全文获取类型
收费全文 | 18288篇 |
免费 | 1806篇 |
国内免费 | 2588篇 |
专业分类
22682篇 |
出版年
2024年 | 103篇 |
2023年 | 436篇 |
2022年 | 508篇 |
2021年 | 647篇 |
2020年 | 731篇 |
2019年 | 865篇 |
2018年 | 723篇 |
2017年 | 857篇 |
2016年 | 805篇 |
2015年 | 785篇 |
2014年 | 983篇 |
2013年 | 1448篇 |
2012年 | 796篇 |
2011年 | 997篇 |
2010年 | 827篇 |
2009年 | 1060篇 |
2008年 | 1154篇 |
2007年 | 1057篇 |
2006年 | 957篇 |
2005年 | 773篇 |
2004年 | 713篇 |
2003年 | 627篇 |
2002年 | 515篇 |
2001年 | 468篇 |
2000年 | 467篇 |
1999年 | 413篇 |
1998年 | 318篇 |
1997年 | 276篇 |
1996年 | 243篇 |
1995年 | 227篇 |
1994年 | 220篇 |
1993年 | 192篇 |
1992年 | 178篇 |
1991年 | 172篇 |
1990年 | 139篇 |
1989年 | 121篇 |
1988年 | 101篇 |
1987年 | 98篇 |
1986年 | 106篇 |
1985年 | 70篇 |
1984年 | 80篇 |
1983年 | 64篇 |
1982年 | 97篇 |
1981年 | 55篇 |
1980年 | 68篇 |
1979年 | 38篇 |
1978年 | 36篇 |
1977年 | 15篇 |
1976年 | 14篇 |
1974年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
黄土高原半干旱偏旱区草粮轮作田土壤水分恢复效应模拟 总被引:1,自引:0,他引:1
采用实地调查和EPIC模型相结合的方法,模拟研究了黄土高原半干旱偏旱区不同草粮轮作模式的土壤水分恢复效应.结果表明:多年生苜蓿地和苜蓿翻耕后种植粮食作物农田0~10 m土层土壤湿度调查值与模拟值间的相关系数均超过0.9(P<0.01),相对均方根误差在0.05~0.16,相对误差均低于10%;模拟与实测的土壤湿度剖面分布变化趋势一致.在黄土高原半干旱偏旱区,苜蓿地深层土壤干层恢复难度较大,在种植苜蓿期间应避免8~10 m土层土壤湿度降到5.7%以下.苜蓿地适宜种植年限为4~6 a,最长不应超过8 a.苜蓿翻耕后适宜采用马铃薯→马铃薯→春小麦轮作模式进行土壤水分恢复,32~33 a后可以再次种植苜蓿. 相似文献
162.
Nancy L. Cowger Kim C. O'Connor Timothy G. Hammond Daniel J. Lacks Gabriel L. Navar 《Biotechnology and bioengineering》1999,64(1):14-26
In previous publications, we reported the benefits of a high‐aspect rotating‐wall vessel (HARV) over conventional bioreactors for insect‐cell cultivation in terms of reduced medium requirements and enhanced longevity. To more fully understand the effects that HARV cultivation has on longevity, the present study characterizes the mode and kinetics of Spodoptera frugiperda cell death in this quiescent environment relative to a shaker‐flask control. Data from flow cytometry and fluorescence microscopy show a greater accumulation of apoptotic cells in the HARV culture, by a factor of at least 2 at the end of the cultivation period. We present a kinetic model of growth and bimodal cell death. The model is unique for including both apoptosis and necrosis, and further, transition steps within the two pathways. Kinetic constants reveal that total cell death is reduced in the HARV and the accumulation of apoptotic cells in this vessel results from reduced depletion by lysis and secondary necrosis. The ratio of early apoptotic to necrotic cell formation is found independent of cultivation conditions. In the model, apoptosis is only well represented by an integral term, which may indicate its dependence on accumulation of some factor over time; in contrast, necrosis is adequately represented with a first‐order term. Cell‐cycle analysis shows the percent of tetraploid cells gradually decreases during cultivation in both vessels. For example, between 90% and 70% viability, tetraploid cells in the HARV drop from 43 ± 1% to 24 ± 4%. The data suggests the tetraploid phase as the likely origin for apoptosis in our cultures. Possible mechanisms for these changes in bimodal cell death are discussed, including hydrodynamic forces, cell–cell interactions, waste accumulation, and mass transport. These studies may benefit insect‐cell cultivation by increasing our understanding of cell death in culture and providing a means for further enhancing culture longevity. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 14–26, 1999. 相似文献
163.
Influence of Surfactants on Lipase Fat Digestion in a Model Gastro-intestinal System 总被引:1,自引:0,他引:1
Pedro M. Reis Thomas W. Raab Jean Y. Chuat Martin E. Leser Reinhard Miller Heribert J. Watzke Krister Holmberg 《Food biophysics》2008,3(4):370-381
In the present study, we use a model gastro-intestinal system to study the influence of different food-grade surface-active
molecules (Sn-2 monopalmitin, β-lactoglobulin, or lysophosphatodylcholine) on lipase activity. The interfacial activity of
lipase and surfactants are assessed with the pendant drop technique, a commonly used tensiometry instrument. A mathematical
model is adopted which enables quantitative determination of the composition of the water–oil interface as a function of bulk
surfactant concentration in the water–oil mixtures. Our results show a decrease in gastric lipolysis when interfacially active
molecules are incorporated into a food matrix. However, only the Sn-2 monopalmitin caused a systematic decrease in triglyceride
hydrolysis throughout the gastro-intestinal tract. This effect is most likely due to exclusion of both lipase and triglyceride
from the water–oil interface together with a probable saturation of the solubilization capacity of bile with monoglycerides.
Addition of β-lactoglobulin or lysophopholipids increased the hydrolysis of fat after the gastric phase. These results can
be attributed to an increasing interfacial area with lipase and substrate present at the interface. Otherwise, β-lactoglobulin,
or lysophopholipids reduced fat hydrolysis in the stomach. From the mathematical modeling of the interface composition, we
can conclude that Sn-2 monopalmitin can desorb lipase from the interface, which, together with exclusion of substrate from
the interface, explains the gradually decreased triglyceride hydrolysis that occurs during the digestion. Our results provide
a biophysics approach on lipolysis that can bring new insights into the problem of fat uptake. 相似文献
164.
165.
On semiparametric transformation cure models 总被引:4,自引:0,他引:4
166.
Multivariate failure time data are frequently encountered in longitudinal studies when subjects may experience several events or when there is a grouping of individuals into a cluster. To take into account the dependence of the failure times within the unit (the individual or the cluster) as well as censoring, two multivariate generalizations of the Cox proportional hazards model are commonly used. The marginal hazard model is used when the purpose is to estimate mean regression parameters, while the frailty model is retained when the purpose is to assess the degree of dependence within the unit. We propose a new approach based on the combination of the two aforementioned models to estimate both these quantities. This two-step estimation procedure is quicker and more simple to implement than the EM algorithm used in frailty models estimation. Simulation results are provided to illustrate robustness, consistency, and large-sample properties of estimators. Finally, this method is exemplified on a diabetic retinopathy study in order to assess the effect of photocoagulation in delaying the onset of blindness as well as the dependence between the two eyes blindness times of a patient. 相似文献
167.
Hydrogen sulfide (H2S) is the most undesirable inorganic gas in biogas from anaerobic digestion (AD). However, H2S production in AD is complex and understanding of its processes is still limited. This study performed six controlled batch anaerobic co-digestion experiments to investigate H2S production. Materials were obtained from four field anaerobic digester systems and co-digestion feedstocks from agroindustry. An additional precipitation experiment was conducted to further examine H2S production dynamics. Digesters containing highly soluble, carbohydrate-based wastes had a high H2S final specific production (FSP) value. Additionally, the FSP values were negatively correlated with the initial Fe(II):S ratios in the digester liquid of the batch tests. The precipitation experiment indicated that iron sulfide precipitation was preferred in the presence of an anaerobic community. The H2S production as a time series was successfully modeled using a generalized additive model (R2 > 0.82). This study revealed that sulfate, phosphorus, and iron concentrations are important predictors and potential inhibitors of H2S production in AD. Further examination of real-time H2S modeling in AD is warranted. 相似文献
168.
Celina B. Baines Justin M. J. Travis Shannon J. McCauley Greta Bocedi 《Evolution; international journal of organic evolution》2020,74(10):2238-2249
Empirical studies have documented both positive and negative density-dependent dispersal, yet most theoretical models predict positive density dependence as a mechanism to avoid competition. Several hypotheses have been proposed to explain the occurrence of negative density-dependent dispersal, but few of these have been formally modeled. Here, we developed an individual-based model of the evolution of density-dependent dispersal. This model is novel in that it considers the effects of density on dispersal directly, and indirectly through effects on individual condition. Body condition is determined mechanistically, by having juveniles compete for resources in their natal patch. We found that the evolved dispersal strategy was a steep, increasing function of both density and condition. Interestingly, although populations evolved a positive density-dependent dispersal strategy, the simulated metapopulations exhibited negative density-dependent dispersal. This occurred because of the negative relationship between density and body condition: high density sites produced low-condition individuals that lacked the resources required for dispersal. Our model, therefore, generates the novel hypothesis that observed negative density-dependent dispersal can occur when high density limits the ability of organisms to disperse. We suggest that future studies consider how phenotype is linked to the environment when investigating the evolution of dispersal. 相似文献
169.
The basic reproductive ratio, R0, is a central quantity in the investigation and management of infectious pathogens. The standard model for describing stochastic epidemics is the continuous time epidemic birth-and-death process. The incidence data used to fit this model tend to be collected in discrete units (days, weeks, etc.), which makes model fitting, and estimation of R0 difficult. Discrete time epidemic models better match the time scale of data collection but make simplistic assumptions about the stochastic epidemic process. By investigating the nature of the assumptions of a discrete time epidemic model, we derive a bias corrected maximum likelihood estimate of R0 based on the chain binomial model. The resulting 'removal' estimators provide estimates of R0 and the initial susceptible population size from time series of infectious case counts. We illustrate the performance of the estimators on both simulated data and real epidemics. Lastly, we discuss methods to address data collected with observation error. 相似文献
170.
一类一级饱和反应系统的极限环 总被引:4,自引:0,他引:4
本文研究生化反应中一类饱和反应的数学模型:应用微分方程定性理论,完整地解决了该系统极限环的存在性、唯一性和不存在性等问题. 相似文献