RAMAN THERMOMETRY OF GRAPHENE BASED THERMAL MATERIALS

With the growing demand for high performance computing, we are pushing for higher performance integrated circuits at an ever faster rate. Recent advances in semiconductor production technology sees transistors with a 5 nm process devices being produced for consumer use. This enabled engineers to pack tens of billions of transistors in a package no larger than a fingernail. However, that brings up a problem that we have been long battling against. How can we get rid of the heat produced by these billions of transistors. The current electronic performance is bottle-necked by the ability of the package taking heat away from the transistors. Traditional methods call for mounting the die onto metal using thermal paste or solder, which were good enough to conduct the heat for dissipation with previous technologies. But with the growing power density of modern integrated circuits, that can be problematic. Simply, packages made of metal do not take away the heat fast enough. This thesis investigates thermal properties of graphene based materials. Using the method of Raman thermometry, we can observe the temperature of the materials whilst applying a heat flux at the same time. Using finite-element analysis, our computational model maps our experimental data and extracts the properties of the thermal interface and the material itself. Unlike conventional methods of measuring heat conductivity, raman thermometry is not as limited to the size and continuity of the material. This thesis will be looking at several materials that are difficult to characterize with conventional methods, by observing the temperature of the substrate and the thin film on top. It is also possible for us to calculate the thermal interface resistance. In this thesis, several graphene derived materials will be investigated, such as graphene grown on metal foams, free standing graphene foams and graphene oxide papers. The results of the experiments show the thermal conductivity of the current graphene based foams can have a thermal conductivity of 630 W/mK for the solid portion of the graphene-metal foam structure, but 1.9 W/mK for the bulk material

Magnetic Frustration and Iron-Vacancy Ordering in Iron-Chalcogenide

We show that the magnetic and vacancy orders in the 122 $(A_{1-y}Fe_{2-x}Se_2)$ iron-chalcogenides can be naturally derived from the $J_1-J_2-J_3$ model with $J_1$ being the ferromagnetic (FM) nearest neighbor exchange coupling and $J_{2}, J_3$ being the antiferromagnetic (AFM) next and third nearest neighbor ones respectively, previously proposed to describe the magnetism in the 11(FeTe/Se) systems. In the 11 systems, the magnetic exchange couplings are extremely frustrated in the ordered bi-collinear antiferromagnetic state so that the magnetic transition temperature is low. In the 122 systems, the formation of iron vacancy order reduces the magnetic frustration and significantly increases the magnetic transition temperature and the ordered magnetic moment. The pattern of the 245 iron-vacancy order ($\sqrt{5}\times \sqrt{5}$) observed in experiments is correlated to the maximum reduction of magnetic frustration. The nature of the iron-vacancy ordering may hence be electronically driven. We explore other possible vacancy patterns and magnetic orders associated with them. We also calculate the spin wave excitations and their novel features to test our model.Comment: Figures are modified and more discussion is adde

An Empirical study of EFL Learners' Dictionary use in Chinese–English translation

This article reports on the results of a study which investigated English as Foreign Language (EFL) learners' use of an electronic dictionary in a L1–L2 translation task. Forty-seven university graduate students from a Chinese university were asked to translate a Chinese passage into English on computers with the support of an embedded dictionary. Screen recorders were used to record their dictionary use behavior and a follow-up interview was conducted to tap into the thinking processes behind their behavior. The results of the study show that when translating, EFL learners demonstrate preferences for L2 equivalents and content words in their lookups, and reveal specific problems such as a preoccupation with L2 equivalents and lack of awareness of other lexical information, which may hinder correct application of dictionary information. This study suggests that dictionary use behavior may affect the development of students' ability to translate and requires attention from both EFL learners and teachers. It is further suggested that translation teachers should alert learners to the importance of checking other lexical information in a dictionary in their translation practice.Keywords: dictionary use preferences, dictionary use processes, efl learners, translation task, interview, screen recording, log files