全文获取类型
收费全文 | 435022篇 |
免费 | 3671篇 |
国内免费 | 1074篇 |
专业分类
化学 | 207862篇 |
晶体学 | 5693篇 |
力学 | 17545篇 |
综合类 | 15篇 |
数学 | 43130篇 |
物理学 | 118135篇 |
无线电 | 47387篇 |
出版年
2020年 | 3253篇 |
2019年 | 3533篇 |
2018年 | 4621篇 |
2017年 | 4590篇 |
2016年 | 6961篇 |
2015年 | 4142篇 |
2014年 | 6788篇 |
2013年 | 16863篇 |
2012年 | 12599篇 |
2011年 | 15615篇 |
2010年 | 11300篇 |
2009年 | 11678篇 |
2008年 | 15593篇 |
2007年 | 16225篇 |
2006年 | 15198篇 |
2005年 | 14021篇 |
2004年 | 12811篇 |
2003年 | 11558篇 |
2002年 | 11505篇 |
2001年 | 12444篇 |
2000年 | 10063篇 |
1999年 | 8046篇 |
1998年 | 7230篇 |
1997年 | 7150篇 |
1996年 | 6601篇 |
1995年 | 6183篇 |
1994年 | 6235篇 |
1993年 | 5795篇 |
1992年 | 6387篇 |
1991年 | 6526篇 |
1990年 | 6216篇 |
1989年 | 6023篇 |
1988年 | 5935篇 |
1987年 | 5328篇 |
1986年 | 5081篇 |
1985年 | 6521篇 |
1984年 | 6876篇 |
1983年 | 5793篇 |
1982年 | 6021篇 |
1981年 | 5857篇 |
1980年 | 5749篇 |
1979年 | 5761篇 |
1978年 | 6185篇 |
1977年 | 6013篇 |
1976年 | 6096篇 |
1975年 | 5669篇 |
1974年 | 5791篇 |
1973年 | 5917篇 |
1972年 | 4152篇 |
1971年 | 3556篇 |
排序方式: 共有10000条查询结果,搜索用时 8 毫秒
181.
Silver nitrate-acetonitrile and π iodine-benzene complexes in thermotropic liquid crystals have been studied by 1H, 2H, and 13C NMR spectroscopy and by optical microscopy. Evidence for at least two silver complexes in each liquid crystal is presented. 相似文献
182.
W. Fallmann P. Hudek I. Kostic A. Neubauer D. Pum I. Rangelow K. Riedling F. Rüdenauer U. B. Sleytr G. Stangl 《e & i Elektrotechnik und Informationstechnik》1998,115(7-8):349-353
This contribution describes the preparation, based upon a chemically amplified novolak resist (CAR), electron beam lithography, and ECR plasma etching, of structures with a high aspect ratio (10∶1) and lateral dimensions in the sub-micrometer range (150nm–300nm) which may serve as collector surfaces for sub-μm dust particles in a space experiment. 相似文献
183.
Bouferrache B. Krim G. Mejdoub N. Quanjer P.H. Freville M. Libert J.P. 《IEEE transactions on bio-medical engineering》1998,45(11):1305-1312
Dynamic modeling of lung C18O diffusion is used to measure the C18O transfer factor (TLco) of 14 newborns aged 1-4 mo. The model equation is based on the alveolar fractions of C18O and on changing alveolar ventilation induced by the rebreathing conditions. The model does not involve the volume of the rebreathing bag which is usually needed when applying rebreathing technique add which is a source of error. The equation is discretized and solved for recorded data obtained with equipment adapted to use in newborns. A least-square parameter calculation technique is applied to estimate TLco. Results show a strong relationship between this index and the biometrical ones and confirm those found in the literature featuring that the measurement duration can be considerably shortened 相似文献
184.
Pallares J. Marsal L.F. Correig X. Calderer J. Alcubilla R. 《Electron Devices, IEEE Transactions on》1998,45(1):54-61
This paper addresses the problem of the space charge region Shockley-Read-Hall (SRH) recombination currents in heterojunctions with one noncrystalline side. A formulation which generalizes previous works is discussed. The approach is based on the drift-diffusion model with a thermionic-field emission boundary condition. The main physical parameters which determine the relative contribution of each zone of the space charge region (SCR) to the total recombination current are identified. The general analysis is applied for the first time to amorphous/crystalline heterojunctions and design criteria are established to minimize the total recombination current 相似文献
185.
When the statistics of the noise are non-Gaussian, analytic expressions for the probability of false alarms in detection systems are rarely available. Monte Carlo estimation techniques are therefore typically necessary. The author presents an importance sampling biasing distribution which renders exponential savings over standard Monte Carlo simulations. Two important features of this biasing strategy are that no importance sampling parameters need to be determined and no additional computations are required for implementation 相似文献
186.
The loss performance of tagged and normal ATM cells at a first-in-first-out (FIFO) buffer is studied. The authors show that if a partial buffer sharing mechanism is adopted the loss probability of normal cells can be firmly guaranteed, regardless of the traffic intensity of tagged cells 相似文献
187.
This paper describes a fully integrated digital-spread spectrum transceiver chip fabricated through MOSIS in 1.2 μm CMOS. It includes a baseband spread spectrum transmitter and a coherent intermediate frequency (IF) receiver consisting of a Costas loop, an acquisition loop for the pseudo-noise (PN) sequence, and a clock recovery loop with a 406.4 MHz onchip numerically controlled oscillator (NCO). The transceiver is capable of operating at a maximum IF sampling rate of 50.8 MS/s and a maximum chip rate of 12.7 R Mchips/s (Mcps) with selectable data rates of 100, 200, 400, and 800 kbps. At the maximum operating speed of 50.8 R MS/s, it dissipates 1.1 W. In an additive white Gaussian noise channel the IF receiver achieves a receiver output SNR within 1 dB of theory and can acquire code with a wide range of input SNR from -17 dB to over 30 dB. The transceiver chip has been interfaced to an RF up/down converter to demonstrate a wireless voice/data/video link operating in the 902-928 MHz band 相似文献
188.
The data recovered from compact discs are contaminated with errors that exhibit a bursty behavior. A channel model is developed to characterize the error bursts and good-data gaps that occur in actual data recovered from compact discs. The probability of having m erroneous symbols in a data block of length n symbols is calculated using the channel model. This probability can be used to evaluate the performance of the error-correction code (ECC) used in the compact disc system 相似文献
189.
G. Price 《Accreditation and quality assurance》1996,1(2):57-66
National measurement systems are infrastructures to ensure, for each nation, a consistent and internationally recognised
basis for measurement. Such complex systems have historical, technical, legal, organisational and institutional aspects to
connect scientific metrology with practical measurements. Underlying any valid measurement is a chain of comparisons linking
the measurement to an accepted standard. The ways the links are forged and the etalons (measurement standards) to which they
connect are defining characteristics of all measurement systems. This is often referred to as traceability which aims at basing
measurements in common measurement units – a key issue for the integration of quantitative chemical analysis with the evolving
physical and engineering measurement systems. Adequate traceability and metrological control make possible new technical capabilities
and new levels of quality assurance and confidence by users in the accuracy and integrity of quantitative analytical results.
Traceability for chemical measurements is difficult to achieve and harder to demonstrate. The supply of appropriate etalons
is critical to the development of metrology systems for chemical analysis. An approach is suggested that involves the development
of networks of specialised reference laboratories able to make matrix-independent reference measurements on submitted samples,
which may then be used as reference materials by an originating laboratory using its practical measurement procedures.
Received: 31 July 1995 Accepted: 19 August 1995 相似文献
190.
V. F. Shevchenko A. A. Petrov V. G. Petrov 《Journal of Infrared, Millimeter and Terahertz Waves》1993,14(9):1755-1768
A basic principle of pulse radar reflectometry is considered in this paper. A numerical analysis is performed in order to study errors appearing due to the expansion of a microwave pulse reflected from the plasma. A block diagram of the basic pulse radar scheme and its overall performance is presented. The first experimental results obtained on T11-M tokamak are discussed. An improved pulse radar scheme with cross-detection is considered. The results of bench test experiments and future applications of this scheme are discussed. 相似文献