全文获取类型
收费全文 | 17318篇 |
免费 | 2157篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 1361篇 |
综合类 | 403篇 |
化学工业 | 8708篇 |
金属工艺 | 252篇 |
机械仪表 | 392篇 |
建筑科学 | 652篇 |
矿业工程 | 168篇 |
能源动力 | 103篇 |
轻工业 | 1802篇 |
水利工程 | 116篇 |
石油天然气 | 69篇 |
无线电 | 336篇 |
一般工业技术 | 2895篇 |
冶金工业 | 162篇 |
原子能技术 | 25篇 |
自动化技术 | 2035篇 |
出版年
2023年 | 625篇 |
2022年 | 360篇 |
2021年 | 706篇 |
2020年 | 672篇 |
2019年 | 581篇 |
2018年 | 553篇 |
2017年 | 391篇 |
2016年 | 616篇 |
2015年 | 786篇 |
2014年 | 817篇 |
2013年 | 1414篇 |
2012年 | 545篇 |
2011年 | 428篇 |
2010年 | 783篇 |
2009年 | 929篇 |
2008年 | 438篇 |
2007年 | 388篇 |
2006年 | 282篇 |
2005年 | 276篇 |
2004年 | 223篇 |
2003年 | 204篇 |
1998年 | 182篇 |
1997年 | 133篇 |
1996年 | 197篇 |
1995年 | 186篇 |
1994年 | 160篇 |
1993年 | 219篇 |
1992年 | 155篇 |
1991年 | 122篇 |
1990年 | 156篇 |
1989年 | 174篇 |
1988年 | 136篇 |
1987年 | 161篇 |
1986年 | 182篇 |
1985年 | 166篇 |
1984年 | 168篇 |
1983年 | 176篇 |
1982年 | 155篇 |
1981年 | 202篇 |
1980年 | 165篇 |
1979年 | 169篇 |
1977年 | 147篇 |
1976年 | 149篇 |
1975年 | 202篇 |
1974年 | 188篇 |
1973年 | 365篇 |
1972年 | 212篇 |
1971年 | 150篇 |
1970年 | 144篇 |
1968年 | 153篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Summary The effect of a thermal gradient on the transverse vibration of a prestressed rectangular plate is investigated by the method of matched asymptotic expansions. This class of heated plate is characterised by changing its Young's modulus with temperature. Analytical results for the eigenvalues are presented for fully-clamped and fully-hinged rectangular plates when the bending rigidity is small compared to the in-plane loading. To leading order in (where 2 denotes the normalized bending rigidity), the eigenvalues of an ideal membrane are obtained, independent of thermal effects.Thermal gradient effects occur in the first order correction of eigenvalues for a clamped plate while the eigenvalues of a hinged plate are affected by thermal gradient only to second order. In particular, Schneider's results are recovered when thermal gradient effects are absent.Nomenclature
W'
bending deflection
-
D(x)
flexural rigidity
-
D
0
reference flexural rigidity
-
x',y'
rectangular co-ordinate
-
E
modulus of elasticity
-
E
1
reference modulus of elasticity
-
t'
time
-
h'
height of plate
-
a'
length of plate
-
b'
width of plate
-
T
temperature
-
T
0
reference temperature
-
slope of variation ofE withT
-
parameter
-
L
characteristic length
-
N
0
characteristic in-plane force
-
m
mass per unit area
-
characteristic frequency
-
outer solution
-
inner solution
-
small parameter 相似文献
122.
Dipl.-Ing. P. Raitsios Prof. Dr.-Ing. A. Safacas 《Electrical Engineering (Archiv fur Elektrotechnik)》1992,75(6):411-417
Übersicht Die vorliegende Arbeit befaßt sich mit der gesamten Stromdichte- und Magnetfeldverteilung in vielen flachen leitenden Platten konstanter Permeabilität, die bei einer Anordnung aus diesen Platten und vielen parallelen stromführenden Leitern angeordnet in mehreren Lagen entsteht. Unter Anwendung der Maxwellschen Differentialgleichungen und durch Einführung des Vektorpotentials werden allgemeine Gleichungen für die Komponenten der magnetischen Induktion im zweidimensionalen Feldraum aufgestellt. Die Stromdichteverteilung in den leitenden Platten, in denen Wirbelströme entstehen, wird aus dem Vektorpotential hergeleitet. Zunächst werden die Platten unendlich lang angenommen und die entsprechenden Gleichungen in Integralform aufgestellt, für deren Lösung die Gauss-Laguerresche Methode angewandt wird. Dann werden die Platten beiderseits durch Eisenwände begrenzt, womit die Gleichungen unter Berücksichtigung der Randbedingungen in Summenform ausgedrückt werden.
Liste der Symbole A Vektorpotential - B Magnetische Induktion - I 1 Stromstärke eines Leiters - N 1 Anzahl der parallelen Leiter - N 2 Anzahl der parallelen Platten - Permeabilität des Plattenmaterials - r Relative Permeabilitätskonstante des Plattenmaterials - Leitfähigkeit - b Breite des Hauptstreukanals - i Stromdichte der Platten samt Wirbelströme - h 1 Höhe der Leiter - a 1 Breite der Leiterlagen - a Plattendicke - s Breite des Nebenstreukanals - c Abstand der letzten Platte von der Eisenwand - g Plattenhöhe - x, z, , Dimensionen - Q, K Integrationskonstanten - k ganze Zahl 相似文献
Distribution of current and magnetic field density in flat conductive plates in an array composed from plates and many parallel conductors
Contents In the present research work the overall distribution of current and magnetic field density in many flat plates of constant magnetic permeability has been investigated, which applies to an array composed from the flat plates and many parallel conductors installed in many layers. Use of Maxwell's differential equations and vector potential results in general equations for the components of magnetic density in two dimensional field space. The distribution of current density in the conductive plates, where eddy currents are taken into account, is obtained from the vector potential. Initially the flat plates are considered being of infinite length, and with appropriate arrangement of the boundary conditions, the model applies to flat plates limited to both sides by iron partitions. Examples are given for both cases, where for the plates of infinite length the Gauss-Laguerre method is applied.
Liste der Symbole A Vektorpotential - B Magnetische Induktion - I 1 Stromstärke eines Leiters - N 1 Anzahl der parallelen Leiter - N 2 Anzahl der parallelen Platten - Permeabilität des Plattenmaterials - r Relative Permeabilitätskonstante des Plattenmaterials - Leitfähigkeit - b Breite des Hauptstreukanals - i Stromdichte der Platten samt Wirbelströme - h 1 Höhe der Leiter - a 1 Breite der Leiterlagen - a Plattendicke - s Breite des Nebenstreukanals - c Abstand der letzten Platte von der Eisenwand - g Plattenhöhe - x, z, , Dimensionen - Q, K Integrationskonstanten - k ganze Zahl 相似文献
123.
Dr. R. Fazio 《Acta Mechanica》1992,95(1-4):1-7
Summary In the present paper we point out that the correct way to solve the Blasius problem by numerical means is to reformulate it as free boundary value problem. In the new formulation the truncated boundary (instead of infinity) is the unknown free boundary and it has to be determined as part of the numerical solution. Taking into account the partial inavariance of the mathematical model at hand with respect to a stretching group we define a non-iterative transformation method. Further, we compare the improved numerical results, obtained by the method in point, with analytical and numerical ones. Moreover, the numerical results confirm that the question of accuracy depends on the value of the free boundary. Therefore, this indicates that boundary value problems with a boundary condition at infinity should be numerically reformulated as free boundary value problems. 相似文献
124.
Prof. Dr. A. Kaveh 《Acta Mechanica》1986,62(1-4):189-196
Summary A combinatorial method is presented for examining the rigidity of planar structures. In this approach, an expansion process is used for the formation of a statically determinate substructure, known as a -tree of a structure. The algorithm of Lováz and Yemini, and the method of Sugihara are employed for the recognition of the elementary subgraphs, during this process.With 4 Figures 相似文献
125.
Summary Three real gas isentropic exponentsk
Tv,k
rv,k
pT are introduced, which when used in place of the classical isentropic exponentk=c
p/c in the ideal gas isentropic change equations, the latter may describe very accurately the isentropic change of real gases. The usual practice of employing exponentk may lead to considerably incorrect results even when the value ofk corresponds to the correct local value ofc
p/c
v of the real gas under examination. The numerical values of the new exponents are calculated in the case of real air for temperatures from 150 K to 450 K and pressures from 1 bar to 1000 bar. It is seen that at low temperatures and high pressures the values of the new exponents differ considerably from the values of the classical exponentk. Therefore, the error resulting by approximating, as is usually the case, the behaviour of real gases by the ideal gas isentropic change equations in a stepwise fashion with exponentk instead of the new exponents, is considerable. It follows that exponentk, which appears in various relations in thermodynamics, fluid mechanics, gasdynamics, heat transfer etc., should be suitably replaced by combinations of the three exponents. Related numerical examples, made in the case of real air, showed that the use ofk leads (in the temperature and pressure ranges examined) to a 5% error in the calculation of blowby rate in internal combustion enginers, high pressure compressors or steam turbines and to a 50% error in the calculation of the isentropic expansion or compression.Nomenclature
A
ij,B
i,N
ij,O
ij,Q
ij
Coefficients
-
c
Velocity
-
c
p
Specific heat under constant pressure
-
c
v
Specific heat under constant volume
-
h
Specific enthalpy
-
k
Isentropic exponent,k=c
p/c
v
-
k
pT
Real gas isentropic exponent corresponding to the pair of variablesp,T
-
k
pv
Real gas isentropic exponent corresponding to the pair of variablesp, v
-
k
Tv
Real gas isentropic exponent corresponding to the pair of variablesT,v
-
M
Mach number,M=c/
-
p
Pressure
-
p
c
Pressure at the critical point
-
R
Constant of the air,R=287.22 J/kg K
-
s
Specific entropy
-
T
Temperature
-
T
c
Temperature at the critical point
-
v
Specific volume
-
v
c
Specific volume at the critical point
-
z
Compressibility factor
-
Sound velocity
- T
Temperature increment
With 14 Figures 相似文献
126.
Dr. J. Lewandowski 《Acta Mechanica》1987,68(1-2):21-31
Summary The propagation of acoustic plane and line-polarized waves in an isotropic solid containing random cavities is studied theoretically and experimentally. The effect of the cavities on the energy density and macroscopic propagation parameters of the acoustic waves is considered and the relationships between the cavities volume concentration, their average dynamic shape factor and the dynamic overall stiffness moduli of the solid are derived in the long wave approximation. 相似文献
127.
Dr. Qingyun Tang Assist. Prof. Dr. Ioannis V. Pavlidis Dr. Christoffel P. S. Badenhorst Prof. Dr. Uwe T. Bornscheuer 《Chembiochem : a European journal of chemical biology》2021,22(16):2584-2590
Halide methyltransferases (HMTs) enable the enzymatic synthesis of S-adenosyl-l -methionine (SAM) from S-adenosyl-l -homocysteine (SAH) and methyl iodide. Characterisation of a range of naturally occurring HMTs and subsequent protein engineering led to HMT variants capable of synthesising ethyl, propyl, and allyl analogues of SAM. Notably, HMTs do not depend on chemical synthesis of methionine analogues, as required by methionine adenosyltransferases (MATs). However, at the moment MATs have a much broader substrate scope than the HMTs. Herein we provide an overview of the discovery and engineering of promiscuous HMTs and how these strategies will pave the way towards a toolbox of HMT variants for versatile chemo- and regioselective biocatalytic alkylations. 相似文献
128.
129.
Summary Copolymers containing -S-CO-S-and/or -O-CO-S-groups have been synthesized mainly by interfacial polycondensation.Different chemical structures were obtained by reacting 1,3-benzene dithiol (BDT) respectively with phosgene alone, phosgene and bisphenol-A (BPA), bischloroformate of BPA, BPA polycarbonate oligomers and by reacting phosgene with the products of BPA polycarbonate degraded with BDT. The chemical structures of the copolymers were investigated by IR, 1H-NMR and 13C-NMR; molecular weights were determined by viscometry and vapor pressure osmometry. Although no attempt was made to find the optimum conditions for high molecular weight, some copolymers with fairly high mol. weight were obtained. 相似文献
130.
Dr.-Ing. Werner Liedy 《化学,工程师,技术》1993,65(2):167-173
Influence of Product Properties on Drying. A number of items of information are required for selection and design of a drying process. Thermodynamic equilibrium as well as heat and mass transfer are the traditional prerequisites for describing the drying operation. In the development of a drying process further specific properties of the product must be known. These properties, such as chemical reactions or sticking properties, greatly reduce the number of possible drying processes. For this reason, the measurement of these properties plays an important role in the development of a drying process. Many of the bench scale tests in use do not have a physical basis, but have been developed empirically because the physical context is often unknown. Yet a number of physically well-defined methods are available for product characterization. In practical work a combination of empirical and scientifically defined methods is used. 相似文献