Joana Catarina Mendes

Property mapping of polycrystalline diamond coatings over large area

Large-area polycrystalline diamond (PCD) coatings are important for fields such as thermal management, optical windows, tribological moving mechanical assemblies, harsh chemical environments, biological sensors, etc. Microwave plasma chemical vapor deposition (MPCVD) is a standard technique to grow high-quality PCD films over large area due to the absence of contact between the reactive species and the filament or the chamber wall. However, the existence of temperature gradients during growth may compromise the desired uniformity of the final diamond coatings. In the present work, a thick PCD coating was deposited on a 100-mm silicon substrate inside a 915-MHz reactor; the temperature gradient resulted in a non-uniform diamond coating. An attempt was made to relate the local temperature variation during deposition and the different properties of the final coating. It was found that there was large instability inside the system, in terms of substrate temperature (as high as ΔT = 212 °C), that resulted in a large dispersion of the diamond coating’s final properties: residual stress (∼15.8 GPa to +6.2 GPa), surface morphology (octahedral pyramids with (111) planes to cubo-octahedrals with (100) flat top surfaces), thickness (190 μm to 245 μm), columnar growth of diamond (with appearance of variety of nanostructures), nucleation side hardness (17 GPa to 48 GPa), quality (Raman peak FWHM varying from 5.1 cm−1 to 12.4 cm−1 with occasional splitting). This random variation in properties over large-area PCD coating may hamper reproducible diamond growth for any meaningful technological application.

Measuring Torque and Temperature in a Rotating Shaft Using Commercial SAW Sensors

Real-time monitoring of torque in a rotating shaft is not easy to implement with technologies such as optic fiber sensors or strain gages. Surface acoustic wave (SAW) sensors are wireless and passive and can be used to monitor strain in moving parts. Commercial solutions (sensors, antennas and interrogation unit) can easily be purchased from some companies; however, they are not customized and may not meet the specificity of the measurements. In order to evaluate the adequacy of commercial off-the-shelf (COTS) solutions, temperature and strain sensors fabricated by SENSeOR (Besançon, France) were mounted on a load cell. The sensors were calibrated using a thermal chamber and a universal testing machine. The load cell was then assembled together with a steel shaft that rotated at different speeds inside an oven. The commercial antennas were replaced with an RF (radio frequency) coupler and the sensors were interrogated with the commercial interrogation unit. The influence of rotation in the accuracy on the measurements, as well as the adequacy of the sensors structure, was evaluated. It can be concluded that SAW sensors can be used to measure temperature or torque in a rotating environment; however, some customization of the components is required in order to overcome the limitations posed by COTS sensing solutions.

Memristor state-space embedding

This paper presents a procedure for the determination of the dimensionality of the state space of a memristive device. The state space dimensionality of a device corresponds to the minimum number of time delayed values/derivatives of the voltage and current required to represent the device dynamics for a specified set of inputs. The algorithm is based on the observed time domain voltage-current (i.e. input-output) data which is obtained by measurement. The determination of the state space dimensionality is important to achieve a single-valued input-output multivariate mapping between the device outputs as a function of the embedding variables. In this paper, this will be accomplished using an embedding technique, based on the false nearest neighbor principle.

Thermal analysis of high power LEDs using different PCB materials

Visible Light Communication is an emerging technology in Optical Wireless Communication where the Light Emitting Diodes (LEDs) are utilized for transmission of data. The luminance of the LEDs increases with the forward current; however, for high current levels the heat generated by non-radiative recombination of carriers increases the LED junction temperature, decreasing the efficiency and the lifetime of the devices. Therefore, the design of high power LED driver circuits needs to take into account the unavoidable local heating produced by high power components. In this paper, we performed thermal analysis of an LED driver mounted on Printed Circuit Boards (PCBs) using Thermal Risk Management (TRM) software and studied the heat dissipated by various components on PCBs composed of different dielectric materials such as FR4 and Diamond. Based on TRM simulations, the replacement of FR4 with diamond caused the junction temperature to decrease from 64°C to 45.2°C for a forward current level of 350 mA. The replacement of FR4 with diamond thus effectively improves the thermal management of LEDs and corresponding driver circuits.

HFCVD nanostructured diamond films deposited by a combination of seeding suspensions and novel nucleation process

The ultrasonic seeding of a substrate with diamond suspensions enriches the surface with nanometre-sized seeds that coalesce and form a closed conformal film during early stages of diamond growth. To get insight on seeds early growth and evaluate the seeding efficiency of different suspensions, silicon samples were exposed to diamond growth conditions before seeding; this leaves a thin carbon film on the substrate surface. Following this step samples were seeded with commercial nanodiamond suspensions, exposed again to growth conditions and characterised by SEM. Results showed that seeding suspensions played a role depending on particle size and nature of dispersing medium. Seeding density was larger and more uniform in samples pre-exposed to diamond growth conditions. The carbon film deposited during the pre-treatment improves deagglomeration of nanodiamond seeds via a more effective interaction between substrate surface and seeds. This procedure represents a viable way to grow thin conformal diamond coatings by HFCVD.

Diamond / SiC heterojunctions

Diamond and SiC are wide bandgap (WBG) materials which can be used to fabricate high power devices with improved performance. The combination of these materials into one single device is expected to bring some benefits, like a better thermal management with a corresponding increase in the operating power. Diamond films deposited by Chemical Vapor Deposition (CVD) can be doped with boron, making them p-type semiconductors. Diamond films deposited on foreign substrates are intrinsically polycrystalline, so the quality of the interface, determined by deposition conditions and seeding method, plays a critical role in the heterojunction characteristics, impacting both reverse current and breakdown voltage. This work reports the fabrication and characterization of p-diamond / n-SiC heterojunctions. P-type polycrystalline diamond (PCD) films were deposited directly on the surface on n-type SiC commercial wafers by Hot Filament CVD (HFCVD) using different seeding techniques. I-V characteristics of the obtained heterojunctions were measured at room temperature and the quality and morphology of the diamond films were assessed by scanning electronic microscopy (SEM) and Raman spectroscopy. The influence of the different seeding techniques on the I-V characteristics is discussed.

Deposition of Carbon Nanotube Films on Polyamide and Polypropylene Substrates: A Computer Simulation Approach

In this work we study hydroxylated carbon nanotube (CNT) assembly on polyamide (PA) and polypropylene (PP) polymers activated by UV radiation from a theoretical and experimental perspective. Molecular computer simulation was done to understand the stable conformations and bulk properties (molecular dynamics) of the polymers before and after exposure to UV radiation at the molecular level. Our experiments suggest that PA presents more -OH active groups, producing a more hydrophilic surface, whereas PP exhibits less potential UV activation. These results suggest that it is possible a facile covalent functionalization method to tune organic polymer surface properties through SWCNT anchoring for nanotechnological applications requiring defined surface roughness and chemical functionality on inexpensive polymers.

On the usage of harmonic balance to simulate memristive devices and circuits

This paper proposes the usage harmonic balance method to find the steady state solution on memristive devices and circuits. Harmonic balance is a well-established simulation tool often used to find the periodic steady state solution of circuits involving non-linear elements. It is a particularly efficient method when the focus is on the characterization of the frequency-domain behavior of some quantity in the circuit. This important characteristic arises due to the nature of the mathematical procedures involved, which operate directly on the frequency domain. One such example, is the problem of simulating the frequency behavior of the hysteresis loop area of a memristive device. In this paper, we describe the preliminary analysis involved in harmonic balance simulation and how it applies to the characterization of a memristive device. Simulation results at the end of the paper disclose important considerations on the accuracy of the method and its efficiency when compared to standard time domain methods.