Lower thermal conductivity was measured in a 5 layer sample than an 11 layer sample, which again supports the polymer effect on the thermal transport in few-layer hexagonal boron nitride. / text

For the past decade interest in skutterudites has been significant as a potentially viable material for thermoelectrics. One way to improve the effectiveness of these materials is to lower their thermal conductivity. Lattice thermal conductivity of a series of La- and Yb-filled skutterudite antimonides (with varying filling fraction) has been modeled with different phonon scattering parameters using the debye approximation. It was found that filler atoms both increase point defect scattering and resonance scattering. Subsequently, the thermal conductivity of partially-filled skutterudites AxCo4Sb12, where A = La, Eu and Yb, is analyzed using the Debye model in order to correlate the data with the type of filler atom in evaluating the role of the filler atom in affecting the thermal conductivity. Partial void filling has resulted in relatively high thermoelectric figures of merit at moderately high temperatures. This idea is extended as new materials were synthesized with the intention of filling the voids in the CoGe1.5Se1.5 skutterudite, and analyzing the transport of these novel materials. Results of the analysis of this material are interesting and may indicate an amorphous phase of skutterudite present. Further work is needed to explore fully the implications of this new skutterudite and to fully understand its properties.

The present dissertation investigates the relationship between the structure and thermoelectric properties of ZnO based materials, with a focus on trivalent element doping on engineering the microstructure and altering the electrical and thermal transport properties. Within the solubility range, the addition of trivalent elements, such as In3+, Fe3+ and Ga3+, is observed to increase the electrical conductivity of ZnO and decrease the thermal conductivity. / Engineering and Applied Sciences

Thermal conductivity is a crucial parameter for nuclear fuel, as it sets an upper limit on reactor operating temperature to have safety margins. Uranium mononitride (UN) is a prospective fuel for fast reactors, for which limited experimental studies have been conducted, compared to the currently dominating light-water reactor fuel, uranium dioxide. The aim of this thesis is to determine the thermal conductivity in UN and to determine its porosity dependence. This was done by manufacturing dense and porous high-purity samples of UN and examining them with laser flash analysis, which with data on specific heat and thermal expansion gives the thermal conductivity. To analyse the result, a theoretical study of the phenomenology of thermal conductivity as well as a review and comparison with previous investigations were carried out. The porosity range was 0.1–31% of theoretical density. Thermal diffusivity data from laser flash analysis, thermal expansion data and specific heat data was collected for 25–1400 C. The laser flash data had high discrepancy at higher temperatures due to thermal instability in the device and deviations due to graphite deposition on the samples, but the low temperature data should be reliable. As the specific heat data was also of poor quality, literature data was used instead. As for the thermal diffusivity data, the calculated thermal conductivity for lower temperatures are more accurate. A modified version of the porosity model by Ondracek and Schulz was used to analyse the porosity dependence of the thermal conductivity, taking into account the different impacts of open and closed porosity. / Värmeledningsförmåga är en avgörande egenskap för kärnbränslen, eftersom det begränsar den maximala drifttemperaturen i reaktorn för att ha säkerhetsmarginaler. Uranmononitrid (UN) är ett framtida bränsle för snabba reaktorer. Jämfört med det dominerande bränslet i lättvattenreaktorer, urandioxid, har endast begränsade experimentella studier gjorts av UN. Målet med detta arbete är att bestämma värmeledningsförmågan i UN och bestämma dess porositetsberoende. Detta gjordes genom att tillverka kompakta och porösa prover av UN och undersöka dem med laserblixtmetoden, vilket tillsammans med värmekapacitet och värmeutvidgning ger värmeledningsförmågan. För att analysera resultatet gjordes en teoretisk studie av värmeledning såväl som en genomgång av och jämförelse med tidigare undersökningar. Provernas porositet sträckte sig från 0.1% till 31% av teoretisk densitet. Värmediffusivitetsdata från laserblixtmetoden, värmeutvidgningsdata och värmekapacitetsdata samlades in för 25–1400 C. Värdena från laserblixtmätningen hade hög diskrepans vid höga temperaturer p.g.a. termisk instabilitet i anordningen och avvikelser p.g.a. grafitavlagring på proverna, men data för låga temperaturer borde vara tillförlitliga. Eftersom resultaten från värmekapacitetsmätningen var av dålig kvalité, användes litteraturdata istället. Som en konsekvens av bristerna i mätningen av värmediffusivitet är presenterade data för värmeledningsförmåga mest exakta för låga temperaturer. En modifierad version av Ondracek-Schulz porositetsmodell användes för att analysera värmeledningsförmågans porositetsberoende genom att ta hänsyn till olika inverkan av öppen och sluten porositet.

The last years a TC-sensing module called HSS-440 has been developed at AppliedSensor. The sensor is used in hydrogen powered cars to detect H2-leakages. TC-sensing is a technique that uses small changes in thermal conductivity when H2 is present to determine concentrations. Today these small changes are estimated with a prediction model that uses several hundreds of parameters. A sensor substrate from a new manufacturer is now introduced. This means an opportunity to look over the current solution. The task for this thesis is to investigate system properties and new solutions regarding a prediction model with minimal need for calibration. System properties are investigated and relations for heat flow and influence of H2 are established. In the process an earlier not known nonlinearity are proved to exist. From this, a new open loop nonlinear greybox model is estimated and the nonlinearity are concluded to improve the model. The model is then closed with an earlier implemented PI-regulator and concluded to be useful for H2-predictions. The new model also utilizes 11 parameters instead of hundreds which is a big improvement. / Sista åren har en sensor, med beteckningen HSS-440, mätandes Termisk konduktivitet utvecklats på AppliedSensor. Sensorn används för att upptäcka läckage av H2-gas i vätgasdrivna bilar. Vid Termisk Konduktivitets mätning används små förändringar av den termiska konduktiviteten, då H2 är närvarande i det omgivande mediumet, som ett mått på koncentrationen. Idag änvänder prediktionsmodellen flera hundra parametrar för att skatta denna koncentration. Nu introduceras ett sensorsubstrat från en ny tillverkare, vilket innebär ett bra tillfälle att se över den gamla lösningen. Syftet med examensarbetet är därför att undersöka nya systemegenskaper i och med introduktionen av det nya sensorsubstratet samt nya lösningar på en prediktionsmodel med ett minimalt behov av kalibrering. Systemegenskaperna undersöks och samband för värmeflöden och H2's påverkan på systemet fastställs. Vid denna undersökning upptäcks en tidigare okänd olinjäritet. Utifrån detta bestämms en ny olinjär greybox modell där den nyfunna olinjäriteten bevisas förbättra modellen. Modellen sluts med en tidigare implementerade PI-regulator och bevisas vara användbar vid H2-prediktion. Den nya modellen använder även bara 11 parametrar istället för flera hundra vilket är en stor förbättring.

Inherent safety is claimed for gas-cooled pebble bed reactors, such as the South African Pebble Bed Modular Reactor (PBMR), as a result of its design characteristics, materials used, fuel type and physics involved. Therefore, a proper understanding of the mechanisms of heat transfer, fluid flow and pressure drop through a packed bed of spheres is of utmost importance in the design of a high temperature Pebble Bed Reactor (PBR). In this study, correlations describing the effective thermal conductivity through packed pebble beds are examined. The effective thermal conductivity is a term defined as representative of the overall radial heat transfer through such a packed bed of spheres, and is a summation of various components of the overall heat transfer. This phenomenon is of importance because it forms an intricate part of the self-acting decay heat removal chain, which is directly related to the PBR safety case. In this study standard correlations generally employed by the thermal fluid design community for PBRs are investigated, giving particular attention to the applicability of the correlations when simulating the effective thermal conductivity in the near-wall region. Seven distinct components of heat transfer are examined namely: conduction through the solid, conduction through the contact area between spheres, conduction through the gas phase, radiation between solid surfaces, conduction between pebble and wall, conduction through the gas phase in the wall region, and radiation between the pebble and wall surface. The effective thermal conductivity models are typically a function of porosity in order to account for the pebble bed packing structure. However, it is demonstrated in this study that porosity alone is insufficient to quantify the porous structure in a randomly packed bed. A new Multi-sphere Unit Cell Model is therefore developed, which accounts more accurately for the porous structure, especially in the near-wall region. Conclusions on the applicability of the model are derived by comparing the simulation results with measurements obtained from various experimental test facilities. This includes the PBMRs High Temperature Test Unit (HTTU) situated on the campus of the North-West University in Potchefstroom in South Africa. The Multi-sphere Unit Cell Model proves to encapsulate the impact of the packing structure in a more fundamental way and can therefore serve as the basis for further refinement of models to simulate the effective thermal conductivity. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010

Inherent safety is claimed for gas-cooled pebble bed reactors, such as the South African Pebble Bed Modular Reactor (PBMR), as a result of its design characteristics, materials used, fuel type and physics involved. Therefore, a proper understanding of the mechanisms of heat transfer, fluid flow and pressure drop through a packed bed of spheres is of utmost importance in the design of a high temperature Pebble Bed Reactor (PBR). In this study, correlations describing the effective thermal conductivity through packed pebble beds are examined. The effective thermal conductivity is a term defined as representative of the overall radial heat transfer through such a packed bed of spheres, and is a summation of various components of the overall heat transfer. This phenomenon is of importance because it forms an intricate part of the self-acting decay heat removal chain, which is directly related to the PBR safety case. In this study standard correlations generally employed by the thermal fluid design community for PBRs are investigated, giving particular attention to the applicability of the correlations when simulating the effective thermal conductivity in the near-wall region. Seven distinct components of heat transfer are examined namely: conduction through the solid, conduction through the contact area between spheres, conduction through the gas phase, radiation between solid surfaces, conduction between pebble and wall, conduction through the gas phase in the wall region, and radiation between the pebble and wall surface. The effective thermal conductivity models are typically a function of porosity in order to account for the pebble bed packing structure. However, it is demonstrated in this study that porosity alone is insufficient to quantify the porous structure in a randomly packed bed. A new Multi-sphere Unit Cell Model is therefore developed, which accounts more accurately for the porous structure, especially in the near-wall region. Conclusions on the applicability of the model are derived by comparing the simulation results with measurements obtained from various experimental test facilities. This includes the PBMRs High Temperature Test Unit (HTTU) situated on the campus of the North-West University in Potchefstroom in South Africa. The Multi-sphere Unit Cell Model proves to encapsulate the impact of the packing structure in a more fundamental way and can therefore serve as the basis for further refinement of models to simulate the effective thermal conductivity. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010

Thermal conductivity measurements are made on a variety of systems in order to probe low energy quasiparticle excitations. In particular, thermal conductivity measurements were made on the iron based superconducting material LaFePO at temperatures from 60 mK to 1 K and in fields from 0 T to 5 T in order to shed light on the symmetry of the superconducting order parameter. A substantial non-zero electronic contribution to the thermal conductivity is observed and interpreted as sub-gap electronic quasiparticles which is clear evidence for a nodal gap symmetry. A high scattering rate and non-T3 temperature dependence of the conductivity is evidence against the d-wave scenario. However, the field dependence does seem to suggest that the anisotropic s+- picture is a likely candidate for the order parameter, although more theoretical work is required to confirm this. Thermal conductivity measurements were also made on the spin-ice system Ho2Ti2O7 between 50 mK and 1.4 K in applied magnetic fields from 0 T to 8 T in an attempt to observe the much debated magnetic monopole-like quasiparticles. An applied magnetic field of 8 T was applied along to [111] direction as to fully polarize the magnetic moments in order to extract the phonon contribution of the thermal conductivity. The low field thermal conductivity reveals evidence for an additional heat transfer mechanism that also scatters phonons which is magnetic in nature. This is taken to be evidence for the existence of monopole-like excitations out of the spin-ice ground state and is described by existing Debye-Huckel theory. Thermal transport was used in conjunction with charge conductivity to study the unconventional quantum critical point (QCP) in the heavy-Fermion superconductor beta-YbAlB4 at temperatures down to 60 mK and in fields up to 2 T. The results show that the Wiedemann-Franz law (WFL) is obeyed down to the lowest measured temperatures indicating that the Landau quasiparticles remain intact near the QCP. A small suppression of the Wiedemann-Franz ratio (L/L0 = kappa / sigma T L0) is seen at finite temperatures (T < 1 K) with minimal dependence on magnetic field. Comparing with other similar quantum critical systems, it becomes apparent that inelastic scattering events have little effect on the transport and are mainly field independent in beta-YbAlB4. An overview of the design for a new thermal conductivity mount is also presented. The design hinges around the idea of building the experiment mount into a small copper box rather than on an open frame. Not only does this provide mechanical stability for safe transportation, it also reduces the noise caused by electromagnetic interference (EMI) in the sample thermometers by more than a factor of ten over the old wire frame design.