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   8573篇
  免费   1035篇
  国内免费   479篇
  2023年   116篇
  2022年   132篇
  2021年   210篇
  2020年   349篇
  2019年   377篇
  2018年   417篇
  2017年   368篇
  2016年   392篇
  2015年   350篇
  2014年   442篇
  2013年   672篇
  2012年   275篇
  2011年   410篇
  2010年   336篇
  2009年   417篇
  2008年   450篇
  2007年   446篇
  2006年   405篇
  2005年   400篇
  2004年   296篇
  2003年   291篇
  2002年   269篇
  2001年   184篇
  2000年   159篇
  1999年   160篇
  1998年   158篇
  1997年   126篇
  1996年   102篇
  1995年   149篇
  1994年   147篇
  1993年   104篇
  1992年   106篇
  1991年   95篇
  1990年   76篇
  1989年   88篇
  1988年   72篇
  1987年   62篇
  1986年   54篇
  1985年   64篇
  1984年   62篇
  1983年   37篇
  1982年   50篇
  1981年   38篇
  1980年   43篇
  1979年   32篇
  1978年   15篇
  1977年   21篇
  1976年   22篇
  1975年   8篇
  1973年   8篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
91.
This article examines an important class of information system that serves as the foundation for corporate energy and greenhouse gas (GHG) accounting: energy and carbon management systems (ECMS). Investors, regulators, customers, and employees increasingly demand that organizations provide information about their organizational energy use and GHG emissions. However, there is little transparency about how organizations use ECMS to meet such demands. To shed light on ECMS implementation and application, we collected extensive qualitative interview data from two service‐sector organizations: one that uses a spreadsheet‐based ECMS and another that implemented an ECMS provided by a third‐party vendor. Our analysis of collected data revealed numerous challenges in the areas of business processes, managerial capabilities, data capture and integration, and data quality. Though our study is built on only two organizations and requires confirmation in large‐sample surveys, we provide several recommendations for organizations regarding ECMS. We also provide suggestions for future studies to build on our tentative results.  相似文献   
92.
BackgroundThis study aimed to track the toxic ions released by MTA Fillapex, BioRoot RCS, and an experimental tricalcium silicate-based sealer (CEO) into local and distant tissues as well as to investigate their potential adverse effects. In addition, the chemical constituents of the sealers were also evaluated. The main components of the dry powders, pastes, and mixed sealers were characterized.Material and methodsDry powder and sealer discs were each set for 72 h and their main components were characterized by energy dispersive X-ray spectroscopy. Polyethylene tubes filled with sealers were used to measure silicon and calcium ions. Polyethylene tubes filled with sealers or empty tubes were implanted into the dorsal connective tissue of Wistar rats. On days 7, 15, 30, and 45, the animals were euthanized and their brains, livers, kidneys, and subcutaneous tissues were removed and processed to determine the concentrations of chromium, cobalt, copper, lead, iron, magnesium and nickel using an inductively coupled plasma optical emission spectrometer.ResultsThe main compounds in all sealers were carbon, oxygen, silicon, and calcium. MTA Fillapex release more Si while highest levels of Si were found in presence of BioRoot. The release of Si and Ca ions promoted by MTA Fillapex raise by time. No traces of cobalt, chromium, or magnesium were detected in any tissue. Irrespective of the sealer, no traces of copper and lead were found in the subcutaneous tissue; however, they were observed in the organs. The highest concentration of iron was identified in the liver. All sealers exhibited similar nickel traces in the brain, kidney, and liver except for MTA Fillapex, which demonstrated levels higher than CEO in the subcutaneous tissue on day 7. Tracing nickel ions over time revealed that lowest concentrations were found in subcutaneous tissue.ConclusionTaken together, our data demonstrate that CEOs have chemical compositions similar to those of other commercial sealers. Furthermore, none of them exhibited a threat to systemic health. Moreover, the minimal amounts of iron and nickel detected were not related to the sealers.  相似文献   
93.
Water sorption isotherms of superabsorbent polymers were measured, and their affinity for water was evaluated by solution thermodynamics. The results provide basic data for the functional packaging of food to control the water content of food during its transportation or storage. Water activity above 0.9 was measured by adding a specific amount of water to the samples, while that below 0.9 was measured with apparatus for evaluating water sorption isotherms. Thus, water sorption isotherms for superabsorbent polymers were obtained up to a water activity of approximately 0.98. The amount of water sorbed by the superabsorbent polymers was influenced by the type of functional groups in the polymers, and not by the degree of cross-linking in the polymers. The integral Gibbs free energy, which is the most suitable parameter for evaluating the affinity of a material for water, was evaluated from the water sorption isotherms by using solution thermodynamics.  相似文献   
94.
Measurements were made of the differences between sunflower and climbing hempweed leaves for development, fecundity and egg hatchability of Diacrisia casignetum Kollar. Developmental time and weight gains of sunflower-fed larvae were respectively longer and greater than those of hempweed-fed ones. The development time and weight gain, attributable to differences in host plants, were statistically heterogeneous (P<0.05-<0.10). Relative fecundity and egg hatchability of sunflower reared insects were greater than those of hempweed reared ones, and these were also statistically heterogeneous (P<0.1 and <0.05, respectively). Hempweed leaves contained relatively high titres of phenols and low ratios of proteins to phenols. Ovaries were heavier in insects reared on sunflower than on hempweed, whereas the level of phenols incorporated into the ovary was higher in insects fed on hempweed than those on sunflower. The differences in these parameters may be due to variations in nutritional quality of these hosts and incorporation of ingested phenolic materials to ovary and insect biomass.
Résumé L'étude a porté sur l'évaluation de l'influence des feuilles de tournesol (Helianthus annuus) et de Mikana cordata sur le développement, la fécondité et l'éclosion des oeufs de D. casignetum Kollaz. La durée de développement et le gain de poids sur tournesol étaient plus importants que sur M. cordata. La durée de développement était statistiquement hétérogène (P<0.05), ce qui peut être dû à des différences entre plantes. La fécondité et le taux d'éclosion chez les insectes élevés sur tournesol étaient plus élevés que ceux observés sur M. cordata, dont les résultats étaient aussi hétérogènes (P<0.1 et<0.05 respectivement). Les feuilles de M. cordata contenaient des taux relativement élevés de phénols et des rapports protéines sur phénols bas. Tout ceci suggère que les différences observées sont attribuables à des variations dans la qualité nutritive de ces hôtes et dans l'incorporation à la biomasse de l'insecte de matériaux phénologiques ingérés.
  相似文献   
95.
A G Anderson  J Hermans 《Proteins》1988,3(4):262-265
A direct attack on the protein-folding problem has been initiated with the free energy perturbation methods of molecular dynamics. The complete conformational probability map for the alanine dipeptide is presented. This work uses the SPC model for the explicit hydration of the dipeptide. Free energy differences for the four observed minima (beta, alpha R, alpha L, C7ax) are given, and the free energy barriers between minima are outlined.  相似文献   
96.
The soil is probably the most diverse habitat there is, with organisms ranging in sizes from less than 1 μm to several metres in length. However, it is increasingly evident that we know little about the interactions occurring between these organisms, the functions that they perform as individual species, or together within their different feeding guilds. These interactions between groups of organisms and physical and chemical processes shape the soil as a habitat and influence the nature of the soil food web with consequences for the above‐ground vegetation and food web. Protists are known as one of the most abundant groups of bacterivores within the soil; however, they are also consumers of a number of other food sources. Even though they are responsible for a large proportion of the mineralisation of bacterial biomass and have a large impact on the C and N cycles within the soil they are regularly overlooked when investigating the complete soil food web. Recently, stable isotopes have been used to determine trophic interactions and here we describe how this technique has been used to highlight linkages between protists and the soil food web.  相似文献   
97.
Von Hippel and colleagues have highlighted the crucial role of users in innovation in different industries and types of products. They describe the innovation process in terms of the distinct domains of knowledge that producers and users possess. Producers have knowledge about technical solutions and users about their needs, the context of use, and their own capabilities as users. Both sets of knowledge are characterized by "stickiness": They move relatively freely within their own domain but are difficult to transfer outside of it.
In the case of radical innovations for sustainable consumption, the problem of "sticky information" is compounded. Both producers and consumers need to reach out of their conventional competencies and search for new solutions. "Societal actors," such as government bodies or environmental experts, can show the way to such solutions, but this new knowledge needs to be integrated with the "sticky" knowledge about everyday practices in production and consumption.
In the present article we attempt to conceptualize the role and interaction of user and producer knowledge with the knowledge of environmental experts in housing energy innovations. We do so by applying the user−producer interaction framework to a case study on the introduction of low-energy housing concepts in Finland. On the basis of this analysis, we draw conclusions on the potential and limitations of today's practices in the field. For example, we suggest that user involvement can help to enhance the acceptance of low-energy solutions but that the methods for involving users need to be adapted to the particular circumstances in each industry.  相似文献   
98.
ABSTRACT. Mating pair formation in Paramecium primaurelia was shown to be inhibited by incubating mating-competent mating type (mt) I and mt U cells with Limulus polyphemus agglutinin (LPA) or wheat germ agglutinin (WGA). Preincubation of LPA and WGA with their specific binding-monosaccharides, N-acetylneuraminic acid (NeuAc) and N-acetylglucosamine (GlcNAc), respectively, prevented the lectin effect on pair formation. Addition either of NeuAc or GlcNAc resulted in a reversal of cell pairing inhibition by LPA or WGA, respectively. Both NeuAc and GlcNAc monosaccharides inhibited pair formation when their concentration exceeded a threshold value. The pattern of the relative distribution of NeuAc and GlcNAc molecules on the cell surface was analyzed using fluorescence resonance energy transfer techniques combined with imaging systems. Mt I1 cells showed a high lectin-binding site density localized just on the surface region engaged in conjugation. The pattern of mt I cells, consisting of a quite homogeneous lectin-binding site density spread on the cell surface, was also common to nonmating-competent cells and to immature cells. These findings suggest that in P. primaurelia pair formation involves both NeuAc and GlcNAc residues and that the development of mating-competence is related to a modification in NeuAc and GlcNAc relative distribution on the cell surface of mt 11 cells.  相似文献   
99.
The Population Study of Women in Gothenburg, Sweden is an ongoing prospective study of female residents who were recruited from the local registry in 1968–1969 when they were 38–60 years old. The data presented here were collected from 361 healthy women who underwent a baseline physical examination including a supplementary dietary history interview and returned for a second general health examination 6 years later. This report identifies a subgroup of 57 women who were sedentary during their leisure time and appear to have been particularly susceptible to gaining weight as a function of the fat content of their diets. Specifically, longitudinal analysis of body weights in the whole sample revealed a statistical interaction between leisure-time physical activity and habitual dietary fat intake (energy-%), as reported at the baseline examination, in the prediction of subsequent weight change. Further stratified analysis suggested that weight changes were significantly dependent on dietary fat intake among the sedentary women only. High energy intake also predicted weight gain in the sedentary group, although the predictive value for a high-fat diet was of marginal significance after adjusting for total energy consumption. These results suggest that sedentary recreational activity plus a low-fat diet may have a combined contribution to weight change that is not equivalent to the sum of the separate effects. Such a synergy between two modifiable lifestyle factors seems highly relevant for prevention of obesity.  相似文献   
100.
To explore the logic of evolutionary explanations of obesity we modelled food consumption in an animal that minimizes mortality (starvation plus predation) by switching between activities that differ in energy gain and predation. We show that if switching does not incur extra predation risk, the animal should have a single threshold level of reserves above which it performs the safe activity and below which it performs the dangerous activity. The value of the threshold is determined by the environmental conditions, implying that animals should have variable ‘set points’. Selection pressure to prevent energy stores exceeding the optimal level is usually weak, suggesting that immediate rewards might easily overcome the controls against becoming overweight. The risk of starvation can have a strong influence on the strategy even when starvation is extremely uncommon, so the incidence of mortality during famine in human history may be unimportant for explanations for obesity. If there is an extra risk of switching between activities, the animal should have two distinct thresholds: one to initiate weight gain and one to initiate weight loss. Contrary to the dual intervention point model, these thresholds will be inter-dependent, such that altering the predation risk alters the location of both thresholds; a result that undermines the evolutionary basis of the drifty genes hypothesis. Our work implies that understanding the causes of obesity can benefit from a better understanding of how evolution shapes the mechanisms that control body weight.  相似文献   
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