全文获取类型
收费全文 | 46053篇 |
免费 | 10856篇 |
国内免费 | 17696篇 |
专业分类
测绘学 | 7862篇 |
大气科学 | 5401篇 |
地球物理 | 9055篇 |
地质学 | 32745篇 |
海洋学 | 8789篇 |
天文学 | 593篇 |
综合类 | 3979篇 |
自然地理 | 6181篇 |
出版年
2024年 | 532篇 |
2023年 | 1347篇 |
2022年 | 2696篇 |
2021年 | 3395篇 |
2020年 | 2677篇 |
2019年 | 3151篇 |
2018年 | 2661篇 |
2017年 | 2512篇 |
2016年 | 2613篇 |
2015年 | 3089篇 |
2014年 | 2948篇 |
2013年 | 3523篇 |
2012年 | 3788篇 |
2011年 | 4156篇 |
2010年 | 3847篇 |
2009年 | 3801篇 |
2008年 | 3556篇 |
2007年 | 3513篇 |
2006年 | 3589篇 |
2005年 | 3194篇 |
2004年 | 3024篇 |
2003年 | 2035篇 |
2002年 | 1961篇 |
2001年 | 1646篇 |
2000年 | 1293篇 |
1999年 | 668篇 |
1998年 | 498篇 |
1997年 | 410篇 |
1996年 | 311篇 |
1995年 | 232篇 |
1994年 | 190篇 |
1993年 | 379篇 |
1992年 | 363篇 |
1991年 | 177篇 |
1990年 | 109篇 |
1989年 | 107篇 |
1988年 | 48篇 |
1987年 | 75篇 |
1986年 | 66篇 |
1985年 | 27篇 |
1984年 | 35篇 |
1983年 | 25篇 |
1982年 | 22篇 |
1981年 | 16篇 |
1979年 | 29篇 |
1978年 | 19篇 |
1977年 | 26篇 |
1976年 | 20篇 |
1954年 | 30篇 |
1937年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
191.
Fuzzy Optimum Model of Semi-Structural Decision for Lectotype Optimization of Offshore Platforms 总被引:8,自引:0,他引:8
In the process of concept design of offshore platforms, it is necessary to select the best from feasible alternatives through comparison and filter. The criterion set, used to evaluate and select the satisfying alternative, consists of many qualitative and quantitative factors. Therefore, the selection is a problem of multicriteria and semi- structural decision- making. Different from traditional methods in semi- structural decision- making, a new framework and methodology is presented in this paper for evaluation of offshore platform alternatives. First, the criterion set is established for the evaluation of alternatives. Next, the approach is studied to construct the relative membership degree matrix, in which both qualitative and quantitative factors are consistent with the uniform calculating standard. And then a new weight-assessing method is developed for calculation of the weights based on the relative membership degree matrix. Finally, a multi-hierarchy fuzzy optimum model is adopted to select t 相似文献
192.
东营城区高精度三维地震采集方法研究 总被引:3,自引:0,他引:3
根据东营城区内建筑物较多,环境干扰较大,地下构造复杂,断裂非常发育,目的层埋藏深,资料信噪比较低的特点,以地震老资料和钻井资料为基础,建立地震地质模型,利用射线追踪技术选取合适的观测系统参数。充分利用卫星数字地图,没计了灵活多变的观测系统,采用可控震源与炸药震源配合使用方法,及时对城区内地下面元的覆盖次数、方位角和炮检距的分布进行质量监控,利用现场处理系统对资料进行及时分析,提高了地震资料品质。所获得的东营城区高精度三维地震采集资料显示,其浅中层分辨率和中深层信噪比都有了明显改善,取得了良好的地质效果。 相似文献
193.
The incompressible viscous uniform and shear flow past a circular cylinder is studied. The two-dimensional Navier-Stokes equations are solved by a finite element method. The governing equations are discretized by a weighted residual method in space. The stable three-step scheme is applied to the momentum equations in the time integration. The numerical model is firstly applied to the computation of the lid-driven cavity flow for its validation. The computed results agree well with the measured data and other numerical results. Then, it is used to simulate the viscous uniform and shear flow over a circular cylinder for Reynolds numbers from lO0 to lO00. The transient time interval before the vortex shedding occurs is shortened considerably by introduction of artificial perturbation. The computed Strouhal number, drag and lift coefficients agree well with the experimental data. The computation shows that the finite element model can be successfully applied to the viscous flow problem. 相似文献
194.
介绍了自适应电流保护原理在胜利海上平台供电系统中的应用。应用该原理的电流保护克服了传统电流保护一成不变的定值、受系统运行方式和短路类型的影响较大等缺点,它充分发挥了微机的记忆、逻辑判断、数学运算等强大的功能,提供实时计算保护装设处到系统等效电源间的阻抗,通过不断监视负荷电流来自动整定过流保护,并引入了反时限特性,解决了实际存在的技术问题,使得保护装置灵敏可靠。 相似文献
195.
196.
The effects of different lateral confinement stress on the fatigue behavior of and cumulative damage to plain concrete are investigated experimentally. Eighty 100 mm x 100 mm x 100 mm specimens of ordinary strength concrete are tested under constant- or variable-amplitude fatigue loading and lateral confinement pressure in two orthogonal directions. A fatigue equation is developed by modifying the classical Aas-Jakobsen S-N equation for taking into account the effect of the confined stress on fatigue strength of plain concrete. The results of variable-amplitude fatigue tests indicate that the linear damage theory proposed by Palmgren and Miner is unreasonable in the biaxial stress state. A nonlinear cumulative damage model that could model the effects of the magnitude and sequence of variable-amplitude fatigue loading and lateral confinement pressure is proposed on the basis of the evolution laws of the residual strains in the longitudinal direction during fatigue tests. The residual fatigue life predic 相似文献
197.
198.
The effect of the cone spacing of a conieal structure on the ice force is studied by model experiments. The ice force reduction coefficient presented in this paper expresses the relationship between the ice force and the arrangement of cones. The experiments prove that the mode of the ice failure before the boundary of upward-downward bending cone (UDBC) is crushing. A conclusion can also be drawn from the experiments that the ice force on the boundary of UDBC is by far less than that on a vertical pile with the same diameter. Moreover, the ice force frequencies on upright and inve-rted cones are obtained under the field condition of the platform JZ20-2, respectively. The results show that the alterna-tion of the ice force on UDBC can hardly induce resonance of platform JZ20-2. 相似文献
199.
茎柔鱼(Dosidicus gigas)是我国远洋渔业的重要捕捞对象。当前针对茎柔鱼渔场分布及其与环境关系的研究多集中于秘鲁海域,针对赤道海域茎柔鱼特定种群小型群体资源分布及其渔场环境特征研究较少。根据2019年12月至2020年2月茎柔鱼生物学数据,2019年12月至2020年4月生产和环境数据,运用胴长-体重关系拟合、地统计插值、广义可加模型(GAM)探究其资源分布及渔场环境状况。结果表明:东太平洋赤道海域茎柔鱼胴长范围为136~407 mm,体重范围为117~1557 g;2019年12月至2020年4月各月渔获量呈先增加后减小趋势,2月渔获量最高;CPUE曲线除2月增加外,总体呈下降趋势;渔场集中分布于0°~3°S、105°W~114°W海域,不同月份渔场重心经向变化明显;渔场最适SST范围是24.5~25.5 °C,最适Chl-a范围是0.16~0.20 mg/m3,月份是影响茎柔鱼CPUE的主要因子。研究表明:该海域茎柔鱼渔获主要为小型群体;小型群体生长发育期(2–3月)对渔场分布有重要影响,生长发育期前茎柔鱼集群度高,生长发育期后逐渐分散活动;单一影响因子与茎柔鱼CPUE相关性不显著,综合考虑其他环境因素及其交互影响是今后的研究方向。 相似文献
200.