Showing papers by "Kenneth J. D. MacKenzie published in 2008"
TL;DR: In this paper, the authors demonstrate the important role of the aluminate species in the formation of aluminosilicate inorganic polymers, and demonstrate that severe mechanochemical processing (ball-milling for 60h or vibro-melting for 15min) leads to the formation four-, five-and sixfold coordinated Al as in metakaolinite, and enables viable polymers to be formed at 60°C which show the characteristic XRD, 27 Al and 29 Si MAS NMR spectra.
Abstract: Attempts to produce fully reacted aluminosilicate inorganic polymers (judged on the basis of their hardening at ambient temperatures, amorphous XRD patterns and their 27 Al and 29 Si NMR spectra) from the crystalline 2:1 layer lattice mineral pyrophyllite were unsuccessful. Although dehydroxylation of this pyrophyllite at 800 °C produces significant changes in the Al coordination, the dehydroxylate does not form a viable geopolymer, probably because of its retention of a crystalline 2:1 layer structure which encloses the Al–O sheet by Si–O sheets and prevents alkaline attack to form soluble aluminate species. Disruption of the crystalline 2:1 layer lattice structure by severe mechanochemical processing (ball-milling for 60 h or vibro-milling for 15 min) leads to the formation of four-, five- and sixfold coordinated Al as in metakaolinite, and enables viable inorganic polymers to be formed at 60 °C which show the characteristic XRD, 27 Al and 29 Si MAS NMR spectra. These results illustrate the important role of the aluminate species in the formation of aluminosilicate inorganic polymers.
63 citations
TL;DR: In this article, the effect of Zr content on the crystal structure and electrical properties of barium zirconate titanate (Ba(Zr,Ti)O3) was studied by X-ray diffraction and dielectric, ferroelectric and impedance spectroscopy.
Abstract: The effect of Zr content on the crystal structure and electrical properties of barium zirconate titanate (Ba(Zr,Ti)O3) was studied by X-ray diffraction and dielectric, ferroelectric and impedance spectroscopy. An increase of Zr content into BaTiO3 leads to a reduction in its c-parameter and an increase in its a-parameter, resulting in a change from tetragonal to cubic symmetry of the BaTiO3 unit cell. The Curie temperatures are lowered and the relative permittivity values are decreased with increasing Zr content. The presence of BaZrO3 secondary phases has the affect of decreasing tanδ. A higher applied electric field is required during the polarization process because of the effect of domain-wall pinning caused by oxygen vacancies. Impedance spectroscopy studies of Ba(Ti0.95Zr0.05)O3 ceramics show a decrease in the bulk resistance with increasing temperature, indicating a typical negative temperature coefficient of resistance.
53 citations
TL;DR: In this paper, a new planetary mill with a centrifugal factor of 28 g was used to produce nanostructured β-sialon precursors from the relevant mixtures of Si 3 N 4, AlN and Al 2 O 3 by mechanical milling.
Abstract: Nanostructured β-sialon precursors were produced from the relevant mixtures of Si 3 N 4 , AlN and Al 2 O 3 by mechanical milling in a new planetary mill with a centrifugal factor of 28 g, at milling times 2 O 3 crystallites were formed with new boundaries and dislocations, but the α-Si 3 N 4 particles were more resistant to deformation. Milling at the higher acceleration conditions produces only a small increase in the crystal lattice parameters of the precursor components, but FTIR and 27 Al and 29 Si solid-state MAS NMR spectroscopy indicates the formation of new tetrahedral AlO 4 units and Si–O–Al bonds, possibly in solid solution or partially reacted hybrid particles of Al 2 O 3 and β-Si 3 N 4 . The total oxygen content in the activated sialon precursor mixture can be limited by replacement of alumina by AlN.
36 citations
TL;DR: The crystal structure of Al4B2O9, synthesized from Al(NO)(3)center dot 9H(2)O and B(OH) via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with (11) B and Al-27 MAS NMR spectroscopy as mentioned in this paper.
Abstract: The crystal structure of Al4B2O9, synthesized from Al(NO)(3)center dot 9H(2)O and B(OH)(3) via a sol-gel process, is studied and characterized by Rietveld refinements and grid search analyses combined with (11) B and Al-27 MAS NMR spectroscopy. The aluminum borate with a unit-cell composition of Al32B16O72 is closely related to the boralsilite (Al32B12Si4O74) Structure with Si replaced by B and to mullite (Al4+2xSi2-2xO10-x) It crystallizes in the monoclinic space group C2/m, a = 14.8056(7) angstrom, b = 5.5413(2) angstrom, c = 15.0531(6) angstrom, beta = 90.913(2)degrees, Z = 8 for Al4B2O9. The main structural units are isolated chains of edge-sharing AlO6-octahedra running parallel to b that is a characteristic feature of the mullite-type crystal structures. The octahedral chains are crosslinked by AlO4, AlO5, BO3, and BO4 groups with two B atoms and one O atom (O5') disordered on interstitial positions. Al-27 and B-11 NMR studies confirm the presence of sixfold (octahedral), fivefold, and fourfold (tetrahedral) coordinated Al (sixfold: [fourfold + fivefold] =similar to 50%:50%) and of threefold and fourfold coordinated B (similar to 80%:20%).
33 citations
TL;DR: In this paper, a series of synthetic calcium silicates has been produced comprising nano-sized plates, referred to as nano-structured calcium silicate, NCS, which appears itself X-ray amorphous but contains impurities of calcite.
Abstract: A series of synthetic calcium silicates has been produced comprising nano-sized plates. The starting calcium silicate referred to as nano-structured calcium silicate, NCS, appears itself X-ray amorphous but contains impurities of calcite. These impurities decompose during the dry thermal conversion of the material into wollastonite. NCS can be enriched with calcium or silicon, respectively. The silicon enriched NCS can be hydrothermally transformed into a sheet material related to Ca7Si16O38(OH)2. The X-ray pattern of this material was sufficient to allow the calculation of its unit cell with a comparatively high figure of merit. The calcium enriched NCS can form two very distinctly different ceramic materials upon hydrothermal treatment, one a band material (formed below 200 °C) and the other a rose petal shaped material (formed above 210 °C). The X-ray diffraction patterns could not be resolved. The fit between the observed and calculated patterns was less than 50% as expressed in comparatively low figures of merit (unusually <20), which is attributed to calcium carbonate impurities in the samples disrupting the long range order. The patterns of these calcium enriched samples could be best compared to those of tobermorite or truscotite.
7 citations
TL;DR: In this article, the effect of the processing parameters on the ferroelectric, dielectric and pyroelectric properties of PZT-type perovskite structures was determined.
5 citations
TL;DR: In this paper, the formation mechanism of β-SiAlON was investigated using X-ray diffraction and 29 Si and 27 Al NMR spectroscopy, and it was revealed that most of the Si and Al atoms react to form β-siAlON via amorphous forms of Si-Al-O-N and Ο-Si-AlON.
Abstract: β-SiAlON was synthesized from a zeolite by NH 3 gas nitridation and its formation mechanism was investigated using X-ray diffraction and 29 Si and 27 Al NMR spectroscopy. It was revealed that most of the Si and Al atoms react to form β-SiAlON via amorphous forms of Si-Al-O-N and Ο-SiAlON. Nitridation using NH 3 gas is an effective means of preventing mullite formation and promoting the introduction of nitrogen into aluminosilicate materials at lower temperatures than temperatures required by the carbothermal reduction nitridation process. Further, the NMR spectra showed that the siliceous part of the system changed into low z-value of Si 6-z Al z O z N 8-z (β-SiAlON) and the incorporation of Al components into the β-SiAlON was promoted in the later stages of the reaction.
5 citations
TL;DR: Water evolution behavior from reaction-bonded aluminum oxide (RBAO) powder precursor was studied by the Carl-Fischer titration method as discussed by the authors, where RBAO mixtures were milled for 4 and 8h in absolute ethanol and water-ethanol mixtures containing 5 and 10% water.
Abstract: Water evolution behavior from reaction-bonded aluminum oxide (RBAO) powder precursor was studied by the Carl-Fischer titration method. RBAO mixtures were milled for 4 and 8 h in absolute ethanol and water–ethanol mixtures containing 5 and 10 vol% water. The water content was then determined under dynamic heating conditions from 20°C to 250°C and static heating at 250°C. When the aluminum content is increased from 30 to 60 vol%, the water content of the milled powder precursor mixtures also increased. The higher water contents observed in powders milled in absolute ethanol are discussed in terms of the possible interaction of the ethanol with the surface of the aluminum particles.
1 citations
TL;DR: In this article, an asymmetric Schottky contact at the BaTiO3/Ag and Ag/Ba 0.xSr1-0.xTiO 3 interfaces was found to cause a hysteresis shift along the polarization axis when driven by an alternating electric field.
Abstract: Compositionally graded Ba(1-x)SrxTiO3 thick films were fabricated by the airflow deposition technique. Films displayed hysteresis translation along the polarization axis when driven by an alternating electric field. The trend of the hysteresis shift as a function of temperature is similar to the difference of DC currents measured when a positive and negative DC electric field is applied. The study suggests that the origin of this phenomenon lies in an asymmetry of the film conductance due to the presence of asymmetric Schottky contacts at the BaTiO3/Ag and Ag/Ba0.xSr1-0.xTiO3 interfaces.
1 citations