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   2790篇
  免费   54篇
  国内免费   30篇
测绘学   258篇
大气科学   173篇
地球物理   567篇
地质学   1026篇
海洋学   65篇
天文学   656篇
综合类   24篇
自然地理   105篇
  2022年   26篇
  2021年   31篇
  2020年   37篇
  2019年   31篇
  2018年   152篇
  2017年   123篇
  2016年   170篇
  2015年   83篇
  2014年   145篇
  2013年   165篇
  2012年   124篇
  2011年   128篇
  2010年   110篇
  2009年   131篇
  2008年   105篇
  2007年   73篇
  2006年   70篇
  2005年   61篇
  2004年   41篇
  2003年   42篇
  2002年   54篇
  2001年   38篇
  2000年   39篇
  1999年   47篇
  1998年   36篇
  1997年   38篇
  1996年   28篇
  1995年   28篇
  1994年   36篇
  1993年   30篇
  1992年   31篇
  1991年   41篇
  1990年   28篇
  1989年   41篇
  1988年   48篇
  1987年   52篇
  1986年   36篇
  1985年   35篇
  1984年   61篇
  1983年   58篇
  1982年   34篇
  1981年   21篇
  1980年   27篇
  1979年   16篇
  1978年   19篇
  1976年   12篇
  1974年   11篇
  1973年   12篇
  1972年   14篇
  1971年   10篇
排序方式: 共有2874条查询结果,搜索用时 171 毫秒
211.
Summary A number of field and laboratory tests have been carried out on more than 15 coal seams of compressive strengths ranging from 19 MPa to 44 MPa to evolve methods which would help in the selection of suitable coaling machines for hard coal seams. The effect of physico-mechanical properties on cuttability were studied in the laboratory for all these coal seams to identify the relevant parameters affecting the specific energy of coal cuttability. These data were subjected to regression analysis to find the best fit for estimation of laboratory specific energy of coal samples on the basis of simple laboratory and field tests for the strength parameters. Field studies were also conducted over a large number of active mechanized coal faces to study in situ cuttability along with the geo-mining conditions of the site. The field and the laboratory data so generated were correlated and an attempt is made to establish a relationship for estimating the field specific energy for a particular capacity of coaling machines by considering the geo-mining domain of the field in totality.  相似文献   
212.
Summary In this paper the generalized thermoelastic longitudinal waves and the temperature field set up due to coupling of the displacement and the temperature fields, with heat wave travelling with certain finite velocity, in an unbounded medium are studied. The thermoelastic displacement potential and the temperature field at any point are obtained in terms of the surface integrals involving the potential, the temperature and their normal derivatives.Notation x i the cartesian coordinate system,i=1,2,3 - n (u i) the displacement vector - (/x i) the del operator - t /t the derivative with respect to time - T o the temperature corresponding to the natural stat of zero stress and strain - T Absolute temperature - c e the specific heat - , Lamé's constants - 0 the density - coefficient of linear thermal expansion - K thermal conductivity coefficient - kk u - 0 the relaxation time  相似文献   
213.
Summary A number of resistivity profiles over quartz reefs have been taken using Wenner and Schlumberger configurations around Dhanbad in order tofind the potential of these for water accumulation. These reefs give rise to both resistivity highs and lows as compared to the background. The results are discussed in the light of known theories. Some suitable sites for water accumulation have been delineated.  相似文献   
214.
Summary Least square criterion has been introduced to specify the order of isostatic compensation of mountains. The hydrostatic stability of the mountains is of basic significance in plate tectonics. However different concepts of isostasy differ in the evaluation of its order. An elegant and rapid technique is developed which extends the symmetric matrix method of root estimation byNegi andGarde [1]3) to provide a stability test through the mathematical expectation of the root increment. Application is illustrated for Rocky Mountains, United States.  相似文献   
215.
Summary The present paper studies the mechanical response of a piezoelectric composite transducer—with an elastic compliance partly constant and partly variable with time—acted upon by suitable thermal field besides the electrical voltage. The Laplace transform has been applied to solve the problem.  相似文献   
216.
Summary A magnetic survey carried out in Sua area of the Palamau district, Bihar (India) has shown very interesting anomalies. The survey has been conducted specially to obtain the extension of the magnetite deposits which are being quarried at Sua. The interpretation of magnetic anomalies have clearly brought out the following facts: a) the magnetite is extending both sides of the quarry to a considerable distance, b) the nature of the anomaly indicates that the magnetite occurs in the form of veins. It has also been possible to find out the thickness of the veins and the depth at which they occur.  相似文献   
217.
Summary Thermal demagnetization of large number of samples form the Isle of Mull have been carried out. The rocks belong to the Tertiary age. The results indicate that the rocks have been magnetized in the prevalling geomagnetic field at the time of formation. Some of the rocks indicate movement during cooling.  相似文献   
218.
Summary Theoretical expressions are derived for the displacement, velocity and stress in the time domain induced by an axially symmetric shearing stress applied at the inner surface of a spherical cavity in a hornogeneous, isotropic, elastic medium of infinite extent. Theoretical seismograms are computed for a step source and for three sources with exponential decay in time. A satisfactory time-dependence of the source can be obtained by combining the step source with one or more exponentially decaying sources.  相似文献   
219.
V. P. Singh 《水文研究》1995,9(7):783-796
Error equations for the kinematic wave and diffusion wave approximations with lateral inflow neglected in the momentum equation are derived under simplified conditions for space-independent flows. These equations specify error as a function of time in the flow hydrograph. The kinematic wave, diffusion wave and dynamic wave solutions are parameterized through a dimensionless parameter γ which is dependent on the initial conditions. This parameter reflects the effect of initial flow depth, channel-bed slope, lateral inflow, infiltration and channel roughness when the initial condition is non-vanishing; it reflects the effect of bed slope, channel roughness and acceleration due to gravity when the initial condition is vanishing. The error equations are found to be the Riccati equation. The structure of the error equations in the case when the momentum equation neglects lateral inflow is different from that when the lateral inflow is included.  相似文献   
220.
The effect of heat sinks on the free-convection flow of an incompressible viscous fluid past an impulsively started infinite vertical porous plate is considered. Finite difference method has been used to obtain the numerical solution of the governing equations. The influence of the various parameters, occurring into the problem, on the velocity field, the temperature field and on their related quantities is extensively discussed with the help of graphs and tables.  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