Production and characterization of a three-dimensional cellular metal-filled ceramic composite

Silicon carbide in the form of a foam network was vacuum infiltrated with aluminum alloy A356 to produce a new Interpenetrating Composite material. The foam, once infiltrated with a second phase transforms into a composite where two distinct, continuous, three-dimensional network structures are formed. The advantage of this metal matrix composite is its high strength-to-weight ratio for use in lightweight applications such as electronic packaging materials. The electroless nickel coating and vacuum infiltration procedures are developed. Materials characterization of the composite is evaluated by microstructural and compositional analysis, and density, porosity, and nano-indentation measurements. Selected experimental mechanical and thermal property measurements are performed to understand its properties and compare against theoretical models. Results show the final composite to have lower density than conventional electronic base plate packaging materials with low porosity. The composite has an increased Young's modulus and flexural strength to that of the unreinforced alloy and comparable impact toughness to composites with 50–70 vol% SiC particles but with only 12 vol% SiC. The fracture surface of the matrix illustrates conventional fibrous fracture and brittle cleavage whilst the reinforcement struts show signs of layer de-bonding from their SiC layered structure

Dry wear and friction properties of an A356/SiC foam interpenetrating phase composite

The dry sliding wear and friction behaviors of A356 aluminum alloy and a hybrid composite of A356 aluminum alloy and silicon carbide foam in the form of an interpenetrating phase composite were evaluated using a ball-on-disk apparatus at ambient conditions. The stationary 6.35 mm alumina ball produced a wear track (scar) diameter of 7 mm on the rotating specimen surface. Three different loads; 5 N, 10 N and 20 N were applied at a constant sliding speed of 33 mm/s for both materials. Wear tracks were characterized with a scanning electron microscope and measured with an optical surface profilometer. In general, this novel A356/SiC foam composite reduced the friction coefficient and wear rate from that of the base alloy for all loading conditions. In addition, as the load increased, the friction coefficient and wear rate decreased for both materials. The results indicate the composite could be used in light-weight applications where moderate strength and wear properties are needed

Neutral thioether and selenoether macrocyclic coordination to Group 1 cations (Li–Cs) – synthesis, spectroscopic and structural properties

The complexes [M(L)][BArF] (BArF = tetrakis{3,5-bis(trifluoromethyl)-phenyl}borate), L = [18]aneO4S2 (1,4,10,13-tetraoxa-7,16-dithiacyclooctadecane): M = Li–Cs; L = [18]aneO2S4 (1,10-dioxa-4,7,13,16-tetrathiacyclooctadecane): M = Li, Na, K; L = [18]aneO4Se2 (1,4,10,13-tetraoxa-7,16-diselenacyclooctadecane): M = Na, K, as well as [Na(18-crown-6)][BArF], are obtained in good yield as crystalline solids by reaction of M[BArF] with the appropriate macrocycle in dry CH2Cl2. X-ray crystallographic analyses of [Li([18]aneO4S2)][BArF] and [Li([18]aneO2S4)][BArF] show discrete distorted octahedral cations with hexadentate coordination to the macrocycle. The heavier alkali metal complexes all contain hexadentate coordination of the heterocrown, supplemented by M?F interactions via the anions, producing extended structures with higher coordination numbers; Na: CN = 7 or 8; K: CN = 8; Rb: CN = 9; Cs: CN = 8 or 10. Notably, all of the structures exhibit significant M–S/Se coordination. The crystal structures of the potassium and rubidium complexes show two distinct [M(heterocrown)]+ cations, one with M?F interactions to two mutually cis [BArF]? anions, and the other with mutually trans [BArF]? anions, giving 1D chain polymers. Solution multinuclear (1H, 13C, 7Li, 23Na, 133Cs) NMR data show that the macrocyclic coordination is retained in CH2Cl2 solution

The effect of active toe movement (AToM) on calf pump function and deep vein thrombosis in patients with acute foot and ankle trauma treated with cast - a prospective randomized study

Background Patients with foot and ankle trauma treated with cast are advised to perform toe movements to prevent venous thromboembolism (VTE). Our aim was to determine the effect of active toe movement on asymptomatic deep vein thrombosis (DVT) and venous calf pump function. Methods Patients aged 18–60 years with acute foot and ankle trauma requiring below knee non weight bearing cast were randomized to intervention (regular active toe movement) or control groups (n = 100). Patients had bilateral lower limb venous ultrasound to assess for DVT on discharge from clinic. Patients requiring chemical thromboprophylaxis were excluded. Results 78 completed the study. 27% sustained asymptomatic DVT, with no statistically significant difference in calf pump function or DVT incidence between groups. All DVT's occurred in the injured lower limb. Conclusion Active toe movement is not a viable strategy for thromboprophylaxis in patients with acute foot and ankle trauma treated with cast

Formation mechanism of porous reaction-bonded silicon nitride with interconnected pores in the presence of MgO

In porous reaction bonded silicon nitride, whiskers normally grow in globular clusters as the dominant morphology and deteriorate the pore interconnectivity. However, the ceramic microstructure was significantly transformed with the addition of MgO; specifically, the morphology was modified to a combination of matte and hexagonal grains. Microstructural observation along with thermodynamic studies suggest that MgO interfered with the presence and nitridation of SiO(g). Consequently, rather than being involved in the whiskers’ formation, surface silica instead reacted with volatile MgO to form intermediate products. Through these reactions, whisker formation was blocked, and a porous interconnected structure formed which was confirmed by 3D tomography. After heat-treatment at 1700 °C, β-Si3N4 crystallized in a glassy matrix containing magnesium. Resulting samples had an open-pore structure with porosity of 74-84 vol. %, and density of 0.48-0.75 g.cm-3. Combination of high porosity and pore size of <40 μm led to compressive strengths of 1.1 to 1.6 MPa

Trends in the elastic response of binary early transition metal nitrides

Motivated by an increasing demand for coherent data that can be used for selecting materials with properties tailored for specific application requirements, we studied elastic response of nine binary early transition metal nitrides (ScN, TiN, VN, YN, ZrN, NbN, LaN, HfN, and TaN) and AlN. In particular, single crystal elastic constants, Young's modulus in different crystallographic directions, polycrystalline values of shear and Young's moduli, and the elastic anisotropy factor were calculated. Additionally, we provide estimates of the third order elastic constants for the ten binary nitrides.Comment: 10 pages, 7 figure

Enhanced UV/blue fluorescent sensing using metal-dielectric-metal aperture nanoantenna arrays

Subwavelength aperture antenna arrays are designed and fabricated for potential applications in fluorescence sensing in the near UV/blue range. They are designed using finite-difference time-domain (FDTD) simulation, fabricated using focused ion beam etching and characterised using angular Fourier spectroscopy. The aperture arrays are formed in the top layer of an aluminum-silica-aluminum trilayer and produce a maximum simulated field intensity enhancement of 5.8 times at 406 nm and highly directive emission with a beamwidth of 8.3 deg. The normal incidence reflection response has been measured and shows reasonable agreement with modelled results. In addition, to investigate higher field intensity enhancements, bowtie aperture arrays are simulated and the influence of parameters such as dielectric gap, position of dipole source, and aperture shape and size are discussed and show enhancements up to 67 times are possible

Changes in labial capillary density on ascent to and descent from high altitude

Present knowledge of how the microcirculation is altered by prolonged exposure to hypoxia at high altitude is incomplete and modification of existing analytical techniques may improve our knowledge considerably. We set out to use a novel simplified method of measuring in vivo capillary density during an expedition to high altitude using a CytoCam incident dark field imaging video-microscope. The simplified method of data capture involved recording one-second images of the mucosal surface of the inner lip to reveal data about microvasculature density in ten individuals. This was done on ascent to, and descent from, high altitude. Analysis was conducted offline by two independent investigators blinded to the participant identity, testing conditions and the imaging site. Additionally we monitored haemoglobin concentration and haematocrit data to see if we could support or refute mechanisms of altered density relating to vessel recruitment. Repeated sets of paired values were compared using Kruskall Wallis Analysis of Variance tests, whilst comparisons of values between sites was by related samples Wilcoxon Signed Rank Test. Correlation between different variables was performed using Spearman’s rank correlation coefficient, and concordance between analysing investigators using intra-class correlation coefficient. There was a significant increase in capillary density from London on ascent to high altitude; median capillaries per field of view area increased from 22.8 to 25.3 (p=0.021). There was a further increase in vessel density during the six weeks spent at altitude (25.3 to 32.5, p=0.017). Moreover, vessel density remained high on descent to Kathmandu (31.0 capillaries per field of view area), despite a significant decrease in haemoglobin concentration and haematocrit. Using a simplified technique, we have demonstrated an increase in capillary density on early and sustained exposure to hypobaric hypoxia at thigh altitude, and that this remains elevated on descent to normoxia. The technique is simple, reliable and reproducible