Todd M. Alam

Modifier Coordination and Phosphate Glass Networks

The addition of up to approximately 16 mole% Cs{sub 2}O to vitreous P{sub 2}O{sub 5} reduces the glass transition temperature (T{sub g}) by 150 K, whereas further additions up to 50 mole% produce little additional change in T{sub g}. {sup 31}P magic angle spinning nuclear magnetic resonance spectra indicate that the phosphate network is progressively dipolymerized over the entire range of compositions. The property trend is explained by a transition in the Cs{sup +} coordination environment, from isolated Cs-polyhedra below {approximately}16 mole% Cs{sub 2}O to a corner-sharing Cs-polyhedral sub-structure in the glasses with greater Cs{sub 2}O contents. This modifier transition does not occur in Al-phosphate glasses. {sup 27}Al MAS NMR spectra indicate that the average Al coordination number decreases with increasing Al{sub 2}O{sub 3} content to avoid the formation of Al-O-Al bonds in these binary phosphate glasses.

Local structure and connectivity in lithium phosphate glasses: a solid-state 31P MAS NMR and 2D exchange investigation

High resolution solid-state 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the local structure and connectivity for three xLi2O·(1 - x)P2O5 glasses. The principal components of the 31P chemical shift anisotropy (CSA) tensors for the different phosphate tetrahedron sites (Qn) and the relative populations of each Qn species were determined from simulation of the MAS spectra. The medium range structure and connectivity between similar or different Qn units in these lithium phosphate glasses were probed using novel two-dimensional (2D) MAS exchange experiments. Radio-frequency dipolar recoupling (RFDR) techniques allowed the reintroduction of the through space 31P-31P dipolar-dipolar interaction even in the presence of fast MAS. For these binary phosphate glasses the connectivity and medium range structure for different Qn species is described well by a statistical distribution, and is consistent with a model of random depolymerization.

6Li, 7Li nuclear magnetic resonance investigation of lithium coordination in binary phosphate glasses

{sup 6}Li and {sup 7}Li solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the local coordination environment of lithium in a series of xLi{sub 2}O {center_dot} (1-x)P{sub 2}O{sub 5} glasses, where 0.05 {le} x {le} 0.55. Both the {sup 6}Li and {sup 7}Li show chemical shift variations with changes in the Li{sub 2}O concentration, but the observed {sup 6}Li NMR chemical shifts closely approximate the true isotropic chemical shift and can provide a measure of the lithium bonding environment. The {sup 6}Li NMR results indicate that in this series of lithium phosphate glasses the Li atoms have an average coordination between four and five. The results for the metaphosphate glass agree with the coordination number and range of chemical shifts observed for crystalline LiPO{sub 3}. An increase in the {sup 6}Li NMR chemical shift with increasing Li{sub 2}O content was observed for the entire concentration range investigated, correlating with increased cross-linking of the phosphate tetrahedral network by O-Li-O bridges. The {sup 6}Li chemical shifts were also observed to vary monotonically through the anomalous glass transition temperature (T{sub g}) minimum. This continuous chemical shift variation shows that abrupt changes in the Li coordination environment do not occur as the Li{sub 2}O concentration is increased, and such abrupt changes can not be used to explain the T{sub g} minimum.

Chemometric Analysis of NMR Spectroscopy Data: A Review

Abstract The application of chemometric or multivariate analysis techniques to nuclear magnetic resonance (NMR) spectroscopic data is reviewed. Descriptions of the different processing and data manipulation procedures being utilized to produce reproducible input data sets for chemometric analysis are discussed. A brief review of some of the standard supervised and unsupervised chemometric classification methods as applied to NMR data is presented. The application of spectral resolution algorithms in the decomposition of NMR data to obtain pure component spectra and concentrations is also described. Specific examples of the use of chemometrics in NMR for a wide range of different fields are presented. The limitations, advantages and future directions of chemometric analysis in NMR are also discussed.

Aging Characteristics of a Hybrid Sol-gel Pb(Zr, Ti)O 3 Precursor Solution

The {open_quotes}aging{close_quotes} characteristics of an acetic acid/methanol solvent-based lead zirconate titanate (PZT) precursor solution, prepared by the Inverted Mixing Order (IMO) process, have been studied for an extended period of time. The changes in film properties were characterized using optical microscopy, optical scattering, and ferroelectric testing. Films generated from the IMO process exhibit an increase in thickness as a function of solution age due to chemical {open_quotes}aging{close_quotes} (esterification) of the precursor solution. This increased thickness results in a decrease in the microstructural uniformity, which affects the electrical and optical properties. In order to understand and eventually control this phenomenon, we have quantified the {open_quotes}aging{close_quotes} of this solution using a variety of analytical methods, including {sup 1}H NMR spectroscopy, pH measurements, and Fourier transform infrared (FTIR) spectroscopy. It is of note that we have discovered a method which circumvents this {open_quotes}aging{close_quotes} problem by removal of the volatile material, forming an IMO powder which can be redissolved to produce high quality PZT thin films whenever desired. {copyright} {ital 1997 Materials Research Society.}

17O NMR investigation of oxidative degradation in polymers under γ-irradiation

Abstract The γ -irradiated-oxidation of pentacontane (C 50 H 102 ) and the polymer polyisoprene was investigated as a function of oxidation level using 17 O nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that by using 17 O labeled O 2 gas during the γ -irradiation process, details about the oxidative degradation mechanisms can be directly obtained from the analysis of the 17 O NMR spectra. Production of carboxylic acids is the primary oxygen-containing functionality during the oxidation of pentacontane, while ethers and alcohols are the dominant oxidation product observed for polyisoprene. The formation of ester species during the oxidation process is very minor for both materials, with water also being produced in significant amounts during the radiolytic oxidation of polyisoprene. The ability to focus on the oxidative component of the degradation process using 17 O NMR spectroscopy demonstrates the selectivity of this technique over more conventional approaches.

Dynamics of Uranyl Peroxide Nanocapsules.

Discrete aqueous metal oxide polyionic clusters that include aluminum polycations, transition-metal polyoxometalates, and the actinyl peroxide clusters have captivated the interest of scientists in the realm of both their fundamental and applied chemistries. Yet the counterions for these polycations or polyanions are often ignored, even though they are imperative for solubility, crystallization, purification, and even templating cluster formation. The actinyl peroxide clusters have counterions not only external, but internal to the hollow peroxide capsules. In this study, we reveal the dynamic behavior of these internal alkali counterions via solid-state and liquid NMR experiments. These studies on two select cluster geometries, those containing 24 and 28 uranyl polyhedra, respectively, show that the capsules-like clusters are not rigid entities. Rather, the internal alkalis both have mobility inside the capsules, as well as exchange with species in the media in which they are dissolved. The alkali mobilities are affected by both what is inside the clusters as well as the composition of the dissolving medium.

Properties and Structure of Cesium Phosphate Glasses

Abstract A series of Cs-phosphate glasses, xCs2O(1−x)P2O5, where 0⩽x⩽0.60, were prepared. The glass transition temperature (Tg) decreases with the initial addition of Cs2O to P2O5, from 637 K at x=0 to 472 K at x=0.16. There is little change in Tg with further additions of Cs2O up to x=0.60. The 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectra show that Cs2O additions systematically convert the cross-linked ultraphosphate network of ν-P2O5 to a chain-like metaphosphate structure as x approaches 0.50. The 133Cs MAS NMR spectra show a 90 ppm increase in isotropic chemical shift (δiso) with increasing Cs2O content, which indicates a decrease in the average electron density around the Cs+ ions, more covalent Cs–O bonding, and a shorter average Cs–O bond length. The physical properties and spectroscopic results are interpreted using a structural model that considers the effects of composition on the average coordination environment of Cs+ ions.

Molecular Dynamics Simulation of the Structure and Properties of Lithium Phosphate Glasses

A new forcefield model was developed for modeling phosphate materials that have many important applications in the electronics and biomedical industries. Molecular dynamics simulations of a series of lithium phosphate glass compositions were performed using the new forcefield model. A high concentration of three member rings (P{sub 3}O{sub 3}) was found in the glass of intermediate composition (0.2 Li{sub 2}O {center_dot} 0.8 P{sub 2}O{sub 5}) that corresponds to the minimum in the glass transition temperature curve for the compositional series.

Aqueous compatibility of group IIIA monomers and Nb-polyoxoanions

Heteropolyanions, [GaNb18O54](15-) and [AlNb18O54](15-), have been synthesized and characterized. These represent the first examples of introducing group III elements into the heteropolyniobate family; achieved via compatible alkaline aqueous chemistry of group IIIA monomers and Nb-polyoxoanions. The open structure of the polyniobate results in expanded coordination of the tetrahedral Ga in aqueous medium.