I. W. M. Brown

Bio: I. W. M. Brown is an academic researcher from Industrial Research Limited. The author has contributed to research in topics: Sialon & Sintering. The author has an hindex of 16, co-authored 43 publications receiving 1496 citations. Previous affiliations of I. W. M. Brown include MacDiarmid Institute for Advanced Materials and Nanotechnology.

High- T c superconducting phases in the series Bi 2.1 (Ca, Sr) n+l Cu n O 2n+4+δ

Abstract: Following the report of Maeda et al.1 on high-Tc superconductivity in samples of nominal composition BiSrCaCu2Ox we have identified three distinct phases in the homologous series Bi2.1(Ca, Sr)n+lCunO2n+4+δ with n = 1,2 and 3. These have zero resistance Tcs of 80 K, 91 K and 105 K respectively. The structures in the homologous series appear to be based on alternating double bismuth Bi2O2 layers and perovskite (Sr, Ca)O–CuO2 layers, with higher members obtained by intercalating additional 3.1 A Ca-CuO2 bilayers. The structures can be indexed on a pseudo-tetragonal subcell with a = b = 5.4 A and c = 24.4 A (n = 1), 30.76 A (n = 2) or 36 A (n = 3). We were unable to prepare the n =4 member. In each structure a 2× superlattice structure in the c-direction arises as a natural consequence of a 19/4 incommensurate structure in the b direction, which accounts for 2.1 Bi atoms in the unit formula. The critical temperature, Tc, for these phases is sharply dependent on the Sr/Ca ratio and oxygen stoichiometry, as determined by heat treatment.

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: I, Metakaolinite

Abstract: Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid-state high resolution 29Si and 27AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI-0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1-0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well-defined X-ray pattern and the persistence of ∼10% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new 29Si and 27AI NMR data.

Outstanding Problems in the Kaolinite‐Mullite Reaction Sequence Investigated by 29Si and 27Al Solid‐state Nuclear Magnetic Resonance: 11, High‐Temperature Transformations of Metakaolinite

Abstract: Solid-state 29Si and 27Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al2O3· 2SiO2 composition. (4) The spinel phase contains insufficient Si to be detected by 29Si NMR but has a 27Al NMR spectrum consistent with γ-Al2O3. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.

The influence of oxygen on the physical properties of the superconducting series Bi2.1(CaχSr1−χ)n+1Cun+4+δ

Abstract: The sensitivity of Tc to oxygen sorption and the Ca to Sr ratio has been studied for the homologous series Bi2.1(CaχSr1−χ)n+1CunO2n+4+δ for n=1, 2 and 3. The n=0 and n=∞ members have been produced as single phase samples but are found to be semiconducting. While single phase n=2 material has been prepared, the n=1 member always displays a small n=2 contribution to the X-ray diffraction pattern. n=3 is only found in multiphase samples. Single phase superconducting samples were equilibrated under a fixed partial pressure of oxygen at temperatures between 300 and 850°C and quenched in liquid nitrogen. It is shown that Tc can be controlled in a reversible and systematic way by varying the oxygen partial pressure and the annealing temperature and consequently the oxygen stoichiometry. The site energy per oxygen sorbed for n=2 is 3.24 eV, only a little less than that for YBa2Cu3O7. However, the volume change on sorption is ten times smaller than that for YBa2Cu3O7, and is manifested as a changing c-axis with the a-axis invariant. This can be interpreted as implying that oxygen sorption occurs within the Bi-O layers. Tc is found to be maximised for a Ca to Sr ratio of 1 to 2 for both n=1 and 2. Thermal expansion data for n=2 material is also reported to illustrate that oxygen sorption does effect the physical properties of these materials.

Crystalline phase formation in metakaolinite geopolymers activated with NaOH and sodium silicate

Abstract: The effect of the NaOH content and the presence of sodium silicate activators on the formation of crystalline phases from metakaolinite-based geopolymers were studied by X-ray powder diffraction (XRD), Rietveld quantitative XRD, solid-state MAS NMR and SEM in samples synthesized with varying NaOH contents and different curing times at 40 °C. Geopolymers activated with NaOH alone with Si/Na ratios of 4/4 or less formed the crystalline zeolite Na–A (Na96Al96Si96O384·216H2O), but at ratios >4/4 nanosized crystals of another zeolite (Na6[AlSiO4]6·4H2O) were formed. The Si/Na ratio of 4/4 produces a product of greatest crystallinity. The addition of sodium silicate in addition to NaOH significantly reduces crystallite formation. The network units of all the materials containing NaOH and sodium silicate are essentially the same, namely, tetrahedral [SiO4] units coordinated through four bridging oxygens to four aluminium atoms [denoted as Q4 Si(4Al) units]. A templating function of the various silicate units of the sodium silicate molecules is suggested to occur in geopolymerization, which differs from the reaction route operating when NaOH alone is used as the activator. This templating function is responsible for the suppression of crystallization and the increase in strength of the geopolymers activated with sodium silicate.

B–N compounds for chemical hydrogen storage

Abstract: Hydrogen storage for transportation applications requires high volumetric and gravimetric storage capacity. B-N compounds are well suited as storage materials due to their light weight and propensity for bearing multiple protic (N-H) and hydridic (B-H) hydrogens. This critical review briefly covers the various methods of hydrogen storage, and then concentrates on chemical hydrogen storage using B-N compounds. The simplest B-N compound, ammonia borane (H3NBH3), which has a potential 19.6 wt% hydrogen storage capacity, will be emphasised (127 references).