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441.
Howell G.M. Edwards Dennis W. Farwell Jose-Maria Alia Mauro V. de Ameida 《Analytica chimica acta》2005,532(2):177-186
The Raman spectrum of guarana, an important product of the Amazonian rain forest, has been investigated; the therapeutic properties of guarana and it's extracts have been realised for some time and have been attributed to guaranine, which could be a complex of caffeine and tannins or to a new xanthine natural product. The purpose of this study is two-fold: firstly, to provide molecular structural information about guarana seeds and their extracts and, secondly, to test the viability of using the technique as a method of verification of genuine guarana extracts from synthetic composites. Raman spectroscopy shows that the composition of the guarana is very similar for the whole seed and for the outer and inner portions of the dissected seed, which are closely similar also to the ground commercial powders produced in the Amazon for the distributors. The results indicate that Fourier-transform Raman spectroscopy could be used for the monitoring of quality control of guarana products in the phytopharmaceutical industry. 相似文献
442.
James A.S. Howell Denis T. Dixon John C. Kola 《Journal of organometallic chemistry》1984,266(1):69-82
(η4-enone)Fe(CO)2L complexes (enone = benzylideneacetone, cinnamaldehyde; L = PPh3-xMex (x = 0–2), P(OPh)3) exist as interconverting isomeric mixtures in solution in which L occupies either the axial or basal position of a square pyramidal structure. The ratio of isomers is dependent on the steric properties of both L and the enone. 相似文献
443.
13C-NMR spectroscopy has been utilized to characterize Saran polymers containing polyene segments generated by incorporating phenylacetylene (PA) units into either poly(vinylidene chloride) (PVDC) or a typical Saran copolymer of 91% vinylidene chloride (VDC) and 9% methyl acrylate (MA). The incorporation of PA could not be defined in the usual statistical way because the presence of the PA double bond in the polymer backbone appeared to cause the dehydrohalogenation of units next to it. Thus, sequences of PA next to VDC were not observed. Rather, sequences of olefinic units next to VDC units were present at a level equal to the level of PA incorporation. The level of unsaturation in the PA copolymers is approximately four times the level of PA incorporation. These observations are consistent with the random incorporation into the copolymer of PA which then initiates the dehydrohalogenation in adjacent VDC units. This dehydrohalogenation reaction appears to propagate from one unit to the next along the backbone of the polymer such that polyene segments containing the PA unit are formed. 相似文献
444.
In stuttered repetitions of a syllable, the vowel that occurs often sounds like schwa even when schwa is not intended. In this article, acoustic analyses are reported which show that the spectral properties of stuttered vowels are similar to the following fluent vowel, so it would appear that the stutterers are articulating the vowel appropriately. Though spectral properties of the stuttered vowels are normal, others are unusual: The stuttered vowels are low in amplitude and short in duration. In two experiments, the effects of amplitude and duration on perception of these vowels are examined. It is shown that, if the amplitude of stuttered vowels is made normal and their duration is lengthened, they sound more like the intended vowels. These experiments lead to the conclusion that low amplitude and short duration are the factors that cause stuttered vowels to sound like schwa. This differs from the view of certain clinicians and theorists who contend that stutterers actually articulate /schwa/'s when these are heard in stuttered speech. Implications for stuttering therapy are considered. 相似文献
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448.
D P Egolf B T Haley H C Howell S Legowski V D Larson 《The Journal of the Acoustical Society of America》1989,85(1):454-467
Suppressing unstable acoustic feedback in hearing aids will first require knowledge of the open-loop transfer functions of such systems. Reported herein is a mathematical technique for simulating the open-loop transfer function of an in situ eyeglass-type hearing aid. In particular, a computer program was developed that characterized the hearing aid as a serial connection of two-port blocks, each representing one individual component of a hearing aid. Included, for example, were two-port blocks representing the microphone, amplifier, receiver, sound tubes leading to the eardrum (including the ear canal itself), earmold vent, and external pathway from the vent outlet back to the microphone. The computer program was validated by replicating laboratory data derived from an experiment involving a nonstandard manikin fitted with a nonstandard artificial ear. Next, the open-loop transfer function of an eyeglass-type hearing aid in situ on the manikin was simulated via the computer program. Unfortunately, those computer-generated data were not replicated in the laboratory due to the difficulty encountered in actually measuring the open-loop transfer function. Nevertheless, investigators were able to utilize those data to predict, within +/- 25 Hz, the "squeal" frequency of unstable acoustic feedback. 相似文献
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In this work we have applied Fourier-transform Raman spectroscopy to the analysis of several archival samples of brazilwoods from different geographical origins and of different ages. Samples of Caesalpinia echinata Lam. (from Brazil, South America), Caesalpinia sappan L. (East Indies, Asia), Haematoxylum brasiletto Karsten (Central America) and Haematoxylum campechianum L. (North America) were analysed in order to isolate key Raman biomarker bands which could provide the basis for an identification protocol. Previously recorded Raman spectra of brazilin and brazilein pigments extracted from genuine brazilwood of Brazilian origin provided a foundation for the nondestructive spectral discrimination between a sample of false 'brazilwood', which consisted of an inferior wood substratum stained with genuine pigment, and the true specimens. The provision of well-documented specimens of determinable age from the archival collection facilitated the evaluation of the effects of temporal degradation on the observed spectra, which could be used to further test the experimental protocols for nondestructive verification of samples in the archive with questionable assignment or provenance. 相似文献