全文获取类型
收费全文 | 647731篇 |
免费 | 9763篇 |
国内免费 | 1947篇 |
专业分类
电工技术 | 11588篇 |
综合类 | 489篇 |
化学工业 | 99858篇 |
金属工艺 | 25549篇 |
机械仪表 | 20120篇 |
建筑科学 | 14821篇 |
矿业工程 | 3646篇 |
能源动力 | 17267篇 |
轻工业 | 53293篇 |
水利工程 | 6893篇 |
石油天然气 | 12938篇 |
武器工业 | 34篇 |
无线电 | 73323篇 |
一般工业技术 | 129172篇 |
冶金工业 | 121295篇 |
原子能技术 | 14322篇 |
自动化技术 | 54833篇 |
出版年
2021年 | 6013篇 |
2020年 | 4651篇 |
2019年 | 5792篇 |
2018年 | 10017篇 |
2017年 | 9883篇 |
2016年 | 10716篇 |
2015年 | 6916篇 |
2014年 | 11082篇 |
2013年 | 31130篇 |
2012年 | 17711篇 |
2011年 | 23430篇 |
2010年 | 18810篇 |
2009年 | 21136篇 |
2008年 | 21580篇 |
2007年 | 21163篇 |
2006年 | 18689篇 |
2005年 | 16954篇 |
2004年 | 15992篇 |
2003年 | 15769篇 |
2002年 | 15162篇 |
2001年 | 14854篇 |
2000年 | 14005篇 |
1999年 | 14485篇 |
1998年 | 37236篇 |
1997年 | 26374篇 |
1996年 | 20221篇 |
1995年 | 15073篇 |
1994年 | 13378篇 |
1993年 | 13328篇 |
1992年 | 9662篇 |
1991年 | 9234篇 |
1990年 | 9098篇 |
1989年 | 8823篇 |
1988年 | 8541篇 |
1987年 | 7320篇 |
1986年 | 7273篇 |
1985年 | 8254篇 |
1984年 | 7684篇 |
1983年 | 6980篇 |
1982年 | 6547篇 |
1981年 | 6700篇 |
1980年 | 6302篇 |
1979年 | 6103篇 |
1978年 | 6219篇 |
1977年 | 7257篇 |
1976年 | 9554篇 |
1975年 | 5448篇 |
1974年 | 5118篇 |
1973年 | 5268篇 |
1972年 | 4447篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A dental student developed swelling of the lips and peri-oral urticaria, immediately after placement of a dental rubber dam during a chairside demonstration of clinical dentistry. A provisional diagnosis of a Type I hypersensitivity reaction to natural rubber latex was made and an appointment organised for specialist investigations. The student reported an atopic history and allergy to peanuts. A Radioallergosorbent Test (RAST), to measure Ig-E antibodies to latex, was positive. The dental student was given advice concerning the avoidance of natural rubber latex. Dental staff must be aware of potentially serious reactions to natural rubber latex in the dental surgery. 相似文献
942.
In order to observe the transport ability of peritoneum to small molecular substances, peritoneal equilibration test (PET) was performed in 52 CAPD patients. By analysing the relationship between peritoneal transport function and dialysis adequacy, we found the average urea KT/V and Cr were significantly lower in high and low transport groups (n = 6 and n = 2) than in high average and low average groups (n = 35 and n = 9). According to the results of PET, we adjusted the dialysis program of 11 patients and the dialysis adequacy was markedly improved. We concluded that PET was helpful for selecting and adjusting CAPD program, and discussed some questions which should be payed more attention in PET operation. 相似文献
943.
J Brake TJ Walsh CE Benton JN Petitte R Meijerhof G Pe?alva 《Canadian Metallurgical Quarterly》1997,76(1):144-151
The relative importance of fat and lean tissue mass in determining bone mineral mass among postmenopausal women was examined in this 1-year longitudinal study. Fifty postmenopausal Caucasian women entered the study; 45 of them completed a 1-year follow-up. Dual-energy X-ray absorptiometry was employed for measuring total and regional bone mineral density (BMD) and bone mineral content (BMC), fat tissue mass (FTM), lean tissue mass (LTM), and body weight. Results from linear regression analysis using the cross-sectional data (n = 50) of the study indicated that LTM explained a larger percentage of variation in bone mineral mass than did FTM. FTM and LTM were found to be moderately correlated (r = 0.55); when FTM was entered in the same predicting regression models, LTM was a significant predictor (p < 0.05) of the total and regional BMC, but not BMD. The percent FTM (and inversely %LTM) was correlated with BMD and BMC, but significant correlation was primarily found only for total body BMD (or BMC). Weight was the best predictor of total body BMD and BMC. Longitudinally (n = 45), annual changes in both FTM and weight were significantly associated with annual changes in regional BMD after adjustment for initial bone mineral values (p < 0.05). We conclude that bone mineral mass is more closely related to LTM than to FTM, while annual changes in regional BMD are more closely correlated with changes in FTM in healthy postmenopausal women. Meanwhile, increased body weight is significantly associated with increased bone mineral mass. 相似文献
944.
J Nesvera M Pátek J Hochmannová Z Abrhámová V Becvárová M Jelínkova J Vohradsky 《Canadian Metallurgical Quarterly》1997,179(5):1525-1532
The complete nucleotide sequence (4,826 bp) of the cryptic plasmid pGA1 from Corynebacterium glutamicum was determined. DNA sequence analysis revealed four putative coding regions (open reading frame A [ORFA], ORFA2, ORFB, and ORFC). ORFC was identified as a rep gene coding for an initiator of plasmid replication (Rep) according to the high level of homology of its deduced amino acid sequence with the Rep proteins of plasmids pSR1 (from C. glutamicum) and pNG2 (from Corynebacterium diphtheriae). This function was confirmed by deletion mapping of the minimal replicon of pGA1 (1.7 kb) which contains only ORFC. Deletion derivatives of pGA1 devoid of ORFA exhibited significant decreases in the copy number in C. glutamicum cells and displayed segregational instability. Introduction of ORFA in trans into the cells harboring these deletion plasmids dramatically increased their copy number and segregational stability. The ORFA gene product thus positively influences plasmid copy number. This is the first report on such activity associated with a nonintegrating bacterial plasmid. The related plasmids pGA1, pSR1, and pNG2 lacking significant homology with any other plasmid seem to be representatives of a new group of plasmids replicating in the rolling-circle mode. 相似文献
945.
For part I see, ibid., p. 134, 1998. The basic approach outlined in the previous article is applied to the difficult problem of computing the optical modes of a vertical-cavity surface-emitting laser. The formulation utilizes a finite difference equation based upon the lowest order term of an infinite series solution of the scalar Helmholtz equation in a local region. This difference equation becomes exact in the one-dimensional (1-D) limit, and is thus ideally suited for nearly 1-D devices such as vertical-cavity lasers. The performance of the resulting code is tested on both a simple cylindrical cavity with known solutions and an oxide-confined vertical-cavity laser structure, and the results compared against second-order-accurate code based upon Crank-Nicolson differencing 相似文献
946.
Analysis of low bit rate image transform coding 总被引:8,自引:0,他引:8
Calculations based on high-resolution quantizations prove that the distortion rate D(R¯) of an image transform coding is proportional to 2-2R when R¯ is large enough. In wavelet and block cosine bases, we show that if R¯<1 bit/pixel, then D(R¯) varies like R¯1-2γ, where γ remains of the order of 1 for most natural images. The improved performance of embedded codings in wavelet bases is analyzed. At low bit rates, we show that the compression performance of an orthonormal basis depends mostly on its ability to approximate images with a few nonzero vectors 相似文献
947.
Li Wang ping Mechanics Department Huazhong University of Science Technology Wuhan P.R.China Yeo K.S. Khoo B.C. Mechanical Production Engineering Department National University of Singapore 《水动力学研究与进展(B辑)》1999,(3)
1. INTRODUCTIONThemaingoalofthispaperistoexplorethepossibilitytolearnmoreaboutthemechanismofturbulentboundarylayerflowinteractionsanditseffectsoncompliantwallperformance.Therearecertainprerequisiteconditionstofurtherthestudyonthemechanism,i.e.theco… 相似文献
948.
949.
Local autonomous dynamic channel allocation (LADCA) including power control is essential to accommodating the anticipated explosion of demand for wireless. The authors simulate call performance for users accessing channels in a regular cellular array with a base located at the center of each hexagon. The computer model includes stochastic channel demand and a propagation environment characterized by attenuation with distance as well as shadow fading. The study of LADCA shows that distributed power control and channel access can be combined in an access management policy that achieves satisfactory system capacity and provides desired call performance. The authors report: LADCA/power control is observed to be stable alleviating a major concern about users unaware of the signal to interference problems their presence on a channel might cause to others. There can be substantial inadvertent dropping of calls in progress caused by originating calls. Modeling user time dynamics is essential. LADCA contrasts very favorably with fixed channel allocation (FCA) in a comparative example 相似文献
950.
I. S. W. B. Prasetya 《Formal Aspects of Computing》1994,6(4):466-470
UNITY, introduced by Chandy and Misra [ChM88], is a programming logic intended to reason about temporal properties of distributed programs. Despite the fact that UNITY does not have the full power of, for example, linear temporal logic, it enjoys popularity due to its simplicity.There was however a serious problem with the Substitution Rule. The logic is incomplete without the rule, and with the rule it is inconsistent. 相似文献