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   252篇
  免费   1篇
电工技术   3篇
化学工业   7篇
机械仪表   1篇
建筑科学   1篇
轻工业   8篇
无线电   2篇
一般工业技术   5篇
冶金工业   201篇
自动化技术   25篇
  2022年   1篇
  2021年   5篇
  2020年   1篇
  2018年   3篇
  2014年   1篇
  2013年   3篇
  2012年   2篇
  2011年   4篇
  2010年   3篇
  2009年   4篇
  2008年   2篇
  2007年   5篇
  2006年   1篇
  2005年   4篇
  2003年   2篇
  2002年   2篇
  2001年   1篇
  2000年   2篇
  1999年   2篇
  1998年   63篇
  1997年   41篇
  1996年   17篇
  1995年   15篇
  1994年   14篇
  1993年   11篇
  1992年   2篇
  1990年   3篇
  1989年   2篇
  1988年   4篇
  1987年   3篇
  1986年   3篇
  1985年   3篇
  1983年   2篇
  1981年   4篇
  1980年   3篇
  1977年   2篇
  1976年   10篇
  1975年   1篇
  1967年   1篇
  1955年   1篇
排序方式: 共有253条查询结果,搜索用时 15 毫秒
1.
Understanding consumer behaviour is of vital importance to consumer-oriented e-business models today. In this paper, we study the relationships between consumer perceptions of risk and trust and the attitude towards purchasing at a consumer-to-consumer electronic marketplace (EM). Typical for EM settings is that consumer behaviour is subject to perceptions of the selling party as well as of the institutional structures of the intermediary that is operating the EM. Building upon the well-established literature of trust, we consider the concepts of intermediary trust and seller trust. We extend this categorisation by introducing the concepts of intermediary risk and seller risk. We developed measurement instruments for intermediary risk and seller risk. All measurement scales have acceptable alphas and are unidimensional. An empirical study is conducted to explore the relationships between the risk and trust types and consumer purchase attitude. The results reveal significant, direct effects of seller trust and seller risk. Second-order effects of intermediary trust and intermediary risk are investigated and reported. The paper concludes with general observations and recommendations for research and practice.  相似文献   
2.
Rats bled to a severe condition of volume-controlled hemorrhagic shock were randomly assigned to one of the following treatments: (1) saline, 1 ml/kg i.v.; (2) saline, 0.2 ml/kg per min i.v. for 10 min; (3) ACTH-(1-24), 160 micrograms/kg i.v.; 4) methylprednisolone, 40 mg/kg i.v.; (5) methylprednisolone, 80 mg/kg i.v.; (6) aprotinin, 10,000 KIU/kg i.v.; (7) norepinephrine, 5 micrograms/kg per min i.v. for 10 min; (8) norepinephrine, 10 micrograms/kg per min i.v. for 10 min. All rats treated with saline or with either of the two doses of methylprednisolone, and half of the rats treated with aprotinin, died within the subsequent 2 h. On the other hand, rats treated with norepinephrine, at either dose, or with ACTH-(1-24) were all still alive 2 h later, a similar improvement in cardiovascular and respiratory parameters being obtained with the two treatments. The effect of ACTH on mean arterial pressure was however more sustained throughout the observation period. These results further support the potential usefulness of ACTH-(1-24) as first-aid treatment in cases of severe blood losses.  相似文献   
3.
4.
Ruminant livestock systems are a significant source of greenhouse gases (GHGs). Thus far, mitigation options for GHG emissions mainly focused on a single gas, and are treated as isolated activities. The present paper proposes a framework for a farm level approach for the full accounting of GHG emissions. The methodology accounts for the relevant direct and indirect emissions of methane, nitrous oxide and carbon dioxide, including carbon sequestration. Furthermore, the potential trade-off with ammonia volatilisation and nitrate leaching are taken into account. A ruminant livestock farm is represented with a conceptual model consisting of five pools: animal, manure, soil, crop and feed. The carbon and nitrogen inputs, throughputs and outputs are described, and the direct emissions are related to the carbon and nitrogen flows. The indirect emissions included in the methodology are mainly carbon dioxide emissions from energy use and nitrous oxide emissions related to imported resources and nitrogen losses. The whole farm approach is illustrated with a case of two dairy farms with contrasting livestock density and grassland management. It is shown that the inclusion of carbon sequestration and all indirect emissions have a major impact on the GHG budget of the farm. For one farm, the effect of four mitigation options on the GHG emissions was quantified. It was concluded that a whole farm approach of full accounting contributes to a better insight in the interactions between the carbon and nitrogen flows and the resulting emissions, within and outside the farm boundaries. Consequently, the methodology can be used to develop efficient and effective mitigation strategies.  相似文献   
5.
Overlapping features between primary sclerosing cholangitis (PSC and autoimmune hepatitis (AIH) have previously been noted. To assess systematically similarities between these disorders, we have evaluated 114 PSC patients (36 women; 78 men), all confirmed by endoscopic retrograde cholangiography (ERC), according to a scoring system proposed by The International Autoimmune Hepatitis Group for the diagnosis of AIH. The scoring system attributes positive or negative scores to the parameters sex, ratio of elevation of serum levels of alkaline phosphatase (ALP) vs. aminotransferase, serum levels of immunoglobulins and autoantibodies, viral markers, history of drug and alcohol intake, genetic factors, liver histology, and response to therapy. Two of the PSC patients (2%) obtained scores above 15 before treatment, satisfying the diagnostic criterion of "definite" AIH. Thirty-eight patients (33%) scored between 10 and 15 points and could be classified as "probable" AIH. The serum level of immunoglobulin G (IgG) was elevated in 68 patients (61% of 111 cases tested), and positive titers of antinuclear antibodies (ANA) or smooth muscle antibodies (SMA) were detected in 24 patients (22% of 111 cases tested). Thirty-five of the PSC patients (33% of 105 evaluable biopsy specimens) obtained positive scores for histological features similar to those of AIH, but the total score for histology was in the negative range in 72 patients (69%) because of the presence of biliary changes. The frequent finding of high scores in PSC patients underlines the similarities PSC may have with AIH. A modification of the scoring system, in particular by increasing the negative score for histological biliary changes, would improve its potential to discriminate between AIH and PSC.  相似文献   
6.
Topological on-chip photonics based on tailored photonic crystals(PhCs)that emulate quantum valley-Hall effeas has recently gained widespread interest owing to its promise of robust unidirectional transport of classical and quantum information.We present a direct quantitative evaluation of topological photonic edge eigenstates and their transport properties in the telecom wavelength range using phase-resolved near-field optical microscopy.Experimentally visualizing the detailed sub-wavelength structure of these modes propagating along the interface between two topologically non-trivial mirror-symmetric lattices allows us to map their dispersion relation and differentiate between the contributions of several higher-order Bloch harmonics.Selective probing of forward-and backward-propagating modes as defined by their phase velocities enables direct quantification of topological robustness.Studying near-field propagation in controlled defects allows us to extract upper limits of topological protection in on-chip photonic systems in comparison with conventional PhC waveguides.We find that protected edge states are two orders of magnitude more robust than modes of conventional PhC waveguides.This direct experimental quantification of topological robustness comprises a crucial step toward the application of topologically protected guiding in integrated photonics,allowing for unprecedented error-free photonic quantum networks。  相似文献   
7.
A growing number of practitioners and academics endorse that the ability of organizations to foster, develop and use the innovative potential of their employees contributes to organizational success. Yet empirical investigation of individual innovation processes is lacking. In this research we address the question of whether both more flexibility in an employees’ job design and commitment‐oriented HRM activities promote individual innovative work behaviour. Findings suggest that a multifunctional job design and the perceived HRM system promote employee involvement in innovative activities through increased feelings of ownership for work‐related issues and problems.  相似文献   
8.
Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry is used to determine the stoichiometry and oxidation states of the metal centers in several iron-sulfur proteins. Samples are introduced into the ESI source under nondenaturing conditions in order to observe intact metal-containing protein ions. The stoichiometry and oxidation state of the metal or metal-sulfur cluster in the protein ion can be derived from the mass spectrum. Mononuclear metal-containing proteins and [4Fe-4S] centers are very stable and yield the molecular ion with little or no fragmentation. Proteins that contain [2Fe-2S] clusters are less stable and yield loss of one or two sulfur atoms from the molecular species, although the molecular ion is more abundant than the fragment peaks. [3Fe-4S]-containing proteins are the least stable of the species investigated, yielding abundant peaks corresponding to the loss of one to four sulfur atoms in addition to a peak representing the molecular ion. Isotope labeling experiments show that the sulfur loss originates from the [3Fe-4S] center. Negative ion mode mass spectra were obtained and found to produce much more stable [3Fe-4S]-containing ions than obtained in positive ion mode. ESI analysis of the same proteins under denaturing conditions yields mass spectra of the apo form of the proteins. Disulfide bonds are observed in the apoprotein mass spectra that are not present in the holoprotein. These result from oxidative coupling of the cysteinyl sulfur atoms that are responsible for binding the metal center. In addition, inorganic sulfide is found to incorporate itself into the apoprotein by forming sulfur bridges between cysteine residues.  相似文献   
9.
Detergent-based vaginal microbicides, in addition to their high contraceptive failure rates, cause mucosal erosion and local inflammation that might increase the risk of heterosexual human immunodeficiency virus (HIV) transmission. In a systematic effort to identify a microbicide contraceptive potentially capable of preventing the sexual transmission of HIV as well as providing fertility control, a series of novel aryl phosphate derivatives of 5-bromo-6-methoxy-3'-azido-3'-deoxythymidine (AZT; zidovudine) were synthesized and examined for dual anti-HIV and sperm-immobilizing activity (SIA). Whereas AZT displayed potent anti-HIV activity (IC50 = 0.006 microM) but lacked SIA (EC50 > 300 microM), two 5-bromo-6-methoxy-aryl phosphate derivatives of AZT, compounds WHI-05 and WHI-07, exhibited potent anti-HIV activity as well as SIA. The IC50 (HIV) and EC50 (SIA) values for WHI-07 were 439-fold and 13.5-fold lower, respectively, than those for the detergent-based virucidal spermicide, nonoxynol-9 (N-9). Sperm motion kinematics using computer-assisted sperm motion analysis combined with confocal laser scanning microscopy, high-resolution low-voltage scanning, and transmission electron microscopy demonstrated that both WHI-05 and WHI-07 cause a complete and irreversible loss of sperm motility in a concentration- and time-dependent fashion without concomitantly affecting the sperm acrosomal membrane integrity. In experiments designed to assess the fertilizing capacity of treated sperm, preincubation of sperm with either compound resulted in a concentration-dependent loss of the ability to adhere to and penetrate zona-free hamster eggs as well as inhibition of binding to human zona. WHI-07 applied intravaginally prior to artificial insemination of epididymal sperm drastically reduced fertility in hormonally primed CD-1 mice. Unlike the intravaginal application of N-9, repetitive intravaginal application of WHI-07 did not damage the vaginal epithelium or cause local inflammation. Structure-function relationship analyses showed that the addition of bromo-methoxy functional groups to AZT was essential for, and the aryl phosphate derivatization contributory to, the SIA of both compounds. Compounds WHI-05 and WHI-07 may be useful as dual-function vaginal contraceptives for women who are at high risk for acquiring HIV/acquired immunodeficiency virus syndrome by heterosexual vaginal transmission.  相似文献   
10.
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