Brian H. O'Connor

Chemical optimisation of the compressive strength of aluminosilicate geopolymers synthesised by sodium silicate activation of metakaolinite

High strength cements can be synthesised by alkali activation of materials rich in Al2O3 and SiO2. In this study, amorphous aluminosilicate polymers produced by sodium silicate activation of metakaolinite were studied, with particular reference to chemical optimisation of the compressive strength according to the relative concentrations of Si, Al and Na in the polymer. The sodium silicate was manufactured from silica fume and sodium hydroxide. The compressive strengths of polymers with Si∶Al molar ratios of 1.0–3.0 and Na∶Al molar ratios of 0.5–2.0 were considered. The polymers were cured at 75 °C for 24 h and their compressive strengths measured after aging for 7 days. The strength was found to depend systematically on the relative amounts of Si, Al and Na, with the maximum being 64 ± 3 MPa for an Si∶Al∶Na molar ratio of 2.5∶1∶1.3. X-Ray diffraction/scattering data indicate qualitatively that the bonding network in the amorphous aluminosilicate alters systematically with composition.

Application of the Rietveld Refinement Procedure in Assaying Powdered Mixtures

Results are given of an assessment of a Rietveld-type X-ray powder diffraction pattern fitting structure refinement technique for assaying powdered mixtures as an alternative to conventional discrete peak empirical methods of the type described by Klug and Alexander (1974) and Chung (1974). The values obtained for a mixture of corundum and α -quartz, following calibration of the instrument with a profile of the former, indicate that this technique has excellent potential as an analytical tool.

Comparative evaluation of the March and generalized spherical harmonic preferred orientation models using X-ray diffraction data for molybdite and calcite powders

Preferred crystallographic orientation, i.e. texture in crystalline materials powder diffraction data, can cause serious systematic errors in phase composition analysis and also in crystal structure determination. The March model [Dollase (1986). J. Appl. Cryst. 19, 267–272] has been used widely in Rietveld refinement for correcting powder diffraction intensities with respect to the effects of preferred orientation. In the present study, a comparative evaluation of the March model and the generalized spherical harmonic [Von Dreele (1997). J. Appl. Cryst. 30, 517–525] description for preferred orientation was performed with X-ray powder diffraction data for molybdite (MoO3) and calcite (CaCO3) powders uniaxially pressed at five different pressures. Additional molybdite and calcite powders, to which 50% by weight silica gel had been added, were prepared to extend the range of preferred orientations considered. The patterns were analyzed initially assuming random orientation of the crystallites and subsequently the March model was used to correct the preferred orientation. The refinement results were compared with parallel refinements conducted with the generalized spherical harmonic [Sitepu (2002). J. Appl. Cryst. 35, 274–277]. The results obtained show that the generalized spherical harmonic description generally provided superior figures-of-merit compared with the March model results.

Infiltration-processed, functionally graded aluminium titanate/zirconia–alumina compositePart I Microstructural characterization and physical properties

A novel route for processing aluminium titanate (AT)/(alumina–zirconia (AZ)) with graded microstructure and properties is described. This process offers a simple means of tailoring the composition and microstructure of ceramic materials. The processing involves infiltrating porous AZ preforms with a solution of TiCl4, followed by sintering at 1550°C for 3 h. The resultant material has a homogeneous core encased with a graded and heterogeneous layer of AT/AZ. Analyses by X-ray diffraction and energy-dispersive spectrometry have revealed the existence of concentration gradients, the AT content decreasing with increasing sample depth. The presence of both AT and zirconia inhibits the growth of alumina grains through a pinning mechanism. The existence of microcracking in AT and zirconia grains has been revealed by scanning electron microscopy. The graded material displays gradual changes in thermal expansion values due to the presence of AT which gradually reduces in amount from the surface to the core. The inclusion of zirconia has a favourable effect on the thermal stability of AT against phase decomposition.

Characterization of the pore structure of metakaolin-derived geopolymers by neutron scattering and electron microscopy

The pore–solid structure of selected high-compressive-strength metakaolin geopolymers has been characterized to facilitate quantitative prediction of their physical properties. Geopolymers are multiphase materials with pore widths ranging from subnanometre to several tenths of a millimetre. Ultramicrotoming of resin-embedded grains was found to be an effective method for producing electron-transparent sections. Scanning and transmission electron microscopy showed the existence of a bi-level pore system and heterogeneity of the pore morphology. Ultra-small-angle neutron scattering, of sufficiently thin specimens, was found to be useful in detecting the length scales on which statistically significant structural changes occur as the geopolymer chemical composition is varied. Contrast variation experiments confirmed that the small-angle neutron scattering from an Si:Al:Na = 2.5:1:1.2 geopolymer before and after dehydration was dominated by scattering from pores. These experiments suggested the presence of closed (under current experimental conditions) pores in the dehydrated geopolymer. A three-phase analysis was developed for this system, and the scattering of the solid, open pore and closed pore phases was determined as a function of scattering length density ρ. The scattering from all three phases had the same q dependence over the range of likely ρ within the uncertainties. A lower limit of 4.21 (6) × 1010 cm−2 was determined for the scattering length density ρw of the nondehydrated geopolymer by assuming the pore fluid to be water. This scattering length density is significantly higher than the expected value of approximately 3.4 × 1010 cm−2. Small-angle neutron scattering from the dehydrated and nondehydrated Si:Al:Na = 2.5:1:1.2 geopolymer showed that dehydration does not cause a severe change in morphology of the nanoporosity on the length scale probed.

A comparative study of single-line and Rietveld strain-size evaluation procedures using MgO ceramics

Strain–size evaluations from diffraction line broadening for MgO ceramic materials have been compared using single-line integral-breadth and Rietveld procedures with the Voigt function. Diffraction data were measured by Bragg–Brentano X-ray diffractometry (XRD), without incident beam monochromatization, and neutron diffractometry (ND) to encompass near-surface and bulk effects, respectively. The specimens consisted of sets of MgO ceramics and MgO–Y2O3 ceramic composites sintered over a range of temperatures. An MgO ceramic sintered at 1723 K for 2 h exhibited slightly less XRD broadening than the standard LaB6 NIST 660 SRM, and was therefore selected to make instrument profile corrections for both XRD and ND data. It was found for both data types that: (a) sintering initially relieves residual strain present in the MgO powder used to sinter the ceramics and also promotes grain growth; (b) residual strain of the MgO ceramic minimizes as the sintering temperature increases, and then increases with further rise in the sintering temperature, presumably as a result of intragranular interactions associated with grain growth; and (c) introduction of the second phase (Y2O3) increases strain and inhibits crystal growth. The single-line and Rietveld methods gave similar strain values from both the XRD and ND data within the limits of experimental error, but there were substantial differences between the single-line and Rietveld size estimates determined with the XRD and ND data.

Use of Rietveld Pattern Fitting to Determine the Weight Fraction of Crystalline Material in Natural Low Quartz Specimens

Quantisation of low-quartz in crystalline mixtures has been performed by X-ray powder diffractometry (XRPD) for many years. Conventional methodology, using discrete-peak integrated intensities, is frequently performed employing calibrations prepared with natural low-quartz specimens. Previous research showed that from a conventional XRPD study of low-quartz specimens, the amorphous content of a suite of natural low-quartz specimens ranged from 1% to 28%. That study thereby underlined the importance in XRPD calibration of characterising the amorphous content (or its complementary quantity “weight fraction of crystalline material, WCFM” employed here). This paper describes an application of pattern-fitting Rietveld analysis for characterising the WCFM in a suite of low-quartz specimens. The results gave WCFM values ranging from 0.91 (esd, σ= 0.02) to 1.00(0.02) for six specimens for which the SiO 2 chemical content ≥ 99.5%.

Effect of post-sintering heat-treatments on the erosive wear behaviour of liquid-phase-sintered aluminas

The influence of microstructure on the indentation-strength and solidparticle erosive wear behaviour of a liquid-phase-sintered (LPS)alumina subjected to coarsening, quenching and crystallisationheat-treatments were investigated. Strength as a function of cracksize using Vickers indentations of varying loads was assessed. Theshort-crack toughness curves (T-curves) of the materials wereevaluated from indentation-strength data which is pertinent to wearproperties, since wear is governed by fracture characteristics atsmall flaw sizes. The effects of impact angle and particle velocityon erosive wear rates were also analysed. The relationship betweenshort-crack toughness behaviour and erosion resistance are discussedwith reference to the material microstructures and phase composition.

QUANTITATIVE DETERMINATION OF PHASES PRESENT IN OXIDISED CHALCOCITE

A sample of chalcocite (Cu2S) of particle size 45–75 μm was heated in air at 10°C min−1 in a simultaneous TG-DTA apparatus. The phase compositions of the products at various temperatures were quantitatively determined by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and wet chemical analyses. Copper(II) sulfate, of amount 1.7% by mass, was observed at 435°C and increased rapidly in concentration to 56% at 570°C. From 570–670°C, there was a rapid decrease in CuSO4 content to 9.8% as the phase converted to CuSO4·CuO, with the CuSO4 not being detected at 775°C. From 435–570°C, Cu2O formed, but at a rather slower rate, reaching 47% at 570°C. The Cu2O level then decreased to 38% over the range 570–670°C. CuSO4·CuO was first detected at 570°C by FTIR, although it was not detected by XRD at this temperature. The content of this species reached 41% at 670°C, decreased to 24% at 775°C, and was not detected at 840°C. CuO first appeared at 670°C and rose steadily in concentration until at 840°C it was the only compound present.

Diffraction study on the thermal stability of Ti3SiC2/TiC/TiSi2 composites in vacuum

Titanium silicon carbide (Ti3SiC2) possesses a unique combination of properties of both metals and ceramics, for it is thermally shock resistant, thermally and electrically conductive, damage tolerant, lightweight, highly oxidation resistant, elastically stiff, and mechanically machinable. In this paper, the effect of high vacuum annealing on the phase stability and phase transitions of Ti3SiC2/TiC/TiSi2 composites at up to 1550° C was studied using in‐situ neutron diffraction. The role of TiC and TiSi2 on the thermal stability of Ti3SiC2 during vacuum annealing is discussed. TiC reacts with TiSi2 between 1400–1450°C to form Ti3SiC2. Above 1400° C, decomposition of Ti3SiC2 into TiC commenced and the rate increased with increased temperature and dwell time. Furthermore, the activation energy for the formation and decomposition of Ti3SiC2 was determined.