全文获取类型
收费全文 | 2364篇 |
免费 | 12篇 |
国内免费 | 1篇 |
专业分类
电工技术 | 4篇 |
化学工业 | 57篇 |
金属工艺 | 8篇 |
机械仪表 | 4篇 |
建筑科学 | 5篇 |
能源动力 | 3篇 |
轻工业 | 9篇 |
水利工程 | 1篇 |
石油天然气 | 1篇 |
武器工业 | 2篇 |
无线电 | 21篇 |
一般工业技术 | 64篇 |
冶金工业 | 2142篇 |
原子能技术 | 3篇 |
自动化技术 | 53篇 |
出版年
2023年 | 2篇 |
2022年 | 8篇 |
2021年 | 10篇 |
2020年 | 5篇 |
2019年 | 8篇 |
2018年 | 23篇 |
2017年 | 11篇 |
2016年 | 15篇 |
2015年 | 8篇 |
2014年 | 7篇 |
2013年 | 16篇 |
2012年 | 13篇 |
2011年 | 13篇 |
2010年 | 12篇 |
2009年 | 11篇 |
2008年 | 13篇 |
2007年 | 7篇 |
2006年 | 10篇 |
2004年 | 3篇 |
2003年 | 5篇 |
2002年 | 7篇 |
2001年 | 1篇 |
1999年 | 69篇 |
1998年 | 624篇 |
1997年 | 359篇 |
1996年 | 265篇 |
1995年 | 155篇 |
1994年 | 118篇 |
1993年 | 128篇 |
1992年 | 21篇 |
1991年 | 34篇 |
1990年 | 26篇 |
1989年 | 39篇 |
1988年 | 43篇 |
1987年 | 38篇 |
1986年 | 25篇 |
1985年 | 38篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 10篇 |
1981年 | 15篇 |
1980年 | 15篇 |
1978年 | 3篇 |
1977年 | 40篇 |
1976年 | 88篇 |
1975年 | 2篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1966年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有2377条查询结果,搜索用时 15 毫秒
991.
J Bancroft L Williamson P Warner D Rennie SK Smith 《Canadian Metallurgical Quarterly》1993,55(2):133-145
Four groups of women were compared in terms of their perimenstrual symptoms, reported menstrual blood loss and period pain, and neuroticism scores: three patient groups were referred to a Gynaecology Outpatient Clinic because of menorrhagia (N = 101), PMS (N = 104), dysmenorrhea (N = 56), and a control group (N = 105). The three patient groups showed considerable overlap in a number of symptoms. This has led us to postulate three factors contributing to perimenstrual complaints: a) a 'timing factor' linked to the ovarian cycle; b) a 'menstruation factor,' associated with the buildup of the endometrium and its shedding; and c) a 'vulnerability factor,' one aspect of which, 'neuroticism,' was measured in this study. Depressive symptoms, which were the most important in leading women to seek help for their PMS, were related to all three factors. Depressive mood changes seemed to be linked to the 'timing factor' but were noticeably worse and more prolonged in women with high neuroticism, heavy bleeding, or severe pain. One premenstrual symptom, food craving, was of considerable interest. This was weakly related to neuroticism, not apparently affected by the 'menstruation factor' and differed in severity between those in the PMS group and the other three groups. It is potentially relevant that both carbohydrate craving and depression are linked to serotonergic changes in the brain, which may prove to be particularly marked in the late luteal phase. 相似文献
992.
Capacitation of spermatozoa, a complex process occurring after sperm ejaculation, is required to obtain fertilization of the oocyte in vivo and in vitro. Although most of the biochemical/ biophysical events that occur during capacitation in vitro have been characterized, the molecular mechanisms underlying these complex events are still obscure. Increases of intracellular free Ca2+ concentrations ([Ca2+]i) and protein tyrosine phosphorylation have previously been demonstrated during in vitro capacitation of human spermatozoa. In the present study we investigated the relationship between extracellular/intracellular Ca2+, protein tyrosine phosphorylation, and tyrosine kinase and phosphatase activities during sperm capacitation. We report that the increase in tyrosine phosphorylation of several protein bands that occurs during sperm capacitation is independent of the presence of Ca2+ in the external medium and, at least partially, of the increase in [Ca2+]i occurring during the process. Indeed, the spontaneous increase in phosphorylation was still present in Ca(2+)-free/EGTA-containing-medium and in the presence of the intracellular Ca2+ chelator BAPTA/AM. Moreover, phosphorylation of proteins and protein tyrosine kinase (PTK) activity was enhanced if spermatozoa were incubated in Ca(2+)-free medium, suggesting the presence of Ca(2+)-inhibited tyrosine kinase(s) in human sperm. This hypothesis is further substantiated by the lower tyrosine phosphorylation observed after incubation with the ionophore A23187 and the endoplasmic Ca(2+)-ATPase inhibitor thapsigargin, which promote Ca2+ influx in human sperm. The ability of the cells to undergo acrosome reaction in response to progesterone, which can be considered a functional endpoint of capacitation, was highly compromised when spermatozoa were incubated in Ca(2+)-free medium or in the presence of EGTA, confirming that Ca2+ is required for sperm capacitation. Conversely, in the presence of erbstatin, a inhibitor of tyrosine kinase activity, which blunts tyrosine phosphorylation during capacitation, response to progesterone was maintained, suggesting that tyrosine phosphorylation must be kept at a low level (physiologically by the presence of Ca2+ in the external medium, or pharmacologically by the presence of erbstatin) in order to obtain response to progesterone. This mechanism may be important in vivo during sperm transit in the female genital tract to ensure appropriate timing of full capacitation in the proximity of the oocyte. 相似文献
993.
994.
What visual information do we use to guide movement through our environment? Self-movement produces a pattern of motion on the retina, called optic flow. During translation, the direction of movement (locomotor direction) is specified by the point in the flow field from which the motion vectors radiate - the focus of expansion (FoE) [1-3]. If an eye movement is made, however, the FoE no longer specifies locomotor direction [4], but the 'heading' direction can still be judged accurately [5]. Models have been proposed that remove confounding rotational motion due to eye movements by decomposing the retinal flow into its separable translational and rotational components ([6-7] are early examples). An alternative theory is based upon the use of invariants in the retinal flow field [8]. The assumption underpinning all these models (see also [9-11]), and associated psychophysical [5,12,13] and neurophysiological studies [14-16], is that locomotive heading is guided by optic flow. In this paper we challenge that assumption for the control of direction of locomotion on foot. Here we have explored the role of perceived location by recording the walking trajectories of people wearing displacing prism glasses. The results suggest that perceived location, rather than optic or retinal flow, is the predominant cue that guides locomotion on foot. 相似文献
995.
MH Kim SJ Myung DW Seo SK Lee YS Kim MH Lee BM Yoo MI Min 《Canadian Metallurgical Quarterly》1998,30(7):601-604
The precision and the diagnostic performance of the Boehringer Mannheim CEDIA DAU LSD assay was evaluated. The assay was performed in the semi-quantitative mode on a Hitachi 917 analyzer. Within-run coefficients of variation (CVs) of the semiquantitative values for 0.25 and 1.0 ng/mL were 11.2 and 6.2%, respectively. Day-to-day CVs for the same concentrations were 12.6 and 8.6%. We analyzed 318 urine samples by CEDIA, DPC Coat-A-Count RIA and Behring EMIT II. Confirmation was performed by GC-MS, after extraction on Bond Elut Certify columns. Two hundred sixty-three samples were negative by all methods. Twenty-five samples were positive by all immunoassays, 19 of which were confirmed by gas chromatography-mass spectrometry (GC-MS). One sample was falsely negative by CEDIA. Three samples were positive by EMIT and CEDIA, but negative by RIA and GC-MS. Twenty-six samples were positive by EMIT alone, but they were not confirmed by GC-MS. The LSD CEDIA assay seems to be less specific than DPC RIA but more specific than the EMIT LSD assay. 相似文献
996.
A Malley N Pangares SK Mayo M Zeleny-Pooley JV Torres E Benjamin MK Axthelm 《Canadian Metallurgical Quarterly》1993,22(2-3):80-85
These studies defined SRV-2 envelope peptides 96-102, 127-152, and 233-249 as T cell epitopes that induce significant T cell proliferation. Peripheral blood lymphocytes of Celebes macaques (Macaca nigra) exposed to SRV-2 and currently virus- antibody+, cultured with SRV-2 virus show strongly suppressed T cell responses and have two immunoregulatory T cell populations. 相似文献
997.
998.
Many xenobiotics are considered reproductive toxins because of their ability to interact with the nuclear estrogen receptors (ERalpha and ERbeta). However, there is evidence that these xenobiotics can regulate gene expression in the reproductive targets by mechanisms that do not involve these ERs. To examine this further, we compared the effects of estrogenic (o,p'-DDT [1-(o-chlorophenyl)-1-(p-chlorophenyl)2,2,2-trichloroethane] and Kepone, chlordecone) and nonestrogenic (p,p'-DDD [1,1-dichloro-2,2-bis(p-chlorophenyl)ethane], a metabolite of p,p'-DDT) xenobiotics with those of 17beta-estradiol (E2) and 4-hydroxyestradiol-17beta (4-OH-E2), a catechol metabolite of E2, on uterine expression of lactoferrin (LF) and progesterone receptor (PR). These genes are estrogen responsive in the mouse uterus. Normally, LF is expressed in the uterine epithelium, whereas PR is expressed in both the epithelium and stroma in response to estrogenic stimulation. Ovariectomized mice were injected with xenobiotics (7.5 mg/kg), E2 (10 microg/kg), 4-OH-E2 (10 microg/kg), or the vehicle (oil, 0.1 ml/mouse), and uterine tissues were processed for Northern blot and in situ hybridization. The pure antiestrogen ICI-182780 (ICI; 1 or 20 mg/kg) was used to interfere with estrogenic responses that were associated with the ERs. The results of Northern and in situ hybridization demonstrated increased uterine levels of PR and LF messenger RNAs (mRNAs) by all of these xenobiotics, but quantitatively the responses were much lower than those induced by E2 or 4-OH-E2. The results further showed that the E2-inducible epithelial LF mRNA accumulation was markedly abrogated by pretreatment with ICI (20 mg/kg). In contrast, this treatment retained the epithelial expression of PR mRNA, but down-regulated the stromal expression. In contrast, ICI had negligible effects on LF and PR mRNA responses to 4-OH-E2, indicating that this catechol estrogen exerted its effects primarily via a mechanism(s) other than the ERs. The heightened accumulation of LF mRNA in the epithelium in response to Kepone and o,p'-DDT was also severely compromised by pretreatment with ICI, but this antiestrogen had little effect on responses to p,p'-DDD. Similar to E2, Kepone increased the expression of PR mRNA in both uterine epithelium and stroma. However, pretreatment with ICI decreased stromal cell expression, whereas epithelial cell expression remained unaltered or increased. These responses were not noted in mice treated with o,p'-DDT or p,p'-DDD. Collectively, the results demonstrate that catechol estrogens or xenobiotics can alter uterine expression of estrogen-responsive genes by mechanisms that are not totally mediated by the classical nuclear ERs, and these alterations are cell type specific. We conclude that an interaction of a compound with the nuclear ERalpha and/or ERbeta is not an absolute requirement for producing specific estrogen-like effects in the reproductive target tissues. 相似文献
999.
This study examined phosphatidylinositol 4-phosphate (PtdIns4P) synthesis in caveolae that have been suggested to be discrete signaling microdomains of the plasma membrane and are enriched in the marker protein caveolin. Caveolin-rich light membranes (CLMs) were isolated from A431 cells by detergent-free, discontinuous density-gradient centrifugation method. The CLM fraction was separated from the bulk of the cellular protein and was greatly enriched in PtdIns, PtdIns4P, and phosphatidylinositol 4, 5-bisphosphate (PtdIns(4,5)P2) and an adenosine-sensitive type II PtdIns 4-kinase activity. Preparation of CLMs by an OptiPrep-based cell fractionation procedure confirmed the co-localization of PtdIns 4-kinase and caveolin. Electron microscopy confirmed that an anti-caveolin antiserum immunopurified vesicles from CLMs that were within the size range described for caveolae in other systems. Co-immunoprecipitated PtdIns 4-kinase activity could utilize endogenous PtdIns, present within the caveolae-like vesicles, to produce PtdIns4P. The addition of recombinant phosphatidylinositol transfer protein increased PtdIns 4-kinase activity both in immunoisolated caveolae and CLMs. However, less than 1% of the total cellular PtdIns and PtdIns 4-kinase activity was present in caveolae-like vesicles, indicating that non-caveolar light membrane rafts are the main site for cellular PtdIns4P production. 相似文献
1000.