74 research outputs found
The effect of thermal annealing in a hydrogen atmosphere on tungsten deposited on 6H-SiC
Tungsten (W) film was deposited on a bulk single crystalline 6HeSiC substrate and annealed in H2
ambient at temperatures of 700 C, 800 C and 1000 C for 1 h. The resulting solid-state reactions, phase
composition and surface morphology were investigated by Rutherford backscattering spectrometry
(RBS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM) analysis
techniques. These results are compared with the vacuum annealed results reported in our earlier work.
As-deposited RBS results indicated the presence of W and O2 in the deposited thin film, the GIXRD
showed the presence ofW,WO3, W5Si3 andWC. RBS results indicated the interaction betweenWand SiC
was accompanied by the removal of oxygen at 700 C. The GIXRD analysis indicated the presence of
W5Si3 and WC in the samples annealed at 700 C. At temperatures of 800 C and 1000 C, Wannealed in
a H2 ambient further reacted with the SiC substrate and formed a mixed layer containing silicide phases
and carbide phases, i.e.W5Si3, WSi2, WC and W2C. The SEM micrographs of the as-deposited samples
indicated the W thin film had a uniform surface with small grains. Annealing at 800 C led to the
agglomeration of W grains into clusters making the surface rough.National Research Foundation (NRF) (Grant number: 88661), South Africa.http://www.journals.elsevier.com/vacuum2017-07-30hb2016Physic
Iodine assisted retainment of implanted silver in 6H-SiC at high temperatures
The effect of high temperature thermal annealing on the retainment and diffusion behaviour of iodine (I)
and silver (Ag) both individually and co-implanted into 6H-SiC has been investigated using RBS, RBS-C
and heavy ion ERDA (Elastic Recoil Detection Analysis). Iodine and silver ions at 360 keV were both individually
and co-implanted into 6H-SiC at room temperature to fluences of the order of 1 1016 cm 2. RBS
analyses of the as-implanted samples indicated that implantation of Ag and of I and co-implantation of
131I and 109Ag at room temperature resulted in complete amorphization of 6H-SiC from the surface to a
depth of about 290 nm for the co-implanted samples. Annealing at 1500 C for 30 h (also with samples
annealed at 1700 C for 5 h) caused diffusion accompanied by some loss of both species at the surface
with some iodine remaining in the iodine implanted samples. In the Ag implanted samples, the RBS spectra
showed that all the Ag disappeared. SEM images showed different recrystallization behaviour for all
three sets of samples, with larger faceted crystals appearing in the SiC samples containing iodine. Heavy
Ion ERDA analyses showed that both 109Ag and 131I remained in the co-implanted SiC samples after
annealing at 1500 C for 30 h. Therefore, iodine assisted in the retainment of silver in SiC even at high
temperature.National Research Foundation (NRF)http://www.elsevier.com/locate/nimbhb201
Migration behaviour of Europium implanted into single crystalline 6H-SiC
Migration behaviour of Europium (Eu) implanted into 6H-SiC was investigated using Rutherford backscattering spectroscopy (RBS), RBS in a channelling mode (RBS-C) and scanning electron microscopy (SEM). Eu ions of 360 keV were implanted into 6H-SiC at 600 °C to a fluence of 1 × 1016 cm−2. The implanted samples were sequentially annealed at temperatures ranging from 1000 to 1400 °C, in steps of 100 °C for 5 h. RBS-C showed that implantation of Eu into 6H-SiC at 600 °C retained crystallinity with some radiation damage. Annealing of radiation damage retained after implantation already took place after annealing at 1000 °C. This annealing of radiation damage progressed with increasing annealing temperature up to 1400 °C. A shift of Eu towards the surface took place after annealing at 1000 °C. This shift became more pronounced and was accompanied by loss of Eu from the surface at annealing temperatures >1000 °C. This shift was accompanied by broadening of Eu peak/Fickian diffusion after annealing at temperatures >1100 °C. The migration of Eu occurring concurrently with the annealing of radiation damage was explained by trapping and de-trapping of Eu by radiation damage.The National Research Foundation (NRF) (grant no: 94104) of South Africahttp://www.journals.elsevier.com/vacuum2018-07-30hj2017Physic
Optimisation of the synthesis of ZrC coatings in a radio frequency induction-heating chemical vapour deposition system using response surface methodology
A chemical vapour deposition process using radio frequency induction heating operating at atmospheric pressure
was developed for the deposition of ZrC coatings. The precursors utilised in this process were zirconium tetrachloride
and methane as zirconium and carbon sources respectively, in an excess of hydrogen. Additionally, a
stream of argon was used to, first, remove oxygen from the reactor and then to sweep the vapourised ZrCl4 at
300 °C to the reaction chamber. The ZrC coatings were deposited on graphite substrates at substrate temperatures
in the range of 1200 °C–1600 °C. The molar ratio of CH4/ZrCl4 was varied from 6.04 to 24.44. Before the
start of the deposition process, thermodynamic feasibility analysis for the growth of ZrC at atmospheric pressure
was also carried out. Response surface methodology was applied to optimise the process parameters for the deposition
of ZrC coatings. A central composite design was used to investigate the effects of temperature and molar
ratio of CH4/ZrCl4 on the growth rate, atomic ratio of C/Zr and crystallite size of ZrC coatings. Quadratic statistical
models for growth rate and crystallite size were established. The atomic ratio of C/Zr followed a linear trend. It
was found that an increase in substrate temperature and CH4/ZrCl4 ratio resulted in increased growth rate of
ZrC coatings. The carbon content (and concomitantly the atomic ratio of C/Zr) in the deposited coatings increased
with temperature and molar ratio of CH4/ZrCl4. The substrate temperature of 1353.3 °C and the CH4/ZrCl4 molar
ratio of 10.41 were determined as the optimal condition for growing near-stoichiometry ZrC coatings. The values
were 1.03, 6.05 μm/h and 29.8 nm for C/Zr atomic percentage ratio, growth rate and average crystallite size
respectively.University of Pretoria, Busitema University and African Union.http://www.elsevier.com/locate/tsf2018-02-28hb2017Chemical EngineeringPhysic
Interface reactions between Pd thin films and SiC by thermal annealing and SHI irradiation
The solid-state reactions between Pd thin films and 6H-SiC substrates induced by thermal
annealing, room temperature swift heavy ion (SHI) irradiation and high temperature SHI
irradiation have been investigated by in situ and real-time Rutherford backscattering
spectrometry (RBS) and Grazing incidence X-ray diffraction (GIXRD). At room temperature,
no silicides were detected to have formed in the Pd/SiC samples. Two reaction growth zones
were observed in the samples annealed in situ and analysed by real time RBS. The initial
reaction growth region led to formation of Pd3Si or (Pd2Si+Pd4Si) as the initial phase(s) to
form at a temperature of about 450 °C. Thereafter, the reaction zone did not change until a
temperature of 640 °C was attained where Pd2Si was observed to form in the reaction zone.
Kinetic analysis of the initial reaction indicates very fast reaction rates of about 1.55×1015
at.cm-2/s and the Pd silicide formed grew linear with time. SHI irradiation of the Pd/SiC
samples was performed by 167 MeV Xe26+ ions at room temperature at high fluences of
1.07×1014 and 4×1014 ions/cm2 and at 400 °C at lower fluences of 5×1013 ions/cm2. The
Pd/SiC interface was analysed by RBS and no SHI induced diffusion was observed for room
temperature irradiations. The sample irradiated at 400 °C, SHI induced diffusion was
observed to occur accompanied with the formation of Pd4Si, Pd9Si2 and Pd5Si phases which
were identified by GIXRD analysis.http://www.elsevier.com/locate/nimb2017-03-31hb2016Physic
Effect of Xe ion (167 MeV) irradiation on polycrystalline SiC implanted with Kr and Xe at room temperature
The effect of swift heavy ion (Xe 167 MeV) irradiation on polycrystalline SiC individually
implanted with 360 keV Kr and Xe ions at room temperature to fluences of 2×1016 cm-2 and
1×1016 cm-2 respectively, was investigated using transmission electron microscopy (TEM),
Raman spectroscopy and Rutherford backscattering spectrometry (RBS). Implanted
specimens were each irradiated with 167 MeV Xe+26 ions to a fluence of 8.3×1014 cm-2 at
room temperature. It was observed that implantation of 360 keV Kr and Xe ions individually
at room temperature amorphized the SiC from the surface up to a depth of 186 and 219 nm
respectively. Swift heavy ion (SHI) irradiation reduced the amorphous layer by about 27 nm
and 30 nm for the Kr and Xe samples respectively. Interestingly, the reduction in the
amorphous layer was accompanied by the appearance of randomly oriented nanocrystals in
the former amorphous layers after SHI irradiation in both samples. Previously, no similar
nanocrystals were observed after SHI irradiations at electron stopping powers of 33 keV/nm
and 20 keV/nm to fluences below 1014 cm-2. Therefore, our results suggest a fluence threshold for the formation of nanocrystals in the initial amorphous SiC after SHI irradiation.
Raman results also indicated some annealing of radiation damage after swift heavy ion
irradiation and the subsequent formation of small SiC crystals in the amorphous layers. No
diffusion of implanted Kr and Xe was observed after swift heavy ion irradiation.National Research Foundation (NRF)http://iopscience.iop.org0022-37272016-10-20hb201
A call to action: A need for initiatives that increase equitable access to COVID-19 therapeutics
Structural racism is endemic in the United States and causes inequitable health outcomes that have been amplified throughout the COVID-19 pandemic. Non-Hispanic Black, Hispanic/Latino, and Native American individuals have been disproportionately affected, and are twice as likely to be hospitalized or die from COVID-19 or related morbidities when compared to White Americans. Social determinants of health inequities contribute to these disparate outcomes, given that minoritized individuals are more likely to occupy essential worker roles and to live in high-density settings. Despite their higher risk of severe COVID-19 illness, racially and ethnically minoritized individuals are less likely to receive potentially lifesaving COVID-19 therapeutics.3 While several state health departments attempted to implement race-conscious interventions and narrow the disparities, these efforts have been met with fallacious claims of ‘reverse racism’ and the reversal of the proposed implementations
Interfacial reactions and surface analysis of W thin film on 6H-SiC
Tungsten (W) thin film was deposited on bulk single crystalline 6H-SiC substrate and annealed in vacuum
at temperatures ranging from 700 to 1000 C for 1 h. The resulting solid-state reactions, phase composition
and surface morphology were investigated by Rutherford backscattering spectroscopy (RBS), grazing
incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM). XRD was used to identify
the phases present and to confirm the RBS results. The RBS spectra were simulated using the RUMP
software in order to obtain the deposited layer thickness, composition of reaction zone and detect phase
formation at the interface. RBS results showed that interaction between W and SiC started at 850 C. The
XRD analysis showed that WC and CW3 were the initial phases formed at 700 and 800 C. The concentration
of the phases was however, too low to be detected by RBS analysis. At temperatures of 900 and
1000 C, W reacted with the SiC substrate and formed a mixed layer containing a silicide phase (WSi2)
and a carbide phase (W2C). The SEM images of the as-deposited samples showed that the W thin film
had a uniform surface with small grains. The W layer became heterogeneous during annealing at higher
temperatures as the W granules agglomerated into island clusters at temperatures of 800 C and higher.National Research Foundation (NRF), South Africa.http://www.elsevier.com/locate/nimb2017-03-31hb2016Physic
Migration behaviour of selenium implanted into polycrystalline 3C–SiC
Please read abstract in the article.The National Research Foundation and The World Academy of Science.http://www.journals.elsevier.com/vacuum2021-05-01hj2020Physic
Influence of the substrate gas-inlet gap on the growth rate, morphology and microstructure of zirconium carbide films grown by chemical vapour deposition
The influence of the gap between the gas inlet and the substrate in an in-house developed
thermal chemical vapour deposition (CVD) reactor, on the growth rate, surface morphology,
phase composition and microstructure of deposited ZrC films was investigated by X-ray
diffraction (XRD) and scanning electron microscopy (SEM). The ZrC films were grown on
high density graphite substrates at different substrate-inlet gaps, viz. 70 mm, 90 mm, 120 mm,
145 mm and 170 mm, at substrate temperatures of 1200 °C and 1400 °C. The growth rate of
ZrC films prepared at 1400 °C was observed to be higher than at 1200 °C., and was found to decrease with increase in substrate-inlet gap at both temperatures. The boundary layer
thickness increased with an increase in substrate-inlet gap. The diffusion coefficients of the
reactants were found to be 0.176 cm2/s and 0.200 cm2/s for the ZrC films deposited at 1200
°C and 1400 °C respectively. A model illustrating the diffusion of source materials through
the boundary layer to the reacting surface was also given. The XRD results of ZrC films
showed that at both 1200 °C and 1400 °C the (111) plane was the less preferred orientation,
while (200) and (220) were the preferred planes. The degree of preferred orientation of ZrC
films was found to decrease with increasing substrate-inlet gap. SEM results indicated that as
the substrate-inlet gap increased from 70 mm to 170 mm for 1400 °C, the films became more
uniform with increased particle agglomeration. The cauliflower-like clusters of particles grew
larger in size and covered the whole surface. By contrast, at 1200 °C the surface crystallites
had complex facets that decreased in size as the substrate-inlet gap increased from 70 mm to
170 mm.University of Pretoria, Busitema University and African Union.http://www.elsevier.com/locate/ceramint2018-01-31hb2017Chemical EngineeringPhysic
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