稀土改性剂对玻璃纤维-SiO2/聚四氟乙烯复合材料性能的影响

采用不同的稀土改性剂（La、Ce、Pr、Nd和Sm）对SiO₂和玻璃纤维（GF）进行改性处理,SiO₂含量为55wt%,GF含量为2wt%。填料改性处理后与聚四氟乙烯（PTFE）分散液在高速分散机中混合,然后通过热压烧结法制得GF-SiO₂/PTFE复合材料。考察了不同的稀土改性剂对GF-SiO₂/PTFE复合材料吸水性、介电性能、热膨胀系数和力学性能的影响。采用FTIR手段对稀土La改性剂,未改性的填料和改性后的填料结构进行了测试。并用SEM和EDS对复合材料的断口形貌及表面处理前后填料的形貌进行观察和能谱分析。结果表明：由于稀土La的电子层结构比其他轻稀土更稳定,对阴离子的吸引作用比其他轻稀土强,稀土La改性剂比其他的稀土改性剂能更好地促进填料GF与PTFE界面粘合,改善GF-SiO₂/PTFE复合材料性能。GF-SiO₂/PTFE复合材料的介电性能受到La含量的影响,当La含量为0.3wt%时介电性能最佳。     

Torsional rod wave scattering from tapering between regions of different cross-sections

Previously (Engan 1998) we have studied the scattering of torsional waves in circular rods with a free surface where the rod diameter is subject to abrupt diameter changes. Here, we discuss the scattering of torsional waves from tapered regions and, in particular, transmission of the fundamental torsional mode. The calculations are based on a mode expansion technique in which propagating as well as cut-off modes are taken into account. The tapered region is approximated by numerous steps separated by uniform rod parts arranged so that they closely render the taper shape. Transfer matrices for all steps and uniform parts are combined to yield the desired transmission coefficient through the taper region. Numerical results of the frequency variation of fundamental mode transmission are presented, depending on geometrical parameters, such as taper length and the diameters of the two uniform adjacent rod parts. The dependence of the transmission on taper shape is demonstrated by calculating the response of various shapes.     

In vivo 1H MRS of normal breast and breast tumors using a dedicated double breast coil

OBJECTIVE: To characterize sleep patterns of patients with juvenile rheumatoid arthritis (JRA). METHODS: Sixteen patients with JRA aged 12+/-4 years and 9 controls aged 11+/-3 years underwent a comprehensive evaluation by self-report questionnaire and formal all night polysomnographic recordings. Multiple sleep latency test was performed in 7 patients. RESULTS: Patients had 90% more arousals and awakenings (p<0.01) and the median length of occurrences of uninterrupted sleep in stages 2 and 3 and rapid eye movement (REM) sleep was 60% shorter than in controls (p<0.01). The overall amount of sleep stage shift from deeper to lighter sleep was 23.5+/-10.8 events in patients compared to 14.9+/-4.0 in controls (p<0.05). In 15 of 16 patients 15% of non-REM sleep consisted of alpha-delta (alpha-rating) sleep, compared with less than 1% in controls (p<0.001). Multiple sleep latency test for patients was 10.3+/-2.6 min. There were no differences between JRA and controls in self-reported questions. However, patients reported longer afternoon naps, 1.8+/-1.3 h compared to 0.3+/-0.8 h in controls (p<0.05). CONCLUSION: Objective polysomnographic evidence of abnormal sleep has been confirmed in patients with JRA. Sleep disturbance was associated with daytime sleepiness as evidenced by abnormal multiple sleep latency test and longer afternoon naptime.     

Subset selection in noise based on diversity measure minimization

We develop robust methods for subset selection based on the minimization of diversity measures. A Bayesian framework is used to account for noise in the data and a maximum a posteriori (MAP) estimation procedure leads to an iterative procedure which is a regularized version of the focal underdetermined system solver (FOCUSS) algorithm. The convergence of the regularized FOCUSS algorithm is established and it is shown that the stable fixed points of the algorithm are sparse. We investigate three different criteria for choosing the regularization parameter: quality of fit; sparsity criterion; L-curve. The L-curve method, as applied to the problem of subset selection, is found not to be robust, and we propose a novel modified L-curve procedure that solves this problem. Each of the regularized FOCUSS algorithms is evaluated through simulation of a detection problem, and the results are compared with those obtained using a sequential forward selection algorithm termed orthogonal matching pursuit (OMP). In each case, the regularized FOCUSS algorithm is shown to be superior to the OMP in noisy environments.     

ORGANOLEPTIC THRESHOLD VALUES OF SOME ORGANIC ACIDS IN BEER

In a Norwegian all-malt pilsener beer organoleptic threshold values were determined for the following acids: acetic, propionic, n-butyric, isobutyric, n-valeric, isovaleric, caproic, caprylic, pelargonic, capric, undecanoic, lauric, citric, lactic, tartaric, malic, malonic and succinic.     

Propagation and optical interaction of guided acoustic waves intwo-mode optical fibers

Acoustic modes propagating in cylindrical solid rods are considered. A review of the properties of several mode groups is given, and computed results are shown for a wide range of material properties. The lower mode of each group is relevant for an all-fiber-optic frequency shifter recently demonstrated. For these modes, dispersion relations, mode patterns, power relations, and acoustooptic coupling strength are calculated. Some experimental results are compared with the calculations, and alterations to improve the frequency shifter are proposed     

A CHEMOMETRIC STUDY OF SOME BEER FLAVOUR COMPONENTS

Laboratory and full scale fermentation experiments have been carried out in Norwegian breweries varying the yeast strains and temperature conditions. Gas chromatographic data were submitted to factor analyses. One factor consisting of phenylethanol and the acetate esters was influenced by temperature. Another factor consisting of fatty acids or their derivatives is dependant on scale. When the yeast strain was changed the influence of the yeast tended to dominate the influence of the brewery.     

Optimized signal expansions for sparse representation

Traditional signal decompositions such as transforms, filterbanks, and wavelets generate signal expansions using the analysis-synthesis setting: the expansion coefficients are found by taking the inner product of the signal with the corresponding analysis vector. In this paper, we try to free ourselves from the analysis-synthesis paradigm by concentrating on the synthesis or reconstruction part of the signal expansion. Ignoring the analysis issue completely, we construct sets of synthesis vectors, which are denoted waveform dictionaries, for efficient signal representation. Within this framework, we present an algorithm for designing waveform dictionaries that allow sparse representations: the objective is to approximate a training signal using a small number of dictionary vectors. Our algorithm optimizes the dictionary vectors with respect to the average nonlinear approximation error, i.e., the error resulting when keeping a fixed number n of expansion coefficients but not necessarily the first n coefficients. Using signals from a Gaussian, autoregressive process with correlation factor 0.95, it is demonstrated that for established signal expansions like the Karhunen-Loeve transform, the lapped orthogonal transform, and the biorthogonal 7/9 wavelet, it is possible to improve the approximation capabilities by up to 30% by fine tuning of the expansion vectors     

WORT COMPOSITION AND BEER FLAVOUR. I. THE INFLUENCE OF SOME AMINO ACIDS ON THE FORMATION OF HIGHER ALIPHATIC ALCOHOLS AND ESTERS

Wort, to which was added various amounts of glucose solution containing either (NH₄)₂SO₄ or one of the amino acids alanine, leucine, isoleucine, valine or proline, was fermented, and in the resulting beers the concentrations of ethyl acetate, isoamyl acetate, n-propanol, isobutanol, and amyl alcohols were determined by gas chromatography. The nitrogen source had some effect on the formation of higher alcohols, in addition to the effect due to conversion of some of the amino acids to the corresponding alcohols via the Ehrlich pathway. Ethyl acetate formation was not significantly affected by the nitrogen source, and iso-amyl acetate formation increased when amyl alcohol formation increased.