Synthesis Of Graphene Nanomaterials And Their Application In Electrochemical Energy Storage

The need to store and use energy on diverse scales in a modern technological society necessitates the design of large and small energy systems, among which electrical energy storage systems such as batteries and capacitors have attracted much interest in the past several decades. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors, with fast power delivery and long cycle life are complementing or even replacing batteries in many applications. The rapid development of miniaturized electronic devices has led to a growing need for rechargeable micro-power sources with high performance. Among different sources, electrochemical micro-capacitors or micro-supercapacitors provide higher power density than their counterparts and are gaining increased interest from the research and engineering communities. Rechargeable Li ion batteries with high energy and power density, long cycling life, high charge-discharge rate (1C - 3C) and safe operation are in high demand as power sources and power backup for hybrid electric vehicles and other applications. In the present work, graphene-based graphene materials have been designed and synthesized for electrochemical energy storage applications, e.g., conventional supercapacitors (macro-supercapacitors), microsupercapacitors and lithium ion batteries. Factors influencing the formation and structure of graphitic petals grown by microwave plasma-enhanced chemical vapor deposition on oxidized silicon substrates were investigated through process variation and materials analysis. Insights gained into the growth mechanism of these graphitic petals suggest a simple scribing method can be used to control both the location and formation of petals on flat Si substrates. Transitional metal oxides and conducting polymers have been coated on the graphitic petal-based electrodes by facile chemical methods for multifunctional energy storage applications. Detailed electrochemical characterization (e.g., cyclic voltammetry and constant galvanostatic charge/discharge) has been carried out to evaluate the performance of electrodes

An investigation of diamond thin film deposition on steel substrates

The motivation behind this work was the exploration of the possibility of diamond deposition on steel substrates for low friction and low wear applications. Materials such as tungsten carbide are commercially available as diamond coated tools, where the diamond coating greatly extends the tool lifetime and performance. The diamond deposition on steel differs in terms of limitations to the diamond deposition on tungsten carbide. The main limitations of steel are its sensitivity to elevated temperatures which are commonly used for diamond deposition and a large difference in the thermal expansion coefficients of steel and diamond. Overcoming those challenging limitations would result in an introduction of competitive products for many applications. This project was a pioneering work in diamond deposition on steel substrates at Aston University in co-operation with Teer Coatings Ltd (Miba goup). The main focus was on the use of an interlayer as a facilitator of enhanced diamond growth and its adhesion towards the steel substrate. Particular attention was given to amorphous carbon coating being a buffer layer for subsequent diamond growth, followed by the investigation of diamond film growth on tungsten coated steel substrates. Interlayers were deposited using the magnetron sputtering technique at Teer Coatings. Diamond thin films were deposited at Aston University using microwave plasma chemical vapour deposition (CVD) with methane and hydrogen as a deposition gas mixture. Investigation of diamond growth from amorphous carbon films coated on steel substrates was found despite the initial promising results to provide low diamond nucleation coverage resulting samples with a sparse population of diamond crystals. The focus of the study changed into an investigation of diamond growth on steel substrates coated with metallic interlayers. As an enhancement for diamond nucleation a pre-treatment of seeding the substrates with nanocrystalline diamond particles, transferred onto the substrates by immersion into a diamond suspension, was developed and used further in this work. Tungsten coating was chosen as the main interlayer material for its diffusion barrier properties, carbide formation, specific thermal expansion coefficient and no inclination to hydrogen embrittlement. The direct tungsten deposition onto a substrate was found problematic and was initially solved by the development of a structured CrW interlayer (1 μm thick) on which an optimization of diamond CVD deposition conditions was performed. The need for a reliable temperature measurement resulted in creation of a setup with thermocouple mounted at the bottom of a substrate holder and a suitable calibration of the setup to be able to calculate the temperature of the substrate surface. CrW was found to have poor adhesion properties and a new MoW interlayer (1 μm thick) possessing excellent adhesion characteristics was developed. The diamond films deposited using previously optimised diamond deposition conditions was found to be at 785 °C. The ≈250 nm thick diamond films showed a good adhesion strength while the MoW interlayer was proved to be an effective diffusion barrier. The previously optimised diamond deposition conditions were found to deteriorate the steel substrate’s properties and further low temperature diamond deposition conditions were optimised for diamond growth at 650 °C. The resulting ≈250 nm thick films showed poor diamond adhesion characteristics due to weaker bonding between diamond and the substrate. The steel substrate did not undergo any softening during the diamond deposition. The effect of different diamond deposition temperatures, as well as the different thickness of the MoW interlayer on stress within diamond film, was studied. Lowest amount of compressive stress of 1.6 GPa was found for a sample coated with the thickest MoW (8.3 μm) and diamond deposition conditions at 650 °C. The sample showed superior adhesion upon Rockwell C indentation, while poor adhesion was observed by means of scratch testing using WC ball as an indenter

Revestimentos multicamada de diamante CVD micro/nanocristalino para biotribologia

Doutoramento em Ciência e Engenharia de MateriaisIn the present work multilayered micro/nanocrystalline (MCD/NCD) diamond coatings were developed by Hot Filament Chemical Vapour Deposition (HFCVD). The aim was to minimize the surface roughness with a top NCD layer, to maximize adhesion onto the Si3N4 ceramic substrates with a starting MCD coating and to improve the mechanical resistance by the presence of MCD/NCD interfaces in these composite coatings. This set of features assures high wear resistance and low friction coefficients which, combined to diamond biocompatibility, set this material as ideal for biotribological applications. The deposition parameters of MCD were optimized using the Taguchi method, and two varieties of NCD were used: NCD-1, grown in a methane rich gas phase, and NCD-2 where a third gas, Argon, was added to the gas mixture. The best combination of surface pre-treatments in the Si3N4 substrates is obtained by polishing the substrates with a 15 μm diamond slurry, further dry etching with CF4 plasma for 10 minutes and final ultrasonic seeding in a diamond powder suspension in ethanol for 1 hour. The interfaces of the multilayered CVD diamond films were characterized with high detail using HRTEM, STEM-EDX and EELS. The results show that at the transition from MCD to NCD a thin precursor graphitic film is formed. On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, as a result of the richer atomic hydrogen content and of the higher substrate temperature for MCD deposition. At those transitions, WC nanoparticles were found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate. In order to study the adhesion and mechanical resistance of the diamond coatings, indentation and particle jet blasting tests were conducted, as well as tribological experiments with homologous pairs. Indentation tests proved the superior behaviour of the multilayered coatings that attained a load of 800 N without delamination, when compared to the mono and bilayered ones. The multilayered diamond coatings also reveal the best solid particle erosion resistance, due to the MCD/NCD interfaces that act as crack deflectors. These results were confirmed by an analytical model on the stress field distribution based on the von Mises criterion. Regarding the tribological testing under dry sliding, multilayered coatings also exhibit the highest critical load values (200N for Multilayers with NCD-2). Low friction coefficient values in the range μ=0.02- 0.09 and wear coefficient values in the order of ~10-7 mm3 N-1 m-1 were obtained for the ball and flat specimens indicating a mild wear regime. Under lubrication with physiological fluids (HBSS e FBS), lower wear coefficient values ~10-9-10-8 mm3 N-1 m-1) were achieved, governed by the initial surface roughness and the effective contact pressure.No presente trabalho desenvolveram-se revestimentos de diamante micro/nanocristalino (MCD/NCD) em multicamadas obtidos por deposição química em fase vapor (CVD) assistida por filamento quente. Pretendeu-se minimizar a rugosidade através de um camada superficial de NCD, maximizar a adesão com um filme inicial de MCD sobre substratos cerâmicos de nitreto de silício (Si3N4) e incrementar a resistência mecânica pela presença de interfaces MCD/NCD nestes revestimentos compósitos. Este conjunto de características garante elevada resistência ao desgaste e baixo coeficiente de atrito, o que somado à biocompatibilidade do diamante, configuram este material como ideal para aplicações em biotribologia. Os parâmetros de deposição do MCD foram otimizados usando o método de Taguchi e utilizaram-se duas variedades de NCD: NCD-1 crescido numa atmosfera com sobressaturação de metano e NCD-2 crescido na presença de árgon. A melhor combinação de pré-tratamentos nos substratos de Si3N4 consiste num polimento com suspensão de diamante (15 μm), seguido de ataque por plasma de CF4 durante 10 minutos e riscagem em suspensão de pó de diamante em etanol durante 1 hora. As interfaces das multicamadas de diamante foram caracterizadas em detalhe por HRTEM, STEM-EDX e EELS. Os resultados mostram que na transição de diamante MCD para NCD ocorre a formação de um filme fino de carbono amorfo, inexistente na transição de NCD para MCD, como resultado da maior percentagem de hidrogénio atómico na mistura de gases e do incremento da temperatura do substrato para a deposição de MCD. Uma característica comum nas interfaces nos dois tipos de NCD é a presença de partículas esféricas de carboneto de tungsténio, devido à contaminação pelos filamentos, estando também presentes na interface entre a camada de MCD e o substrato de nitreto de silício. A adesão e resistência mecânica dos filmes de diamante foram avaliadas por ensaios de indentação, erosão com partículas de carboneto de silício e ensaios tribológicos em movimento recíproco, com pares próprios. Por indentação verificou-se que as multicamadas suportam uma carga de 800N, sem delaminação, valor superior ao atingido pelas mono- e bicamadas. Nos ensaios de erosão, as multicamadas apresentaram igualmente melhor comportamento, devido à ação das interfaces MCD/NCD como defletoras das fissuras, sendo estes resultados confirmados por uma análise de distribuição de tensões de von Mises. As multicamadas apresentam também as cargas críticas de delaminação máximas nos ensaios tribológicos a seco (200 N para multicamadas com NCD-2). Os valores do coeficiente de atrito variam na gama μ=0.02-0.09, para coeficientes de desgaste ~10-7 mm3 N-1 m-1 para a esfera e placa, indicando um regime de desgaste moderado. Sob lubrificação de líquidos fisiológicos (HBSS e FBS) descem para ~10-9-10-8 mm3 N-1 m-1, valores determinados pela rugosidade de partida e pelo regime de pressão de contato efetiva

Feature Papers in Electronic Materials Section

This book entitled "Feature Papers in Electronic Materials Section" is a collection of selected papers recently published on the journal Materials, focusing on the latest advances in electronic materials and devices in different fields (e.g., power- and high-frequency electronics, optoelectronic devices, detectors, etc.). In the first part of the book, many articles are dedicated to wide band gap semiconductors (e.g., SiC, GaN, Ga2O3, diamond), focusing on the current relevant materials and devices technology issues. The second part of the book is a miscellaneous of other electronics materials for various applications, including two-dimensional materials for optoelectronic and high-frequency devices. Finally, some recent advances in materials and flexible sensors for bioelectronics and medical applications are presented at the end of the book

Towards an on-chip power supply: Integration of micro energy harvesting and storage techniques for wireless sensor networks

The lifetime of a power supply in a sensor node of a wireless sensor network is the decisive factor in the longevity of the system. Traditional Li-ion batteries cannot fulfill the demands of sensor networks that require a long operational duration. Thus, we require a solution that produces its own electricity from its surrounding and stores it for future utility. Moreover, as the sensor node architecture is developed on complimentary metal-oxide-semiconductor technology (CMOS), the manufacture of the power supply must be compatible with it. In this thesis, we shall describe the components of an on-chip lifetime power supply that can harvest the vibrational mechanical energy through piezoelectric microcantilevers and store it in a reduced graphene oxide (rGO) based microsupercapacitor, and that is fabricated through CMOS compatible techniques. Our piezoelectric microcantilevers confirm the feasibility of fabricating micro electro- mechanical-systems (MEMS) size two-degree-of-freedom systems which can solve the major issue of small bandwidth of piezoelectric micro-energy harvesters. These devices use a cut-out trapezoidal cantilever beam to enhance the stress on the cantilever’s free end while reducing the gap remarkably between its first two eigenfrequencies in 400 - 500 Hz and 1 - 2 kHz range. The energy from the M-shaped harvesters will be stored in rGO based microsupercapacitors. These microsupercapacitors are manufactured through a fully CMOS compatible, reproducible, and reliable micromachining processes. Furthermore, we have also demonstrated an improvement in their electrochemical performance and yield of fabrication through surface roughening from iron nanoparticles. We have also examined the possibility of integrating these devices into a power management unit to fully realize a lifetime power supply for wireless sensor networks

Electrodeposition of Diamond-like Carbon Films

Electrodeposition of diamond-like carbon (DLC) films was studied on different substrates using two different electrochemical methods. The first electrochemical method using a three-electrode system was studied to successfully deposit hydrogenated DLC films on Nickel, Copper and Brass substrates. The as-deposited films were characterized by scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) and cyclic voltammetry (CV). A variety of experimental parameters were shown to affect the deposition process. The second electrochemical method was developed for the first time to deposit hydrogen free DLC films on Ni substrates through a two-electrode system. The as-deposited films were characterized by Raman spectroscopy and FTIR. According to Raman spectra, a high fraction of diamond nanocrystals were found to form in the films. Several possible mechanisms were discussed for each deposition method. An electrochemical method was proposed to deposit boron-doped diamond films for future work

Tribochemical Studies of Hard Carbon Films as a Function of Load and Environment

Hydrogen-free, hard carbon thin films are exciting material coatings candidates as solid lubricants. Two examples, ultrananocrystalline diamond (UNCD) and tetrahedral amorphous carbon (ta-C), are particularly promising, because their exceptional mechanical and tribological properties are combined with extremely smooth surfaces. However, their tribological performance can be seriously affected by variations in humidity. These materials do not perform well in vacuum or inert environments. The mechanisms controlling the friction and wear of UNCD and ta-C are not well understood because of a fundamental lack of physical understanding of the surface interactions. The aim of this thesis is to elucidate the fundamental mechanisms of friction and wear in UNCD and ta-C films. An experimental protocol is defined to examine the relationship between the sliding environment, tribological performance, and mechanical and chemical changes to the films. Self-mated reciprocating tribometry in controlled environments measure UNCD and ta-C friction as a function of load and relative humidity (RH). Scanning white light interferometry measures the post-mortem height profile. Finally, chemical changes inside the wear track are characterized by x-ray photoelectron emission microscopy combined with near-edge x-ray absorption fine structure (X-PEEM-NEXAFS) spectromicroscopy. Results for ta-C and UNCD show that both films, like single crystal diamond, perform better at lower loads or with higher amounts of RH in the environment. Previous hypotheses for this suggested that lubrication for these films either comes in the form of graphitization (converting carbon from diamond-type bonding to graphite-like bonding) or by passivation (the termination of broken carbon bonds by species in the environment, such as water). All spectroscopic evidence shows no evidence of graphitization, but support the passivation hypothesis. Furthermore, the spectroscopy shows that the passivation is in the form of hydroxyl groups, most likely from water. This affects the run-in (period at the start of sliding of high friction as asperities are being smoothed) behavior of these films. The level of passivation also controls whether the films have high or low friction