全文获取类型
收费全文 | 7417篇 |
免费 | 97篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 57篇 |
综合类 | 2篇 |
化学工业 | 690篇 |
金属工艺 | 60篇 |
机械仪表 | 148篇 |
建筑科学 | 200篇 |
矿业工程 | 10篇 |
能源动力 | 120篇 |
轻工业 | 389篇 |
水利工程 | 37篇 |
石油天然气 | 25篇 |
无线电 | 543篇 |
一般工业技术 | 833篇 |
冶金工业 | 3763篇 |
原子能技术 | 77篇 |
自动化技术 | 564篇 |
出版年
2021年 | 60篇 |
2020年 | 44篇 |
2019年 | 56篇 |
2018年 | 51篇 |
2017年 | 55篇 |
2016年 | 51篇 |
2015年 | 42篇 |
2014年 | 80篇 |
2013年 | 217篇 |
2012年 | 96篇 |
2011年 | 177篇 |
2010年 | 140篇 |
2009年 | 126篇 |
2008年 | 184篇 |
2007年 | 166篇 |
2006年 | 148篇 |
2005年 | 137篇 |
2004年 | 117篇 |
2003年 | 118篇 |
2002年 | 154篇 |
2001年 | 113篇 |
2000年 | 111篇 |
1999年 | 224篇 |
1998年 | 1097篇 |
1997年 | 663篇 |
1996年 | 426篇 |
1995年 | 239篇 |
1994年 | 245篇 |
1993年 | 252篇 |
1992年 | 69篇 |
1991年 | 96篇 |
1990年 | 73篇 |
1989年 | 94篇 |
1988年 | 112篇 |
1987年 | 69篇 |
1986年 | 74篇 |
1985年 | 85篇 |
1984年 | 77篇 |
1983年 | 57篇 |
1982年 | 60篇 |
1981年 | 76篇 |
1980年 | 77篇 |
1979年 | 59篇 |
1978年 | 54篇 |
1977年 | 152篇 |
1976年 | 303篇 |
1975年 | 33篇 |
1974年 | 47篇 |
1973年 | 42篇 |
1972年 | 29篇 |
排序方式: 共有7518条查询结果,搜索用时 15 毫秒
61.
62.
Piers Wilson 《Network Security》2003,2003(8):16-18
Rogue servers are a constant threat and pose a variety of problems for the IT security team. This article summarises the threats that come from not managing rogue servers and gives prevention and detection tips. 相似文献
63.
64.
CONTEXT: Adolescent smoking prevalence is tracked annually and has increased since 1991. In contrast, little is known about trends in smoking among college students, a group that has previously been more resistant to tobacco use than other young adults. OBJECTIVE: To examine changes in cigarette smoking among college students between 1993 and 1997 and among different types of students and colleges. DESIGN: Self-administered survey (Harvard School of Public Health College Alcohol Study). SETTING: One hundred sixteen nationally representative 4-year colleges. SUBJECTS: A total of 15103 randomly selected students in 1993 (70% response rate) and 14251 students in 1997 (60% response rate). MAIN OUTCOME MEASURES: Self-reports of cigarette smoking in the past 30 days and in the past year, age at smoking first cigarette, and number of attempts to quit. RESULTS: Over 4 years, the prevalence of current (30-day) cigarette smoking rose by 27.8%, from 22.3% to 28.5% (P<.001). The increase was observed in 99 of 116 colleges and was statistically significant (P<.05) in 27 (23%) of them. Current smoking increased across all student subgroups (defined by sex, race/ethnicity, and year in school) and in all types of colleges. Smoking is rising faster in public schools (from 22.0% to 29.3%) than in private schools (from 22.9% to 26.8%). Eleven percent of college smokers had their first cigarette and 28% began to smoke regularly at or after age 19 years, by which time most were already in college. Half of current smokers tried to quit in the previous year; 18% had made 5 or more attempts to quit. CONCLUSIONS: Cigarette use is increasing on campuses nationwide in all subgroups and types of colleges. Substantial numbers of college students are both starting to smoke regularly and trying to stop. National efforts to reduce smoking should be extended to college students. 相似文献
65.
RB Singh H Mori J Chen S Mendis M Moshiri S Zhu SH Kim RG Sy AM Faruqui 《Canadian Metallurgical Quarterly》1996,3(6):489-494
PURPOSE: Two types of glass wool were used to remove leukocytes in semen for evaluation of reactive oxygen species production by spermatozoa in oligozoospermic patients with leukocytospermia. METHODS: Semen samples were prepared using fine-structure glass wool (SpermFertil) and coarse-structure glass wool. In each treatment group, native semen was evaluated for sperm concentration, percentage motility, viability, leukocyte concentration, and production of reactive oxygen species. RESULTS: Electron microscopically, SpermFertil showed a higher number of leukocytes attached to the fibers compared to coarse-structure glass wool. Leukocytes in native semen and after glass wool filtration as determined by peroxidase cytochemistry confirmed this observation. Reactive oxygen species decreased from 45.303 counts/10(7) viable cells in native semen to 15.806 counts/10(7) cells in coarse structure wool and 7.465 counts/10(7) cells in Spermfertil, respectively. CONCLUSIONS: Removal of leukocytes from semen of oligozoospermic patients by means of glass wool filtration is a useful method to distinguish production of reactive oxygen species by leukocytes versus sperm cells. 相似文献
66.
67.
Serum lipids and incidence of coronary heart disease. Findings from the Systolic Hypertension in the Elderly Program (SHEP) 总被引:3,自引:0,他引:3
PH Frost BR Davis AJ Burlando JD Curb GP Guthrie JL Isaacsohn S Wassertheil-Smoller AC Wilson J Stamler 《Canadian Metallurgical Quarterly》1996,94(10):2381-2388
BACKGROUND: The association of serum lipids with coronary heart disease has been studied extensively in middle-aged men and, to a lesser extent, in similar women. Less well defined are lipid variables predictive of CHD in individuals of age > or = 60 years. METHODS AND RESULTS: The Systolic Hypertension in the Elderly Program recruited 4736 persons (mean age, 72 years; 14% were black; and 43% were men). Mean systolic and diastolic blood pressures were 170 and 77 mm Hg, respectively. Baseline mean total cholesterol was 6.11 mmol/L (236 mg/dL); HDL cholesterol, 1.39 mmol/L (54 mg/dL); and non-HDL cholesterol, 4.72 mmol/L (182 mg/dL). Triglyceride levels were 1.62 mmol/L (144 mg/dL) for fasting participants and 1.78 mmol/L for the total group. LDL cholesterol, estimated in fasting samples with triglycerides of < 4.52 mmol/L, averaged 3.98 mmol/L (154 mg/dL). Mean follow-up was 4.5 years. In multivariate Cox regression analyses, baseline total, non-HDL, and LDL cholesterol levels and the ratios of total, non-HDL, and LDL to HDL cholesterol were significantly related to CHD incidence. HDL cholesterol and triglycerides were not significant in these analyses. In fasting participants with triglyceride levels of < 4.52 mmol/L, a 1.03 mmol/L (40 mg/dL) higher baseline total, non-HDL, or LDL cholesterol was associated with a 30% to 35% higher CHD event rate. CONCLUSIONS: The results of this study support the concept that serum lipids are CHD risk factors in older Americans. 相似文献
68.
The biological consequences of O6-methylguanine (m6G) in DNA are well recognized. When template m6G is encountered by DNA polymerases, replication is hindered and trans-lesion replication results in the preferential incorporation of dTMP opposite template m6G. Thus, unrepaired m6G in DNA is both cytotoxic and mutagenic. Yet, cell lines tolerant to m6G in DNA have been isolated, which indicates that some cellular DNA polymerases may replicate m6G-containing DNA with reasonable efficiency. Previous reports suggested that mammalian pol beta could not replicate m6G-containing DNA, but we find that pol beta can catalyze trans-lesion replication; however, the lesion must reside in the optimal context for pol beta activity, single- or short nucleotide gapped substrates. Primed single-stranded DNA templates, with or without template m6G, were poor substrates for pol beta as reported in earlier studies. In contrast, trans-lesion replication by bacteriophage T4 DNA polymerase was observed for primed single-stranded DNA templates. Replication of m6G-containing DNA by T4 DNA polymerase required the gp45 accessory protein that clamps the polymerase to the DNA template. The rate-limiting step in replicating m6G-containing DNAs by both DNA polymerases tested was incorporation of dTMP across from the lesion. 相似文献
69.
RB Nagle 《Canadian Metallurgical Quarterly》1996,15(4):473-482
The human prostate is composed of a series of tubular-alveolar glands and their ducts surrounded by a fibro-muscular stroma. The parenchymal glands secrete the seminal fluid and are anatomically arranged into the central, peripheral, and transitional zones. In this chapter the pattern of intermediate filament expression by the various epithelial components of the ducts, tubuloalveolar glands, and stroma are described. The changes which occur during malignant transformation from normal glands to prostatic intraepithelial neoplasia and subsequent invasive carcinoma are presented. The usefulness of cytokeratin markers in the diagnosis of prostate carcinoma is also discussed. 相似文献
70.
Wilson B.C. Sevick E.M. Patterson M.S. Chance B. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1992,80(6):918-930
The application of optical spectroscopy and imaging in living tissue is complicated by multiple scattering of light. In spectroscopy, this scattering causes uncertainty in the pathlength traveled by photons in the tissue, while images suffer reduced resolution and contrast. Picosecond light sources and fast detectors have made it possible to address these problems by direct measurement of the photon time-of-flight. Diffusion models of light propagation can be used to relate the measured distribution of photon transit times to the scattering and absorption coefficients of the tissue. The advantages of absolute absorption measurement are demonstrated for two problems: determination of hemoglobin oxygenation in tissue and in vivo measurement of the uptake of an exogenous chromosphere such as photosensitizer. Optical imaging may also be improved by the elimination of multiply scattered photons or by selective detection of photons arriving from a given region of the tissue. The potential advantages of these techniques are discussed and illustrated with experimental data 相似文献