首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   134547篇
  免费   11514篇
  国内免费   6657篇
电工技术   8521篇
综合类   10001篇
化学工业   15241篇
金属工艺   6507篇
机械仪表   6605篇
建筑科学   6826篇
矿业工程   4210篇
能源动力   2869篇
轻工业   12775篇
水利工程   3600篇
石油天然气   3077篇
武器工业   1570篇
无线电   9800篇
一般工业技术   10246篇
冶金工业   32066篇
原子能技术   1616篇
自动化技术   17188篇
  2024年   675篇
  2023年   1741篇
  2022年   3723篇
  2021年   4665篇
  2020年   3299篇
  2019年   2268篇
  2018年   2859篇
  2017年   3418篇
  2016年   3554篇
  2015年   4761篇
  2014年   5298篇
  2013年   6201篇
  2012年   9389篇
  2011年   9918篇
  2010年   7852篇
  2009年   7322篇
  2008年   7279篇
  2007年   7121篇
  2006年   6112篇
  2005年   7637篇
  2004年   5739篇
  2003年   4213篇
  2002年   2751篇
  2001年   2494篇
  2000年   1713篇
  1999年   1346篇
  1998年   8736篇
  1997年   5281篇
  1996年   3461篇
  1995年   1995篇
  1994年   1529篇
  1993年   1619篇
  1992年   360篇
  1991年   419篇
  1990年   421篇
  1989年   386篇
  1988年   382篇
  1987年   299篇
  1986年   267篇
  1985年   219篇
  1984年   78篇
  1983年   96篇
  1982年   167篇
  1981年   215篇
  1980年   263篇
  1979年   76篇
  1978年   109篇
  1977年   761篇
  1976年   1592篇
  1975年   111篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
There is now ample evidence that during uterine contractions the utero-placental blood flow diminishes leading to a temporary decrease of fetal oxygenation. Although the fetus lives at a body temperature of 38 degrees Celsius, the adaptive mechanisms seem to be well attuned to sustain this relative oxygen deprived situation during pregnancy and also during labour. Nevertheless in clinical obstetrical practice the fetus can dy in utero or survive with neurologic handicaps. This is a rare event in term low risk babies with a bodyweight appropriate for gestational age (AGA) in contrast to obstetrical pathology that can jeopardize especially infants small for gestational age (SGA).  相似文献   
852.
Diphtheria antitoxin was determined in serum from 44 pregnant women, of whom 26 had received one injection of diphtheria toxoid during pregnancy. Their infants were vaccinated with a combined diphtheria-tetanus vaccine at 3, 5 and 12 months of age. This vaccination schedule has been used in Sweden since 1986, replacing the old schedule of vaccination at 3, 4.5 and 6 months of age originally designed for diphtheria-tetanus-pertussis vaccine, which had not been used after cessation of general vaccination against pertussis in 1979. Serum samples from the infants were obtained at 3, 7 and 18 months of age. After 2 injections infants of mothers with high antitoxin titers, > or = 0.1 IU/ml, tended to have lower antitoxin titers than infants of mothers with low antitoxin concentrations (P = 0.067). All children had, however, antitoxin above the minimum protective level of 0.01 IU/ml. Median antitoxin titers were 1.6 IU/ml in both groups after the third booster injection. Four infants of mothers who had been vaccinated during pregnancy and who had titers of > or = 0.4 IU/ml did not reach the 0.1 IU/ml level after 2 injections: all 4 responded with high antitoxin titers after the third dose. Thus all infants were primed by 2 doses of vaccine, irrespective of maternal antibody concentration. The repressive effect of maternal antibody on titers noted after 2 doses was no longer observed after the third, booster dose.  相似文献   
853.
Since 1987 The National AIDS Council in Mexico (CONASIDA), has conducted research projects among female commercial sex workers (CSW), who are seen at the AIDS Information Centers. Through these studies, we have been able to find out how different issues influence commercial sex work in Mexico City. Since the beginning of our own studies in 1989, HIV prevalence in this particular group has remained low (0.04%-0.2%). Factors found to be associated with condom use were: educational level, not having children, more experience in sex work and younger age. Condom use was reported by 88% of the women; however, qualitative information and high STD prevalence rates suggest a much lower condom use. Mandatory HIV testing is not useful as a preventive measure among sex workers. Persuasive and structural interventions must be implemented in accordance with local and legal situations.  相似文献   
854.
We have investigated the stimulative effects of mast cell growth factor (MGF) in primary acute myeloid leukemia (AML) in vitro. MGF stimulated DNA synthesis of purified leukemic blasts in eight out of 10 cases and colony formation in four cases in serum-free (SF) culture. MGF synergized with interleukin-3 (IL-3; four out of 10 cases), granulocyte-macrophage colony-stimulating factor (GM-CSF; three out of 10 cases), granulocyte colony-stimulating factor (G-CSF; six out of 10 cases), macrophage colony-stimulating factor (M-CSF; one out of 10 cases) and erythropoietin (EPO; one out of 10 cases) when added to culture in combination. Synergistic effects of MGF in combination with other CSFs were also seen in the colony assay. Antibodies against GM-CSF, M-CSF, G-CSF, and IL-6 did not inhibit the MGF response, suggesting that the stimulative effect of MGF was not mediated through autocrine release of those cytokines. Cell recovery data in liquid cultures that contained MGF, IL-3, or MGF + IL-3, indicated that both MGF and IL-3 augmented the maintenance of clonogenic cells as compared to nonsupplemented cultures, but the effect of the combination of IL-3 + MGF did not show synergy. In contrast, activation of DNA synthesis by MGF was abrogated in the presence of tumor necrosis factor (TNF; four out of 10 cases) and interleukin-4 (IL-4; two out of 10 cases). Fluorescence-activated cell sorting (FACS) analysis with anti c-kit antibodies revealed MGF receptor expression in eight out of nine cases, often in a subpopulation of the cells. Scatchard analysis of MGF receptors in two cases indicated the presence of 1460 and 41,500 (mean) binding sites, respectively, of high affinity (Kd 40-160 pmol/l). The MGF dose-response curve in the presence of IL-3 or GM-CSF resulted in a higher plateau of DNA synthesis, however no shift in the dose response was apparent. The respective reciprocal dose response relations to GM-CSF, IL-3, or G-CSF were similarly elevated when MGF was added. MGF did not alter IL-3 and GM-CSF receptor expression, nor did IL-3, GM-CSF, G-CSF, TNF, or IL-4 influence MGF binding to AML cells.(ABSTRACT TRUNCATED AT 400 WORDS)  相似文献   
855.
The purpose of this study was to examine the relationship of job satisfaction to job difficulty and worthiness, reported by the clinical psychologists who worked at medical institutions. A mail survey of 748 clinical psychologists working at hospital and clinics revealed the following results. First, job satisfaction was related more to job situation than job content. Also, job satisfaction was low among those who worked at non-university psychiatric or geriatric hospitals, who did not practice psychotherapy, and who were not in education, research, or management. In addition, compared to the high job satisfaction group, the low group perceived more incongruity between institutional climate and their psychological work, felt being less valued by other professionals, were less able to function fully as a practicing psychologist, and experienced more role ambiguity as a clinical psychologist in a hospital setting.  相似文献   
856.
This study defines the current modes of treatment of patients with uterine fibromas with a review of the literature. Progesterone treatments appear to be principally used in cases of minor functional symptomatology and we discuss recent studies of mifepristone. GnRH agonists are particularly effective in preoperative treatment for conservative surgery. The indications and results of hysteroscopic resection and laparoscopic myomectomy are compared to those of classic myomectomy and hysterectomy. The indications for myolysis are discussed.  相似文献   
857.
858.
Coat colors in the chestnut horse, the yellow Labrador retriever, the red fox, and one type of yellow mouse are due to recessive alleles at the extension locus. Similarly, dominant alleles at this locus are often responsible for dark coat colors in mammals, such as the melanic form of the leopard, Panthera pardus. We show here that the murine extension locus encodes the melanocyte-stimulating hormone (MSH) receptor. In mice, the recessive yellow allele (e) results from a frameshift that produces a prematurely terminated, nonfunctioning receptor. The sombre (Eso and Eso-3J) and tobacco darkening (Etob) alleles, which both have dominant melanizing effects, results from point mutations that produce hyperactive MSH receptors. The Eso-3J receptor is constitutively activated, while the Etob receptor remains hormone responsive and produces a greater activation of its effector, adenylyl cyclase, than does the wild-type allele.  相似文献   
859.
An 11-year global WHO campaign for eradication of smallpox finished in October 1977 as the result of Edward Jenner's primary success in 1796, who for the first time applied human vaccination against variola virus (VARV). The 200th anniversary of this happening is a good occasion to summarize the current status of the knowledge about the role of B and T lymphocytes in the control of orthopoxvirus infections. This short review concentrates on general characteristics of orthopoxviruses and the immune response to infection, mainly by vaccinia virus (VV) and ectromelia virus (EV).  相似文献   
860.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号