全文获取类型
收费全文 | 2830篇 |
免费 | 2篇 |
专业分类
化学工业 | 17篇 |
机械仪表 | 1篇 |
建筑科学 | 1篇 |
矿业工程 | 1篇 |
轻工业 | 5篇 |
无线电 | 6篇 |
一般工业技术 | 3篇 |
冶金工业 | 2796篇 |
自动化技术 | 2篇 |
出版年
2020年 | 3篇 |
2018年 | 2篇 |
2017年 | 2篇 |
2016年 | 1篇 |
2015年 | 1篇 |
2014年 | 1篇 |
2013年 | 3篇 |
2010年 | 2篇 |
2008年 | 2篇 |
2005年 | 3篇 |
2003年 | 3篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 84篇 |
1998年 | 916篇 |
1997年 | 492篇 |
1996年 | 334篇 |
1995年 | 185篇 |
1994年 | 139篇 |
1993年 | 148篇 |
1992年 | 17篇 |
1991年 | 43篇 |
1990年 | 32篇 |
1989年 | 33篇 |
1988年 | 33篇 |
1987年 | 44篇 |
1986年 | 36篇 |
1985年 | 20篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 10篇 |
1981年 | 16篇 |
1980年 | 20篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 56篇 |
1976年 | 134篇 |
1975年 | 5篇 |
1955年 | 1篇 |
排序方式: 共有2832条查询结果,搜索用时 0 毫秒
131.
Mikael Wahlström Miika Aittala Helinä Kotilainen Tiina Yli-Karhu Janne Porkka Esa Nykänen 《Virtual Reality》2010,14(3):197-211
Several studies indicate that virtual reality (VR) systems are useful for end-user participation in an environmental design process. However, these systems can be costly and thus support for the decision whether to invest in a VR of some type is useful. This study presents a novel method for analysing the usefulness of a VR system for the purpose of end-user participation. We collected qualitative end-user opinion data in the real environment and then contrasted this data with the capabilities of a VR system. Additionally, to better understand the capabilities of the VR used, we examined how the end-users perceive the used virtual environment, which in this case was CAVE, an immersive VR system where projectors are directed to the walls of a room-sized cube. In this way, we analysed whether the same functions and elements identified by end-users on the actual wards could also be evaluated in the CAVE. Eleven nurses and 11 patients participated in the study by evaluating a bathroom and/or four patient rooms modelled by the CAVE and the actual hospital wards. The CAVE was convenient for evaluating most issues identified by the study participants in the actual hospital wards, i.e. aesthetics; correct location of equipment, supplies and materials; distraction by or the good companion of other patients as well as window position and size and living/workspace. However, it was not possible to evaluate with full certainty the possibilities for bracing against grab bars or other objects in the VR, and this was found to be relevant to the independent functioning of patients with limited mobility. Also, due to the relatively low luminance levels of projectors, evaluations regarding lighting were considered unreliable. Moreover, end-users were not always certain about the sizes and sufficiency of space in the CAVE. Solutions to overcome these limitations were proposed. 相似文献
132.
133.
134.
135.
R Ramasamy R Kanagaratnam PD Chandanie K Kulachelvy MS Ramasamy PM Dharmasena 《Canadian Metallurgical Quarterly》1999,1453(1):115-125
We describe here a new type of mitochondrial mutation (dum24; for dark uniparental minus inheritance) of the unicellular photosynthetic alga Chlamydomonas reinhardtii. The mutant fails to grow under heterotrophic conditions and displays reduced growth under both photoautotrophic and mixotrophic conditions. In reciprocal crosses between mutant and wild-type cells, the meiotic progeny only inherit the phenotype of the mating-type minus parent, indicating that the dum24 mutation exclusively affects the mitochondrial genome. Digestion with various restriction enzymes followed by DNA gel blot hybridizations with specific probes demonstrated that dum24 cells contain four types of altered mitochondrial genomes: deleted monomers lacking cob, nd4, and the 3' end of the nd5 gene; deleted monomers deprived of cob, nd4, nd5, and the 5' end of the cox1 coding sequence; and two types of dimers produced by end-to-end fusions between monomers similarly or differently deleted. Due to these mitochondrial DNA alterations, complex I activity, the cytochrome pathway of respiration, and presumably, the three phosphorylation sites associated with these enzyme activities are lacking in the mutant. The low respiratory rate of the dum24 cells results from the activities of rotenone-resistant NADH dehydrogenase, complex II, and alternative oxidase, with none of these enzymes being coupled to ATP production. To our knowledge, this type of mitochondrial mutation has never been described for photosynthetic organisms or more generally for obligate aerobes. 相似文献
136.
Infants today lose maternal measles antibody sooner than in the past. This is related to demographic changes in maternal immunization. Data for rates of decay of maternal antibody and seroconversion after measles vaccination for infants born to naturally immune (Group 1) or vaccinated (Group 2) mothers have been used to evaluate two vaccination schedules: Regime 1, measles-mumps-rubella (MMR) at 1 year of age and Regime 2, monovalent measles at 6 months followed by MMR at 15 months of age. Regime 2 costs less because MMR can be administered at 15 months with the last pentavalent booster. Months of protection/1000 children aged 0-15 months (child-months of protection) were estimated for infant populations ranging from 0 to 100% Group 1 for Regimes 1 and 2. Regime 1 provides more child-months of protection only for 100% Group 1 populations. For the study population Regime 2 provided at least 17% more child-months of protection than Regime 1. Regime 2 provides increased medical and financial benefits in proportion to the number of Group 2 infants in the population and thus is ever more advantageous for today's increasingly vaccinated populations. 相似文献
137.
138.
PM Napoli 《The Journal of communication》1999,49(4):7-34
This article presents an examination of the diversity principle in communications policy. Given the Federal Communication Commission's recent emphasis on diversity as a policy objective, diversity assessments must return to the forefront of communications policy analysis. To provide an analytical framework for such research, the diversity principle is broken into three distinct components (source, content, and exposure diversity), and multiple subcomponents. In the spirit of the FCC's empirical commitment to the diversity principle, this analysis focuses on diversity as a measurable concept, with potentially interacting dimensions and numerous means of assessment. 相似文献
139.
The purpose of this study was to compare effects on rat bone and uterus of estrogen depletion and exposure to the coplanar PCB-congener 3,3',4,4',5-pentachlorobiphenyl (PCB #126) which exhibits anti-estrogenic properties. Half of the rats were ovariectomized (n = 20) and the other half were sham-operated. Ten of the ovariectomized rats and ten of the sham operated were exposed to PCB #126 (ip injections) for 3 months (total dose: 384 microgram/kg body wt). The remaining control rats were injected with corn oil (vehicle). The rats were killed and the tibiae and uteri were dissected. The left tibia was used for measurements of weight, length, and bone mineral density and the right for histomorphometrical analysis. The uteri were analyzed with respect to estrogen receptor content. PCB #126 exposure did not affect bone mineral density or trabecular bone volume of tibia in sham-operated rats. In ovariectomized rats PCB #126 exposure resulted in a decreased length and an increased bone mineral density of tibia. An obvious PCB #126 induced increase in osteoid surface was observed in sham-operated rats. The cortical thickness and the organic content of the tibia were also increased in these rats. In estrogen deprived tissue like the uteri of ovariectomized rats, PCB #126 showed weak estrogen agonistic activity. The observed effects of PCB #126 on bone and uterine tissues differed between ovariectomized and sham-operated rats. 相似文献
140.
A Grakoui DL Donermeyer O Kanagawa KM Murphy PM Allen 《Canadian Metallurgical Quarterly》1999,162(4):1923-1930
We examined the role of the peptide/MHC ligand in CD4+ T cell differentiation into Th1 or Th2 cells using a TCR alphabeta transgenic mouse specific for hemoglobin (Hb)(64-76)/I-Ek. We identified two altered peptide ligands of Hb(64-76) that retain strong agonist activity but, at a given dose, induce cytokine patterns distinct from the Hb(64-76) peptide. The ability of these peptides to produce distinct cytokine patterns at identical doses is not due to an intrinsic qualitative property. Each peptide can induce Th2 cytokines at low concentrations and Th1 cytokines at high concentrations and has a unique range of concentrations at which these distinct effects occur. The pattern of cytokines produced from limiting dilution of naive T cells demonstrated that the potential to develop an individual Th1 or Th2 cell is stochastic, independent of Ag dose. We propose that the basis for the observed effects on the Th1/Th2 balance shown by the altered peptide ligands and the amount of Ag dose involves the modification of soluble factors in bulk cultures that are the driving force that polarize the population to either a Th1 or Th2 phenotype. 相似文献