全文获取类型
收费全文 | 412732篇 |
免费 | 5817篇 |
国内免费 | 1386篇 |
专业分类
电工技术 | 7704篇 |
综合类 | 349篇 |
化学工业 | 61994篇 |
金属工艺 | 14285篇 |
机械仪表 | 11870篇 |
建筑科学 | 11637篇 |
矿业工程 | 869篇 |
能源动力 | 11117篇 |
轻工业 | 43797篇 |
水利工程 | 3312篇 |
石油天然气 | 1835篇 |
武器工业 | 13篇 |
无线电 | 55388篇 |
一般工业技术 | 76614篇 |
冶金工业 | 74951篇 |
原子能技术 | 5568篇 |
自动化技术 | 38632篇 |
出版年
2021年 | 2669篇 |
2019年 | 2434篇 |
2018年 | 3838篇 |
2017年 | 3904篇 |
2016年 | 4144篇 |
2015年 | 3426篇 |
2014年 | 5714篇 |
2013年 | 19282篇 |
2012年 | 10202篇 |
2011年 | 14680篇 |
2010年 | 11283篇 |
2009年 | 12884篇 |
2008年 | 13914篇 |
2007年 | 14285篇 |
2006年 | 12777篇 |
2005年 | 11961篇 |
2004年 | 11574篇 |
2003年 | 11279篇 |
2002年 | 10913篇 |
2001年 | 11131篇 |
2000年 | 10271篇 |
1999年 | 10769篇 |
1998年 | 24010篇 |
1997年 | 17587篇 |
1996年 | 13903篇 |
1995年 | 10979篇 |
1994年 | 9771篇 |
1993年 | 9354篇 |
1992年 | 7121篇 |
1991年 | 6790篇 |
1990年 | 6397篇 |
1989年 | 6106篇 |
1988年 | 5860篇 |
1987年 | 4884篇 |
1986年 | 4834篇 |
1985年 | 5978篇 |
1984年 | 5599篇 |
1983年 | 4832篇 |
1982年 | 4496篇 |
1981年 | 4447篇 |
1980年 | 4208篇 |
1979年 | 4080篇 |
1978年 | 3771篇 |
1977年 | 4548篇 |
1976年 | 6097篇 |
1975年 | 3115篇 |
1974年 | 3011篇 |
1973年 | 2890篇 |
1972年 | 2318篇 |
1971年 | 2003篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
A novel, heat-resistant and Pronase-sensitive, inhibitor of eukaryotic DNA topoisomerase I has been purified from Xenopus laevis ovaries. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the most purified fraction revealed three bands with apparent molecular masses of 25, 28.5, and 33.5 kDa. The 25- and 33.5-kDa peptides recovered from an SDS-PAGE gel inhibited X. laevis DNA topoisomerase I. The purified inhibitor was specific to DNA topoisomerase I and did not inhibit other DNA enzymes tested. The inhibitor blocked the catalytic activity of DNA topoisomerase I by interacting with the enzyme, rather than by competing for binding sites on substrate DNA. Binding of DNA topoisomerase I to substrate DNA was blocked by the inhibitor, as was the cleavage reaction catalyzed by DNA topoisomerase I. Inhibition of DNA topoisomerase I was relieved by divalent cations Ca2+, Mg2+, or Mn2+. 相似文献
902.
K Takayama H Ueno XH Pei Y Nakanishi J Yatsunami N Hara 《Canadian Metallurgical Quarterly》1998,5(3):361-368
Adenovirus mediated transfer of growth-inhibiting molecules, such as p53 shows promise as an effective method of suppressing the growth of cancer cells. As the basis for in vivo studies, we examined transfection efficiency using 15 human lung cancer cell lines that differ in their endogenous p53 status. When infected with an adenovirus expressing bacterial beta-galactosidase, the different cell lines showed different levels of beta-galactosidase activity. We found a correlation between the level of integrin alpha v beta 5, which is thought to be an adherence receptor for adenoviruses, and the expression level of the transferred gene, suggesting that gene expression is largely dependent on the infection efficiency. Growth inhibition was induced in all cell lines tested following infection with an adenovirus containing p53, regardless of the genetic status of their endogenous p53 provided a sufficient amount of p53 protein was expressed. Our results (1) confirm that the examination of the susceptibility of target cancer cells to an adenovirus is important when considering performing adenovirus-mediated gene transfer and for evaluating its therapeutic effects; and (2) suggest that the quantification of integrin alpha v beta 5 may be a good way of predicting the susceptibility of cells to adenoviral vectors. 相似文献
903.
Danqing Yi Changhai Li Zonghe Lai O. M. Akselsen J. H. Ulvensoen 《Metallurgical and Materials Transactions A》1998,29(1):119-129
Ternary alloying of MoSi2 with adding a series of transition elements was investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission
electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Iron, Co, Ni, Cr, V, Ti, and Nb were chosen as alloying
elements according to the AB2 structure map or the atomic size factor. The studied MoSi2 base alloys were prepared by the arc melting process from high-purity metals. The EDS analysis showed that Fe, Co, and Ni
have no solid solubility in as-cast MoSi2, while Cr, V, Ti, and Nb exhibit limited solid solubilities, which were determined to be 1.4±0.7, 1.4±0.4, 0.4±0.1, and 0.8±0.1.
Micro-structural characterization indicated that Mo-Si-MVIII (MVIII=Fe, Co, Ni) and Mo-Si-Cr alloys have a two-phase as-cast microstructure, i.e., MoSi2 matrix and the second-phase FeSi2, CoSi, NiSi2, and CrSi2, respectively. In as-cast Mo-Si-V, Mo-Si-Ti, and Mo-Si-Nb alloys, besides MoSi2 and C40 phases, the third phases were observed, which have been identified to be (Mo, V)5Si3, TiSi2, and (Mo, Nb)5Si3. 相似文献
904.
J. P. Freidberg 《Journal of Fusion Energy》1998,17(2):75-79
In many academic institutions plasma science is currently viewed as a basic physics discipline encompassing a broad range of applications including fusion, astrophysics, space physics, low temperature plasma physics for the semiconductor industry, and environmental remediation of nuclear and chemical waste. Although the applications are broad, it is accurate to state that the major development of the field has been driven by the scientific needs of a single program, fusion. As such, plasma science and engineering has played an important role in graduate education since the early days of the fusion program, late 50's, early 60's. 相似文献
905.
JF Weel RW van der Hulst Y Gerrits GN Tytgat A van der Ende J Dankert 《Canadian Metallurgical Quarterly》1996,34(9):2158-2162
Combination therapies that include metronidazole (MTZ) are the most successful therapies used in eradicating Helicobacter pylori. In this study, the prevalence and the relevance of heterogeneity in susceptibility to MTZ among H. pylori populations of 156 patients were evaluated. The results of this study show that 37 patients (24%) were infected with MTZ-resistant H. pylori (MIC > or = 8 micrograms/ml). Furthermore, 33% (52 of 156) of the patients were found to be infected with H. pylori populations heterogeneous for their susceptibility to MTZ. The reassessment of the MICs of MTZ for these 52 H. pylori populations revealed MTZ resistance in 28 of them, increasing the number of MTZ-resistant H. pylori populations among the 156 patients to 65 (42%). Out of 20 isolates, 2 (10%) heterogeneous in their susceptibility to MTZ also appeared to be heterogeneous at the genome level as determined by randomly amplified polymorphic DNA fingerprinting. In conclusion, the results show the limitations and risk of possible misinterpretations when only a single colony, picked from the primary H. pylori populations isolated from patients, is analyzed for its susceptibility to MTZ. 相似文献
906.
Heterochromatin effects on the frequency and duration of LCR-mediated gene transcription 总被引:3,自引:0,他引:3
E Milot J Strouboulis T Trimborn M Wijgerde E de Boer A Langeveld K Tan-Un W Vergeer N Yannoutsos F Grosveld P Fraser 《Canadian Metallurgical Quarterly》1996,87(1):105-114
Organ transplantation is associated with an early bone loss that subsequently increases the risk of osteopenia and bone fractures. It is not known whether bone loss continues in long-term survivors, but persistent bone demineralization could further jeopardize an already diminished bone mass. To better define long-term bone status of kidney transplant recipients (KTR), we conducted cross-sectional and longitudinal evaluations of bone mineral density (BMD) in 70 KTR with a mean posttransplantation time of 8.1 years. BMD was determined by dual-energy X-ray absorptiometry and was repeated in 55 of the patients after a mean follow-up period of 22 +/- 5 months. Lumbar and femoral osteopenia, defined as a BMD lower than 2 standard deviations from mean value of sex- and age-matched controls, was present in 28.6% and 10.5% of patients, respectively. There was a significant negative correlation between cumulative prednisone dose and adjusted lumbar as well as femoral BMD (R = 0.45, P < 0.001 and R = 0.29, P < 0.05, respectively). Five patients had a vertebral BMD below a fracture threshold of 0.777 g/cm2. Vertebral fractures (VF) were found in four men and were associated with higher prednisone dosage (P < 0.05), larger cumulative prednisone dose (P < 0.05), and significantly lower BMD values. According to World Health Organization recent criteria for women, prevalences of lumbar and femoral osteopenia and lumbar and femoral osteoporosis in female patients reach 75%, 65%, 33%, and 10%, respectively. The longitudinal part of the study showed a persistent pathological lumbar demineralization process. Over the study period, BMD, expressed as a percentage of that of controls, decreased from 89 +/- 14% to 86 +/- 14% (P < 0.001). Annual change of bone mass was -1.7 +/- 2.8% per year. Accelerated vertebral bone loss (defined as a decrease of BMD, expressed as a percentage of that of controls, of more than 1% per year) was present in 56% of patients and was associated with higher prednisone dosage (P < 0.01). In conclusion, although VF are relatively infrequent in long-term survivors of renal transplantation, osteopenia is a frequent finding, and a substantial proportion of women present lumbar osteoporosis. An ongoing demineralization process of the spine is also demonstrated and probably contributes to long-term spinal osteoporosis incidence. Prednisone dosage remains the most constantly isolated risk factor. 相似文献
907.
EH Jeffery K Abreo E Burgess J Cannata JL Greger 《Canadian Metallurgical Quarterly》1996,48(6):649-665
Although the full mechanisms are not yet elucidated, research into the mechanism of toxicity of aluminum (Al) on bone formation and remodeling and on hematopoietic tissue is ongoing. In contrast little information exists on the interactive effects of systemic Al and the kidney. In bone, both clinically and experimentally, high doses of Al inhibit remodeling, slowing both osteoblast and osteoclast activities and producing osteomalacia and adynamic bone disease. In contrast, while very low levels of Al are mitogenic in bones of experimental animals, the effect of low levels of Al in humans is unknown. Aluminum has been shown to have its mitogenic action at the osteoblast, but whether the effect on resorption is viz osteoblast-directed changes in osteoclast activity has not yet been determined. Parathyroid hormone (PTH) levels are disrupted by Al in humans and animals. Whether altered PTH levels play a major or even a minor role in Al-dependent osteotoxicity requires clarification. In hematopoietic tissue, Al causes a microcytic anemia, not reversible by iron. Friend leukemia cells treated with Al have been reported to accumulate excess iron, without incorporating it into ferritin or heme. It is not yet known which steps in iron metabolism are disrupted by Al, if they involve a single mechanism of action, or even if this disruption in iron metabolism accounts for the anemia seen in Al toxicosis. In kidney, research is needed to evaluate Al nephrotoxicity; there are almost no studies in this area. Furthermore, research is needed to evaluate mechanisms of renal Al excretion, presently shown by one study to occur at the distal tubule. Such studies might well throw light on whether Al plays a role in aggravating renal insufficiency, or whether the role of the kidney in Al toxicosis is limited to the causative effect of renal compromise on Al accumulation. In summary, while a number of mechanisms have been proposed for the toxic action of Al, no single mechanism emerges to explain these diverse effects of systemic Al. Recommendations for future research are presented and summarized in Table 1. 相似文献
908.
J Holroyd 《Canadian Metallurgical Quarterly》1996,44(1):33-51
Clinical and experimental research literature indicates hypnosis is very useful for severe and persistent pain, yet reviews suggest hypnosis is not widely used. To encourage more widespread clinical application, the author reviews recent controlled clinical studies in which hypnosis compares favorably with other interventions; links advances in understanding endogenous pain modulation to a neurophysiologic view of hypnosis and hypnoanalgesia; relates the neurophysiology of hypnoanalgesia to management of chronic pain; challenges the view that hypnotic pain control is only for the highly hypnotizable patient; and raises issues about how people learn to control pain with hypnosis. Training in hypnotic analgesia may usefully enhance nervous system inhibitory processes that attenuate pain. 相似文献
909.
S Cohrs G Herrendorf J Wiltfang A Rodenbeck J Kinkelbur L Adler G Hajak 《Canadian Metallurgical Quarterly》1996,146(13-14):366-368
10 patients with obstructive sleep apnea syndrome (OSAS) have been treated with the new surgical procedure functional palatoraphy and modified genioplasty. 5 months after surgery 7 patients with an apnea hypopnea index under 10 were cured. Three therapy refractory patients were all overweight with a body mass index of more than 29 kg/m2. Excessively overweight patients should therefore not be operated. Following the selection criteria we introduced an effective new treatment method for OSAS. 相似文献
910.
Abrial A. Bouvier J. Fournier J.-M. Senn P. 《Solid-State Circuits, IEEE Journal of》1993,28(7):725-729
A half-flash, subranging, 8-b, 13.5-MHz, video ADC (analog-to-digital converter) using overlapped architecture that combines the advantages of both flash and half-flash converters is described. Its conversion rate is that of a flash, without any multiplexing and with a low number of comparators. Its low power consumption and the small silicon area required for its implementation enable it to be integrated in mixed digital/analog circuits such as a video acquisition circuit devoted to visiophony applications. It has been manufactured using a CMOS 1-μm technology with two polysilicon and two metallization layers 相似文献