全文获取类型
收费全文 | 28877篇 |
免费 | 1524篇 |
国内免费 | 130篇 |
专业分类
电工技术 | 470篇 |
综合类 | 234篇 |
化学工业 | 4826篇 |
金属工艺 | 793篇 |
机械仪表 | 660篇 |
建筑科学 | 788篇 |
矿业工程 | 148篇 |
能源动力 | 502篇 |
轻工业 | 3994篇 |
水利工程 | 211篇 |
石油天然气 | 135篇 |
武器工业 | 10篇 |
无线电 | 1533篇 |
一般工业技术 | 5293篇 |
冶金工业 | 7307篇 |
原子能技术 | 164篇 |
自动化技术 | 3463篇 |
出版年
2023年 | 303篇 |
2022年 | 268篇 |
2021年 | 610篇 |
2020年 | 505篇 |
2019年 | 426篇 |
2018年 | 905篇 |
2017年 | 817篇 |
2016年 | 899篇 |
2015年 | 778篇 |
2014年 | 908篇 |
2013年 | 1732篇 |
2012年 | 1336篇 |
2011年 | 1342篇 |
2010年 | 1139篇 |
2009年 | 1087篇 |
2008年 | 1221篇 |
2007年 | 1168篇 |
2006年 | 828篇 |
2005年 | 664篇 |
2004年 | 487篇 |
2003年 | 452篇 |
2002年 | 418篇 |
2001年 | 331篇 |
2000年 | 322篇 |
1999年 | 448篇 |
1998年 | 2282篇 |
1997年 | 1373篇 |
1996年 | 960篇 |
1995年 | 580篇 |
1994年 | 471篇 |
1993年 | 427篇 |
1992年 | 171篇 |
1991年 | 179篇 |
1990年 | 153篇 |
1989年 | 158篇 |
1988年 | 150篇 |
1987年 | 159篇 |
1986年 | 138篇 |
1985年 | 217篇 |
1984年 | 180篇 |
1983年 | 165篇 |
1982年 | 131篇 |
1981年 | 175篇 |
1980年 | 172篇 |
1979年 | 121篇 |
1978年 | 105篇 |
1977年 | 222篇 |
1976年 | 480篇 |
1975年 | 85篇 |
1973年 | 111篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Pre-pruning and Post-pruning are two standard techniques for handling noise in decision tree learning. Pre-pruning deals with noise during learning, while post-pruning addresses this problem after an overfitting theory has been learned. We first review several adaptations of pre- and post-pruning techniques for separate-and-conquer rule learning algorithms and discuss some fundamental problems. The primary goal of this paper is to show how to solve these problems with two new algorithms that combine and integrate pre- and post-pruning. 相似文献
83.
To investigate the energetic costs of lactation in a female mammal in relation to previous reproductive history, we compared the performance of adult female Long-Evans rats that had previously bred (multiparous) with young females that had not previously given birth (primiparous). All litters were standardized to 10 +/- 1 young. We compared maternal production (growth of pups), body mass, and energy intake (food consumption) of mothers, as well as their energy expenditure (resting oxygen consumption). The mass of litters at birth and the growth of pups during lactation did not differ according to reproductive history of the mothers. The body mass of primiparous mothers was less than that of multiparous mothers, and primiparous mothers showed an increase in mass during early lactation. To accomplish the essentially identical production of offspring under these circumstances, the primiparous mothers consumed and expended more energy than the multiparous mothers. This remarkable performance of first-time mothers results in an overall efficiency of energy allocation to reproduction amounting to only 25%, compared with 38% in multiparous mothers. The energetic inefficiency of primiparous female lactation results largely from the excessive expenditures associated with physiological and behavioral performances of first-time reproduction, together with a small component of additional expenditure due to further growth by the primiparous mothers. We suggest that this inefficiency probably contributes to the observed low reproductive success of novice breeders; furthermore, active restraint of fecundity may be an evolutionary response to the constraints of the energetic inefficiency of primiparous breeding by female mammals. 相似文献
84.
R Zahn A Vogt K Seidl S Schuster H Gülker KW Heinrich M Gottwik K Neuhaus J Senges 《Canadian Metallurgical Quarterly》1997,86(9):712-721
Balloon angioplasty as the treatment of first choice in the setting of an acute myocardial infarction (AMI) is gaining widespread acceptance because of favourable results from specialised centres concerning high patency rates and low mortality. This study reports the results of angioplasty for AMI at large community hospitals during 1992-1995. 4625 procedures were performed at 68 centres of the Arbeitsgemeinschaft Leitender Kardiologischer Krankenhaus?rzte (ALKK). The age of the patients was 60.8 +/- 11.3 years, with 75.1% men. The infarct related artery was the left anterior descendent in 43%, the right coronary artery in 37%, the circumflex artery in 16%, a bypass graft in 2.3% and the left main stem in 1.4% of patients. The success rate (residual stenosis < 50%) of the intervention was 86%. There was a wide range of procedures per centre, with a median of 40 AMI angioplasties per year and centre. The amount of angioplasties for AMI in relation to all angioplasties performed during this period rose from 5.2% in 1992 to 5.9% in 1995 (p = 0.01). Local complications at the puncture site occurred in 3.2%, with the need for a surgical intervention in 1.1% of patients. In 273 (5.9%) of the patients a second angioplasty was performed during the hospital stay. Aortocoronary bypass surgery was performed in 3% of the patients. Hospital mortality was 9.5% (438/4625 patients). The mortality rate remained constant during the years investigated (1992: 10.6%; 1993: 8.6%; 1994: 9.7%; 1995: 9.8%; p = ns). Higher mortality was observed in older patients, patients with multiple vessel disease, the left anterior descending artery or a bypass graft as infarct related artery as well as in patients with failed reperfusion (residual stenoses > 50%). Hospitals with a case load of more than 40 angioplasties for AMI per year showed a lower mortality as compared to the others. In clinical practice at large community hospitals results of angioplasty for AMI concerning mortality, complications and technical success rate are comparable to those of highly specialised centres. The absolute numbers of angioplasties for AMI increased constantly over the years. 相似文献
85.
Reliable Computing - 相似文献
86.
M Küchle A Amberg P Martus NX Nguyen GO Naumann 《Canadian Metallurgical Quarterly》1997,81(10):862-866
AIM/BACKGROUND: The pseudoexfoliation (PEX) syndrome is frequently associated with impairment of the blood-aqueous barrier. This study analysed if this might stimulate secondary cataract following cataract extraction. METHODS: This historical cohort study included 197 eyes of 197 patients (99 with and 98 without PEX) that underwent extracapsular cataract extraction with posterior chamber lens implantation (PMMA optic) between 1985 and 1991. Secondary cataract was defined as opacification of the axial posterior capsule and decrease of visual acuity by two or more lines. Mean follow up was 23.8 months. For statistical analysis, the Kaplan-Meier method and multivariate Cox regression analysis were used. RESULTS: Secondary cataract was observed within 24 months in 35% (SD 7%) of all eyes, and was significantly more frequent in eyes with PEX (45 (11)%) than in eyes without PEX (24 (9)%, p < 0.03). Eyes with diabetes mellitus (n = 32) showed a significantly lower frequency of secondary cataract (11 (11)%) than eyes without diabetes mellitus (39 (8)%, p < 0.01). The influences of sex, open angle glaucoma, type of cataract, surgeon, positioning of IOL, and phacoemulsification versus nuclear expression on secondary cataract did not reach statistical significance. CONCLUSION: The higher frequency of secondary cataract could be considered as another potential complication of cataract surgery in eyes with PEX. 相似文献
87.
BACKGROUND: To assess the relationship between placental inflammation, umbilical artery Doppler waveforms and fetal biophysical profile score, umbilical artery Doppler studies and fetal biophysical evaluations were performed in 24 preterm pregnants with premature rupture of membranes (PPROM). SUBJECTS: After delivery, the placentas were microscopically examined and two subgroups were formed including noninflamed or inflamed placentas. RESULTS: In the first group, which includes 14 cases with no histological signs of placental inflammation, we found increased systolic/diastolic ratio only in one patient, whereas in the second group including ten cases with microscopically proven inflammation, nine were found to have increased systolic/diastolic ratios (p < 0.05). Mean systolic/diastolic ratio in the first and the second groups were 2.74 +/- 0.18 and 4.64 +/- 0.93 respectively (p < 0.001). Mean biophysical profile score was 9 +/- 1.04 in the first group and 7 +/- 1.05 in the second group (p < 0.001). CONCLUSION: Abnormal biophysical profile scores along with increased arterial systolic/diastolic ratios have been shown to be the markers of impending clinical infection. 相似文献
88.
A theoretical model is presented by which a true expression of pregnancy rate resulting from stimulated cycles can be calculated. This includes the transfer of both fresh and cryopreserved embryos. It is concluded that the total reproductive potential of a single cycle of stimulation can only be evaluated by including pregnancies arising from all fresh and frozen embryo transfers resulting from that cycle. 相似文献
89.
Apigenin is a plant flavonoid that has been shown to significantly inhibit ultraviolet-induced mouse skin tumorigenesis when applied topically and may be an alternative sunscreen agent for humans. A long-term goal of our laboratory is to elucidate the molecular mechanism or mechanism by which apigenin inhibits skin tumorigenesis. In a previous publication, we characterized the mechanism by which apigenin induced G2/M arrest in keratinocytes. More recent studies in our laboratory have provided evidence that apigenin can induce G1 arrest in addition to arresting cells at G2/M. Here we describe the mechanism of the apigenin-induced G1 arrest in human diploid fibroblasts (HDF). Treatment of asynchronous HDF for 24 h with 10-50 microM apigenin resulted in dose-dependent cell-cycle arrest at both the G0/G1 and G2/M phases as measured by flow cytometry. The G0/G1 arrest was more clearly defined by using HDF that were synchronized in G0 and then released from quiescence by replating at subconfluent densities in medium containing 10-70 microM apigenin. The cells were analyzed for cell-cycle progression or cyclin D1 expression 24 h later. A dose of apigenin as low as 10 microM reduced the percentage of cells in S phase by 20% compared with control cultures treated with solvent alone. Western blot analysis of apigenin-treated HDF indicated that cyclin D1 was expressed at higher levels than in untreated cells, which signifies that they were arrested in G1 phase rather than in a G0 quiescent state. The G1 arrest was further studied by cyclin-dependent kinase 2 (cdk2) immune complex-kinase assays of apigenin-treated asynchronous HDF, which demonstrated a dose-dependent inhibition of cdk2 by apigenin. Inhibition of cdk2 kinase activity in apigenin-treated cells was associated with the accumulation of the hypophosphorylated form of the retinoblastoma (Rb) protein as measured by western blot analysis. The cdk inhibitor p21/WAF1 was also induced in a dose-dependent manner, with a 22-fold induction of p21/WAF1 in 70 microM apigenin-treated cells. In conclusion, apigenin treatment produced a G1 cell-cycle arrest by inhibiting cdk2 kinase activity and the phosphorylation of Rb and inducing the cdk inhibitor p21/WAF1, all of which may mediate its chemopreventive activities in vivo. To our knowledge this is the first report of a chemopreventive agent inducing p21/WAF1, a known downstream effector of the p53 tumor suppressor protein. 相似文献
90.
Characterization of the epidermal growth factor receptor in pig oviduct and endometrium 总被引:1,自引:0,他引:1
K Wollenhaupt U Tiemann R Einspanier F Schneider W Kanitz KP Brüssow 《Canadian Metallurgical Quarterly》1997,111(2):173-181
Two different plaque variants of Japanese encephalitis virus were selected from a wild-type Taiwanese isolate using Vero cells. One variant was found to exhibit small plaque morphology with retarded virus replication kinetics in Vero cells, and was demonstrated to be resistant to monoclonal antibody (mAb) E3.3 neutralization. The other variant showed large plaque morphology, was sensitive to mAb E3.3 neutralization, and manifested reduced virulence in mice on both intracranial and intraperitoneal inoculations. These two variants propagated in Vero cells retained high levels of infectivity but had relatively low HA titers as compared with the parent strain. The envelope sequences of these two variants showed four amino acid differences at residues E-85 (Glu/Arg), E-306 (Glu/Gly), E-331 (Ser/Arg), and E-387 (Met/Arg). Our results indicated the neutralizing epitope of Japanese encephalitis virus did not overlap with virus virulence determinant. 相似文献