全文获取类型
收费全文 | 525889篇 |
免费 | 7771篇 |
国内免费 | 3036篇 |
专业分类
电工技术 | 10601篇 |
技术理论 | 6篇 |
综合类 | 2345篇 |
化学工业 | 80625篇 |
金属工艺 | 22943篇 |
机械仪表 | 16359篇 |
建筑科学 | 14512篇 |
矿业工程 | 3052篇 |
能源动力 | 12952篇 |
轻工业 | 47364篇 |
水利工程 | 5229篇 |
石油天然气 | 10220篇 |
武器工业 | 178篇 |
无线电 | 61052篇 |
一般工业技术 | 98198篇 |
冶金工业 | 97937篇 |
原子能技术 | 11003篇 |
自动化技术 | 42120篇 |
出版年
2021年 | 4477篇 |
2019年 | 3870篇 |
2018年 | 6389篇 |
2017年 | 6343篇 |
2016年 | 6808篇 |
2015年 | 5111篇 |
2014年 | 8266篇 |
2013年 | 23060篇 |
2012年 | 13092篇 |
2011年 | 17758篇 |
2010年 | 14624篇 |
2009年 | 16192篇 |
2008年 | 16907篇 |
2007年 | 16704篇 |
2006年 | 15177篇 |
2005年 | 14057篇 |
2004年 | 13037篇 |
2003年 | 12885篇 |
2002年 | 12968篇 |
2001年 | 12913篇 |
2000年 | 11868篇 |
1999年 | 12610篇 |
1998年 | 30122篇 |
1997年 | 21793篇 |
1996年 | 16918篇 |
1995年 | 12779篇 |
1994年 | 11353篇 |
1993年 | 11059篇 |
1992年 | 8158篇 |
1991年 | 8045篇 |
1990年 | 7458篇 |
1989年 | 7397篇 |
1988年 | 7261篇 |
1987年 | 6161篇 |
1986年 | 6117篇 |
1985年 | 7104篇 |
1984年 | 6609篇 |
1983年 | 6050篇 |
1982年 | 5637篇 |
1981年 | 5786篇 |
1980年 | 5550篇 |
1979年 | 5314篇 |
1978年 | 5376篇 |
1977年 | 6377篇 |
1976年 | 8758篇 |
1975年 | 4794篇 |
1974年 | 4573篇 |
1973年 | 4590篇 |
1972年 | 3916篇 |
1971年 | 3602篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
NIKIÉT. Translated from Atomnaya Énergiya, Vol. 75, No. 4, pp. 265-269, October, 1993. 相似文献
992.
Z. -G. Fan Y. -X. Zhuang G. Yang R. Shao G. -F. Zhang 《Journal of Alloys and Compounds》1993,200(1-2):33-36
The oxygen absorption process in highly aligned YBa2Cu3Ox samples in air and in pure oxygen was studied at several constant temperatures. The process can be expressed in two steps, a chemical reaction controlled step and a diffusion controlled step. The kinetic equations are as follows respectively:
(1 − at) ln(1 − at) + at = kDt
The activation energy was calculated using an Arrhenius equation. The technical conditions of heat treatment for oxygen absorption of highly aligned YBa2Cu3Ox crystals are discussed. 相似文献
993.
Pattern classification is frequently performed using the k-nn algorithm or a neural network. The choice of parameters for the former is often difficult and the amount of data which has to be stored in the classifier can be high. Neural network classifiers can overcome some of these problems but learning is often unreliable and slow. An alternative which combines some of the best features of the k-nn and neural network classifier is described by the authors. The classifier is called a packed hyper-ellipsoid classifier 相似文献
994.
A novel vector algorithm for Reed-Muller (RM) expansions is proposed which can save as many as a factor of 2n-1 memory elements compared with previous matrix algorithms 相似文献
995.
The authors have implemented chirped distributed feedback (DFB) grating lasers with phase shifts [λ/8, 2λ/8 and 3λ/8] distributed continuously over various axial distances, using their recently proposed method for producing bent optical waveguides. The singlemode stability of the lasers with distributed phase shifts was found to be considerably higher than for lasers with abrupt phase shifts 相似文献
996.
N. Ravishankar H. I. Aaronson K. Chattopadhyay 《Metallurgical and Materials Transactions A》1994,25(12):2631-2637
The early stages in the formation of α1 plates in the ordered β′ matrix of a Cu-39 pct Zn alloy have been studied with transmission electron microscopy (TEM). The
early stage plates were found to be free of stacking faults. Their contrast features are indeed those normally expected from
small coherent plates. Electron microdiffraction confirms that these plates have the 9R structure characteristic of α1 with the same lattice parameters and orientation relationships reported by previous investigators at much later stages of
growth. Similarly, the 9R structure was disordered, even though it was formed in an ordered matrix, again repeating previous
results obtained at a later stage of growth. These results further support the view that the α1 Cu-Zn plates form by a diffusion-controlled mechanism.
This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation
of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S
Phase Transformations Committee. 相似文献
997.
AM De Mattos MA Head J Everett J Hosenpud R Hershberger A Cobanoglu G Ott R Ratkovec DJ Norman 《Canadian Metallurgical Quarterly》1994,57(4):626-630
To determine if cardiac allograft outcome is improved among patients with fewer HLA-DR mismatches with their donors, we studied 132 recipients of a primary cardiac allograft who were transplanted between December 1985 and December 1991. These recipients and their donors all had high-confidence-level serological HLA-DR typing, previously shown to correlate highly with DNA DR typing. Patients were divided in two groups based on the HLA-DR mismatch with their donors. Group I consisted of 78 patients with 1 or zero DR mismatch and group II of 54 patients with 2 DR mismatches. Allograft outcome measurements included incidence of moderate rejection, incidence of allograft vasculopathy at 12 months, cardiac function measured as left ventricular ejection fraction (LVEF) and cardiac index (CI), and actuarial graft survival up to 7 years. Groups I and group II were not different with regard to recipient age, donor age, ischemia time, pulmonary vascular resistance, sex, or PRA greater than 0%. Group II had a higher incidence of moderate rejection on the first-week biopsy (47% vs. 25%, P = 0.019), and during the first month (84% vs. 58%, P = 0.006), but no difference was found in frequency of rejection from months 2 to 12. LVEF was not different in the groups at any point. CI was better in group I at 12 months (2.76 vs. 2.5, P = 0.03). No statistically significant difference was found in incidence of allograft vasculopathy (17% vs. 26%, P = 0.204). Actual graft survival at 1 year was better for group I (91% vs. 74%, P = 0.008), and actuarial graft survival at 6 years also favored group I (76% vs. 56%, P = 0.04). Using high-confidence-level serological HLA-DR typing assignments we demonstrated that HLA-DR mismatching correlates highly with cardiac allograft outcome. Implications are that heart transplant survival could be improved if prospective matching were feasible and prioritized or if immunosuppression were tailored to the HLA-DR match. 相似文献
998.
H Rosling 《Canadian Metallurgical Quarterly》1993,90(46):4069-4075
Health conditions have improved worldwide; therefore, the division into developed and developing countries no longer holds true. International organizations tend to divide countries into three groups. An increasing number of people are born in middle income countries where health conditions continue to improve. However, in a number of the least developed countries, mortality is on the rise in a spiral of economic stagnation, environmental problems, social misery, and ethnic/civil conflicts. This requires the medical assistance of international agencies from abroad. Since the 1960s there has been a drastic decline of child mortality in developing countries, especially in Asia. 1 billion people live in countries with child mortality under 20/1000, almost 3 billion live in countries with child mortality ranging 20-100/1000, and over 1 billion live in the least developed countries with child mortality over 100/1000. Unicef divides countries into developed, developing, and least developed countries, while the World Bank groups them as high-, medium-, and low-income countries. Thailand's child mortality is the same as that of Russia, while Cuba has a lower rate than Washington, D.C. On the other hand, Singapore is a developed high-income country with one of the world's healthiest populations. Stagnation and conflicts in the former socialist countries mean that many Asian and Latin American countries have better health status than some parts of Europe. Despite Africa's high mortality, its population growth is the highest in the world: in 20 years its population has doubled. Economic stagnation and the debt burden in many of these countries has resulted in ethnic conflicts and the collapse of social institutions: Somalia, southern Sudan, Rwanda, Liberia, Angola, perhaps Zaire, and Mozambique. The organization Physicians Beyond Borders is an example of extending humanitarian help and combatting social collapse in the least developed countries. 相似文献
999.
K Tordjman N Stern G Ouaknine Y Yossiphov N Razon M Nordenskj?ld E Friedman 《Canadian Metallurgical Quarterly》1993,77(3):765-769
The majority of pituitary tumors are of monoclonal origin; however, the molecular basis for their formation is poorly understood. Somatic mutations in the alpha-subunit of the GTP-binding protein, Gs alpha (gsp oncogene) have been found in about one third of GH-secreting tumors. Mutations in another alpha-subunit of a GTP-binding protein, Gi2 alpha (gip mutations) have been described in other endocrine tumors. In this study, we examined 21 nonfunctioning pituitary tumors and 4 macroprolactinomas for gsp mutations and 27 nonfunctioning tumors and 4 macroprolactinomas for gip mutations. Using the polymerase chain reaction and denaturing gradient gel electrophoresis, 2 nonfunctioning pituitary tumors displayed migration abnormalities when the Gs alpha-gene was analyzed. Sequence analysis of these abnormally migrating polymerase chain reaction products revealed two previously known gsp mutations: arginine at codon 201 altered to cysteine, and glutamine at codon 227 changed to leucine. No gip mutations could be demonstrated. These findings emphasize the monoclonal origin of nonfunctioning pituitary tumors and suggest that cAMP may play a role in tumorigenesis of nonfunctioning pituitary tumors. 相似文献
1000.
Y Ohe ER Podack KJ Olsen Y Miyahara K Miura H Saito Y Koishihara Y Ohsugi T Ohira K Nishio 《Canadian Metallurgical Quarterly》1993,67(5):939-944
HuIL-6 cDNA, cloned into a neomycin resistant conferring expression vector, BMGNeo, was transfected into Lewis Lung Carcinoma (LLC) cells. LLC cells (5 x 10(6) ml-1) transfected with IL-6 cDNA (LLC-IL6) secreted IL-6 into the culture supernatant at a concentration of 9.9 ng ml-1 within 48 h. When 1,000,000 of untransfected LLC, BMGNeo vector transfected LLC (LLC-Neo) or LLC-IL6 cells were transplanted into C57BL/6 mice subcutaneously, the mean +/- s.d. of survival times of these mice were 33.3 +/- 9.7, 34.3 +/- 7.1 and 17.0 +/- 3.1 days, respectively. The survival time of LLC-IL6 cells transplanted mice was significantly shorter than that of LLC (P < 0.01) or LLC-Neo (P < 0.01) cells transplanted mice without a measurable difference of tumour size. Plasma concentration of IL-6 steadily increased in LLC-IL6 transplanted mice. Body weight and serum albumin were significantly lower in LLC-IL6 transplanted mice than in LLC transplanted mice. Mouse IL-1 alpha and mouse TNF-alpha were not detected in the plasma of LLC-IL6 transplanted mice. These data suggested that secretion of IL-6 from LLC cells was unable to alter net tumour growth rate but rather caused a state similar to cachexia without detectable increase of IL-1 alpha and TNF-alpha in the plasma. This state may be responsible for the shortened survival of LLC-IL6 tumour-bearing mice. 相似文献