全文获取类型
收费全文 | 18594篇 |
免费 | 1625篇 |
国内免费 | 1211篇 |
专业分类
电工技术 | 837篇 |
技术理论 | 1篇 |
综合类 | 3805篇 |
化学工业 | 1705篇 |
金属工艺 | 627篇 |
机械仪表 | 1038篇 |
建筑科学 | 914篇 |
矿业工程 | 335篇 |
能源动力 | 500篇 |
轻工业 | 608篇 |
水利工程 | 581篇 |
石油天然气 | 1303篇 |
武器工业 | 249篇 |
无线电 | 1453篇 |
一般工业技术 | 2948篇 |
冶金工业 | 488篇 |
原子能技术 | 370篇 |
自动化技术 | 3668篇 |
出版年
2024年 | 58篇 |
2023年 | 165篇 |
2022年 | 282篇 |
2021年 | 303篇 |
2020年 | 394篇 |
2019年 | 443篇 |
2018年 | 467篇 |
2017年 | 501篇 |
2016年 | 544篇 |
2015年 | 514篇 |
2014年 | 817篇 |
2013年 | 1190篇 |
2012年 | 1249篇 |
2011年 | 1234篇 |
2010年 | 995篇 |
2009年 | 990篇 |
2008年 | 1024篇 |
2007年 | 1130篇 |
2006年 | 1086篇 |
2005年 | 922篇 |
2004年 | 833篇 |
2003年 | 759篇 |
2002年 | 653篇 |
2001年 | 535篇 |
2000年 | 530篇 |
1999年 | 534篇 |
1998年 | 463篇 |
1997年 | 420篇 |
1996年 | 388篇 |
1995年 | 386篇 |
1994年 | 305篇 |
1993年 | 258篇 |
1992年 | 224篇 |
1991年 | 201篇 |
1990年 | 164篇 |
1989年 | 135篇 |
1988年 | 101篇 |
1987年 | 58篇 |
1986年 | 43篇 |
1985年 | 35篇 |
1984年 | 25篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 18篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1976年 | 2篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1959年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
41.
Rose Challenger Desmond J. Leach Christopher B. Stride Chris W. Clegg 《人机工程学与制造业中的人性因素》2012,22(3):197-212
Clegg and Spencer's (2007) model of job design synthesizes and extends recent conceptions of the job design process by incorporating variables such as knowledge, motivation, and trust into a cyclical and dynamic system. The objective of this study was to examine the sequential organization of variables that comprise the model, as a basis from which to justify further investigations of the model's dynamic properties. Data were collected via questionnaires from 432 participants in two work organizations. Results obtained from structural equation modeling are broadly supportive of the proposed relationships between the variables. Implications of this initial study for future research and practice are discussed. © 2011 Wiley Periodicals, Inc. 相似文献
42.
Equations of state and contact values of hard-sphere radial distribution functions (rdf's) which are given by a linear combination of the Percus— Yevick and scaled-particle virial expressions are considered. In the one-component case the mixing coefficient() is, in general, a function of the volume fraction. In mixtures the coefficient(
i
,d
i
), in general, depends upon the volume fraction
i
, and diameterd
i
, of each species,i andj. For the contact valuesY
ij
of the rdf's, the mixing coefficients
ij
(
k
) also depend on speciesi andj. Density expansions for the exact for the one-component hard-sphere fluid are obtained and compared with several approximations made in earlier works and in our own work, as well as with simulations. For a mixture, it turns out that one cannot obtain the exact fourth virial coefficient by using a linear combination of the Percus-Yevick and scaled-particle virial expressions forY
ij
unless one allows
ij
to depend on mole fractionsx
i
even at the zeroth order of its density expansion. We also find that
ij
must depend on particle speciesi andj in order to satisfy the exact limits obtained earlier by Sung and Stell. A new equation of state for the binary hard-sphere mixture which satisfies all the exact limits we have considered is suggested.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A. 相似文献
43.
Daniel Ocone 《Mathematics of Control, Signals, and Systems (MCSS)》1988,1(2):183-202
This paper applies the techniques of Malliavin’s stochastic calculus of variations to Zakai’s equation for the one-dimensional
cubic sensor problem in order to study the existence of densities of conditional statistics. Let {X
t} be a Brownian motion observed by a cubic sensor corrupted by white noise, and let
denote the unnormalized conditional estimate of φ(X
i
). If φ1,...,φ
n
are linearly independent, and if
, it is shown that the probability distribution of
admits a density with respect to Lebesgue measure for anyn. This implies that, at any fixed time, the unnormalized conditional density cannot be characterized by a finite set of sufficient
statistics.
Research supported in part by NSF Grant No. MCS-8301880 and by the Institute for Mathematics and It Applications, Minneapolis,
Minnesota. 相似文献
44.
A method capable of estimating the hydrograph from a prescribed storm for a practical mild slope upstream catchment is proposed. This method makes use of two new characteristic parameters, andS, in conjunction with the kinematic wave equation to compute lateral inflows of the main stream of the catchment. The depth profile of overland flow at any instant within the catchment and hydrograph at any location can be easily found. Lag times for individual lateral inflows are then considered and are linearly combined to obtain the hydrograph at the outlet of the catchment or depth profile of the main stream at any instant. The validity of the excess rainfall-surface runoff linear relationship in this study has also been verified with Tatsunokuchiyama catchment, and it shows good results for this computed runoff. 相似文献
45.
多矩阵变量线性矩阵方程(LME)约束解的计算问题在参数识别、结构设计、振动理论、自动控制理论等领域都有广泛应用。本文借鉴求线性矩阵方程(LME)同类约束最小二乘解的迭代算法,通过构造等价的线性矩阵方程组,建立了求多矩阵变量LME的一种异类约束最小二乘解的迭代算法,并证明了该算法的收敛性。在不考虑舍入误差的情况下,利用该算法不仅可在有限步计算后得到LME的一组异类约束最小二乘解,而且选取特殊初始矩阵时,可求得LME的极小范数异类约束最小二乘解。另外,还可求得指定矩阵在该LME的异类约束最小二乘解集合中的最佳逼近解。算例表明,该算法是有效的。 相似文献
46.
The specific heat at constant pressure, C
p, of aluminum measured by Ditmars, Plint, and Shukla has been reduced to the volume V
0 appropriate for 0 K employing the Murnaghan equation. The C
v0 thus obtained is compared with the theoretical C
v0 calculated in the harmonic and the lowest-order anharmonic approximation from three different pseudopotentials (Harrison, Ashcroft, and Dagens-Rasolt-Taylor) as well as a phenomenological Morse potential. The higher-order (
4) anharmonic contributions are calculated from the same nearest-neighbor Morse potential as in the lowest-order anharmonic theory. The role of the vacancy and the higher-order anharmonic contributions to C
v0 has been examined and we conclude that the
4 contributions to C
v0 are much smaller than the vacancy contribution. After removal of the vacancy contribution, the reduced C
v0 is found to be in excellent agreement with the Ashcroft and Harrison pseudopotentials as well as the Morse potential including the
2 and
4 contributions to C
v0. 相似文献
47.
The nonlinear Boussinesq equation is used to understand water table fluctuations in various ditch drainage problems. An approximate solution of this equation with a random initial condition and deterministic boundary conditions, recharge rate and aquifer parameters has been developed to predict a transient water table in a ditch-drainage system. The effects of uncertainty in the initial condition on the water table are illustrated with the help of a synthetic example. These results would find applications in ditch-drainage design.Notation
A
/ tanh t
-
a
lower value of the random variable representing the initial water table height at the mid point
-
a+b
Upper value of the random variable representing the initial water table height at the midpoint
-
B
tanh t
-
C
4/
-
h
variable water table height
- h
mean of the variable water table height
-
h
m
variable water table height at the mid point
- h
m
mean of the variable water table height at the mid point
-
K
hydraulic conductivity
-
L
half spacing between the ditches
-
m
0
initial water table height at the mid point
-
N
Uniform rate of recharge
-
S
specific yield
-
t
time of observation
-
x
distance measured from the ditch boundary
-
(4/SL)(NK)1/2
-
(L/4)(N/K)1/2
-
dummy integral variable 相似文献
48.
This paper investigates a stochastic optimal control problem with delay and of mean-field type, where the controlled state process is governed by a mean-field jump–diffusion stochastic delay differential equation. Two sufficient maximum principles and one necessary maximum principle are established for the underlying system. As an application, a bicriteria mean–variance portfolio selection problem with delay is studied to demonstrate the effectiveness and potential of the proposed techniques. Under certain conditions, explicit expressions are provided for the efficient portfolio and the efficient frontier, which are as elegant as those in the classical mean–variance problem without delays. 相似文献
49.
Eduardo X. Miqueles Tiago A. Coimbra Bruno D. Amaro J.J.S. de Figueiredo 《Computer Physics Communications》2014
Ray tracing technique is an important tool not only to forward but also for inverse problems in Geophysics, which most of the seismic processing steps depend on. However, implementing ray tracing codes can be very time consuming. This article presents a computer library to trace rays in 2.5D media composed by a stack of layers. The velocity profile inside each layer is such that the eikonal equation can be analytically solved. Therefore, the ray tracing within such profile is made fast and accurate. The great advantage of an analytical ray tracing library is the numerical precision of the quantities computed and the fast execution of the implemented codes. Even though ray tracing programs exist for a long time, for example the seis88 package by ?ervený, most of those programs use a numerical approach to compute the ray. Regardless of the fact that numerical methods can solve more general problems, the analytical ones could be part of a more sophisticated simulation process, where the ray tracing time is completely relevant. We demonstrate the feasibility of our codes using several examples (Miqueles et al., 2013) [1]. The library can also be used for other applications besides seismic, e.g., optics and tomography. 相似文献
50.
It is suggested that some relaxation processes observed in crystalline polyethylene are consequences of the diffusive motion of a particular defect called a point dislocation or twist dispiration loop along the polyethylene stems in lamellar crystals. The motion of the defect, characterized by a diffusion coefficient and a mobility, is described by solutions of the Smoluchowski diffusion equation with boundary conditions that constrain the defect to move along routes that produce experimentally observable results. The fact that passage of the defect causes both a 180° rotation of the chain and moves an extra CH2 group in the direction of the chain axis is important to the interpretation of the data according to this model. The diffusion coefficient for a defect is estimated to be around 2 × 10?9 cm2 s?1 at 70°C. This value is shown to be reasonable both from the viewpoint of detailed computer modelling of defect motion and contemporary ideas about scaling. 相似文献