全文获取类型
收费全文 | 2289篇 |
免费 | 13篇 |
专业分类
电工技术 | 10篇 |
化学工业 | 107篇 |
金属工艺 | 10篇 |
机械仪表 | 13篇 |
建筑科学 | 59篇 |
矿业工程 | 1篇 |
能源动力 | 13篇 |
轻工业 | 107篇 |
水利工程 | 9篇 |
石油天然气 | 1篇 |
无线电 | 73篇 |
一般工业技术 | 66篇 |
冶金工业 | 1745篇 |
原子能技术 | 6篇 |
自动化技术 | 82篇 |
出版年
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 5篇 |
2017年 | 9篇 |
2016年 | 6篇 |
2015年 | 8篇 |
2014年 | 10篇 |
2013年 | 31篇 |
2012年 | 23篇 |
2011年 | 28篇 |
2010年 | 27篇 |
2009年 | 24篇 |
2008年 | 30篇 |
2007年 | 30篇 |
2006年 | 31篇 |
2005年 | 18篇 |
2004年 | 20篇 |
2003年 | 22篇 |
2002年 | 12篇 |
2001年 | 9篇 |
2000年 | 17篇 |
1999年 | 61篇 |
1998年 | 546篇 |
1997年 | 330篇 |
1996年 | 210篇 |
1995年 | 126篇 |
1994年 | 98篇 |
1993年 | 124篇 |
1992年 | 24篇 |
1991年 | 37篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 23篇 |
1987年 | 34篇 |
1986年 | 24篇 |
1985年 | 17篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 12篇 |
1981年 | 22篇 |
1980年 | 19篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1977年 | 35篇 |
1976年 | 91篇 |
1975年 | 4篇 |
1974年 | 8篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1965年 | 2篇 |
排序方式: 共有2302条查询结果,搜索用时 15 毫秒
71.
Associations between coping responses, drinking motivations, expectations of meeting social and academic goals, and family of origin problem drinking and measures of college students' quantity/frequency of alcohol use and social complications of alcohol use were investigated in a sample of 218 college students. Positive associations were found between "emotion-focused" forms of coping such as detachment and the criterion measures, whereas "problem-focused coping" was not significantly associated with quantity/frequency of alcohol use or drinking complications. Positive correlations were also found between drinking motives, goal attainment expectancies and family models measures and the criterion measures. Regression models constructed for alcohol quantity/frequency and drinking complications implicated the total number of drinking motives, family models of problem drinking and the coping strategy of self-blame as strongly related to criterion measures. Positive social drinking motives and coping by seeking social support were implicated as possible protective factors. 相似文献
72.
SD Colman SA Rasmussen VT Ho CR Abernathy MR Wallace 《Canadian Metallurgical Quarterly》1996,58(3):484-490
Using loss of heterozygosity analysis, a method designed to detect moderate to large gene deletions, we have identified a new-mutation neurofibromatosis type 1 (NF1) patient who is somatically mosaic for a large maternally derived deletion in the NF1 gene region. The deletion extends at least from exon 4 near the 5' end of the gene to intron 39 near the 3' end. The gene-coding region is, therefore, mostly or entirely deleted, encompassing a loss of > or = 100 kb. We hypothesize that the deletion occurred at a relatively early developmental timepoint, since signs of NF1 in this patient are not confined to a specific body region, as seen in "segmental" NF, and since both mesodermally and ectodermally derived cells are affected. This report provides the first molecular evidence of somatic mosaicism in NF1 and, taken together with a recent report of germ-line mosaicism in NF1, adds credence to the concept that mosaicism plays an important role in phenotypic and genetic aspects of NF1 and may even be a relatively common phenomenon. 相似文献
73.
Simon Richard Proud Mads Olander Rasmussen Inge Sandholt Wycliffe Mutero Assaf Anyamba 《Remote sensing of environment》2010,114(8):1687-1698
In order to obtain high quality data, the correction of atmospheric perturbations acting upon land surface reflectance measurements recorded by a space-based sensor is an important topic within remote sensing. For many years the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) radiative transfer model and the Simplified Method for Atmospheric Correction (SMAC) codes have been used for this atmospheric correction, but previous studies have shown that in a number of situations the quality of correction provided by the SMAC is low. This paper describes a method designed to improve the quality of the SMAC atmospheric correction algorithm through a slight increase in its computational complexity. Data gathered from the SEVIRI aboard Meteosat Second Generation (MSG) is used to validate the additions to SMAC, both by comparison to simulated data corrected using the highly accurate 6S method and by comparison to in-situ and 6S corrected SEVIRI data gathered for two field sites in Africa. The additions to the SMAC are found to greatly increase the quality of atmospheric correction performed, as well as broaden the range of atmospheric conditions under which the SMAC can be applied. When examining the Normalised Difference Vegetation Index (NDVI), the relative difference between SMAC and in-situ values decreases by 1.5% with the improvements in place. Similarly, the mean relative difference between SMAC and 6S reflectance values decreases by a mean of 13, 14.5 and 8.5% for Channels 1, 2 and 3 respectively. Furthermore, the processing speed of the SMAC is found to remain largely unaffected, with only a small increase in the time taken to process a full SEVIRI scene. Whilst the method described within this paper is only applicable to SEVIRI data, a similar approach can be applied to other data sources than SEVIRI, and should result in a similar accuracy improvement no matter which instrument supplies the original data. 相似文献
74.
There is little information concerning the intracellular function of inositol 1,3,4,5,6-pentakis- and hexakisphosphate, despite their being the most abundant inositol polyphosphates. Current opinions that they play passive roles as antioxidants (Graf, E., Mahoney, J. R., Bryant, R. G., and Eaton, J. W. (1987) J. Biol. Chem. 259, 3620-3624) or "housekeeping" molecules (Berridge, M. J., and Irvine, R. F. (1989) Nature 341, 197-205) arises from belief in their metabolic lethargy. However, we have discovered that cell homogenates, incubated with 5 mM fluoride and 5 mM ATP, converted both inositol hexakisphosphate (Km = 2 +/- 0.5 microM, Vmax = 9 +/- 2 pmol/mg of protein/min) and inositol 1,3,4,5,6-pentakisphosphate (Km = 13 +/- 4 microM, Vmax = 11 +/- 5 pmol/mg of protein/min) to more polar products. These reactions were also observed in intact cells treated with 0.5-20 mM fluoride, and the precursor/product relationships were confirmed by comparing the effects of fluoride on cells differentially labeled with [3H]inositol in either short-term or pulse-chase protocols. The novel products were determined to be inositol pyrophosphates because of their relatively specific hydrolysis by tobacco pyrophosphatase and alkaline phosphatase. The pyrophosphates were metabolized rapidly by cell homogenates back to their pentakisphosphate and hexakisphosphate precursors. This endogenous pyrophosphatase activity was inhibited by up to 99% by 5 mM fluoride in vitro. In intact cells incubated with 10 mM fluoride, about 20% of the inositol 1,3,4,5,6-pentakisphosphate pool, and 50% of the inositol hexakisphosphate pool were each converted to pyrophosphate derivatives within 1 h. 相似文献
75.
Current methods used clinically to assess myocardial perfusion are invasive and expensive. As the technology of ultrasound imaging improves, CE may provide a relatively inexpensive, noninvasive means of quantitating myocardial perfusion. Issues regarding stability of microbubble contrast agents must be studied more closely under physiologic conditions. As such, encapsulated microbubbles may provide more stability under physiologic pressures than free gas microbubbles. Introducing high concentrations of contrast, either by hyperconcentrating the contrast agent or by increasing the injection rate, may provide greater stability under physiologic conditions. Further, before quantitative statement of tissue perfusion can be made, the relationship between tracer concentration and system response must be established. Further, a "linear" postprocessing ultrasound setting does not eliminate this requirement as data must still undergo nonlinear transformation during log compression and time-gain compensation. Additionally, issues regarding "electronic thresholding" must be explored more extensively in vivo. Commercial ultrasound scanners, in their present form, may not offer adequate sensitivity for absolute quantitative studies. Further development of modified ultrasound systems may provide sufficient sensitivity for quantitative perfusion imaging. CE offers a potentially powerful tool in the clinical management of patients with ischemic heart disease. Conventional coronary angiography provides information on the size of a lesion, but accompanying tissue perfusion distal to the lesion cannot be determined. Doppler ultrasonography determines velocity of blood flow in large vessels but does not offer the potential to quantitate tissue perfusion. Clearly, CE has a place in the future of diagnostic imaging. The recent work of Ito et al. demonstrated the qualitative potential of CE in the identification of "areas at risk" in patients who had undergone thrombolysis or percutaneous transluminal coronary angioplasty after an acute myocardial infarction. With further improvement in the ultrasound imaging techniques and microbubble stability, CE may offer an inexpensive, noninvasive means of assessing myocardial perfusion. 相似文献
76.
An analogue of "HIV-1 protease" was designed in which the ability to donate important water-mediated hydrogen bonds to substrate was precisely and directly deleted. Chemical ligation of unprotected peptide segments was used to synthesize this "backbone-engineered" enzyme. The functionally relevant amide -CONH- linkage between residues Gly49-Ile50 in each flap of the enzyme was replaced by an isosteric thioester -COS- bond. The backbone-engineered enzyme had normal substrate specificity and affinity (Km). However, the catalytic activity (kcat) was reduced approximately 3000-fold compared to the native amide bond-containing enzyme. Inhibition by the reduced peptide bond substrate analogue MVT-101 was unaffected compared with native enzyme. By contrast, the normally tight-binding hydroxyethylamine inhibitor JG-365 bound to the backbone-engineered enzyme with an approximately 2500-fold reduction in affinity. The reduced catalytic activity of the -Gly49-psi(COS)-Ile50-backbone-engineered enzyme analogue provides direct experimental evidence to support the suggestion that backbone hydrogen bonds from the enzyme flaps to the substrate are important for the catalytic function of the HIV-1 protease. 相似文献
77.
Rasmussen CD Jørgensen MB Carneiro IG Flyvholm MA Olesen K Søgaard K Holtermann A 《Ergonomics》2012,55(2):256-264
Worksite health promotion is seldom offered to workers who are low-educated and multi-ethnic, possibly due to an assumption that they are more reluctant to participate. Furthermore, little has been done to promote health at female-dominated workplaces. The main aim of this study was to investigate differences in participation among immigrant and Danish cleaners throughout a 1-year randomised controlled study tailored to cleaners and carried out in predominantly female workplaces. No significant differences in ethnicity were found in consent and participation throughout the 1-year intervention. Dropout was equally distributed among Danish and immigrant cleaners. This study indicates that a worksite health promotion intervention among a female-dominated, high-risk occupation such as cleaning can be equally appealing for Danes and immigrants. PRACTITIONER SUMMARY: This study provides insight about participation of Danish and immigrant cleaners in a worksite health promotion intervention in a predominantly female occupation. For attaining high participation and low dropout in future worksite health promotion interventions among cleaners, the intervention ought to not only target the ethnic background of the workers, but also to be specifically tailored to the job group. 相似文献
78.
To investigate the feasibility of using tissue obtained from human tumor xenografts for in vitro screening of antineoplastic agents, we grew human tumor colony-forming units (CFU) in semisold agar from xenografts serially passaged in nude mice. Growth of human tumor CFU was accomplished from nine xenografts representing five different histological tumor types (ovarian carcinoma, adenocarcinoma of the colon, malignant melanoma, epidermoid carcinoma of the lung, and malignant astrocytoma). Cloning efficiency ranged from 0.04 to 0.1% and showed significant variability both between tumor types and between individual animals bearing the same type of xenograft. A high percentage of tumor CFU was in S phase [47 +/- 20% (S.D.)] as determined by the thymidine "suicide" technique. The number of tumor CFU observed increased linearly with increasing numbers of cells plated. In vitro drug sensitivity of the tumor CFU was assessed to Adriamycin, cis-platinum, and melphalan. The patterns of drug sensitivity were found to be reproducible and stable over a period of 9 months. Drug sensitivity curves to Adriamycin for five xenografts representing four tumor types showed complex patterns with plateau portions similar to those described for tumor CFU from primary tumors. The rank order of sensitivity of the tumors was compared to that of normal granulocyte-macrophage progenitors and, with the exception of the melanomas, was found to correlate well with clinical experience (order of sensitivity = colon less than ovary less than bone marrow). Growth of human tumor CFU from xenografts represents a reproducible and stable means for the study of the biology of tumor CFU and has potential applications as a means for screening new anticancer agents. 相似文献
79.
Introducing XP into Greenfield Projects: lessons learned 总被引:1,自引:0,他引:1
ThoughtWorks introduced XP into an organization and successfully completed a bleeding-edge technology project with client staff that had no previous experience using an agile development approach. This article illustrates not only how XP helped make the project a success, but also gives other lessons learned regarding the introduction of XP at client sites. 相似文献
80.
We present a simple simulation of road growing dynamics that can generate global features as belt-ways and star patterns observed in urban transportation infrastructure. The road growing dynamics consist of two steps: Identifying the maximum transportation potential bewteen two locations within the city, followed by the generation of the least expensive road between these two locations. The simulation defines a previously missing component for modeling the co-evolution of urban settlement- and road systems as it can directly be coupled to existing urban settlement simulations. 相似文献