Keith R. Carduner

Solid state NMR: Applications in high performance ceramics

Solid state NMR is a new analytical tool that has proven to be both powerful and versatile in the characterization of ceramic systems. In this paper, we review many of the recent applications of NMR in ceramics, with an emphasis on applied research. Since solid-state NMR is a relatively new approach, a brief introduction into the technique is provided. Examples are given to illustrate how NMR can be used to (1) identify both crystalline and amorphous phases, (2) quantitate both crystalline and amorphous phases, (3) determine the structure of both crystalline and amorphous phases, (4) probe local structural order and (5) study the chemistry of ceramic systems. These capabilities are demonstrated in a series of brief applications including (1) a study of structure in a new ceramic material (LaSi3N5), (2) accurate phase composition analyses on commercial Si3N4 powders, (3) determination of structure and curing mechanisms in amorphous SiC fibres, (4) investigation of dispersion aid mechanisms, (5) determination of structure in several SiAION ceramics and (6) identification and quantitation of phases typically associated with the grain boundaries of Y2O3 sintered Si3N4.

The influence of aging on the effectiveness of an organophosphite in suppressing transesterification in polymer blends

SummaryOrganophophites are being used with varying degree of effectiveness to suppress transesterification in polyester blends. Using solid state31p NMR it was discovered that for bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite a conversion of the phosphite group to a phosphonate moiety, probably via hydrolysis, is a prerequisite for an effective inhibition of transesterification. This conversion occurs readily during melt compounding if the polymers are not completely dry. However, if rigorous drying is employed and phosphite conversion does not occur, then transesterification is not arrested. It was also found that over a long period of time the conversion of the phosphite to a phosphonate takes place at room temperature as well. Thus, after aging for about a year, the originally ineffective compound, has become a very effective inhibitor of transesterification in blends containing polyethylene terephthalate, polybutylene terephthalate, polycarbonate, and polyarylate.

Application of tunable diode laser spectroscopy to the real-time analysis of engine oil economy

Tunable Diode Laser Spectroscopy (TDLAS) of oil derived SO2 in automotive exhaust demonstrated acceptable repeatability in determination of oil consumption at steady state engine operating conditions. The response time of the instrument was approximately 30 sec, the time related to the flow rate of the sampling system. Instrument sensitivity is sufficient to measure SO2 levels of 0.1 to 1 ppm required to the oil consumption determination. Typical exhaust gas species were investigated for their interference effects and were observed to have less than a 10% interference on the SO2 signal for mixing ratios with SO2 typical of automotive exhaust. Water, on the other hand, did show a significant, but compensatible interference. Carbon deposition under rich engine conditions was observed and is expected to be a problem for any analytical device and is best solved by using a heated sampling line.

Application of total suppression of spinning sidebands to acquisition of aluminum-27 magic-angle-spun NMR spectra

Abstract A novel application of the total suppression of spinning sidebands (TOSS), a pulse irradiation sequence previously used for removal of sidebands from magic-angle-spinning (MAS) NMR spectra of spin - 1 2 nuclei, can also be used to remove sidebands from MAS NMR spectra of 27A1, a spin - 5 2 quadrupolar nucleus. The absence of sidebands, which present systematic distortions of the spectrum, can potentially lead to more quantitatively representative spectra, especially for species at low concentration. Examples demonstrate substantial improvements in 27A1 MAS NMR spectra of the central transition of the quadrupole-perturbed 27A1 Zeeman resonance line for cases involving different central transition linewidths.

Spectroscopic method for the determination of the compositions of ternary polymer blends

Abstract A spectroscopic method for the determination of polybutyleneterphthalate, polycarbonate and poly(methyl methacrylate)-coated polybutadiene-polystyrene rubber content in polymeric blends of these components is presented. The technique uses i.r. spectroscopy with quantitative calibration by 13 C magic angle spinning nuclear magnetic resonance (MASNMR) data. Several advantages of an i.r. spectral integration treatment are discussed. The primary advantage of the method is the elimination of arbitrary baseline estimates. The technique illustrates the use of MASNMR to acquire quantitative data. The compositions of three samples of an engineering plastic blend were determined using this method. No changes in composition were observed for samples that had been extruded two and three times.

POLYTYPIC TRANSFORMATION AND SINTERING BEHAVIOR IN SILICON CARBIDE MATERIALS

The influence of starting powder morphology and composition, nature of the additives and the sintering process on microstructures of sintered bodies of silicon carbide will be discussed. The phase distribution of the starting powder is determined using pulsed, solid state 29Si magic angle spinning nuclear magnetic resonance (NMR) and x-ray diffraction (XRD); and phase transformation and microstructural development are studied using transmission electron microscopy (TEM). Pure 6H lamellae in β SiC powder have distinctive effect on lamellar formation when alumina is introduced as an additive. Growth of the lamellae is observed on relatively pure 6H grain with a thin alumina layer at the tip to enhance liquid phase grain growth. Polytypic transformation to 4H, 15R and others in the sintered bodies is dependent on the diffusion mechanism of the additive elements into α lamellae during the sintering process. The diffusion of the elements occurs simultaneously and most effectively with partial dislocation propagation to form stacking faults during β-α phase transformation in SiC materials. In order to optimize the microstructure of SiC for various applications, detailed analysis of the β-SiC starting powder is essential. Polytype distribution by TEM, XRD and NMR is a good indicator of impurity elements distribution and final microstructure.

Protonated phosphorus suppression and its application to chemistry in polymer blends

In a recent experiment, we emp loyed 3’P mag ic-angle spinning (MAS) nuclear magnetic resonance to unravel the phosphorus-based chemistry associated with an organophosphi te antioxidant agent in polymer blends during high-temperature (280-300°C) extrusion (I, 2). To produce these blends, a phosphite stabilizer, bis( 2,4-di-t-butylphenyl) pentaerythritol d iphosphate (BTBP), is first compounded, as a concentrate, with one of the components of the blend, at a relatively low temperature. The other components of the blend are added in another extrusion step to bring the concentration of the phosphite to the desired final level. The objective in the 31P MAS NMR study was twofold: ( 1) to determine the concentration of the stabilizer both in the concentrate (about 10% wt proposed) and in the final b lend (0.5% wt) and (2) to determine what, if any, chemical reactions are occurring with the BTBP stabilizer during the compounding processes. The sensitivity ofthe isotropic chemical shift of 3’P MAS NMR to local chemical environment makes this technique ideal for following solid-state chemistry (3). MAS NMR was successfully emp loyed to follow the concentration of BTBP throughout processing ( 1) . In addition, interesting phosphorus chemistry was observed. This chemistry was found to be directly related to the effectiveness of the stabilizer in preventing ester exchange reactions during the compounding process (2). In particular, BTBP, whose MAS NMR chemical shift is 115.5 ppm, was observed to be converted either completely or in part to a family of phosphorus species with resonances near 7 ppm. On ly blend batches in which this chemistry occurs were observed to be protected against ester exchange during compounding. The reader is referred to Refs. ( 1, 2) for further details on ester exchange, its measurement, and its relationship to the NMR results. It was desired to identify the phosphorus-containing species resonating near 7 ppm that were the reaction products of BTBP. This was not possible, however, solely on the basis of the observed MAS isotopic chemical shift since the resulting species could have been either orthophosphates or dialkyl phosphonates, considering only the chemical shift. These two classes of phosphorus compounds have overlapping chemical-shift ranges. The first class of species results from oxidation of the phosphite stabilizer while the latter is produced by hydrolysis (4). Phosphorus in phosphates and in phosphonates differs in that only the latter has a direct P-H bond. This suggested that one way to rapidly make an identification was to use the idea embodied in protonated