全文获取类型
收费全文 | 333003篇 |
免费 | 5047篇 |
国内免费 | 1444篇 |
专业分类
电工技术 | 5075篇 |
综合类 | 2509篇 |
化学工业 | 48073篇 |
金属工艺 | 12819篇 |
机械仪表 | 10235篇 |
建筑科学 | 8921篇 |
矿业工程 | 1092篇 |
能源动力 | 8332篇 |
轻工业 | 32152篇 |
水利工程 | 3349篇 |
石油天然气 | 2194篇 |
武器工业 | 5篇 |
无线电 | 42345篇 |
一般工业技术 | 61735篇 |
冶金工业 | 49170篇 |
原子能技术 | 3621篇 |
自动化技术 | 47867篇 |
出版年
2021年 | 1820篇 |
2020年 | 1357篇 |
2019年 | 1679篇 |
2018年 | 16663篇 |
2017年 | 15606篇 |
2016年 | 12544篇 |
2015年 | 2747篇 |
2014年 | 3882篇 |
2013年 | 12151篇 |
2012年 | 9778篇 |
2011年 | 18742篇 |
2010年 | 15467篇 |
2009年 | 13750篇 |
2008年 | 15455篇 |
2007年 | 16408篇 |
2006年 | 8417篇 |
2005年 | 8625篇 |
2004年 | 8291篇 |
2003年 | 8220篇 |
2002年 | 7133篇 |
2001年 | 7121篇 |
2000年 | 6496篇 |
1999年 | 6879篇 |
1998年 | 16439篇 |
1997年 | 11408篇 |
1996年 | 8636篇 |
1995年 | 6563篇 |
1994年 | 5733篇 |
1993年 | 5584篇 |
1992年 | 4018篇 |
1991年 | 3800篇 |
1990年 | 3558篇 |
1989年 | 3376篇 |
1988年 | 3293篇 |
1987年 | 2599篇 |
1986年 | 2501篇 |
1985年 | 3161篇 |
1984年 | 2811篇 |
1983年 | 2571篇 |
1982年 | 2349篇 |
1981年 | 2395篇 |
1980年 | 2204篇 |
1979年 | 2055篇 |
1978年 | 1887篇 |
1977年 | 2183篇 |
1976年 | 2727篇 |
1975年 | 1586篇 |
1974年 | 1490篇 |
1973年 | 1558篇 |
1972年 | 1146篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
H. Safiri M. Ahmadi G.A. Jullien W.C. Miller 《The Journal of VLSI Signal Processing》2002,31(2):91-100
A new algorithm based on Genetic Programming (GP) for the problem of optimization of Multiple constant Multiplication (MCM) by Common Subexpression Elimination(CSE) is developed. This method is used for hardware optimization for DSP systems. A solution based on Genetic Programming is shown in this paper. The performance of the technique is demonstrated in one- and multi-dimensional digital filters with constant coefficients. 相似文献
992.
Dishongh T. Basaran C. Cartwright A.N. Ying Zhao Heng Liu 《Advanced Packaging, IEEE Transactions on》2002,25(3):433-438
In this paper the influence of the temperature cycle time history profile on the fatigue life of ball grid array (BGA) solder joints is studied. Temperature time history in a Pentium processor laptop computer was measured for a three-month period by means of thermocouples placed inside the computer. In addition, Pentium BGA packages were subjected to industry standard temperature cycles and also to in-situ measured temperature cycle profiles. Inelastic strain accumulation in each solder joint during thermal cycling was measured by high sensitivity Moire interferometry technique. Results indicate that fatigue life of the solder joint is not independent of the temperature cycle profile used. Industry standard temperature cycle profile leads to conservative fatigue life observations by underestimating the actual number of cycles to failure. 相似文献
993.
994.
S. Abiraman H. K. Varma T. V. Kumari P. R. Umashankar Annie John 《Bulletin of Materials Science》2002,25(5):419-429
This study investigates quantitatively and qualitatively the sol-gel derived bioactive glass-ceramic system (BGS)—apatite-wollastonite
(AW) type granules in the size range of 0.5–1 mm, as an effective graft material for bone augmentation and restoration. Scanning
electron micrographs (SEM) of the sintered granules revealed the rough material surface with micropores in the range 10–30
μm. X-ray diffraction (XRD) pattern of the granules revealed the presence of crystalline phases of the hydroxyapatite and
wollastonite, and the functional groups of the silicate and phosphates were identified by Fourier transform infrared spectroscopy
(FT-IR). Thein vitro cell culture studies with L929 mouse fibroblast cell line showed very few cells adhered on the BGS disc after 24 h. This
could be due to the highly reactive surface of the disc concomitant with the crystallization but not due to the cytotoxicity
of the material, since the cellular viability (MTT assay) with the material was 80‰ Cytotoxicity and cytocompatibility studies
proved that the material was non-toxic and biocompatible. After 12 weeks of implantation of the BGS granules in the tibia
bone of New Zealand white rabbits, the granules were found to be well osteointegrated, as observed in the radiographs. Angiogram
with barium sulphate and Indian ink after 12 weeks showed the presence of microcapillaries in the vicinity of the implant
site implicating high vascularity. Gross observation of the implant site did not show any inflammation or necrosis. SEM of
the implanted site after 24 weeks revealed good osteointegration of the material with the newly formed bone and host bone.
New bone was also observed within the material, which was degrading. Histological evaluation of the bone healing with the
BGS granules in the tibial defect at all time intervals was without inflammation or fibrous tissue encapsulation. After 2
weeks the new bone was observed as a trabeculae network around the granules, and by 6 weeks the defect was completely closed
with immature woven bone. By 12 weeks mature woven bone was observed, and new immature woven bone was seen within the cracks
of the granules. After 24 weeks the defect was completely healed with lamellar bone and the size of the granules decreased.
Histomorphometrically the area percentage of new bone formed was 67.77% after 12 weeks and 63.37% after 24 weeks. Less bone
formation after 24 weeks was due to an increased implant surface area contributed by the material degradation and active bone
remodeling. The osteostimulative and osteoconductive potential of the BGS granules was established by tetracycline labelling
of the mineralizing areas by 2 and 6 weeks. This sol-gel derived BGS granules proved to be bioactive and resorbable which
in turn encouraged active bone formation. 相似文献
995.
P. Meenakshi S. E. Noorjahan R. Rajini U. Venkateswarlu C. Rose T. P. Sastry 《Bulletin of Materials Science》2002,25(1):25-29
DriedMusa paradiciaca (banana) stem and veins of the leaves, which were hitherto discarded as a waste, were collected and used as starting material
for the preparation of cellulose and cellulose acetate. This cellulose acetate was mixed with polystyrene to form blend of
cellulose acetate-polystyrene in order to provide enhanced stability and extended utility to the end products. The films of
these composites or their individual partners were made separately and studied for their mechanical properties, chemical modification
and morphological changes. We report here that banana stem is good source of cellulose and that cellulose completely undergoes
modification upon esterification. 相似文献
996.
997.
This paper reports investigations on the techniques and economics of hydrogen storage by means of cryoadsorption. Also a comparison with alternative storage methods is included. The hydrogen storage capacity of several adsorbents in the temperature range 65–150 K has been investigated experimentally. Based on these data, economics and operating conditions for minimum total costs of the system are calculated. Utilization-factor and capacity-factor parameters are shown to be decisive for outlining the favourable ranges of application for competitive hydrogen storage methods. 相似文献
998.
J. C. Schulz 《International journal for numerical methods in engineering》1985,21(6):1039-1048
A new hourglassing control technique for quadrilateral and hexahedral elements used in dynamic finite element codes is presented. Based on expansion of the strees in a Taylor series and retention of additional terms beyond the usual constant stress term, this technique has the advantage that actual rather than artificial material properties are used. Simplifications that render this technique competitive with artificial viscosity and stiffness procedures in terms of additional storage and computation requirements are described. 相似文献
999.
A new bipolar differential input/output current-controlled current source (CCCS) is described. The basic cell consists of a translinear array of six transistors with two bipolar inputs, and is suited for the input stage of a differential current-mode operational amplifier. 相似文献
1000.
Dicyanate Semi-Interpenetrating Polymer Networks (semi IPNs) are made by dissolving a thermoplastic in crosslinking dicyanates and then curing the resulting mixture. The semi IPNs produced are strong, with tensile strengths of 10,000 to 12,000 psi, and flexible, with elongations to break of 10 to 17 percent. Dicyanate semi IPNs also have good thermal stability and the softening temperatures of the IPNs are significantly, higher than those of the corresponding thermoplastics. 相似文献