No description available.

No description available.

This research provides a comprehensive investigation in understanding the effect of the addition of graphene nano-platelets (GNP) on the mechanical, tribological and biological properties of glass/ceramic composites. We investigated two kinds of materials namely amorphous matrices like glasses (silica, bioglass) and polycrystalline matrices like ceramics (alumina). The idea was to understand the effect of GNP on these matrices as GNP was expected to behave differently in these composites. Bioglass (BG) was also chosen as a matrix material to prepare BG-GNP composites. GNP can improve the electrical conductivity of BG which can be used further for bone tissue engineering applications. The effect of GNP on both electrical conductivity and bio-activity of BG-GNP composites was investigated in detail. There were three main problems for fabricating these novel nano-composites: 1) Production of good quality graphene; 2) Homogeneous dispersion of graphene in a glass/ceramic matrix and; 3) Retention of the graphitic structure during high temperature processing. The first problem was solved by synthesising GNP using liquid phase exfoliation method instead of using a commercially available GNP. The prepared GNP were ~1 μm in length with a thickness of 3-4 layers confirmed using transmission electron microscopy. In order to solve the second problem various processing techniques were used including powder and colloidal processing routes along with different solvents. Processing parameters were optimised to fabricate glass/ceramic-GNP composite powders. Finally in order to avoid thermal degradation of the GNP during high temperature processing composites were sintered using spark plasma sintering (SPS) technique. Fully dense composites were obtained without damaging GNP during the sintering process also confirmed via Raman spectroscopy. Finally the prepared composites were characterised for mechanical, tribological and biological applications. Interestingly fracture toughness and wear resistance of the silica nano-composites increased with increasing concentration of GNP in the glass matrix. There was an improvement of ~45% in the fracture toughness and ~550% in the wear resistance of silica-GNP composites with the addition of 5 vol% GNP. GNP was found to be aligned in a direction perpendicular to the applied force in SPS. In contrast to amorphous materials fracture toughness and scratch resistance of alumina-GNP composites increased only for small loading of GNP and properties of the composites decreased after a critical concentration. There was an improvement of ~40% in the fracture toughness with the addition of only 0.5 vol% GNP in the alumina matrix while the scratch resistance of the composite increased by ~10% in the micro-ductile region. Electrical conductivity of the BG-GNP composite was increased by ~9 orders of magnitude compared to pure BG. In vitro bioactivity tests performed on BG-GNP composites confirmed that the addition of GNP to BG matrix also improved the bioactivity of the nano-composites confirmed using XRD analysis. Future work should focus on understanding electrical and thermal properties of these novel nano-composites.

For high-resolution X-ray imaging scintillator applications, we have prepared and optically characterized divalent samarium doped fluorochlorozirconate (FCZ:Sm2+) glasses and glass-ceramics. Sm2+ doped FCZ glasses were obtained by adding a reducing agent, NaBH4 into the initial melt to convert some of the Sm3+ to Sm2+. However, the Sm2+ concentration at most was estimated to be only approximately 0.003 %. The as-prepared glass samples were further heat treated to obtain glass-ceramics; the nucleation and growth of BaCl2 nanocrystals were confirmed by powdered X-ray diffraction (XRD) experiments. Depending on the heat treatment conditions (temperature and time), the average nanocrystal size varies from 8 to 170 nm, and the sample contains BaCl2 nanocrystals with the orthorhombic and/or hexagonal structure. The optical absorption spectra for our glass-ceramic samples suggested the substitution of Sm2+ ions into the BaCl2 lattice site. The FCZ:Sm2+ glass-ceramics samples showed strong fluorescence in the red region of spectrum (approximately 8 times that of an as-prepared glass), and the transparency can be very high (transmittance > 80 % for samples with thickness about 0.5 mm) and can be equivalent to that of an as-prepared glass . These two results promise potential as a high-resolution X-ray scintillator due to the emission wavelength range and high transparency. Extensive studies of photoluminescence (PL) spectra at low temperatures (12 -- 200 K) for FCZ:Sm2+ glass-ceramics suggested useful indicators of the crystal structure and average size of embedded BaCl2 nanocrystals. A detailed analysis of the optical spectra has lead to the identification of the origin of the emission peaks and the location of Sm ions at specific crystallographic sites. X-ray induced luminescence (XL) studies have suggested a strong dependence of the fluorescence intensity on the concentration of Sm2+ ions. In addition, for more efficient fluorescence, a sample should be heat treated in a hydrogen containing atmosphere (e.g. H2 + Ar gas), and the heat treatment conditions should be such that the nanocrystals grow in the hexagonal structure.

For high-resolution X-ray imaging scintillator applications, we have prepared and optically characterized divalent samarium doped fluorochlorozirconate (FCZ:Sm2+) glasses and glass-ceramics. Sm2+ doped FCZ glasses were obtained by adding a reducing agent, NaBH4 into the initial melt to convert some of the Sm3+ to Sm2+. However, the Sm2+ concentration at most was estimated to be only approximately 0.003 %. The as-prepared glass samples were further heat treated to obtain glass-ceramics; the nucleation and growth of BaCl2 nanocrystals were confirmed by powdered X-ray diffraction (XRD) experiments. Depending on the heat treatment conditions (temperature and time), the average nanocrystal size varies from 8 to 170 nm, and the sample contains BaCl2 nanocrystals with the orthorhombic and/or hexagonal structure. The optical absorption spectra for our glass-ceramic samples suggested the substitution of Sm2+ ions into the BaCl2 lattice site. The FCZ:Sm2+ glass-ceramics samples showed strong fluorescence in the red region of spectrum (approximately 8 times that of an as-prepared glass), and the transparency can be very high (transmittance > 80 % for samples with thickness about 0.5 mm) and can be equivalent to that of an as-prepared glass . These two results promise potential as a high-resolution X-ray scintillator due to the emission wavelength range and high transparency. Extensive studies of photoluminescence (PL) spectra at low temperatures (12 -- 200 K) for FCZ:Sm2+ glass-ceramics suggested useful indicators of the crystal structure and average size of embedded BaCl2 nanocrystals. A detailed analysis of the optical spectra has lead to the identification of the origin of the emission peaks and the location of Sm ions at specific crystallographic sites. X-ray induced luminescence (XL) studies have suggested a strong dependence of the fluorescence intensity on the concentration of Sm2+ ions. In addition, for more efficient fluorescence, a sample should be heat treated in a hydrogen containing atmosphere (e.g. H2 + Ar gas), and the heat treatment conditions should be such that the nanocrystals grow in the hexagonal structure.

No description available.

Tribological properties of a mica glass-ceramic designed for use in dental applications were assessed experimentally in accord with the ASTM pin on disk technique. The glass ceramic was produced through controlled crystallization of the glass in the system SiO2, Al2O3, CaO, MgO, K2O, and F. Crystallization was accomplished by subjecting the parent glass to a regulated heat treatment that resulted in the nucleation and growth of crystal phases formed in the glass. The tests were conducted by sliding a zirconia ball against the glass and glass ceramic disk. The friction coefficient and wear rate were determined as functions of the applied load, sliding speed and sliding time in ambient laboratory conditions and in acidic and basic solutions. The friction coefficient was monitored during the tests. The wear volumes determined from surface profile traces obtained on the wear tracks after completion of the tests were used for calculation of the wear rates. The wear rates increased with increasing applied load and sliding speed but decreased with increasing sliding distance. Results were correlated with the tribological properties of the parent glass, and tribological properties of selected glasses, glass-ceramics and ceramics reported in the literature.

Bioactive glasses are a type of ceramic material designed to be used as bioresorbable therapeutic bone implants. Thermal treatment of bioactive glass ceramics dictates many important features such as microstructure, degree of crystallinity, mechanical properties, and mineralization. This study investigates the effects of temperature, time, and heating rates on the crystallization kinetics of melt cast bioactive glass 45S5. Bulk crystallization (three dimensional crystallite formation) was found to always occur in bulk bioactive glass 45S5 irrespective of the processing conditions. A comparative study of crack paths in amorphous and crystalline phases of bioactive glass 45S5 revealed crack deflections and higher fracture resistance in partially crystallized bioactive glass. Such toughening is likely attributed to different crystallographic orientations of crystals or residual thermal mismatch strains. Furthermore, in vitro immersion testing of partially crystalline glass ceramic revealed higher adhesion capabilities of the mineralized layer formed on amorphous regions as compared to its crystalline counterpart. / Materials Engineering

Une nouvelle famille de vitrocéramiques, avec une microstructure inédite, a été fabriquée par une cristallisation contrôlée des verres dans le système GeSe2-Sb2Se3-CuI. L'influence de la composition et du processus de cristallisation des verres de base, sur la microstructure et sur l’intensité du photo-courant des vitrocéramiques a été étudiée. Une composition optimisée, le 40GeSe2-40Sb2Se3-20CuI, a été particulièrement étudiée avec des résultats suivants: (1) Après une étude systématique , il a été constaté que cette composition donne la plus forte intensité de photo-courant parmi tous les verres étudiés dans ce système pseudo-ternaire GeSe2-Sb2Se3-Cul. Il a été également démontré que le photo-courant généré par différentes vitrocéramiques est non seulement déterminé par la composition, mais aussi par la microstructure composite de la vitrocéramique, qui est déterminée par le processus de céramisation. Ce processus de céramisation a ensuite été optimisé. Par rapport au procédé de traitement thermique en deux étapes, le procédé en une seule étape à basse température est une stratégie plus appropriée pour obtenir une microstructure efficace, favorisant la séparation des charges, construisant des canaux conducteurs et donnant une intensité de photo-courant élevée dans la vitrocéramique. (2) La microstructure composite inédite, discutée ci-dessus est composée de micro-domaines conducteurs interconnectés, formées par des cristaux Sb2Se3 faiblement conducteur en forme de tiges, couverts par des nano-cristaux de Cu2GeSe3 beaucoup plus conducteurs. Le procédé le plus probable de la photo-génération efficace des charges est le suivant: les photons sont efficacement et essentiellement absorbés par Sb2Se3 ainsi que par Cu2GeSe3. Les hétérojonctions formées par les Sb2Se3 du type n et les Cu2GeSe3 du type p, favorisent la séparation de charges, tandis que les Cu2GeSe3 interconnectées et conductrices fournissent des canaux conducteurs et jouent ainsi le rôle de collecteur efficace de charges. Il en résulte ainsi une très longue durée de vie des porteurs de charge et un fort photo-courant. (3) La formation de nano-hétérojonctions entre les cristaux Sb2Se3 et Cu2GeSe3 dans un seul micro-domaine peut conduire à une séparation efficace des électrons et des trous photo-générés. Par conséquent, pour application photo-catalytique, il n’est pas nécessaire de former des canaux conducteurs (conducteurs interconnectés des micro-domaines) dans l'ensemble de la vitrocéramique. De plus, la formation de ces canaux conducteurs, nécessiterait une augmentation de la durée ou/et la température de recuit, pouvant conduire à une diminution de l'activité photo-catalytique à cause de la taille relativement grande des grains cristallins. Les vitrocéramiques optimisées montrent une bonne capacité de désamination oxydative et une forte activité photo-catalytique en général, démontrant ainsi son potentiel en tant que photo-catalyseur efficace. / A totally new family of glass ceramics with a unique microstructure was fabricated by controlling the crystallization of the GeSe2-Sb2Se3-CuI glass system. The influences of the material composition and the crystallizing process of the precursor glasses on the microstructure and photocurrent of the prepared glass ceramics were investigated. An optimized composition, 40GeSe2-40Sb2Se3-20CuI, was particularly studied with the following significant results: (1) After a systematic study, it was found that this particular composition shows the highest photocurrent density among all studied glasses in the pseudo-ternary GeSe2-Sb2Se3-CuI system. It is also demonstrated that the photocurrent generated by different glass ceramics is not only determined by the composition, but also by the composite microstructure of the glass ceramic, which is determined by the ceramisation process. This process was then carefully studied. Compared with the two-step heat treatment process, the single-step process at a low temperature is a more efficient strategy to build up an efficient composite microstructure, which promotes charge carrier separation and provides a conductive channel, leading to a high photocurrent intensity in the glass ceramic. (2) The above-mentioned unique composite microstructure is composed of interconnected conductive microdomains, formed by low conductive rod-like Sb2Se3 crystals, covered by relatively high conductive Cu2GeSe3 nanocrystals. The most likely process for efficient photogeneration of charges is proposed as follows: photons are efficiently and essentially absorbed by Sb2Se3 as well as by Cu2GeSe3, and then the heterojunction formed by n-type Sb2Se3 and p-type Cu2GeSe3 promotes the charge separation, whereas the oriented and relatively conductive Cu2GeSe3 aggregate provides a conductive channel and plays the role of efficient charge collector. This structure results in exceptionally long lifetime of charge carriers (around 16 µs) and high photocurrent (at least 100 times higher than any of Sb2Se3 and Cu2GeSe3 individually). (3) The formation of nano-heterojunctions between Sb2Se3 and Cu2GeSe3 crystals within a single conductive microdomain can fully lead to an efficient separation of photo-generated electrons and holes. Therefore, for the photocatalytic application, it is unnecessary to form conductive channels (interconnected conductive microdomains) in the whole glass ceramic. Moreover, in order to form conductive channels, the necessary increase of annealing time or/and temperature may decrease the photocatalytic activity due to its relatively large crystal grain size. The optimized glass ceramic exhibits a good oxidative deamination ability and high photocatalytic activity, demonstrating its potential as an efficient photocatalyst.

Nos últimos anos tem sido grande o interesse na tecnologia de fabricação de cerâmicas cossinterizadas a baixas temperaturas (LTCC – Low Temperature Cofired Ceramics), usualmente inferiores a 1000 °C, para fabricação de dispositivos microeletrônicos multicamadas a partir de substratos cerâmicos e vitrocerâmicos. Além do considerável progresso nas comunicações wireless, a indústria eletrônica tem demandado miniaturização, multifuncionalidade e compactação de circuitos e componentes de última geração. Com esse propósito, busca-se desenvolver substratos, sinterizados a baixa temperatura, com baixa perda dielétrica e retração controlada. O diopsídio (CaO.MgO.2SiO2) é considerado de grande potencial para uso como substrato na referida tecnologia para aplicações em altas frequências, pois apresenta baixa constante dielétrica e capacidade de ser sinterizado a temperaturas relativamente baixas. Assim, o objetivo do presente trabalho foi a obtenção de substratos vitrocerâmicos conformados por tape casting a partir de pó de vidro com a composição do diopsídio, passando por tratamentos térmicos de sinterização e cristalização, avaliando o comportamento da retração durante a sinterização para diferentes distribuições de tamanho de partícula. Para isso, fizeram-se moagens do vidro em álcool isopropílico durante 6, 12 e 24 h, em seguida suspensões cerâmicas de pó vítreo foram preparadas e caraterizadas reologicamente buscando-se a máxima concentração de sólidos para a colagem das fitas cerâmicas. Para esse trabalho foi construída uma máquina de tape casting para conformação de fitas finas a partir de suspensões de vidro diopsídio. As fitas coladas foram sinterizadas em tratamentos não isotérmicos com uma taxa de aquecimento de 5°C/min e a retração linear das fitas foi caracterizada simultaneamente. Curvas de retração e densificação foram calculadas usando o modelo de Clusters de sinterização de vidros por escoamento viscoso com cristalização concorrente, e comparadas com os valores experimentais. Finalmente, foram avaliadas as características físicas, térmicas, microestruturais e dielétricas das fitas sinterizadas. A melhor distribuição de tamanho de partícula foi a representada pelo D50 = 5,7 μm, que apresentou as melhores condições reológicas e de dispersão em barbotinas, sendo possível preparar suspensões com concentração de até 49 %vol de sólidos e fitas de espessura acimas de 1 mm. A retração linear ao término da sinterização variou de acordo com a densidade de empacotamento, que por sua vez depende da concentração inicial de sólidos. A maior concentração e densidade de empacotamento foram obtidas para a distribuição de maior tamanho de partícula (D50 = 13,4 μm) e apresentou uma retração linear de 14%. O substratos de diopsídio sinterizado e totalmente cristalizado de 1 mm de espessura e 14% de porosidade apresentou constante dielétrica de 5,3 e perdas dielétricas menores que 0,008 para uma frequência de 30 GHz. Os resultados demostram que o material obtido a partir do processo de manufatura desenvolvido tem grande potencial de inovação. / In recent years there has been a great deal of interest in Low Temperature Cofired Ceramics (LTCC) manufacturing technology, usually below 1000°C, for the manufacture of multilayer microelectronic devices from ceramic and glass-ceramic substrates. In addition to the considerable progress in wireless communications, the electronics industry needs miniaturization, multifunctionality, and integration of the latest components and circuits. Therefore, it is sought to develop dense, sintered at low-temperature substrates, with low loss dielectric and controlled shrinkage. The diopside (CaO.MgO2.SiO2) has a great potential for use as a substrate for LTCC technology for high-frequency applications since it presents low dielectric constant and the capacity of sintered at low temperatures. The aim of this work was to obtain glass-ceramic substrates prepared by tape casting from diopside glass powder through sintering and crystallization thermal treatments, assessing the behavior of shrinkage during sintering for different particles sizes. For this purpose, the glass was milled in isopropyl alcohol for 6, 12 and 24 h, then slurries from glass powder were prepared and rheological characterized, aiming at the using the highest possible solids concentration to tape casting. It was manufactured a tape casting machine to thin tape making from diopside glass powder slurries. The tapes were sintered in non-isothermal treatments with a heating rate of 5°C/min and the linear shrinkage of the tapes was characterized in situ. Shrinkage and densification curves were calculated using the Clusters model of glass sintering with concurrent crystallization compared with the experimental values. Finally, the physical, thermal, microstructural and dielectric characteristics of the sintered tapes were evaluated. The best particle size distribution was represented by the D50 = 5.66 μm, which presented the optimal rheological and dispersion conditions in slurries, is it possible to prepare suspensions with a concentration of up to 49% volume solids and tape of the thickness of 1 mm. The linear shrinkage after sintering varied according to the green density, which in turn depends on the initial solids concentration. The highest concentration and green density were obtained for the largest particle size distribution (D50 = 13.40 μm) and showed a linear shrinkage of 14%. The sintered and fully crystallized diopside substrates of 1 mm thickness and 14% porosity showed a dielectric constant of 5.3 and dielectric losses of less than 0.008 at a frequency of 30 GHz. The results show that the material obtained has great potential for innovation.