1,720,996 research outputs found
Si/SiGe thermoelectric generator
Full text linkThis PhD thesis researches thermoelectric generator (TEG) which transfers wasted heat into electricity by thermoelectric materials.
As a parameter used to characterize thermoelectric materials, figure-of-merit (ZT) models of Si bulk, Si/Si_(1-x) Ge_x bulk and Si bulk/nanowires (NWs) are built via building their models of Seebeck coefficient, electrical conductivity and thermal conductivity in this thesis. ZT of Si bulk is increased by 18% by applying a 3μm thick Si_0.8 Ge_0.2 bulk layer, and it is increased by 1000% by applying 35μm long Si NWs.
TEG’s output power model which takes account of the effects of thermoelectric material, as well as all parasitic effects that affect TEG’s output power. TEG’s output power model demonstrates the output power depends on thermoelectric material’s characteristics and the contact interface quality between thermoelectric material and metal probe. Thermoelectric material’s characteristics are improved by Si NWs, Si_(1-x) Ge_x bulk, Si_(1-x) Ge_x NWs and spin-on-doping (SOD). SOD also improves the contact interface quality between thermoelectric material and metal probe, which also can be improved by sputter coating a layer of metal on thermoelectric material’s surface. Finally, TEG’s output power is increased by an order of 3 by these techniques.Open Acces
User perceptions of wearability of knitted sensor garments for long term monitoring of breathing health: thematic analysis of focus groups and a questionnaire survey
No full textBackground: long term unobtrusive monitoring of breathing patterns can potentially give a more realistic insight into the respiratory health of people with asthma or chronic obstructive pulmonary disease than brief tests performed in medical environments. However, it is uncertain whether users would be willing to wear these sensor garments long term.Objective: our objective was to explore whether users would wear knitted garments with knitted-in breathing sensors long term to monitor their lung health and under what conditions.Methods: multiple knitted breathing sensor garments, developed and fabricated by the research team, were presented during a demonstration. Participants were encouraged to touch and feel the garments and ask questions. This was followed by two semi-structured, independently led focus groups with a total of 16 participants of which 4 had asthma. The focus group conversations were recorded and transcribed. Thematic analysis was carried out by three independent researchers in three phases consisting of: familiarization with the data, independent coding and overarching theme definition. Participants also completed a web-based questionnaire to probe opinion about wearability and functionality of the garments. Quantitative analysis of the sensors’ performance was mapped to participants’ garment preference to support the feasibility of the technology for long term wear.Results: key points extracted from the qualitative data were: 1) garments more likely to be worn if medically prescribed, 2) cotton vest as underwear was preferred, and 3) a breathing crisis warning system was seen as a promising application. The qualitative analysis showed a preference for loose short sleeved T-shirts with a 81% acceptability rate, a 69% acceptability rate for snug fitting garments and 0% for tight-fitting garments. 62% of the participants would wear the knit for the whole day and 81% only during the night if not too hot. The sensitivity demands on the knitted wearable sensors can be aligned with users’ garment preferences.Conclusions: there is an overall positive opinion about wearing a knitted sensor garment over a long period of time for monitoring of respiratory health. The knit cannot be tight but should be able to be worn as a vest as underwear in a breathable material. These requirements can be fulfilled with the proposed garments. Participants with asthma supported using it as a sensor garment connected to an asthma attack alert system
RF Sensors for Monitoring the Electrical Properties of Electrolyte Solutions
Full text linkA radio frequency electrical sensor for the qualitative analysis and monitoring of the
electrical properties of electrolyte solutions is designed, simulated and experimentally tested
in this research. This work is based on the use of planar inductors for the detection of a
change in the concentration of ionic species in a liquid sample. At first a literature review on
the physical chemistry of electrolyte solutions is provided. This will include topics on the
conductivity and relaxation properties of electrolytes. This will be followed by a look at
dielectric spectroscopy sensors, electrochemical sensors and inductive sensing devices. The
principles of electrodynamics and constitutive equations are discussed. Based on these, the
principles of operation of the RF electrical sensors are analysed. Two methods of theoretical
analysis of such structures are investigated. These methods are; analytical solution and finite
element computation method. The former offers greater insight into the system’s parameters
whilst the latter offers more information regarding the whole system. Given the qualitative
nature of the sensors under investigation and finite element approach was selected and used
in latter chapters to obtain grater insight into the behaviour of the system.
Planar inductor coils are designed on an FR4 substrate and packaged using PDMS to be used
as sensors in the monitoring of electrical properties of electrolytes. Experimental results on
these sensors are provided and discussed. The effects of solvent, acidity of the solutions, and
environmental factors on the behaviour of the sensors shall be discussed. This is followed by
finite element simulations of the sensor and the effect of various parameters on the overall
behaviour of the sensing device. A transformer apparatus is also constructed and
experimental data are provided for it. An electrolyte is placed on one of the coils of the
transformer and scattering parameters are looked upon for data analysis. The results obtained
using the FE method, is then used to obtain further information about the principle of
operation of the device
Reconfigurable nanowires for thermo-electric applications
Full text linkNano-structuration of bulk into nanowires and nanosheets has led to massive changes in device behaviour and to opportunities for designing novel devices that exploit these changes. The combination of these two consequences of nanoscale devices in integrated circuits will lead to further enhancements in their performance in the future. In this project, different novel device structures have been designed and their performance investigated using Sentaurus Technology Computer Aided Design (TCAD) software. Sentaurus TCAD is a commercial software package that is used in the semiconductor industry to predict the behaviour of nanoscale devices. Its transport models have been tuned and calibrated over at least the last decade to give realistic performance parameters of novel device structures. One current trend in semiconductor device technology that we have embraced in this work is the replacement of n+/p (p+/n) junctions as contacts to the external world by metal/semiconductor Schottky contacts. This resolves the k_BT/q limitation of diffusive processes leading to better device performance parameters. In this work, doping has been avoided in order to maximise the mobility of the carriers and minimise the challenges with doping control in nanostructures. Carrier modulation techniques have been implemented to control the thermal and electrical conductivities of the novel devices designed in this work. This project focused on two areas of research. One is the solution of the traditional a-symmetric current-voltage characteristics of the p- and n-FET in CMOS. A novel reconfigurable double gate nanowire structure with oxide bridges that controls the symmetry of operation of the p- and n-FET in CMOS was designed. Not only can the approach be integrated in CMOS technology, it is also relatively tolerant to small geometrical deviations. The challenge in this design lies in the oxide bridge perpendicular to the gate length direction. Nano-structuration of Si also leads to a reduction of the thermal conductivity. Although this is a problem for nanowire and nanosheet FETs due to increased self-heating, it also leads to opportunities offered for on-chip thermo-electric power generation, TEG (or thermoelectric cooling (TEC)) and temperature sensors. We propose a carrier modulation technique based on the work function difference between the gate metal and the intrinsic NW - a technique similar to a nanoFET process and thus compatible with CMOS technology - that shows an increase in the TEG’s performance. Another application where thermo-electrics can be used is in on-chip temperature sensing. Currently on-chip temperature sensors suffer from high power consumption and high real estate on the chip. We use a gated reconfigurable Si nanowire FET as temperature sensor that in principle works on the same supply rail as the circuits and is sufficiently small to be placed close to where hotspots will occur. The gate voltage that needs to be applied to keep the current constant as a function of temperature difference is a measure of heating. Due to the weak electrical conductivity of the nanowire, an efficient temperature difference can be maintained between the hotspot and the surrounding thermal mass minimising the need for re-callibration.Open Acces
High capacitance silicon nanowire array electrodes
Full text linkThe interest in the development of improved, alternative and application-specific electrical energy storage solutions presents the opportunity for Si-based device with the functionality of electrochemical capacitors (ECs). Metal-assisted chemical etching (MACE) provides a low temperature and low-cost method of obtaining a high-density array of high aspect ratio silicon nanowires. The high surface area of the silicon nanowire arrays (SiNWA) is utilised to develop a high capacitance electrode, in conjunction with an ionic liquid (IL) electrolyte giving low volatility, high thermal stability, and high chemical stability enabling a higher operating voltage. High silicon reactivity necessitates passivation of the Si surface. A low temperature (120 °C) wet oxidation process provides a highly dense, ultra-thin (~1.4 nm) protective layer that extends the operating voltage and yields a high energy and power density, bringing the SiNWA electrode within the range of ECs. An alternative coating of metal oxide (TiO2) provides further performance improvement, and with energy and power densities of 0.9 and Wh·kg-1 and 2228 W·kg-1 respectively, places the developed SiNWA electrode towards the frontier of EC devices, as per the Ragone plot. Intermittent presence of apparent faradaic peaks observable on the cyclic voltammetry (CV) plots of SiNWA electrodes was analysed and attributed to the presence of deep level traps (DLTs) as a result of residual Ag from the MACE process. Multiple post-etch doping steps to degenerately dope the surface – pinning the Fermi level below the Si valence band – were found to mitigate the effect of the DLTs, improving the capacitive character and cycling stability of the electrodes.Open Acces
Knitted coils as breathing sensors
Full text linkA new implementation of a wearable respiratory inductive plethysmography garment is obtained by knitting a 250 μm thin insulted Cu wire simultaneously with yarn in the round. This was used to integrate a knitted coil in the body of a baby romper suit. During simulated breathing the diameter of knitted coil changes by stretching the knit circularly, causing a variation of the self-inductance of the coil. Coils with 5 rows of integrated metal wire with different stitch types and patterns were investigated to determine their influence on inductance, series resistance and sensitivity. We observed that knit styles that reduce the resistance of the coil, such as lace and jacquard also reduce the inductance and flexibility of the garment. Jacquard with three colours and one metal wire for each colour, gave the highest coil quality factor but also the poorest flexibility. We found that 1/1 rib stitch has the highest self-inductance for all yarn types. Its sensitivity of 0.5 – 0.6 μH/cm is similar to stockinette stitch except when elastic viscose yarn is used. Coils in stockinette stitch and elastic viscose yarn have the highest sensitivity of 0.84 μH/cm. No hysteresis in self-inductance was observed for circumference variations between 44 and 53 cm of the body of the baby romper in 1/1 rib stitch due to the elasticity of knitted garments
Light trapping structures for photovoltaics using silicon nanowires and silicon micro-pyramids
Full text linkThe current photovoltaic industry is dominated by crystalline or poly-crystalline Si in
a planar pn-junction configuration. The use of silicon nanowire arrays (SiNWA) within
this industry has shown great promise due to its application as an anti-reflective layer,
as well as benefits in charge carrier extraction. In this work, we use a metal assisted
chemical etch process to fabricate SiNWAs onto a dense periodic array of pyramids that
are formed using an alkaline etch masked with an oxide layer. The hybrid micro-nano
structure acts as an anti-reflective coating with experimental reflectivity below 1% over
the visible and near-infrared spectral regions. This represents an improvement of up to
11 and 14 times compared to the pyramid array and SiNWAs on bulk, respectively. In
addition to the experimental work, we optically simulate the hybrid structure using the
commercial Lumerical FDTD package. The results of the optical simulations support our
experimental work, illustrating a reduced reflectivity in the hybrid structure. The nanowire
array increases the absorbed carrier density within the pyramid by providing a guided
transition of the refractive index along the light path from air into the silicon. Furthermore,
electrical simulations which take into account surface and Auger recombination show an
effi ciency increase for the hybrid structure of 56% over bulk, 11% over pyramid array and
8.5% over SiNWAs. Opto-electronic modelling was performed by establishing a tool
flow to integrate the eff ective optical simulator Lumerical FDTD with the excellent fabrication and
electrical simulation capability of Sentaurus TCAD. Interfacing between the two packages
is achieved through tool command language and Matlab, off ering fast and accurate electro-optical
characteristics of nano-structured PV devices.Open Acces
Graphene inspired sensing devices
Full text linkGraphene’s exciting characteristics such as high mechanical strength, tuneable electrical prop- erties, high thermal conductivity, elasticity, large surface-to-volume ratio, make it unique and attractive for a plethora of applications including gas and liquid sensing. Adsorption, the phys- ical bonding of molecules on solid surfaces, has huge impact on the electronic properties of graphene. We use this to develop gas sensing devices with faster response time by suspending graphene over large area (cm^2) on silicon nanowire arrays (SiNWAs). These are fabricated by two-step metal-assisted chemical etching (MACE) and using a home-developed polymer-assisted graphene transfer (PAGT) process. The advantage of suspending graphene is the removal of diffusion-limited access to the adsorption sites at the interface between graphene and its support. By modifying the Langmuir adsorption model and fitting the experimental response curves, we find faster response times for both ammonia and acetone vapours. The use of suspended graphene improved the overall response, based on speed and amplitude of response, by up to 750% on average. This device could find applications in biomedical breath analysis for diseases such lung cancer, asthma, kidney failure and more.
Taking advantage of the mechanical strength of graphene and using the developed PAGT process, we transfer it on commercial (CMOS) Ion-Sensitive Field-Effect Transistor (ISFET) arrays. The deposition of graphene on the top sensing layer reduces drift that results from the surface modification during exposure to electrolyte while improving the overall performance by up to about 10^13 % and indicates that the ISFET can operate with metallic sensing membrane and not only with insulating materials as confirmed by depositing Au on the gate surface. Post- processing of the ISFET top surface by reactive ion plasma etching, proved that the physical location of trapped charge lies within the device structure. The process improved its overall performance by about 105 %. The post-processing of the ISFET could be applied for sensor performance in any of its applications including pH sensing for DNA sequencing and glucose monitoring.Open Acces
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