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   914868篇
  免费   89934篇
  国内免费   1333篇
  2018年   19734篇
  2017年   18245篇
  2016年   18236篇
  2015年   15311篇
  2014年   17430篇
  2013年   24535篇
  2012年   31871篇
  2011年   41204篇
  2010年   31349篇
  2009年   26317篇
  2008年   34850篇
  2007年   37341篇
  2006年   23906篇
  2005年   24013篇
  2004年   23843篇
  2003年   23128篇
  2002年   21982篇
  2001年   38680篇
  2000年   38814篇
  1999年   30581篇
  1998年   10538篇
  1997年   10920篇
  1996年   10265篇
  1995年   9899篇
  1994年   9645篇
  1993年   9450篇
  1992年   24874篇
  1991年   24143篇
  1990年   23490篇
  1989年   22857篇
  1988年   21233篇
  1987年   19815篇
  1986年   18357篇
  1985年   18201篇
  1984年   15130篇
  1983年   12632篇
  1982年   9587篇
  1981年   8562篇
  1980年   8017篇
  1979年   13523篇
  1978年   10524篇
  1977年   9476篇
  1976年   8570篇
  1975年   9529篇
  1974年   10142篇
  1973年   10042篇
  1972年   9523篇
  1971年   8703篇
  1970年   6986篇
  1969年   6737篇
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
1.
Insect and veteran trees are important parts of ecosystems and are usually included in ecological studies of forest management. The loss of veteran trees in woodlands and open landscapes would lead to the loss of saproxylic organisms—an important part of biodiversity. Hence, the persistence of many specialized insects depends on the presence of veteran trees scattered in woodlands (e.g. ancient wood pastures, game parks or protected areas), cities, towns and villages (e.g. avenues, parks or chateau parks) or open landscapes (e.g. fishpond dams, solitary trees or fruit orchards). Veteran tree conditions could be fairly well described by three components—diameter, age and microhabitats present. The problem is that diameter belongs to the most studied characteristics, while age and microhabitats, which can be quite complicated to measure, are much less studied. This paper illustrates that, due to this unbalanced use of indicators of veteran-tree conditions, we are still missing some important information on saproxylic species ecology—and sometimes only large trees might be studied, rather than real veterans. Although we already know that veteran trees are essential habitat for a range of saproxylic organisms, there are still gaps in our knowledge of the specific conditions that veteran trees provide. It is vital that these are quantified and understood so that this information can be used to conserve veteran trees and their associated species.  相似文献   
2.
3.
This study is focused on the possible use of Ceratocystis paradoxa MSR2 native biomass for Cr(VI) biosorption. The influence of experimental parameters such as initial pH, temperature, biomass dosage, initial Cr(VI) concentration and contact time were optimized using batch systems as well as response surface methodology (RSM). Maximum Cr(VI) removal of 68.72% was achieved, at an optimal condition of biomass dosage 2g L−1, initial Cr(VI) concentration of 62.5 mg L−1 and contact time of 60 min. The closeness of the experimental and the predicted values exhibit the success of RSM. The biosorption mechanism of MSR2 biosorbent was well described by Langmuir isotherm and a pseudo second order kinetic model, with a high regression coefficient. The thermodynamic study also revealed the spontaneity and exothermic nature of the process. The surface characterization using FT-IR analysis revealed the involvement of amine, carbonyl and carboxyl groups in the biosorption process. Additionally, desorption efficiency of 92% was found with 0.1 M HNO3. The Cr(VI) removal efficiency, increased with increase in metal ion concentration, biomass concentration, temperature but with a decrease in pH. The size of the MSR2 biosorbent material was found to be 80 μm using particle size analyzer. Atomic force microscopy (AFM) visualizes the distribution of Cr(VI) on the biosorbent binding sites with alterations in the MSR2 surface structure. The SEM-EDAX analysis was also used to evaluate the binding characteristics of MSR2 strain with Cr(VI) metals. The mechanism of Cr(VI) removal of MSR2 biomass has also been proposed.  相似文献   
4.
5.
L-asparaginase (ASP) is a key element in the treatment of paediatric acute lymphoblastic leukaemia (ALL). However, hypersensitivity reactions (HSRs) to ASP are major challenges in paediatric patients. Our aim was to investigate genetic variants that may influence the risk to Escherichia coli-derived ASP hypersensitivity. Sample and clinical data collection was carried out from 576 paediatric ALL patients who were treated according to protocols from the Berlin—Frankfurt—Münster Study Group. A total of 20 single nucleotide polymorphisms (SNPs) in GRIA1 and GALNT10 genes were genotyped. Patients with GRIA1 rs4958351 AA/AG genotype showed significantly reduced risk to ASP hypersensitivity compared to patients with GG genotype in the T-cell ALL subgroup (OR = 0.05 (0.01–0.26); p = 4.70E-04), while no such association was found in pre-B-cell ALL. In the medium risk group two SNPs of GRIA1 (rs2055083 and rs707176) were associated significantly with the occurrence of ASP hypersensitivity (OR = 0.21 (0.09–0.53); p = 8.48E-04 and OR = 3.02 (1.36–6.73); p = 6.76E-03, respectively). Evaluating the genders separately, however, the association of rs707176 with ASP HSRs was confined only to females. Our results suggest that genetic variants of GRIA1 might influence the risk to ASP hypersensitivity, but subgroups of patients can differ significantly in this respect.  相似文献   
6.
Capsaicin has known health beneficial and therapeutic properties. It is also able to enhance the permeability of drugs across epithelial tissues. Unfortunately, due to its pungency the oral administration of capsaicin is limited. To this end, we assessed the effect of nanoencapsulation of capsaicin, under the hypothesis that this would reduce its pungency. Core-shell nanocapsules with an oily core and stabilized with phospholipids were used. This system was used with or without chitosan coating. In this work, we investigated the in vitro release behavior of capsaicin-loaded formulations in different physiological media (including simulated saliva fluid). We also evaluated the influence of encapsulation of capsaicin on the cell viability of buccal cells (TR146). To study the changes in pungency after encapsulation we carried out a sensory analysis with a trained panel of 24 students. The in vitro release study showed that the systems discharged capsaicin slowly in a monotonic manner and that the chitosan coating had an effect on the release profile. The cytotoxic response of TR146 cells to capsaicin at a concentration of 500 μM, which was evident for the free compound, was reduced following its encapsulation. The sensory study revealed that a chitosan coating results in a lower threshold of perception of the formulation. The nanoencapsulation of capsaicin resulted in attenuation of the sensation of pungency significantly. However, the presence of a chitosan shell around the nanoformulations did not mask the pungency, when compared with uncoated systems.  相似文献   
7.
8.
9.
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