全文获取类型
收费全文 | 5578篇 |
免费 | 788篇 |
国内免费 | 825篇 |
专业分类
林业 | 393篇 |
农学 | 595篇 |
基础科学 | 2350篇 |
1401篇 | |
综合类 | 1784篇 |
农作物 | 121篇 |
水产渔业 | 156篇 |
畜牧兽医 | 141篇 |
园艺 | 41篇 |
植物保护 | 209篇 |
出版年
2024年 | 20篇 |
2023年 | 176篇 |
2022年 | 284篇 |
2021年 | 264篇 |
2020年 | 314篇 |
2019年 | 293篇 |
2018年 | 179篇 |
2017年 | 325篇 |
2016年 | 347篇 |
2015年 | 261篇 |
2014年 | 321篇 |
2013年 | 368篇 |
2012年 | 504篇 |
2011年 | 472篇 |
2010年 | 358篇 |
2009年 | 343篇 |
2008年 | 299篇 |
2007年 | 355篇 |
2006年 | 295篇 |
2005年 | 247篇 |
2004年 | 173篇 |
2003年 | 166篇 |
2002年 | 118篇 |
2001年 | 86篇 |
2000年 | 83篇 |
1999年 | 78篇 |
1998年 | 78篇 |
1997年 | 69篇 |
1996年 | 59篇 |
1995年 | 43篇 |
1994年 | 40篇 |
1993年 | 39篇 |
1992年 | 28篇 |
1991年 | 19篇 |
1990年 | 29篇 |
1989年 | 19篇 |
1988年 | 20篇 |
1987年 | 9篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1979年 | 2篇 |
1956年 | 1篇 |
排序方式: 共有7191条查询结果,搜索用时 46 毫秒
1.
完整的水轮机特性曲线是水电站运行仿真和过渡过程计算的基础,然而厂家提供的水轮机模型综合特性曲线仅反映了水轮机高效率区域的特性,不能满足水轮机动态过程仿真需求。该研究提出了一种扩展方法:首先以零转速、零流量、飞逸曲线、零开度线、单位力矩交点为边界条件,作为划分特性曲线区域与约束各分区延拓范围的特征点,并依据内特性模型和外特性数据辨识水轮机结构参数,计算边界条件;然后针对不同区域的特点,提出不同拟合方法得到各区的特性曲线;最后对分区界线两侧拟合结果进行平滑连接,形成完整的可用于仿真分析的混流式水轮机特性曲线。对某水轮机进行实例分析,与典型外特性法和内特性法的对比表明,本文方法既保证了外特性试验数据处的拟合精度,又能反映水轮机的水力特性;与典型外特性法进行过渡过程实测反演对比可知,本文所提方法将最大蜗壳压力的相对误差从2.03%降至1.69%,小开度工况下区域平均蜗壳压力的相对误差从3.48%降至1.47%,动态过程时域响应更接近实测结果。本文特性曲线处理方法有利于提高过渡过程计算精度,也可为类似叶片式农业机械的设计及特性曲线处理提供参考。 相似文献
2.
3.
Yuya Kumagai Keigo Toji Satoshi Katsukura Rie Morikawa Toshiki Uji Hajime Yasui Takeshi Shimizu Hideki Kishimura 《Marine drugs》2021,19(4)
More than 7000 red algae species have been classified. Although most of them are underused, they are a protein-rich marine resource. The hydrolysates of red algal proteins are good candidates for the inhibition of the angiotensin-I-converting enzyme (ACE). The ACE is one of the key factors for cardiovascular disease, and the inhibition of ACE activity is related to the prevention of high blood pressure. To better understand the relationship between the hydrolysates of red algal proteins and the inhibition of ACE activity, we attempted to identify novel ACE inhibitory peptides from Pyropia pseudolinearis. We prepared water soluble proteins (WSP) containing phycoerythrin, phycocyanin, allophycocyanin, and ribulose 1,5-bisphosphate carboxylase/oxygenase. In vitro analysis showed that the thermolysin hydrolysate of the WSP had high ACE inhibitory activity compared to that of WSP. We then identified 42 peptides in the hydrolysate by high-performance liquid chromatography and mass spectrometry. Among 42 peptides, 23 peptides were found in chloroplast proteins. We then synthesized the uncharacterized peptides ARY, YLR, and LRM and measured the ACE inhibitory activity. LRM showed a low IC50 value (0.15 μmol) compared to ARY and YLR (1.3 and 5.8 μmol). In silico analysis revealed that the LRM sequence was conserved in cpcA from Bangiales and Florideophyceae, indicating that the novel ACE inhibitory peptide LRM was highly conserved in red algae. 相似文献
4.
通过有限单元法构建大网目底拖网渔具的数学模型,以计算在定常流条件下底拖网渔具的受力情况、网口扩张和渔具的形态变化。为了提高计算效率,提出了适用于拖网的网目群化方法。群化后的拖网模型为单元数6513的model-1模型和单元数1787的model-2模型两种规格,分别进行数值计算,并与水槽物理模型试验结果进行比较。结果表明,model-1模型和model-2模型在相同流速条件下的形态变化趋势基本一致;model-1的阻力计算结果相较于model-2更接近于物理模型试验结果,model-2的计算结果存在16%~26%的相对误差;两种数值模型的网口垂直扩张和水平扩张的计算结果与物理模型试验结果相近,最大相对误差仅5%~10%。数值模拟较好地呈现了大网目底拖网在稳定流场中的状态,可以在一定程度上代替传统的水槽模型试验,为拖网渔具设计和优化提供一种可供选择的数值分析方法。 相似文献
5.
为解决沙棘浆果中含有杂质,导致浆果筛分含杂较高的问题,设计三层结构滚筒筛装置,并确定振动电机、减震弹簧等关键部件参数。其激振力为7 644 N,功率为3.24 kW,减震弹簧刚度8 047.3 N/m。使用EDEM离散元软件对滚筒筛的转速、倾角进行模拟分析。模拟结果表明:当滚筒筛转速、倾角为38 r/min、8°时,其筛分效率、含杂率为84.2%、6.98%。以转速、倾角两个因素为自变量,筛分效率、含杂率为指标进行方差分析,当滚筒筛转速为38 r/min、倾角为8°时是最优参数组合解,且筛分效率、含杂率分别为82%、8.7%。实测值与仿真试验的筛分效率相差2.2%,含杂率的相差1.72%。 相似文献
6.
针对江苏地区油菜免耕播种机作业时存在易堵塞、种床清洁率低等问题,设计一种齿盘式秸秆移位装置,通过运动力学分析确定秸秆移位的重要影响因素,使用离散元仿真软件进行仿真试验,得出秸秆移位装置工作时的最佳参数组合,最后进行台架试验验证仿真试验准确性。仿真试验结果表明秸秆清除率与位移偏角以及前进速度均呈现正相关的关系,当前进速度一定时,位移偏角越大,装置的秸秆清除效果越好;当位移偏角一定时,前进速度越大,装置的秸秆清除效果越好。当前进速度为0.6 m/s、位移偏角为30°时,秸秆清除率达到最大值64.70%。台架验证试验表明,当机具前进速度为0.6 m/s、齿盘位移偏角为30°时秸秆清除效果较好,此时秸秆清除率为60.69%。试验结果表明齿盘式秸秆移位装置的设计基本满足油菜播种的要求。 相似文献
7.
作为拖拉机研发的重要验证手段,拖拉机室内台架试验可方便高效地为拖拉机模拟各种工况下的性能表现。首先探究动力总成控制技术、负载模拟技术、试验数据处理技术和试验结果评价技术4种拖拉机室内台架试验关键技术,结合试验技术的最新研究进展,对4种技术进行详细的阐述和分析。接着从动力总成控制算法、负载模拟算法、试验数据处理方法、试验结果评价指标体系4个方面对拖拉机室内台架试验技术进行讨论与展望。最后得出对动力总成算法进行优化、提高动态负载模拟精度、保持试验数据处理方法的前沿性、对试验结果综合客观评价是拖拉机室内台架试验技术的发展趋势,以期为我国拖拉机室内台架试验技术的发展提供参考。 相似文献
8.
Effects of earthworm casts on soil nutrient dynamics and their responses to changing moisture availability in subtropical ecosystems remain poorly understood. This study aimed to examine short-term carbon(C) and nitrogen(N) dynamics and their interactions with wetting-drying cycles in three different structural forms(i.e., granular, globular, and heap-like) of Amynthas earthworm casts. The rates of C and N mineralization in the earthworm casts were examined under two different wetting-drying cycles(i.e., 2-d and 4-d wetting intervals) using a rainfall simulation experiment. After three simulated rainfall events, subsamples of the earthworm casts were further incubated for 4 d for the determination of CO_2 and N_2O fluxes. The results of this study indicated that the impacts of wetting-drying cycles on the short-term C and N dynamics were highly variable among the three cast forms, but wetting-drying cycles significantly reduced the cumulative CO_2 and N_2O fluxes by 62%–83% and 57%–85%, respectively, when compared to the control without being subjected to any rainfall events. The C mineralization rates in different cast forms were affected by the amount of organic substrates and N content in casts, which were associated with the food preference and selection of earthworms. Meanwhile, the cumulative N_2O fluxes did not differ among the three cast forms. Repeated wetting and drying of casts not only enhanced aggregate stability by promoting bonds between the cast particles, but also inhibited microbial survival and growth during the prolonged drying period, which together hindered decomposition and denitrification. Our findings demonstrated that the interactions between the structural forms, aggregate dynamics, and C and N cycling in the earthworm casts were highly complex. 相似文献
9.
Xuchao ZHU Yin LIANG Zhiyuan TIAN Yi ZHANG Yugang ZHANG Jing DU Xin WANG Yu LI Lili QU Mengmeng DAI 《土壤圈》2021,31(5):715-724
Soil erodibility(K) is a key factor of soil erosion, and its appropriate quantification and interpolation are vitally important for soil and water conservation.The traditional point-represent-polygon approaches and common kriging method for the estimation of K, however, do not sufficiently represent the original data. The objectives of this study were to simulate the spatial distribution of K using a sequential Gaussian algorithm and analyze the uncertainty in evaluating the risk of soil erodibility in southeastern China. We determined 101 sampling points in the area and collected disturbed soil samples from the 0–20 cm layer at each point. Soil properties were determined, and K was calculated using five common models: the EPIC(Erosion/Productivity Impact Calculator),approximate nomograph, Torri, Shirazi, and Wang models. Among the chosen models, the EPIC model performed the best at estimating K(KEPIC), which ranged from 0.019 to 0.060 t ha h(ha MJ mm)~(-1), with a mean of 0.043 t ha h(ha MJ mm)~(-1). The KEPICwas moderately spatially variable and had a limited spatial structure, increasing from south to north in our study area, and all spatial simulations using the cooperative kriging(CK) interpolation and the sequential Gaussian simulation(SGS) with 10, 25, 50, 100, 200, and 500 realizations had acceptable accuracies. The CK interpolation narrowed the range,and the SGS maintained the original characteristics of the calculated data. The proportions of the risk area were 38.0% and 10.1%, when the risk probability for K was 60% and 80%, respectively, and high risk areas were mostly located in the north. The results provide scientific guidance for managing the risk of soil erodibility in southeastern China. 相似文献
10.
Developing a process-based and remote sensing driven crop yield model for maize(PRYM–Maize) and its validation over the Northeast China Plain 下载免费PDF全文
Spatial dynamics of crop yield provide useful information for improving the production. High sensitivity of crop growth models to uncertainties in input factors and parameters and relatively coarse parameterizations in conventional remote sensing(RS) approaches limited their applications over broad regions. In this study, a process-based and remote sensing driven crop yield model for maize(PRYM–Maize) was developed to estimate regional maize yield, and it was implemented using eight data-model coupling strategies(DMCSs) over the Northeast China Plain(NECP). Simulations under eight DMCSs were validated against the prefecture-level statistics(2010–2012) reported by National Bureau of Statistics of China, and inter-compared. The 3-year averaged result could give more robust estimate than the yearly simulation for maize yield over space. A 3-year averaged validation showed that prefecture-level estimates by PRYM–Maize under DMCS8, which coupled with the development stage(DVS)-based grain-filling algorithm and RS phenology information and leaf area index(LAI), had higher correlation(R, 0.61) and smaller root mean standard error(RMSE, 1.33 t ha~(–1)) with the statistics than did PRYM–Maize under other DMCSs. The result also demonstrated that DVS-based grain-filling algorithm worked better for maize yield than did the harvest index(HI)-based method, and both RS phenology information and LAI worked for improving regional maize yield estimate. These results demonstrate that the developed PRYM–Maize under DMCS8 gives reasonable estimates for maize yield and provides scientific basis facilitating the understanding the spatial variations of maize yield over the NECP. 相似文献