Corrosion of hot pressed silicon nitride-based materials by molten copper

The behaviour of several types of silicon nitride-based materials in contact with molten copper has been evaluated. Tests were performed at a temperature of 1400 K for a holding time of 260 h in fluido-dynamic conditions.The extent of corrosion has been evaluated in terms of linear loss, weight loss, morphology and chemical composition of the surfaces remaining in contact with the molten phase, by means of SEM, EPMA and X-ray diffraction. Corrosion resistance has been found to be a function of the chemical composition, type and amount of the secondary phases. Materials produced with MgO as a sintering aid showed poor resistance whereas those with Al₂O₃ exhibited the best performance. In any case the extent of corrosion was very slight and the results revealed that all the tested ceramics may be considered suitable for use in contact with molten copper.     

The influence of microstructure on the thermal conductivity of aluminium nitride

Starting from three different commercial powders, AIN materials were densified by pressureless sintering under various temperature and time values in order to investigate the influence of microstructure on thermal conductivity. The influence of the sintering aids (3 wt% Y₂O₃ and 2 wt% CaC₂) and of the forming processes (cold isostatic pressing and thermocompression of tape cast pieces) were also been evaluated. Thermal conductivity increased with the purity level of the starting powder and with an increasing the sintering temperature and soaking time. The highest thermal conductivity values (196 Wm^–1 K^–1) were obtained with the purest powder and high temperature (1800 °C) sintering over long periods (6 h). No influence on thermal conductivity was detected from the forming technique.     

On the possibility of silicon nitride as a ceramic for structural orthopaedic implants. Part I: processing, microstructure, mechanical properties, cytotoxicity

Notwithstanding the good combination of mechanical and tribological properties, the suitability of silicon nitride for application as prosthesis in bone reconstruction or in articular joints replacements is still controversial. This study aims to design and produce three different silicon nitride-based ceramics and to test the materials. In this Part I the microstructure and mechanical properties evidence outstanding characteristics and the cytotoxicity studies confirm that all the materials are extremely inert and biocompatible. In Part II, the wear performance and the wettability and chemical stability against different aqueous media and physiological solutions are investigated and discussed.     

Oxidation resistance of SIC-AlN ceramics coated by oxidation-assisted-pack cementation process

Liquid-phase-pressureless-sintered SiC-AlN-Y₂O₃ composites were coated by means of modified pack cementation process (OXPAC, OXidation-assisted-PAck Cementation) using rare-earth oxides, RE₂O₃ (RE = Sc, Er, Sm, Lu, Ho), as reactive powders. The coatings, composed by the oxidation products of SiC, AlN and rare-earth silicates, were adherent to the substrate, without porosity and with a thickness of 10 μm. The oxidation resistance of the coated SiC-AlN-Y₂O₃ ceramics was also investigated at 1500 °C for a period of 200 h. The coated samples showed specific weight gain lower than the uncoated and pre-oxidised samples. Furthermore, the specific weight gain linearly increased with the rare-earth cationic radius.     

Dapagliflozin Does Not Modulate Atherosclerosis in Mice with Insulin Resistance

Type 2 diabetes mellitus (T2DM) increases morbimortality in humans via enhanced susceptibility to cardiovascular disease (CVD). Sodium-glucose co-transporter 2 inhibitors (SGLT2i) are drugs designed for T2DM treatment to diminish hyperglycaemia by reducing up to 90% of renal tube glucose reabsorption. Clinical studies also suggest a beneficial action of SGLT2i in heart failure and CVD independent of its hypoglycaemiant effect. In the present study, we explored the effect of SGLT2i dapagliflozin (DAPA) in the metabolism and atherosclerosis in Apoe−/−Irs2+/− mice, which display accelerated atherosclerosis induced by insulin resistance. DAPA treatment of Apoe−/−Irs2+/− mice, which were fed a high-fat, high-cholesterol diet, failed to modify body weight, plasma glucose or lipid. Carbohydrate metabolism characterisation showed no effect of DAPA in the glucose tolerance test (GTT) despite augmented insulin levels during the test. In fact, decreased C-peptide levels in DAPA-treated mice during the GTT suggested impaired insulin release. Consistent with this, DAPA treatment of Apoe−/−Irs2+/− isolated islets displayed lower glucose-stimulated insulin secretion compared with vehicle-treated islets. Moreover, insulin-signalling experiments showed decreased pAKT activation in DAPA-treated adipose tissue indicating impaired insulin signalling in this tissue. No changes were seen in lesion size, vulnerability or content of macrophages, vascular smooth muscle cells, T cells or collagen. DAPA did not affect circulating inflammatory cells or cytokine levels. Hence, this study indicates that DAPA does not protect against atherosclerosis in insulin-resistant mice in hypercholesterolemic conditions.     

An alternative encapsulation approach for production of active chitosan–propolis beads

Encapsulation is a promising technology to carry natural active substances, preventing their loss and maintaining their stability until use. Beads of chitosan‐containing propolis have been prepared using a mono‐pore filter device, which permits the encapsulation of natural polyphenols avoiding heat treatments, high shear rates and the use of toxic solvents. Beads proved to be active against Bacillis cereus, Escherichia coli, Listeria innocua, Pseudomonas fluorescens, Yarrovia lipolytica and three moulds strains; the highest effect was found against Staphylococcus aureus (MIC 0.8 mg beads mL^?1). Results in liquid cultures of S. aureus evidenced that beads were able to release the flavonoids from propolis: the diffusion of the active compounds is a key factor in the exploitation of the microbial activity. The obtained chitosan–propolis beads represent an example of natural antimicrobial delivery system that could be used to prevent the growth of pathogenic/spoilage bacteria in food applications.     

Microstructure and properties of alumina-SiC nanocomposites prepared from ultrafine powders

Alumina-Silicon Carbide nanocomposites were produced and studied under different aspects: characteristics of the starting materials, processing, microstructure and mechanical properties. The raw materials were two kinds of fine SiC powders (30 and 45 nm) and two Al₂O₃ powders (60 and 140 nm). Different compositions (amounts of SiC in the range 0.5–5 vol%) were performed and the characteristics of the resulting materials compared. The oxygen enrichment in SiC nanopowder due to specific powder treatments was controlled, in order to optimize powder processing routes. Densification tests of Al₂O₃-SiC powder mixtures were performed both by pressureless sintering and hot pressing route. The addition of SiC reduced the densification rate and favoured a refinement of the matrix. Improvement of mechanical properties over monolithic alumina was obtained in composites with the 45 nm SiC. The study pointed out that the critical factor for the success of these materials is the choice of the raw SiC powders in terms of grain size and state of agglomeration. The addition of this ultrafine SiC strongly affected the microstructural evolution, even at low volumetric fractions. The results do not substantiate any remarkable effect by dispersoids in the tested nanosize range.