Synthesis and thermal properties of inorganic polymers (geopolymers) for structural and refractory applications from volcanic ash

The volcanic ash occurring as an abundant and readily accessible natural resource in the Central African country of Cameroon was used to synthesize aluminosilicate geopolymers using sodium hydroxide as the sole alkaline activator. Both the curing conditions and the Na₂O/SiO₂ molar ratio were found to influence the development of compressive strength of the geopolymer cement paste, which achieved a maximum strength of 55 MPa at Na₂O/SiO₂ = 0.3. The formation of a mortar by the addition of 40 wt% sand to the optimized geopolymer cement composition reduced the compressive strength to 30 MPa, still within the useful range for construction applications. The geopolymers consist largely of X-ray amorphous material with a small content of crystalline phases. Scanning electron microscopy showed a homogenously distributed mixture of lath-shaped and agglomerated morphologies, with a homogeneous distribution of Si, Al and O in the geopolymer matrix. The geopolymers are relatively stable to heat, shrinking only slowly and retaining about 60% of their as synthesized compressive strength on heating to 900 °C. The FTIR spectra of both the as synthesized and heated geopolymers show two broad absorbance bands, between 820-1250 cm⁻¹ and 450-730 cm⁻¹ assigned to the internal vibrations of Si-O-Si, and Si-O-Al respectively. The compressive strengths and the thermal stability of these materials suggest their suitability for building applications and low-grade refractories.     

Interferometric measurement of J for arbitrary geometry and loading

International Journal of Fracture -     

Simultaneous uptake of ammonium and phosphate ions by composites of γ-alumina/potassium aluminosilicate gel

The simultaneous uptake of ammonium and phosphate ions from solution by composites of γ-alumina/potassium aluminosilicate (KAS) gel has been investigated. The composites were prepared by selective leaching of calcined kaolinite (Al₂Si₂O₅(OH)₄) using KOH solution, followed by neutralization of the leachate at pH 5.5 with nitric acid. The composites were reacted with solutions containing various concentrations of (NH₄)₂HPO₄ at pH 5, 7 and 10 at room temperature for 24 h to examine their uptake of ammonium and phosphate ions. Simultaneous uptake of ammonium and phosphate ions was found, the uptake of ammonium ions being greater than for phosphate ions, especially at pH=7. This observation is considered to result mainly from the porous properties of the composites, which should therefore be controlled to enhance the simultaneous uptake of both ions, especially phosphate ion.     

Microbiological oxidation of 2-methylbutanol of differing enantiomeric ratios by Gluconobacter species

 2-Methylbutanoic acid, an aroma compound of high economic interest, was synthesised using Gluconobacter species for bioconversion. The main focus was on enantioselective effects during metabolism. A preference for the (S)-enantiomer was observed when the microorganisms were fed with 2-methylbutanol of known enantiomeric ratios. Received: 15 December 1997     

Stress relaxation in carbon-black-filled rubber vulcanizates at moderate strains

Stress-relaxation measurements have been made on rubber vulcanizates containing carbon black. Evidence has been produced for the existence of a distinct relaxation process which occurs within the first 60 s of relaxation at room temperature. This process becomes less important as the strain is increased and disappears when the temperature is increased or the rubber is swollen. It is suggested that this relaxation is the result of breakdown of structure formed from the carbon-black particles.     

Demonstration of GaN MIS diodes by using AlN and Ga2O3(Gd2O3) as dielectrics

GaN MIS diodes were demonstrated utilizing AlN and Ga₂O₃(Gd₂O₃) as insulators. A 345 Å of AlN was grown on the MOCVD grown n-GaN in a MOMBE system using trimethylamine alane as Al precursor and nitrogen generated from a SVT RF N₂ plasma. For the Ga₂O₃(Gd₂O₃) growth, a multi-MBE chamber was used and a 195 Å oxide was E-beam evaporated from a single crystal source of Ga₅Gd₃O₁₂. The forward breakdown voltage of AlN and Ga₂O₃(Gd₂O₃) diodes are 5 and 6 V, respectively, which are significantly improved over 1.2 V from that of a Schottky contact. From the C–V measurements, both kinds of diodes showed good charge modulation from accumulation to depletion at different frequencies. The insulator/GaN interface roughness and the thickness of the insulator were measured with X-ray reflectivity.