全文获取类型
收费全文 | 1507440篇 |
免费 | 26999篇 |
国内免费 | 7884篇 |
专业分类
电工技术 | 35466篇 |
技术理论 | 5篇 |
综合类 | 7503篇 |
化学工业 | 275622篇 |
金属工艺 | 64731篇 |
机械仪表 | 42533篇 |
建筑科学 | 47101篇 |
矿业工程 | 11831篇 |
能源动力 | 50655篇 |
轻工业 | 116631篇 |
水利工程 | 15176篇 |
石油天然气 | 38429篇 |
武器工业 | 369篇 |
无线电 | 197422篇 |
一般工业技术 | 282758篇 |
冶金工业 | 150642篇 |
原子能技术 | 33960篇 |
自动化技术 | 171489篇 |
出版年
2021年 | 16709篇 |
2020年 | 12600篇 |
2019年 | 15370篇 |
2018年 | 16151篇 |
2017年 | 15448篇 |
2016年 | 22034篇 |
2015年 | 18332篇 |
2014年 | 29783篇 |
2013年 | 89622篇 |
2012年 | 37071篇 |
2011年 | 49353篇 |
2010年 | 43817篇 |
2009年 | 52041篇 |
2008年 | 45900篇 |
2007年 | 43054篇 |
2006年 | 45890篇 |
2005年 | 39981篇 |
2004年 | 41597篇 |
2003年 | 41423篇 |
2002年 | 40668篇 |
2001年 | 37051篇 |
2000年 | 35638篇 |
1999年 | 34265篇 |
1998年 | 40702篇 |
1997年 | 36842篇 |
1996年 | 33675篇 |
1995年 | 29655篇 |
1994年 | 27995篇 |
1993年 | 27832篇 |
1992年 | 26037篇 |
1991年 | 22983篇 |
1990年 | 23345篇 |
1989年 | 22419篇 |
1988年 | 20984篇 |
1987年 | 19237篇 |
1986年 | 18570篇 |
1985年 | 21911篇 |
1984年 | 22279篇 |
1983年 | 20202篇 |
1982年 | 19200篇 |
1981年 | 19325篇 |
1980年 | 17883篇 |
1979年 | 18492篇 |
1978年 | 17720篇 |
1977年 | 17204篇 |
1976年 | 17918篇 |
1975年 | 15977篇 |
1974年 | 15521篇 |
1973年 | 15573篇 |
1972年 | 13044篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Wavelength selection for low-saturation pulse oximetry 总被引:1,自引:0,他引:1
Mannheimer P.D. Cascini J.R. Fein M.E. Nierlich S.L. 《IEEE transactions on bio-medical engineering》1997,44(3):148-158
Conventional pulse oximeters are accurate at high oxygen saturation under a variety of physiological conditions but show worsening accuracy at lower saturation (below 70%). Numerical modeling suggests that sensors fabricated with 735 and 890 nm emitters should read more accurately at low saturation under a variety of conditions than sensors made with conventionally used 660 and 900 nm band emitters. Recent animal testing confirms this expectation. It is postulated that the most repeatable and stable accuracy of the pulse oximeter occurs when the fractional change in photon path lengths due to perturbations in the tissue (relative to the conditions present during system calibration) is equivalent at the two wavelengths. Additionally, the penetration depth (and/or breadth) of the probing light needs to be well matched at the two wavelengths in order to minimize the effects of tissue heterogeneity. At high saturation these conditions are optimally met with 660 and 900 nm band emitters, while at low saturation 735 and 890 nm provide better performance 相似文献
992.
Yu. N. Denisyuk N. M. Ganzherli I. M. Maurer S. A. Pisarevskaya 《Technical Physics Letters》1997,23(4):279-280
A thick-layer light-sensitive material consisting of glycerin-containing bichromated gelatin has been prepared and used to
record real-time volume holograms with 0.44 μm helium-cadmium laser radiation. The thickness of the layers is 400–600 μm.
The holographic sensitivity of the material is 1 J/cm2. The highest diffraction efficiency of holograms recorded using a symmetric system with parallel beams is 32%.
Pis’ma Zh. Tekh. Fiz. 23, 62–65 (April 12, 1997) 相似文献
993.
994.
The perfectly matched layer for use with the finite-difference time-domain method is adapted to our transmission-line matrix simulation as what we call a heaviside absorbing layer (HAL). It is shown that the reflection coefficient for the wave incident on a HAL is less than that of the wave incident on a matched-load termination at all angles of incidence. Furthermore, the dispersion relation of a transmission-line matrix mesh of a symmetrical condensed node with both electric and magnetic loss stubs is derived. It provides guidance on how to choose the losses of HAL and other simulation parameters properly 相似文献
995.
Space diversity reception and forward-error correction coding are powerful techniques to combat multipath fading encountered in mobile radio communications. In this paper, we analyze the performance of a discrete-time switched diversity system using trellis-coded modulation multiple phase-shift keying (TCM-MPSK) on slow, nonselective correlated Nakagami (1960) fading channels. Analytical upper bounds using the transfer function bounding technique are obtained and illustrated by several numerical examples. A simple integral expression for calculating the exact pairwise error probability is presented. The use of optimum adaptive and fixed switching thresholds is considered. Monte Carlo simulation results, which are more indicative of the exact system performance, are also given 相似文献
996.
A new method based on a two-wire line has been developed for noncontact diagnostics of a low-temperature plasma and its fluxes
in channels with complex profiles. This method provides good spatial and time resolution.
Pis’ma Zh. Tekh. Fiz. 23, 1–7 (July 26, 1997) 相似文献
997.
Valenzuela R.A. Landron O. Jacobs D.L. 《Vehicular Technology, IEEE Transactions on》1997,46(1):203-212
We explore techniques for the measurement of local mean signal strength at 900 MHz and 2 GHz. In particular, we characterize the impact of transmitter and receiver antenna rotation on the estimated local mean. Then, we explore the collection of high resolution data while moving along a linear trajectory and using linear averaging techniques to estimate the local mean. With this information, the best measurement techniques can be chosen depending on the required speed versus accuracy tradeoff. Finally, we use a ray tracing propagation model to evaluate different methods of calculating the local mean signal strength for indoor environments 相似文献
998.
X. Wang S. B. Lambert 《Fatigue & Fracture of Engineering Materials & Structures》1997,20(12):1637-1655
Abstract— A Fourier series approach is proposed to calculate stress intensity factors using weight functions for semi-elliptical surface cracks in flat plates subjected to two-dimensional stress distributions. The weight functions were derived from reference stress intensity factors obtained by three-dimensional finite element analyses. The close form weight functions derived are suitable for the calculation of stress intensity factors for semi-elliptical surface cracks in flat plates under two-dimensional stress distributions with the crack aspect ratio in the range of 0.1 ≤ a/c ≤ 1 and relative depth in the range of 0 ≤ a/t ≤ 0.8. Solutions were verified using several two-dimensional non-linear stress distributions; the maximum difference being 6%. 相似文献
999.
F Pigozzi N Santori V Di Salvo A Parisi L Di-Luigi 《Canadian Metallurgical Quarterly》1997,20(6):505-509
OBJECTIVE: To estimate the frequency of perioperative morbidities in patients who underwent anesthesia and a surgical procedure with no preoperative laboratory testing. MATERIAL AND METHODS: We conducted an electronic database search of medical records of 56,119 patients who underwent surgical or diagnostic procedures and anesthesia at Mayo Clinic Rochester in 1994 and found 5,120 who had no laboratory tests done within 90 days before the procedure. From this group, we randomly selected 1,044 patients (87 from each month) to document the absence of preoperative tests, the presence of preexisting disease (by organ system), the type of anesthetic agent, and the outcomes and tests intraoperatively and postoperatively. RESULTS: The 1,044 patients ranged in age from 0 to 95 years (median age, 21). No deaths or major perioperative morbidities occurred (0.0%; exact 95% confidence interval, 0.00 to 0.35%). Although 10 patients underwent blood typing and screening for antibodies immediately preoperatively, no blood transfusions were necessary. Intraoperatively, 17 laboratory tests and 1 electrocardiogram were obtained, and 3 results were abnormal. Postoperatively, 42 blood tests and 2 electrocardiographic procedures were performed. Five of the 42 blood tests showed abnormal results (hemoglobin levels in 3, serum sodium in 1, and arterial blood gases in 1). One electrocardiogram showed normal findings, and the other revealed normal results except for premature ventricular contractions. No laboratory test done intraoperatively or postoperatively was found to change surgical or medical management substantially. One patient who had unanticipated blood loss during an outpatient procedure was admitted to the hospital for observation. CONCLUSION: All 1,044 patients, 97% of whom were relatively healthy, with no recent laboratory testing safely underwent anesthesia and an operation. We conclude that patients who have been assessed by history and physical examination and determined to have no preoperative indication for laboratory tests can safely undergo anesthesia and operation with tests obtained only as indicated intraoperatively and post-operatively. Current anesthetic and medical practices rapidly identify perioperative indications for laboratory evaluation as they arise. 相似文献
1000.
Color image retrieval based on hidden Markov models 总被引:1,自引:0,他引:1
In this correspondence, a new approach to retrieving images from a color image database is proposed. Each image in the database is represented by a two-dimensional pseudo-hidden Markov model (2-D PHMM), which characterizes the chromatic and spatial information about the image. In addition, a flexible pictorial querying method is used, by which users can paint the rough content of the desired images in a query picture. Image matching is achieved by comparing the query picture with each 2-D PHMM in the database. Experimental results show that the proposed approach is indeed effective. 相似文献