B. T. G. Tan

The chemical nature of the nitrogens in polypyrrole and polyaniline: A comparative study by x‐ray photoelectron spectroscopy

The chemical nature of the nitrogens corresponding to various intrinsic redox states of polypyrrole (PPY) and polyaniline (PAN) has been critically compared using x‐ray photoelectron spectroscopy (XPS) as a primary tool. Proton modifications of nitrogens in PPY give rise to a number of intrinsic redox states analogous to those observed in PAN. The behavior of the corresponding oxidation states in both polymers towards oxidation/reduction, deprotonation/reprotonation, or charge–transfer interactions with electron acceptors are grossly similar. However, the nitrogens of the two oxidized polymer complexes do differ in their thermal degradation behavior which suggests that the oxidized pyrrolylium nitrogens are more susceptible to deprotonation than their oxidized counterparts.

Kalman-filtering speech enhancement method based on a voiced-unvoiced speech model

In this work, we are concerned with optimal estimation of clean speech from its noisy version based on a speech model we propose. We first propose a (single) speech model which satisfactorily describes voiced and unvoiced speech and silence (i.e., pauses between speech utterances), and also allows for exploitation of the long term characteristics of noise. We then reformulate the model equations so as to facilitate subsequent application of the well-established Kalman filter for computing the optimal estimate of the clean speech in the minimum-mean-square-error sense. Since the standard algorithm for Kalman-filtering involves multiplications of very large matrices and thus demands high computational cost, we devise a mathematically equivalent algorithm which is computationally much more efficient, by exploiting the sparsity of the matrices concerned. Next, we present the methods we use for estimating the model parameters and give a complete description of the enhancement process. Performance assessment based on spectrogram plots, objective measures and informal subjective listening tests all indicate that our method gives consistently good results. As far as signal-to-noise ratio is concerned, the improvements over existing methods can be as high as 4 dB.

Amendment of cavity perturbation method for permittivity measurement of extremely low-loss dielectrics

The quality factor of a resonant cavity may increase after introducing an extremely low-loss dielectric, so the conventional cavity perturbation method, widely used in dielectric permittivity measurement, may be invalid for extremely low-loss dielectric samples. After a brief review of the conventional cavity perturbation theory, this paper discusses the change of quality factor of a resonant cavity due to the introduction of a dielectric sample. A new concept, expected quality factor Q/sub 0/ is introduced in this paper to denote the quality factor of a resonant cavity loaded with a strictly no-loss sample, and a calibration procedure is proposed to find the frequency dependence of Q/sub 0/. The conventional resonant perturbation formulas are then amended by substituting the quality factor before the perturbation with the expected quality factor Q/sub 0/ corresponding to the frequency after the perturbation. Experiments show that the accuracy of resonant perturbation method has been greatly increased after the amendment, especially for extremely low-loss dielectric samples.

Preparation of polyanilines doped in mixed protonic acids: Their characterization by X-ray photoelectron spectroscopy and thermogravimetry

Abstract A comparative study of polyaniline chemically prepared and simultaneously doped in protonic acid mixtures is presented. As the resultant polyaniline salts (PANI salts) are insoluble, solid-state techniques such as X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), derivative thermogravimetry (DTG) and elemental analysis have been applied to study the dopant environment as well as the electronic and chemical structure of the polymers. Both XPS and TG/DTG indicate that in the presence of competing acids, the stronger acid component is preferentially incorporated over the others. No advantage can be gained by the use of mixed-acid systems. XPS confirms that ring substitution by the dopant and the degree of protonation can be accurately measured based on the percentage of charged nitrogen in the N 1s envelope. Previous methods based on the total halogen content can lead to serious errors. TG results suggest an upper application temperature limit of about 150°C, at which point elimination of dopant occurs with loss of conductivity.

Minimum-noise-variance beamformer with an electromagnetic vector sensor

We study the performance of the minimum-noise-variance beamformer employing a single electromagnetic (EM) vector sensor that is capable of measuring the complete electric and magnetic fields induced by EM signals at one point. Two types of signals are considered: one carries a single message, and the other carries two independent messages simultaneously. The state of polarization of the interference under consideration ranges from completely polarized to unpolarized. We first obtain explicit expressions for the signal to interference-plus-noise ratio (SINR) in terms of the parameters of the signal, interference, and noise. Then, we discuss some physical implications associated with the SINR expressions. These expressions provide a basis for effective interference suppression as well as generation of dual-message signals of which the two message signals have minimum interference effect on one another. We also analyze the characteristics of the main-lobe and side-lobe of the beampattern of an EM vector sensor and compare them with other types of sensor arrays.

Efficient method for estimating directions-of-arrival of partially polarized signals with electromagnetic vector sensors

We have developed a high-resolution ESPRIT-based method for estimating the directions-of-arrival of partially polarized signals with electromagnetic vector sensors, each of which provides measurements of the complete electric and magnetic fields induced by electromagnetic signals. The method is computationally efficient since unlike many high-resolution methods, it does not involve searching across a multidimensional array manifold. In addition, the method has two variants, of which one is applicable to scenarios where a priori information about the array system, such as the sensor positions, is unavailable.

XPS STUDIES OF PROTON MODIFICATION AND SOME ANION EXCHANGE PROCESSES IN POLYPYRROLE

The N(1s) X-ray photoelectron spectroscopy (XPS) core-level spectrum reveals that proton modifications of the nitrogens in polypyrrole (PPY) can give rise to a number of intrinsic redox states, as in the case of the nitrogen in polyaniline. In PPY, these states can range from that consisting of about 25% oxidized neutral imine-like (=N−) structure observed in this study, to that consisting entirely of reduced amine-like (−NH−) structure or PPY0. The former is obtained by deprotonation of the oxidized and positively charged pyrrolylium nitrogens (−N+H− structure) while the latter is obtained via the reduction of the deprotonated polymer by phenylhydrazine. Protonation of the deprotonated units by acids or oxidation of PPY0 by electron acceptors returns the polymer to its oxidized and positively charged state. The oxidized PPY/halide and PPY/p-chlonil complexes readily undergo anion exchange in protonic acid media or in organic solvents containing an electron acceptor of higher electron affinity than the original oxidizing anion. The anion exchange processes offer an alternative chemical route for the preparation of some conductive and anion specific PPY complexes.

X-RAY PHOTOELECTRON SPECTROSCOPY STUDIES OF DEPROTONATED POLYPYRROLE AND ITS COMPLEXES

Abstract Oxidized polypyrrole (PPY) was subjected to deprotonation by 0.5 M NaOH. The interactions of the deprotonated pyrrolylium nitrogens with various protonic acids and organic electron acceptors were studied by X-ray photoelectron spectroscopy (XPS) and were found to be similar to those observed for the imine nitrogens in the emeraldine oxidation state of polyaniline. However, the cyclic acid/base treatments of the deprotonated PPY were not completely reversible. The interactions with the halobenzoquinones, such as o -chloranil, p -chloranil and o -bromanil, resulted in the formation of positively charged nitrogens and the halogen and benzoquinone anions. In the case of interaction with TCNE, cyano anions were involved while interaction with DDQ involved all three types of anions. The charge transfer (CT) behaviour of the deprotonated pyrrolylium nitrogens towards acid protonation, and ‘doping’ by the organic acceptors, together with the charge neutrality considerations, suggested the presence of localized units positive charge on the pyrrolylium nitrogens.

Novel heteroarylene polyazomethines : their syntheses and characterizations

Novel thiophene or furan-containing polyazomethines were synthesized by chemical oxidative polymerization with ferric(III) chloride. The structures of the undoped and doped polymers were evaluated by FTi.r., elemental analysis, u.v.—vis spectrometry, X-ray photoelectron spectroscopy (XPS) and their thermal stability determined using thermogravimetric analysis (t.g.a.). The conductivities of the undoped polymers were less than 10−8 S cm−1, while those of the doped ones were in the range of 10−7 to 10−6 S cm−1. The low conductivity values were attributed to the low degree of conjugation in the polymers caused by non-planarity of the polymer chains. The thiophene-containing polymers underwent sulfur extrusion during polymerization, resulting in an interruption of conjugation in the polymer backbone with consequent diminished electrical conductivity.

Charge trapping on different cuts of a single‐crystalline α‐SiO2

A scanning electron microscope is employed for the investigation of charging on different cuts of an α‐SiO2. A method for the determination of trapped charges is proposed. Charging on different cuts is observed to decrease in the order of z cut, 30° cut, 45° cut, and 60° cut of the α‐SiO2. This phenomenon is related to permittivity, defect density, and stress of the samples. Details of the experiments and the method of charge determination are given.