Showing papers in "Chemistry of Materials in 1996"
TL;DR: In this paper, a review of the proton conductivity in materials and the elements of proton conduction mechanisms are discussed with a special emphasis on proton chemistry, including structural reorganization and diffusional motion of extended moieties.
Abstract: In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanismsproton transfer, structural reorganization and diffusional motion of extended moietiesare discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggest...
2,039 citations
TL;DR: In this paper, a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties is presented, followed by a brief discussion of some promising experimental techniques in synthesis and measurements.
Abstract: Understanding the correlation between magnetic properties and nanostructure involves collaborative efforts between chemists, physicists, and materials scientists to study both fundamental properties and potential applications. This article introduces a classification of nanostructure morphology according to the mechanism responsible for the magnetic properties. The fundamental magnetic properties of interest and the theoretical frameworks developed to model these properties are summarized. Common chemical and physical techniques for the fabrication of magnetic nanostructures are surveyed, followed by some examples of recent investigations of magnetic systems with structure on the nanometer scale. The article concludes with a brief discussion of some promising experimental techniques in synthesis and measurements.
1,522 citations
TL;DR: In this article, the molecular shapes of covalent organosilanes, quaternary ammonium surfactants, and mixed surfactant in various reaction conditions can be used to synthesize silica-based mesophase configurations.
Abstract: The low-temperature formation of liquid-crystal-like arrays made up of molecular complexes formed between molecular inorganic species and amphiphilic organic molecules is a convenient approach for the synthesis of mesostructure materials. This paper examines how the molecular shapes of covalent organosilanes, quaternary ammonium surfactants, and mixed surfactants in various reaction conditions can be used to synthesize silica-based mesophase configurations, MCM-41 (2d hexagonal, p6m), MCM-48 (cubic Ia3d), MCM-50 (lamellar), SBA-1 (cubic Pm3n), SBA-2 (3d hexagonal P63/mmc), and SBA-3 (hexagonal p6m from acidic synthesis media). The structural function of surfactants in mesophase formation can to a first approximation be related to that of classical surfactants in water or other solvents with parallel roles for organic additives. The effective surfactant ion pair packing parameter, g = V/a0l, remains a useful molecular structure-directing index to characterize the geometry of the mesophase products, and pha...
1,428 citations
TL;DR: In this paper, the template method is used to synthesize nanotubules and fibrils of polymers, metals, semiconductors, carbons, and other materials.
Abstract: This paper reviews a relatively new method for preparing nanomaterials: membrane-based synthesis. This method entails the synthesis of the desired material within the pores of a nanoporous membrane. Because the membranes employed contain cylindrical pores of uniform diameter, monodisperse nanocylinders of the desired material, whose dimensions can be carefully controlled, are obtained. These nanocylinders may be either hollow (a tubule) or solid (a fibril or nanowire). We call this approach the “template” method because the pores in the nanoporous membranes are used as templates for forming the desired material. This template method is a very general approach; it has been used to prepare nanotubules and fibrils of polymers, metals, semiconductors, carbons, and other materials.
1,419 citations
TL;DR: A brief summary of the research activities in the field of organic/inorganic nanocomposite materials and a general background of the sol−gel chemistry are first given in this article.
Abstract: Organic/inorganic hybrid materials prepared by the sol−gel approach have rapidly become a fascinating new field of research in materials science. The explosion of activity in this area in the past decade has made tremendous progress in both the fundamental understanding of the sol−gel process and the development and applications of new organic/inorganic hybrid materials. In this review, a brief summary of the research activities in the field of organic/inorganic nanocomposite materials and a general background of the sol−gel chemistry are first given. The emphasis of this report, however, is placed on the synthesis, structure−property response, and potential applications of the organic/inorganic hybrid networks that possess chemical bonding between the organic and inorganic phases, particularly those systems that were developed in our laboratory since 1985.
1,088 citations
TL;DR: In this article, the shape and size distribution and crystallinity of the Fe3O4 nanoparticles were assessed by transmission electron microscopy and selected area electron diffraction, and very uniform and stable colloidal suspensions of the particles were synthesized.
Abstract: Synthesis of suspensions of nanosize particles of Fe3O4 was carried out in bulk aqueous solutions without the presence of surfactants. The Fe3O4 nanoparticles were oxidized to γ-Fe2O3 by direct aeration of the suspension at 100 °C. The shape and size distribution and crystallinity of the Fe3O4 nanoparticles were assessed by transmission electron microscopy and selected area electron diffraction. Very uniform and stable colloidal suspensions of the Fe3O4 nanoparticles in water could be synthesized. Oxidation of the colloidal system leads to γ-Fe2O3 nanoparticles of much larger size than Fe3O4.
1,009 citations
TL;DR: In this article, the static and dynamic properties of polymer-layered silicate nanocomposites are discussed in the context of polymers in confined media, and both the local and global dynamics of the polymer in the hybrids are dramatically different from those in the bulk.
Abstract: The static and dynamic properties of polymer-layered silicate nanocomposites are discussed in the context of polymers in confined media. Despite the topological constraints imposed by the host lattice, mass transport of the polymer into the silicate layers (at least in the case of essentially non-polar polystyrene) appears to be unhindered and exhibits mobility similar to that of the pure polymer. However, both the local and global dynamics of the polymer in the hybrids are dramatically different from those in the bulk. On a local scale, intercalated polymer chains exhibit higher flexibility along their backbone along with a marked suppression (or even absence) of cooperative dynamics typically associated with the glass transition. On a global scale, relaxation of polymer chains either tethered to or in close proximity (<1 nm as in intercalated hybrids) to the host surface are dramatically altered and parallel those of other intrinsically anisotropic materials such as block copolymers and liquid crystals.
853 citations
TL;DR: In this paper, two classes of organic template-derived amorphous silicas are distinguished by the nature of template-matrix interactions and the extent to which subsequent processing dictates the final pore morphology.
Abstract: This review discusses two classes of organic template-derived amorphous silicas distinguished by the nature of template-matrix interactions and the extent to which subsequent processing dictates the final pore morphology. First we discuss surfactant-templated silicas where the template-matrix interaction is via non-covalent bonding mechanisms and the pore structure is established in the solution stage. We then discuss silicas templated by organic ligands covalently bonded to the siloxane network where subsequent processing strongly influences the final pore structure.
720 citations
TL;DR: In this article, a review of the fast-growing area of catalysis by crystalline mesoporous materials is presented, focusing on acid catalysis, redox catalysis and miscellaneous applications.
Abstract: Crystalline mesoporous molecular sieves may be prepared under a wide range of conditions in the presence of cationic, anionic, gemini, or neutral surfactants. These mesostructured materials include pure and modified silicates, other metallic oxides and sulfides as well as aluminophosphates. Air calcination of silicate-based mesoporous molecular sieves with stable frameworks (MCM-41, MCM-48, SBA-n, MSU-n) affords materials with extremely high surface areas. Their pore size may be adjusted from ca. 20 to more than 100 A using different strategies. Because of their unique flexibility in terms of synthesis conditions, pore size tuning, and framework composition, these materials have been targeted for a number of potential applications, particularly in catalysis. The present review deals with the fast-growing area of catalysis by crystalline mesoporous materials. Three topics are discussed separately: (i) acid catalysis, (ii) redox catalysis, and (iii) miscellaneous applications. Particular attention is put o...
689 citations
618 citations
TL;DR: In this article, a series of mesoporous MCM-41 and HMS silica molecular sieves have been prepared by electrostatic and neutral assembly pathways, respectively, and their properties have been compared by a variety of physical techniques.
Abstract: A series of mesoporous MCM-41 and HMS silica molecular sieves have been prepared by electrostatic and neutral assembly pathways, respectively, and their properties have been compared by a variety of physical techniques. Direct S+I- charge matching between cationic quaternary ammonium ion surfactants (S+) and the anionic silicate precursors (I-) affords long-range hexagonal structures under hydrothermal synthesis conditions (100 °C), but the long-range order is greatly reduced when the synthesis is conducted at ambient temperature. Conversely, owing to weaker assembly forces, counterion-mediated S+X-I+ and neutral S0I0 pathways (where X- is halide and S0 is a primary amine) provide calcined products with the best long-range order when the syntheses are conducted at ambient temperature. In general, MCM-41 silicas formed by electrostatic S+I- or S+X-I+ assembly exhibit greater long-range order than the HMS analogues prepared by the S0I0 assembly. However, the framework-confined pore structures, though depend...
TL;DR: In this article, a detailed summary of the molecular level organization of surfactant monolayers, Langmuir−Blodgett and self-assembled films emphasis is placed on the colloid chemistry underlying the self-assembly of nanostructured materials Specifically, details are provided on (i) the preparation and stabilization of nanoparticle dispersions, (ii) the adsorption and desorption of nanoparticles onto solid surfaces, and (iii) the nanoreactors provided by polar liquids selectively adsorbed onto solid surface from binary polar−apolar liquid
Abstract: The “wet” colloid chemical construction of nanosized or nanostructured materials (ie, those in the 1−100 nm range) has been inspired by biomineralization (the in vivo formation of inorganic crystals and/or amorphous particles in biological systems) and hierarchically organized self-assembly (spontaneous stepwise assembly of functional units) Subsequent to a summary of the molecular level organization of surfactant monolayers, Langmuir−Blodgett and self-assembled films emphasis is placed on the colloid chemistry underlying the self-assembly of nanostructured materials Specifically, details are provided on (i) the preparation and stabilization of nanoparticle dispersions, (ii) the adsorption and desorption of nanoparticles onto solid surfaces, and (iii) the nanoreactors provided by polar liquids selectively adsorbed onto solid surfaces from binary polar−apolar liquid mixtures The “wet” colloid chemical preparation of nanostructured materials in our laboratories is illustrated by the layer-by-layer self
TL;DR: In this paper, the microstructure of various (ordered and disordered) polymer−organically modified layered silicate hybrids, synthesized via static polymer melt intercalation, is examined with X-ray diffraction and transmission electron microscopy.
Abstract: The microstructure of various (ordered and disordered) polymer−organically modified layered silicate hybrids, synthesized via static polymer melt intercalation, is examined with X-ray diffraction and transmission electron microscopy. The ordered intercalates exhibit microstructures very similar to the unintercalated organically modified layered silicate (OLS). Polymer intercalation occurs as a front which penetrates the primary OLS particle from the external edge. The disordered hybrids, on the other hand, exhibit heterogeneous microstructures with increased layer disorder and spacing toward the polymer−primary particle boundary. In these hybrids, individual silicate layers are observed near the edge, whereas small coherent layer packets separated by polymer-filled gaps are prevalent toward the interior of the primary particle. The heterogeneous microstructure indicates that the formation of these disordered hybrids occur by a more complex process than simple sequential separation of individual layers sta...
TL;DR: A series of novel monomers and polymers containing aromatic-donor and o-quinoid-acceptor units was prepared, and the relationship between their spectral and electrochemical properties and their structures was investigated as discussed by the authors.
Abstract: A series of novel monomers and polymers containing aromatic-donor and o-quinoid-acceptor units was prepared, and the relationship between their spectral and electrochemical properties and their structures was investigated. X-ray structure analyses of the monomers possessing thiophene units revealed coplanar conformations, whereas calculations of the monomers containing N-methylpyrrole showed torsional conformations. Cyclic voltammetry showed amphoteric properties for all the monomers and p- and n-doping processes for most of the polymers. The reduction potentials were primarily dependent on the electron-accepting character of the o-quinoid-acceptor units. The electrochemical behavior of the polymers was characterized by cyclic voltammetry and suggested narrow-bandgap systems. The bandgaps determined from optical absorption spectra range from 0.5 to 1.4 eV. The polymer composed of thiophenes and benzo[1,2-c;3,4-c‘]bis[1,2,5]thiadiazole exhibited the narrowest bandgap.
IBM1
TL;DR: In this paper, single crystals of the organic-inorganic layered perovskites (C4H9NH3)2MI4 (M = Ge, Sn, Pb) have been grown from aqueous hydriodic acid solutions.
Abstract: Single crystals of the organic-inorganic layered perovskites (C4H9NH3)2MI4 (M = Ge, Sn, Pb) have been grown from aqueous hydriodic acid solutions. X-ray diffraction, thermal analysis, and photoluminescence spectroscopy were used to compare crystal structure, metal atom lone-pair stereoactivity, and physical properties as a function of group IVB element. The orthorhombic (C4H9NH3)2GeI4 structure, refined in the space group Pcmn, consists of single-layer-thick perovskite sheets of distorted corner-sharing GeI6 octahedra separated by n-butylammonium cation bilayers. (C4H9NH3)2SnI4 and (C4H9NH3)2PbI4 are structurally very similar but adopt the space group Pbca, with a more ideal octahedral iodine coordination around the divalent group IVB atoms. Within the more general tin(II)-based family, (C4H9NH3)2(CH3NH3)n-1SnnI3n+1, a structural comparison between the title semiconducting n = 1 compound and the previously reported semimetallic n = 3 and metallic n → ∞ members demonstrates a correlation between perovskite...
TL;DR: In this article, the authors used high-resolution neutron powder diffraction data taken at 14 temperatures from 0.3 to 693 K to study the structural properties of ZrW2O8 and HfW2Os8.
Abstract: Isostructural ZrW2O8 and HfW2O8 show strong negative thermal expansion from 0.3 K up to their decomposition temperatures of approximately 1050 K. This behavior is especially unusual because these compounds are apparently cubic over their entire existence range. Detailed structural studies of ZrW2O8 were conducted using high-resolution neutron powder diffraction data taken at 14 temperatures from 0.3 to 693 K. Below 428 K, ZrW2O8 adopts the acentric space group P213 and has a well-ordered structure containing corner-sharing ZrO6 octahedra and two crystallographically distinct WO4 tetrahedra. Above the phase transition at 428 K, which appears to be second order, the space group becomes centric Pa3. The structure is now disordered with one oxygen site 50% occupied, suggesting the possibility of high oxygen mobility. Oxygen motion above 428 K is also suggested by dielectric and ac impedance measurements. The negative thermal expansion of ZrW2O8 and HfW2O8 is related to transverse thermal vibrations of bridgi...
TL;DR: In this article, a template carbonization technique was applied to prepare carbon nanotubes and submicron-tubes in one dimensional channels, where an anodic aluminum oxide film that has uniform and straight channels with a diameter at the nanometer level was used as a template.
Abstract: A template carbonization technique was applied to prepare carbon nanotubes and submicron-tubes in one dimensional channels. An anodic aluminum oxide film that has uniform and straight channels with a diameter at the nanometer level was used as a one-dimensional template. The pyrolytic carbon deposition on the channel wall was carried out by exposing the anodic oxide film to propylene at 800 °C. The carbon was then liberated from the anodic oxide film by HF washing. It was found that the resultant carbon is comprised only of uniform hollow tubes with open ends. The infiltration of poly(furfuryl alcohol) into the channels of the film followed by heat treatment led to the formation of bamboolike carbon tubes. For all the carbon tubes obtained here, the outer diameter (30 or 230 nm) and the length (60 or 75 μm) precisely reflect the channel diameter and the thickness of the template used, respectively. Furthermore, in the case of the carbon tubes from propylene, the wall thickness of the tubes was easily cont...
TL;DR: In this article, a thin-film quantum dot composites incorporating bare and overcoated CdSe nanocrystals in a ZnSe matrix were synthesized by electrospray organometallic chemical vapor deposition (ES-OMCVD).
Abstract: Electronic and chemical passivation of CdSe nanocrystals (quantum dots) has been achieved with a thin ZnSe overlayer grown in solution from trioctylphosphine selenide and diethylzinc. Layered particles with a [ZnSe/CdSe] ratio ranging from 0 to ∼5.0 were prepared and characterized by optical absorption spectroscopy, photoluminescence, high-resolution transmission electron microscopy, Auger electron spectroscopy, and X-ray scattering. The overgrown particles were crystalline and displayed band-edge absorption and emission characteristic of the initial CdSe nuclei. Thin-film quantum dot composites incorporating bare and overcoated CdSe nanocrystals in a ZnSe matrix were synthesized by electrospray organometallic chemical vapor deposition (ES-OMCVD). The photoluminescence spectra of the composites with bare CdSe dots were dominated by broad deep-level emission and the photoluminescence yield deteriorated with increasing deposition temperature. In contrast, the composites incorporating the overcoated dots sho...
TL;DR: The porosity and flow characteristics of macroporous polymer monoliths that may be used to prepare separation media, flow-through reactors, catalysts, or supports for solid-phase chemistry can be controlled easily during their preparation.
Abstract: The porosity and flow characteristics of macroporous polymer monoliths that may be used to prepare separation media, flow-through reactors, catalysts, or supports for solid-phase chemistry can be controlled easily during their preparation. Key variables such as temperature, composition of the pore-forming solvent mixture, and content of cross-linking divinyl monomer allow the tuning of average pore size within a broad range spanning 2 orders of magnitude. The polymerization temperature, through its effects on the kinetics of polymerization, is a particularly effective means of control, allowing the preparation of macroporous polymers with different pore size distributions from a single composition of the polymerization mixture. The choice of pore-forming solvent is also important, larger pores being obtained in a poor solvent due to an earlier onset of phase separation. Increasing the proportion of the cross-linking agent present in the monomer mixture not only affects the composition of the final monolit...
TL;DR: In this article, strategies for the preparation of zeolite molecular sieve membranes and layers on different substrates are reviewed, and the resulting nanoporous layers have been applied in highly selective sensors, for the alignment of NLO chromophores and for molecular separations.
Abstract: Strategies for the preparation of zeolite molecular sieve membranes and layers on different substrates are reviewed. Membranes (and continuous thin films) have been synthesized with ZSM-5 (MFI) type molecular sieves and other zeolites, either as free-standing specimens, grown on various supports according to several schemes, or embedded in polymer matrices. The membranes have been studied for molecule-selective separations and for catalytic reactions. Layers of presynthesized molecular sieves have been deposited by means of sol−gel matrixes and other bonding agents, molecular attachment layers, and via sputtering or laser ablation techniques. The resulting nanoporous layers have been applied in highly selective sensors, for the alignment of NLO chromophores and for molecular separations. Finally, oriented molecular sieve layers have been grown on self-assembled organic mono- and multilayers presenting phosphonate groups at the interface. Mixed phosphonate/alkyl films show profound effects on the surface c...
TL;DR: Alumina mesophases have been synthesized by reacting aluminum alkoxides and carboxylic acids with controlled amounts of water in low-molecular-weight alcoholic solvents as discussed by the authors.
Abstract: Alumina mesophases have been synthesized by reacting aluminum alkoxides and carboxylic acids with controlled amounts of water in low-molecular-weight alcoholic solvents. Calcination of these materials yields aluminas that are thermally stable to 1073 K and contain randomly ordered pores. Specific surface areas as high as 710 m2/g and narrow pore size distributions centered at 20 A that do not contain “zeolitic” micropores are exhibited by the calcined solids.
TL;DR: In this paper, photoelectrochemical and photocatalytic degradation experiments carried out in both nitrogen- and oxygen-saturated solutions with an externally applied electrochemical bias provide useful information in optimizing semiconductor concentrations in a composite film.
Abstract: Nanostructured semiconductor films of SnO2, TiO2 and SnO2/TiO2 have been employed for electrochemically assisted photocatalytic degradation of a textile azo dye naphthol blue black (NBB). The degradation rate is significantly higher for SnO2/TiO2 composite films than SnO2 and TiO2 films alone. An effort has been made to correlate the photoelectrochemical behavior of these films to the rate of photocatalytic degradation of NBB. The enhanced degradation rate of NBB using composite semiconductor films is attributed to increased charge separation in these systems. Photoelectrochemical and photocatalytic degradation experiments carried out in both nitrogen- and oxygen-saturated solutions with an externally applied electrochemical bias provide useful information in optimizing semiconductor concentrations in a composite film.
TL;DR: Rietveld analysis of electrochemically grown films reveals pure copper(I) oxide with no copper(II) oxide or copper metal present in the films and a lattice parameter of a = 0.4266 nm as mentioned in this paper.
Abstract: Films of copper(I) oxide can be electrodeposited by reduction of copper(II) lactate in alkaline solution. Rietveld analysis of electrochemically grown films reveals pure copper(I) oxide with no copper(II) oxide or copper metal present in the films and a lattice parameter of a = 0.4266 nm. The cathodic deposition current is limited by a Schottky-like barrier that forms between the Cu2O and the deposition solution. A barrier height of 0.6 eV was determined from the exponential dependence of the deposition current on the solution temperature. At a solution pH of 9 the orientation of the film is [100], while at a solution pH of 12 the orientation changes to [111]. Atomic force images of the [100] oriented films have crystals shaped as four-sided pyramids, while the [111] films have triangular crystals. The grain size for films grown at 65 °C ranges from 2 to 5 μm. A refractive index of 2.6 was measured from the transmission spectrum for wavelengths between 1350 and 2800 nm. The p-type semiconductor has a dire...
TL;DR: In this article, a short-chain alkylammonium smectite clay nanocomposite was shown to completely inhibit exfoliation of the silicate nanolayers in an epoxy matrix.
Abstract: Interfacial effects play a major role in the formation of organic polymer−smectite clay nanocomposites. As little 2 mol % Na+ cations on the exchange sites of a short-chain alkylammonium smectite clay completely inhibits exfoliation of the silicate nanolayers in an epoxy matrix.
TL;DR: In this article, the effects of molecular structure on the electronic structure and redox properties of a series of 22 systematically designed conjugated polyquinolines and polyanthrazolines are explored by cyclic voltammetry and spectroelectrochemistry on thin films.
Abstract: The effects of molecular structure on the electronic structure and redox properties of a series of 22 systematically designed conjugated polyquinolines and polyanthrazolines are explored by cyclic voltammetry and spectroelectrochemistry on thin films. The measured electrochemical bandgap of the series of conjugated polymers was in the range 2.0−3.1 eV and found to be in good agreement with the optical bandgap. The oxidation and reduction potentials, ionization potential, and electron affinity of the series of polymers were correlated with their main structural features. All the polyquinolines and polyanthrazolines had reversible reduction with formal potentials of −1.57 to −2.08 V (versus SCE) which make them excellent n-type semiconducting polymers. Polymers containing anthrazoline units have a higher electron affinity by 0.3−0.4 eV than those containing bis(quinoline) units. On the other hand, thiophene-linked polymers have a lower ionization potential by 0.45−0.5 eV than those with phenylene linkages. ...
TL;DR: A series of bis(2-(3,4-ethylenedioxy)thiophene)-based monomers have been synthesized and fully characterized as discussed by the authors, and they exhibit low redox switching potentials and are quite stable in the conducting state.
Abstract: A series of bis(2-(3,4-ethylenedioxy)thiophene)-based monomers have been synthesized and fully characterized; specifically (E)-1,2-bis(2-(3,4-ethylenedioxy)thienyl)vinylene (BEDOT-V), 1,4-bis(2-(3,4-ethylenedioxy)thienyl)benzene (BEDOT-B), 4,4‘-bis(2-(3,4-ethylenedioxy)thienyl)biphenyl (BEDOT-BP), 2,5-bis(2-(3,4-ethylenedioxy)thienyl)furan (BEDOT-F), 2,5-bis(2-(3,4-ethylenedioxy)thienyl)-thiophene (BEDOT-T), and 2,2‘:5‘,2‘‘-ter(3,4-ethylenedioxy)thiophene, TER-EDOT. The X-ray crystal structures of BEDOT-V and BEDOT-B have been determined. These monomers oxidize and polymerize at low potentials relative to other reported electropolymerizable heterocycles. The electroactive polymers formed exhibit low redox switching potentials and are quite stable in the conducting state. TER-EDOT was found to have the lowest peak oxidation potential of +0.2 V vs Ag/Ag+, making it the most easily oxidized polymerizable thiophene monomer reported. The electronic bandgaps of these EDOT based polymers range from 1.4 to 2.3 eV...
TL;DR: Bone-like crystals were acceleratedly grown in a simulated body fluid on the negatively charged surface of electrically poled hydroxyapatite ceramics, while the crystal growth was completely inhibited on the opposite surface as discussed by the authors.
Abstract: Bone-like crystals were acceleratingly grown in a simulated body fluid on the negatively charged surface of electrically poled hydroxyapatite ceramics, while the crystal growth was completely inhibited on the opposite surface. The poling effect was generalized by the enhanced crystal growth on the poled ferroelectric ceramics of BaTiO3 and CaTiO3.
TL;DR: Giant magnetoresistance (GMR) was found for manganate perovskites of the formula Ln{sub 1-x}A{sub x}MnO{sub 3} (Ln = rare earth, A = dipositive ion) as mentioned in this paper.
Abstract: Giant magnetoresistance (GMR) is found for manganate perovskites of the formula Ln{sub 1-x}A{sub x}MnO{sub 3} (Ln = rare earth, A = dipositive ion). Changes in GMR near the critical temperature and the insulator-metal transition temperature were quantified as was the relationship between the radius of A and GMR. The electron transport properties of these manganates are discussed.
TL;DR: The pyrolysis of polyhedral oligomeric silsesquioxane (POSS) macromers was studied in argon, nitrogen, and under vacuum from 30 to 1000 °C.
Abstract: The pyrolysis of four polyhedral oligomeric silsesquioxane (POSS) macromers, Cy8Si8O11(OH)2, Cy8Si8O11(OSiMe3)2, Cy6Si6O9, and Cy8Si8O12 (where Cy = c-C6H11) and two POSS−siloxane copolymers [Cy8Si8O11−(OSiMe2)nO−] (n = 1, oligomer average 5.4) has been studied in argon, nitrogen, and under vacuum from 30 to 1000 °C. Product gases were analyzed by TGA-FTIR and mass spectroscopy. Analysis of the chars was conducted using cross-polarized (CP) and magic angle spinning (MAS) NMR spectroscopy, X-ray diffraction, density measurements, and gas adsorption analysis. All of the POSS macromers showed a propensity toward sublimation, while the POSS-siloxane copolymers underwent a complex depolymerization−decomposition process. For the copolymer [Cy8Si8O11−(OSiMe2)5.4O−] this process included the evolution of cyclic dimethylsiloxanes at 400 °C, cyclohexyl hydrocarbons from 450 to 550 °C, and H2 liberation from 700 to 1000 °C. Loss of the silsesquioxane “cage” structure occurred upon heating from 450−650 °C and after t...
TL;DR: In this paper, the authors used the scanning tunneling microscope (STM) to study surface and adsorbate structures and their relationship to the electronic and chemical properties of materials.
Abstract: Investigations of long-chain hydrocarbon and substituted hydrocarbon materials physisorbed at the liquid−solid interface have provided a particularly fruitful area of investigation for the scanning tunneling microscope (STM). Studies of this kind provide considerable information concerning surface−adsorbate interactions, the nature of the STM contrast mechanism for molecules adsorbed on surfaces, the effect of the chemical nature of the liquid solvent on the rearrangement and bonding of adsorbates, and the role played by the underlying structure of the surface itself (defects, domains, etc.) in determining the molecular arrangement of these adsorbates. All of these studies are aimed at the elucidation and control of surface and adsorbate structures and their relationship to the electronic and chemical properties of materials. The successful imaging of numerous physisorbed organic films at the solution−solid interface has demonstrated that the STM is sensitive enough to differentiate chemical functional gr...