E. Breval

Formation of Lanxide TM ceramic composite materials

An overview is given of a new process that has been used successfully to make numerous ceramic/metal composite materials by directed oxidation of molten metallic precursors. As an example, the formation of A1 2 O 3 /A1 composites from Al is discussed in detail.

Sodium zirconium phosphate (NZP) as a host structure for nuclear waste immobilization: A review

Sodium zirconium phosphate [NZP] structural family, of which NaZr[sub 2]P[sub 3]O[sub 12] is the parent composition, has been reviewed as a host ceramic waste form for nuclear waste immobilization. NZP compounds are characterized for their ionic conductivity, low thermal expansion and structural flexibility to accommodate a large number of multivalent ions. This latter property of the [NZP] structure allows the incorporation of almost all 42 nuclides present in a typical commercial nuclear waste. The leach studies of simulated waste forms based on NZP have shown reasonable resistance for the release of its constituents. The calculation of dissolution rates of NZP structure has demonstrated that it would take 20,000 times longer to dissolved NZP than quartz.

Reactions of some calcium silicates with metal cations

Abstract The reactions of Pb 2+ , Cd 2+ , Mn 2+ , Zn 2+ , Cu 2+ , Mg 2+ , Co 2+ , or Ni 2+ with calcium silicates such as tobermorite, xonotlite and wollastonite were investigated. Among these cations, Pb 2+ , Cd 2+ , Mn 2+ , Zn 2+ and Cu 2+ appear to replace surface Ca 2+ but it is difficult to delineate the extent of this reaction from that of precipitation as hydroxides, hydroxy carbonates or carbonates and double decomposition reaction with calcite which is present as an impurity. Two unsubstituted tobermorites exhibited an Mg 2+ exchange of only 0.4 meq/g which is due to exchange with surface Ca 2+ . Almost all of the Ca 2+ in tobermorites and xonotlite was irreversibly replaced by Co 2+ or Ni 2+ which led to their amorphization. The mechanism of Co 2+ and Ni 2+ replacement of Ca 2+ is by the breakdown of structural CaO bonds and appears to take place from edge to core in these minerals. The replacement of Ca 2+ by Co 2+ and Ni 2+ in these unsubstituted calcium silicate minerals is irreversible and lead to their amorphization and hence their reactions are not strictly analogous to cation exchange in clays and zeolites.

Nanostructured, nitrogen-doped carbon materials for hydrogen storage

An astonishing feature of carbon materials is the variety of morphological forms created by growth processes. We report here on a growth process originating from CVD diamond synthesis. The addition of nitrogen gas to the microwave plasma ignited in a CH4/H2 mixture has a profound influence on the crystal structure and morphology of diamond and graphite. Graphite appears in several forms: a single-crystal net-like planar form, nanoparticulates, and new nanotubes with perpendicularly stacked graphite rings. Nitrogen incorporation into these forms is approximately 1 at.%. Nanotubes are also formed in the process of etching graphite by nitrogen plasma. Nanotubes and particulates are built from structural units that are graphite nanocrystals with dimensions as small as 2.5×2.5 nm2. The self-assembling tendency of these units results in different forms: rings, nanotubes or flat particles with a diameter of 70, 60 and 25 nm, respectively. Electron diffraction of material which contain 10 at.% N indicates a lattice expansion of graphitic C–N nanocrystals compared to pure graphite. Creation of active sites for hydrogen-molecule adsorption by incorporation of nitrogen atoms into the graphitic network is the subject of speculation. Hydrogen intake for materials grown from the C–H–N gas system is 0.7–0.8 wt.%. Measurements have been performed using a thermogravimetric method at room temperature and under hydrogen pressure of 7 MPa. TGA-150 Cahn microbalance measurements have been corrected for the buoyancy effect.

Diamond formation in air by the Fedoseev-Derjaguin laser process

Abstract Diamond and other high-pressure phases of carbon were synthesized in air by exposing fine particles of carbon black to carbon dioxide and Nd-YAG laser radiation. The high-pressure phases were separated from the carbon black by selective oxidation and were characterized by electron and x-ray diffraction. Formation of cubic diamond, chaoite and graphite was confirmed.

Sol-gel prepared Ni-alumina composite materials

The microstructure of sol-gel prepared Ni-alumina ceramic-metal composites containing up to 50% Ni has been studied with X-ray diffraction, the scanning electron microscope and the transmission electron microscope. The influence of processing temperature upon the size distribution of Ni was established. It was found that increasing the total amount of Ni increases only the number of micrometre-size Ni inclusions in the alumina, whereas the hot-pressing temperature determines the size distribution of Ni. When temperatures much higher than the melting temperature of Ni are used, a large number of Ni inclusions of the order of 10 nm can be found mainly within alumina grains; only a few are formed in grain boundaries and in triple points. When a temperature close to the melting point is used, there are fewer nanometre-size Ni inclusions and a larger number of Ni inclusions of the order of ∼ 100 nm to 1 μm. In this case, the large (∼ 100 nm) and small (∼ 10 nm) Ni inclusions are found in grain boundaries and triple points.

Cation-exchange properties of (Al + Na)-substituted synthetic tobermorites

Tobermorite, Ca5Si6O16(OH)2·4H2O, is a hydrous calcium silicate that has a layer-type of structure similar to that of the 2:1 clay minerals. In its natural form, tobermorite exhibits little or no exchange for alkali cations; synthetic tobermorites, however, exhibit reversible alkali cation exchange and selective cesium uptake upon a coupled substitution of (Al3+ + Na+) for Si4+. Substituted tobermorites were synthesized using aluminosilicate gels, NaOH, and CaO, in Parr bombs at 175°C for 4 days. Unsubstituted tobermorite was synthesized using quartz and CaO in a Parr bomb at 175°C for 20 hr. Two (Al + Na)-substituted tobermorites showed cation-exchange capacities (CEC) of 77 and 71 meq/100 g, whereas an unsubstituted tobermorite showed a CEC of 12 meq/100 g. The substituted tobermorites exhibited selective Cs exchange from either NaCl or CaCl2 solutions. For example, one substituted tobermorite showed a Cs-exchange coefficient (Kd) of 15,100, whereas unsubstituted tobermorite showed a Kd of only 90 from a 0.02 N CaCl2 solution containing 0.0002 moles/liter CsCl. Exchange isotherms for Na+ ⇋ Cs+ showed that Cs+ is preferred over Na+ throughout the exchange in the (Al + Na)-substituted tobermorites. This group of cation exchangers is expected to find applications in radioactive waste disposal.

Nanocomposite aerogels : the SiO2-Al2O3 system

Monolithic nanocomposite aerogels of two aluminosilicate compositions have been prepared and characterized by different techniques. The results show that high surface area and mesoporosity can be preserved in the above nanocomposite aerogels by heating at 1000 °C, unlike the single component aerogels. The presence of alumina as a second phase prevented their densification which resulted in surface areas on the order of 500–600 m 2 /g and mesopores of about 5–6 nm in diameter after heat treatment at 1000 °C. These novel nanocomposite aerogels are expected to find applications at high-temperatures in separations, insulation, catalysis, etc.

Colloidal sol/gel processing of ultra-low expansion TiO2/SiO2 glasses

Abstract A series of titania-silica glasses with 0–9% TiO2 were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of ∼50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens. In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering. The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10−7 and −0.2 × 10−7°C−1 in the range 0 to 9% TiO2. The glass with 7.2% TiO2 exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm.

SrTiO3 glass ceramics

This is the first in a series of two papers describing the crystallization and dielectric properties of glass-ceramics derived from a particular strontium titanium aluminosilicate glass composition. This first paper concerns the development of crystalline phases and microstructure of glass-ceramics prepared under various crystallization conditions. In the following paper, the dielectric properties of these glass-ceramics are described and correlated with the characterization results.Perovskite strontium titanate (SrTiO3) was the primary crystalline phase in glass-ceramics crystallized over the temperature range of 800 to 1100° C. At crystallization temperatures below 950° C, SrTiO3 formed with a spherulitic or dendritic growth habit. X-ray diffraction suggested that the SrTiO3 crystallized in a perovskite-like “precursor” phase which transformed to perovskite SrTiO3 with further crystallization time. However, electron diffraction indicated that this “precursor” phase was cubic perovskite SrTiO3. At higher crystallization temperatures, perovskite SrTiO3 was present as individual crystallites without evidence of the spherulitic habit. The crystallization of SrTiO3 was followed by that of other phases, the hexacelsian and anorthite forms of SrAl3Si2O8, and the rutile and anatase forms of TiO2. The crystallization sequence and microstructure of the glass-ceramics were determined by the competition for strontium and titanium between the crystallizing phases, SrTiO3 and SrAl2Si2O8, and TiO2.