全文获取类型
收费全文 | 3487篇 |
免费 | 113篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 25篇 |
综合类 | 2篇 |
化学工业 | 419篇 |
金属工艺 | 69篇 |
机械仪表 | 54篇 |
建筑科学 | 27篇 |
矿业工程 | 4篇 |
能源动力 | 61篇 |
轻工业 | 303篇 |
水利工程 | 9篇 |
石油天然气 | 18篇 |
无线电 | 103篇 |
一般工业技术 | 249篇 |
冶金工业 | 2095篇 |
原子能技术 | 11篇 |
自动化技术 | 152篇 |
出版年
2024年 | 7篇 |
2023年 | 19篇 |
2022年 | 69篇 |
2021年 | 87篇 |
2020年 | 48篇 |
2019年 | 65篇 |
2018年 | 63篇 |
2017年 | 78篇 |
2016年 | 61篇 |
2015年 | 35篇 |
2014年 | 69篇 |
2013年 | 125篇 |
2012年 | 95篇 |
2011年 | 103篇 |
2010年 | 89篇 |
2009年 | 78篇 |
2008年 | 78篇 |
2007年 | 52篇 |
2006年 | 48篇 |
2005年 | 53篇 |
2004年 | 31篇 |
2003年 | 38篇 |
2002年 | 20篇 |
2001年 | 15篇 |
2000年 | 20篇 |
1999年 | 80篇 |
1998年 | 609篇 |
1997年 | 395篇 |
1996年 | 265篇 |
1995年 | 140篇 |
1994年 | 109篇 |
1993年 | 139篇 |
1992年 | 18篇 |
1991年 | 28篇 |
1990年 | 19篇 |
1989年 | 24篇 |
1988年 | 27篇 |
1987年 | 17篇 |
1986年 | 20篇 |
1985年 | 19篇 |
1983年 | 4篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 3篇 |
1978年 | 6篇 |
1977年 | 63篇 |
1976年 | 125篇 |
1975年 | 6篇 |
1955年 | 2篇 |
排序方式: 共有3601条查询结果,搜索用时 15 毫秒
51.
LC Myers MP Terranova AE Ferentz G Wagner GL Verdine 《Canadian Metallurgical Quarterly》1993,261(5125):1164-1167
The Escherichia coli Ada protein repairs methylphosphotriesters in DNA by direct, irreversible methyl transfer to one of its own cysteines. Upon methyl transfer, Ada acquires the ability to bind specific DNA sequences and thereby to induce genes that confer resistance to methylating agents. The amino-terminal domain of Ada, which comprises the methylphosphotriester repair and sequence-specific DNA binding elements, contains a tightly bound zinc ion. Analysis of the zinc binding site by cadmium-113 nuclear magnetic resonance and site-directed mutagenesis revealed that zinc participates in the autocatalytic activation of the active site cysteine and may also function as a conformational switch. 相似文献
52.
M Goodrich-Tanrikulu AE Stafford JT Lin TA McKeon 《Canadian Metallurgical Quarterly》1996,46(4):382-387
BACKGROUND: From January 1993 to December 1994, we conducted a prospective study to investigate the evolutionary change of rebleeding risk in bleeding peptic ulcers. To obviate possible confounding factors that would influence decision making for discharge of patients, subjects with coexistent acute illnesses, systemic bleeding disorders, alcoholism, and use of nonsteroidal anti-inflammatory drugs were excluded. METHODS: Emergency endoscopies were performed in patients with hematemesis or a melena within 24 hours of admission. Ulcer lesions were divided into six categories according to endoscopic findings. The residual risks of rebleeding of each type of ulcers were calculated for 10 days, and the critical point of acceptable rebleeding risk after discharge was set at 3%. RESULTS: Three hundred ninety-two patients with bleeding peptic ulcers completed the study. The ulcers, characterized by clean bases, red or black spots, adherent clots, nonbleeding visible vessels without local therapy, nonbleeding visible vessels with local therapy, and bleeding visible vessels with local therapy took 0, 3, 3, 4, 4, and 3 days, respectively, to decrease rebleeding risk to below the critical point. All episodes of fatal rebleeding (n = 4) occurred within 24 hours after admission. CONCLUSIONS: Patients with clean-based ulcers can be discharged in the first day of admission. The optimal duration required for hospitalization of patients with ulcers characterized by nonbleeding visible vessels at initial endoscopy is 4 days. The remaining patients with ulcers marked by other bleeding stigmata may be discharged after a 3-day observation. 相似文献
53.
We analysed rates of detection for smear abnormalities in 255,000 women served by the Bristol screening programme. The programme began in 1966 with the aim of eradicating the 30-40 deaths each year in Bristol from cervical cancer. Organisation has been good and population uptake has been high for the past 15 years. Records were computerised in 1977. During the 1988 to 1993 screening round, 225,974 women were tested. New smear abnormalities were found in 15,551, of whom nearly 6000 were referred for colposcopy. These numbers are excessively high in comparison with the incidence of the malignancy we are trying to prevent. The effect of screening on death rates in Bristol is too small to detect. Our conclusion is that despite good organisation of the service, much of our effort in Bristol is devoted to limiting the harm done to healthy women and to protecting our staff from litigation as cases of serious disease continue to occur. The real lesson from 30 years' cervical screening is that no matter how obvious the predicted benefit may seem for any screening test, introduction should never take place without adequate prior evaluation of both positive and negative effects in controlled trials. 相似文献
54.
J Golomb MJ de Leon A Kluger AE George C Tarshish SH Ferris 《Canadian Metallurgical Quarterly》1993,50(9):967-973
OBJECTIVE: To estimate the prevalence of radiographically detectable hippocampal atrophy (HA) in a normal aging sample and to test whether such atrophy is associated with memory dysfunction. DESIGN: One hundred fifty-four medically healthy and cognitively normal elderly persons (aged 55 to 88 years) received magnetic resonance imaging and/or computed tomographic scans designed to identify HA. One hundred forty-five of these subjects also underwent psychometric tests of memory function. Multivariate analyses of variance were used to evaluate differences in memory performance between subjects with and without HA. SETTING: This study was conducted at a research clinic for the investigation of age-associated neuropsychological and neuroradiologic changes. PARTICIPANTS: Based on the following criteria, 154 subjects were consecutively selected from a larger group of elderly research volunteers participating in a study of normal aging: age of 55 years or greater; Global Deterioration Scale score of 2 or less; and Mini-Mental State examination score of 28 or greater. Subjects with evidence for significant medical, psychiatric, or neurologic disease were excluded. MAIN OUTCOME MEASURES: Outcome measurements included individual psychometric test scores and computed tomographic-magnetic resonance imaging hippocampal atrophy ratings. RESULTS: Nearly 33% of the subjects had radiographic evidence for HA. The prevalence of HA increased significantly with age and was more common in male than female subjects. After controlling for age, level of education, and vocabulary, subjects with HA were found to perform more poorly on tests of recent (secondary) verbal memory when compared with subjects without HA (P < .01). No significant differences were found for tests of immediate (primary) memory. CONCLUSION: We conclude that HA is a common accompaniment of normal aging and is associated with mild memory impairment. Additional research is needed to determine whether HA constitutes a significant risk for future dementia. 相似文献
55.
The evidence suggests that enrolled nurses and those working part-time and on night duty consistently attend less continuing professional education than their more senior, full-time and day duty colleagues. This was substantiated in the findings of this study which also highlighted the paucity of opportunities for continuing education among unqualified practitioners. In the light of recent developments within the health services, this inequitable provision of continuing professional education to all nursing practitioners will need to be addressed and resolved. Continuing professional education provided on an arbitrary and random basis will not afford maximum positive outcomes for clients, the service and practitioners and has the potential to increase the levels of frustration among disadvantaged groups of nursing staff. 相似文献
56.
57.
T Toida RE Hileman AE Smith PI Vlahova RJ Linhardt 《Canadian Metallurgical Quarterly》1996,271(50):32040-32047
Two new oligosaccharides were prepared from heparin by its partial depolymerization using heparin lyase I (EC 4.2.2.7) in an attempt to prepare oligosaccharides having intact antithrombin III binding sites. The oligosaccharides were purified by chromatography on the basis of both size and charge and demonstrated a high level of purity by capillary electrophoresis. One- and two-dimensional 1H NMR spectroscopy at 500 MHz revealed the structure of each oligosaccharide. The octasaccharide and decasaccharide are DeltaUAp2S(1-->4)-alpha-DGlcNpS6S(1-->4)-alpha-L-IdoAp (1-->4)-alpha-D -GlcNpAc6S(1-->4)-betaD-GlcAp(1-->4)-alpha-D-GlcNpS 3S6S(1-->4)-alpha- L-IdoAp2S(1-->4)alpha-D-GlcNpS6S (where DeltaUAp is 4-deoxy-alpha-L-threo-hex-enopyranosyluronic acid, GlcNp is 2-amino-2-deoxy-glucopyranose, GlcAp is glucopyranosyluronic acid, S is sulfate and Ac is acetate) and DeltaUAp2S(1-->4)-alpha-D-GlcNpS6S(1-->4)-alpha-L-IdoAp++ +(1-->4)-alpha- D-GlcNpAc6S (1-->4)-beta-D-GlcAp(1-->4)-alpha-D-GlcNpS3S6S(1-->4)-alpha- L-IdoAp2S (1-->4)-alpha-D-GlcNpS6S(1-->4)-alpha-L-IdoAp2S(1-->4)-alpha -D-GlcNpS 6S, respectively. A hexasaccharide containing a similar structural motif to that found in the antithrombin III binding site and having greatly reduced anticoagulant activity was also isolated. The structure of the hexasaccharide is DeltaUAp2S(1-->4)-alpha-D-GlcNpAc6S(1-->4)-beta-D-GlcAp++ +(1-->4)-alpha- D-GlcNpS3S6S(1-->4)-alpha-L-IdoAp(1-->4)-alpha-D-GlcNpS6S . The octasaccharide and decasaccharide correspond to the predominant structural motif found in porcine intestinal mucosal heparin. Sufficient quantities of the decasaccharide were obtained to examine its interaction with antithrombin III using microtitration calorimetry. This decasaccharide bound to antithrombin III with similar avidity as heparin and showed comparable anticoagulant activity, as determined using an antithrombin III dependent anti-factor Xa assay. Interestingly, while both decasaccharide and heparin bound to antithrombin with nanomolar affinity, very little heat of binding was observed. 相似文献
58.
The authors demonstrate the optical generation of extremely narrow linewidth millimetre-wave signals between 40 and 60 GHz using a single-chip semiconductor laser. A dual-mode long-cavity multisection DFB semiconductor laser is driven at a subharmonic of the free-running-mode beat signal frequency to produce phase-locked millimetre waves with a 3 dB linewidth of less than 10 Hz and a 3 dB locking range of ~500 MHz 相似文献
59.
IuM Rozanov AE Pereverzev EV Kaminskaia IuB Vakhtin 《Canadian Metallurgical Quarterly》1996,38(4-5):522-529
Cell heterogeneity of cytochemical characteristics of chromatin in the norm and after provocative factors in vitro (a heating at 45 degrees C, X-irradiation and joint actions of heating and X-ray irradiation) was investigated in the peripheral blood mononuclear cells of sheep and mouse, the lymphoid cell populations of the murine bone marrow, spleen and thymus. As a criterion of cytochemical heterogeneity were used the distinctions in the rate of staining of cell nuclei with DNA-specific dyes, that was registered by means of measurement of coefficient variation of the DNA-histogram basis peak, with a flow cytometry. Heterogeneity of cytochemical characteristics are dealt with showed a variability of spatial organization of interphase nuclei of single cells. The provocative factors bring about a decrease in the level of the cytochemical characteristics heterogeneity in the sheep mononuclear cells. In the murine cell populations studied no such equalizing of the marker of variability was revealed. 相似文献
60.
R. Bonadiman M. D. Lima M. J. de Andrade C. P. Bergmann 《Journal of Materials Science》2006,41(22):7288-7295
In this work, the Catalytic Chemical Vapor Deposition (CCVD) technique was used to synthesize carbon nanotubes (CNT). Natural gas (NG) was employed as a carbon source for the growing of CNT, while magnesium oxide was used as a catalyst support for the nanotubes synthesis. Two systems were utilized. The Fe–Mo/MgO system was obtained by the impregnation technique through the dispersion of iron oxide, which is the catalyst, over magnesia (with molybdenum additions). This system was tested intending to optimize the parameters for the production of single-walled carbon nanotubes (SWCNT). Moreover, Mg1−x
Fe
x
MoO4, which was prepared by the combustion synthesis method, was tested to produce multi-walled carbon nanotubes (MWCNT). The Fe-Mo/MgO tests were carried out under H2/GN and Ar/GN atmospheres at 950 °C, whereas the Mg1−x
Fe
x
MoO4 was submitted to 1,000 °C under H2/GN atmosphere. The Fe–Mo/MgO catalyst produced better results regarding number of CNT and their diameters under Ar/NG atmospheres than under H2/NG atmospheres. The system Mg1−x
Fe
x
MoO4 produced MWCNT according to the expectations. 相似文献