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31.
D. D. Palmer D. K. Rehbein J. F. Smith O. Buck 《Journal of Nondestructive Evaluation》1988,7(3-4):167-174
It has been shown that the acoustic response of imperfect interfaces may be described by a quasi-static spring model. In the present paper, experimental data on the geometry of the contacts between two diffusion-bonded blocks have been used to determine the spring stiffness of such interfaces which have been correlated with experimentally determined ultrasonic reflection coefficients. The correlation between the theoretical reflection coefficient and the spring stiffness was found to be in excellent agreement with experimentally-observed values, if the disbonds are of infinitesimally small thickness. For disbonds of finite thickness, the agreement is less satisfactory. Reasons for the discrepancy in the latter case are unknown at the present time. 相似文献
32.
The effect of positron range on the image-plane resolution of tomographic images is evaluated through calculations based on a model which employs beta-decay energy spectra and an empirical range formula. Predicted range distribution functions are compared with published measurements for three medically important positron emitters: (11 )C, (68)Ga, and (82)Rb. The effect of tomographic slice thickness on point-source annihilation distribution functions is also demonstrated. Line-spread functions are calculated using the model, for the above isotopes as well as for (18)F, (15)O, and (13)N. Image-plane resolution predictions are made for high-resolution positron cameras for various positron emitting isotopes with end-point energies up to 4 MeV. 相似文献
33.
34.
Novel imide-aryl ether benzoxazole copolymers were prepared and their morphology and mechanical properties investigated. A key feature of these copolymers is the incorporation of a benzoxazole moiety by the use of 2,2′-bis[4-(3-aminophenoxy)phenyl]-6,6′-bibenzoxazole or 2,2′-bis[4-(4-aminophenoxy)-phenyl]-6,6′-bibenzoxazole as co-diamines in polyimide syntheses. The preparation of these diamines involved the nucleophilic aromatic substitution of 2,2′-bis(4-fluorophenyl)-6,6′-bibenzoxazole with either 3- or 4-aminophenol in the presence of K2CO3. The diamines were co-reacted with various compositions of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) to synthesize the desired poly(amic acids). Films were cast and cured (350°C) to effect the imide formation, affording films with elongations between 40 and 110% and moduli in the 2000–2750 MPa range. The copolymers exhibited good dimensional (Tg in excess of 300°C) and thermal stability. Wide-angle X-ray diffraction measurements on the copolymers showed that the ordered morphology characteristic of PMDA/ODA polyimide was retained. Improvements in the auto-adhesion were observed, particularly in those copolymers which displayed a Tg. 相似文献
35.
Robert M. Boddey Mark B. Peoples Brian Palmer Peter J. Dart 《Nutrient Cycling in Agroecosystems》2000,57(3):235-270
Biological nitrogen fixation (BNF) associated with trees and shrubs plays a major role in the functioning of many ecosystems, from natural woodlands to plantations and agroforestry systems, but it is surprisingly difficult to quantify the amounts of N2 fixed. Some of the problems involved in measuring N2 fixation by woody perennials include: (a) diversity in occurrence, and large plant-to-plant variation in growth and nodulation status of N2-fixing species, especially in natural ecosystems; (b) long-term, perennial nature of growth and the seasonal or year-to-year changes in patterns of N assimilation; and (c) logistical limitations of working with mature trees which are generally impossible to harvest in their entirety. The methodology which holds most promise to quantify the contributions of N2 fixation to trees is the so-called `15N natural abundance' technique which exploits naturally occurring differences in 15N composition between plant-available N sources in the soil and that of atmospheric N2. In this review we discuss probable explanations for the origin of the small differences in 15N abundance found in different N pools in both natural and man-made ecosystems and utilise previously published information and unpublished data to examine the potential advantages and limitations inherent in the application of the technique to study N2 fixation by woody perennials. Calculation of the proportion of the plant N derived from atmospheric N2 (%Ndfa) using the natural abundance procedure requires that both the 15N natural abundance of the N derived from BNF and that derived from the soil by the target N2-fixing species be determined. It is then assumed that the 15N abundance of the N2-fixing species reflects the relative contributions of the N derived from these two sources. The 15N abundance of the N derived from BNF (B) can vary with micro-symbiont, plant species/provenance and growth stage, all of which create considerable difficulties for its precise evaluation. If the%Ndfa is large and the 15N abundance of the N acquired from other sources is not several 15N units higher or lower than B, then this can be a major source of error. Further difficulties can arise in determining the 15N abundance of the N derived from soil (and plant litter, etc.) by the target plant as it is usually impossible to predict which, if any, non-N2-fixing reference species will obtain N from the same N sources in the same proportions with the same temporal and spatial patterns as the N2-fixing perennial. The compromise solution is to evaluate the 15N abundance of a diverse range of neighbouring non-N2-fixing plants and to compare these values with that of the N2-fixing species and the estimate of B. Only then can it be determined whether the contribution of BNF to the target species can be quantified with any degree of confidence. This review of the literature suggests that while the natural abundance technique appears to provide quantitative measures of BNF in tree plantation and agroforestry systems, particular difficulties may arise which can often limit its application in natural ecosystems. 相似文献
36.
Walter J. Thomas Mohamed A. El-Sawy Edward W. Palmer 《Chemical Engineering Communications》1982,16(1):53-78
Two published theoretical models are examined and applied to experimental results for absorption and desorption. The system used was CO2/H2O and studies were made for liquid film flow down inclined planes. Experimental results give “Reduced” values of mass ransfer rates.
Interferometric studies give interfacial concentration, penetration and film depths, and take-up of carbon dioxide. In the case of desorption the interferograms are distorted by “deflections.”
All the experimental values for absorption and desorption differ from those calculated from theoretical models.
Desorption is not a mirror image of absorption, and it is approximately 75% of the transfer rate of absorption over a wide operating range.
A comparison is made of the behaviour of static pools and flowing liquid films. 相似文献
Interferometric studies give interfacial concentration, penetration and film depths, and take-up of carbon dioxide. In the case of desorption the interferograms are distorted by “deflections.”
All the experimental values for absorption and desorption differ from those calculated from theoretical models.
Desorption is not a mirror image of absorption, and it is approximately 75% of the transfer rate of absorption over a wide operating range.
A comparison is made of the behaviour of static pools and flowing liquid films. 相似文献
37.
Guillermo Palmer 《Polymer》2005,46(19):8169-8177
In this work, the interfacial tension for several polymer pairs, namely polypropylene/polystyrene (PP/PS), PP/polycarbonate (PC), PP/polyamide 6 (PA-6) and low density polyethylene (LDPE) and ethylene vinyl alcohol copolymer (EVOH) was evaluated as a function of temperature using the imbedded fiber retraction method (IFR). Two phenomenological models were used and compared to infer the interfacial tension from the evolution of the imbedded fiber back into a sphere: [Carriere CJ, Cohen A, Arends CB. J Rheol 1989;33:681-9. [1] and Tjahjadi M, Ottino JM, Stone HA. AIChE J 1994;40:385-94. [2]]. It was shown that Carriere et al.'s model overestimates the value of interfacial tension whereas Tjahjadi et al.'s model leads to results that corroborate the ones obtained using other experimental methods. A method to further increase the accuracy of Tjahjadi et al.'s model was proposed. Using this method the interfacial tension for the different polymer pairs studied was evaluated and shown to decrease linearly with increasing temperature. 相似文献
38.
The design of a three-channel solar radiometer used to determine total columnar atmospheric water-vapor amounts is presented. The main channel is located in the 0.94-μm water-vapor band, and two other channels are located in adjacent nonabsorption regions of the solar spectrum and are used to remove scattering effects from the main channel. Water-vapor transmittance is determined by means of a modified Langley approach, and these transmittances are converted to columnar water vapor by means of a band model developed at the University of Arizona. Several cases are presented in which columnar water-vapor amounts are determined through the use of the instrument and method described here. These results are compared with sounding-balloon results. Tests of the method indicate that columnar water vapor may be retrieved with an uncertainty of less than 10%. 相似文献
39.
Examining student satisfaction with wholly online learning 总被引:1,自引:0,他引:1
Learner satisfaction has been shown to be positively correlated with quality of learning outcomes. An understanding of the factors that influence student satisfaction with online learning in a particular context can be used as an input to the appropriate design of learning environments, and for the provision of targeted support to students, with an aim to positively influence the student online learning experience. Following the mandatory inclusion of at least one wholly online unit of study in all undergraduate programs at Deakin University, a large 'experiences of learning online' (ELO) survey was undertaken to gauge students' perceptions of studying in the wholly online mode. A multivariate linear regression of all the questionnaire items was performed against an overall satisfaction item. Five items were found to significantly contribute to a model that explained approximately 70% of reported student satisfaction. Factors that were found to positively influence student satisfaction with studying a wholly online unit primarily related to how confident they felt about their ability to communicate and learn online, having a clear understanding of what was required to succeed in the unit and how well they thought they were performing in the unit. Other results are also reported. 相似文献
40.
Photolithography to produce TiO2 patterns from amorphous films of (5-C5H5)2Ti(N3)2 has been demonstrated. The efficiency of the reaction has been measured yielding a quantum yield of 0.025. The mechanism of the photoreactions of (5-C5H5)2Ti(N3)2 has been studied using Fourier transform-infrared spectroscopy in both a low-temperature 1,2-epoxyethylbenzene glass and as surface films. In each case the primary photochemical process was found to be loss of a single azido group. The result of subsequent photolysis was found to be dependent upon medium and temperature. In the low-temperature glass no further photochemistry was observed. The exhaustive photolysis of films at 20 K, or room temperature, under a vacuum or in air led to loss of all ligands and the formation of TiO2. 相似文献