全文获取类型
收费全文 | 89461篇 |
免费 | 7183篇 |
国内免费 | 3754篇 |
专业分类
电工技术 | 3894篇 |
技术理论 | 10篇 |
综合类 | 7992篇 |
化学工业 | 15658篇 |
金属工艺 | 1587篇 |
机械仪表 | 2009篇 |
建筑科学 | 14125篇 |
矿业工程 | 5114篇 |
能源动力 | 5288篇 |
轻工业 | 5305篇 |
水利工程 | 18833篇 |
石油天然气 | 7751篇 |
武器工业 | 207篇 |
无线电 | 1485篇 |
一般工业技术 | 4056篇 |
冶金工业 | 3330篇 |
原子能技术 | 1121篇 |
自动化技术 | 2633篇 |
出版年
2024年 | 268篇 |
2023年 | 1003篇 |
2022年 | 1961篇 |
2021年 | 2400篇 |
2020年 | 2681篇 |
2019年 | 2205篇 |
2018年 | 2149篇 |
2017年 | 2632篇 |
2016年 | 2927篇 |
2015年 | 2890篇 |
2014年 | 5488篇 |
2013年 | 5581篇 |
2012年 | 6702篇 |
2011年 | 6698篇 |
2010年 | 4941篇 |
2009年 | 5189篇 |
2008年 | 4553篇 |
2007年 | 6033篇 |
2006年 | 5604篇 |
2005年 | 5157篇 |
2004年 | 4030篇 |
2003年 | 3663篇 |
2002年 | 3198篇 |
2001年 | 2572篇 |
2000年 | 2157篇 |
1999年 | 1646篇 |
1998年 | 1188篇 |
1997年 | 979篇 |
1996年 | 756篇 |
1995年 | 684篇 |
1994年 | 500篇 |
1993年 | 383篇 |
1992年 | 303篇 |
1991年 | 238篇 |
1990年 | 176篇 |
1989年 | 188篇 |
1988年 | 115篇 |
1987年 | 126篇 |
1986年 | 95篇 |
1985年 | 95篇 |
1984年 | 89篇 |
1983年 | 54篇 |
1982年 | 27篇 |
1981年 | 9篇 |
1980年 | 16篇 |
1979年 | 17篇 |
1978年 | 3篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1951年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
介绍了标准型直燃机的分类、原理及长庆至呼和浩特输气管道工程冷冻供热站的设计实例 ,重点论述了标准型直燃机在严寒地区使用的利弊 相似文献
42.
北京城市供水安全及对策研究 总被引:3,自引:1,他引:2
“十一五”期间是首都经济社会快速发展的关键时期,也是应对水资源紧缺、保障供水安全的关键时期。确保首都供水安全是北京水务工作的第一要务,北京市水务局针对城市供水存在的主要矛盾和问题,对城市供水水源、水质水量、水厂、管网、设施等情况进行了全面调查,确定了工作思路,并据此提出了保障城市供水安全的目标和措施。 相似文献
43.
The minimum flow requirements in the Svartå River in Sweden are directed at maintaining fishlife and providing suitable dilution for waste flows. The implications of varying the minimum flow requirements in the river are examined using a mixed integer optimisation model. The model is formulated as a modified method-of-weights technique with the economic issues of hydro-electricity generation, irrigation and urban water supply placed in the objective function and the minimum flows specified within the constraint set. The integer component of the model is required to model the operating policy at the major flow regulation facility in the system and the restricted validity of the irrigation permits. Application of the model shows that in dry years where competition between minimum flow levels and the other economic uses, is most intense, the levels achieved by the various economic objectives are only slightly reduced even with significant increases in the minimum flow requirements. Variations in minimum flow requirements of up to 45% only produce changes of 10% or less in the economic objectives. The lack of sensitivity of the objective levels is due primarily to the level of control exerted indirectly on the whole system in dry years by the release regulation policy and the restricted validity of the irrigation permits. In normal to wet years these policies are not as restrictive and more choice is available. In such years, however, there is generally sufficient water to satisfy all requirements and allocation is not a critical issue. The model itself is formulated generally so that a range of scenarios beyond those examined specifically in the paper can be considered. 相似文献
44.
本文介绍一种利用原水中含有的无机盐,加入廉价的化学试剂除去饮用水中微量铀的简便方法。并对其除铀机理进行了探讨,证明除铀主要是以软化水过程中产生的氢氧化镁表面吸附共沉淀机理进行的。 相似文献
45.
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
46.
Thermal sprayed aluminum and zinc provide long-term (> 20 years to first maintenance) corrosion control coatings. However,
this application is usually more expensive than painting or galvanizing if thermal spraying (metallizing) is not integrated
into the design and fabrication phases of new construction and repair projects. Aluminum and zinc metallized coatings are
tough enough to withstand fabrication, transportation, and assembly operations. The improved capabilities and productivity
of metallizing equipment for aluminum and zinc spraying are a major factor in their current cost competitiveness. The net
result is that the cost difference between metallizing, paint, and galvanizing is getting closer every day. Even though the
initial application cost of metallizing may be higher, the life cycle cost (LCC) and average equivalent annual costs (AEAC)
are lower than paint coating systems. Metallizing LCCs, when properly engineered into the construction schedule, are equal
to or less than paint coating LCCs. This article summarizes some metallizing considerations for installing improved corrosion
control coating systems in new construction and in maintenance and repair of infrastructure.
Editor’s Note: The following constants have been used to convert between English and Metric dimensions: 1ft2-0.0929 m2; 1lb/ft2-4.89 kg/m2; 1 mil=0.025mm.
Presented at the 5th National Thermal Spray Conference (NTSC-93), Infrastructure Maintenance and Repair Session, 10 June 1993,
Anaheim, CA. 相似文献
47.
The water vapor (WVP) and oxygen (O2P) permeabilities of beeswax (BW), candelilla wax (CnW), carnauba wax (CrW) and microcrystalline wax (MW), formed as freestanding
films, were determined. CnW and CrW both had small values for O2P (0.29 and 0.26 g·m−1·sec−1·Pa−1 × 10−14, respectively), which are less than half the value for high-density polyethylene and about a decade greater than the value
for polyethylene terephthalate. O2P values for BW and MW were about 6−9× greater than those of CnW and CrW. WVP of CnW was 0.18 g·m−1·sec−1·Pa−1 × 10−12, which is about one-half the value for CrW and MW and about one-third the value for BW. The WVP of CnW was somewhat less
than that of polypropylene and somewhat greater than that of high-density polyethylene. Differences in permeabilities among
the wax films are attributed mainly to differences in chemical composition and crystal type as determined by X-ray diffraction. 相似文献
48.
49.
Simulation of nitrogen dynamics and biomass production in winter wheat using the Danish simulation model DAISY 总被引:14,自引:0,他引:14
S. Hansen H. E. Jensen N. E. Nielsen H. Svendsen 《Nutrient Cycling in Agroecosystems》1991,27(2-3):245-259
A dynamic simulation model for the soil plant system is described. The model includes a number of main modules, viz., a hydrological model including a submodel for soil water dynamics, a soil temperature model, a soil nitrogen model including a submodel for soil organic matter dynamics, and a crop model including a submodel for nitrogen uptake. The soil part of the model has a one-dimensional vertical structure. The soil profile is divided into layers on the basis of physical and chemical soil characteristics. The simulation model was used to simulate soil nitrogen dynamics and biomass production in winter wheat grown at two locations at various levels of nitrogen fertilization. The simulated results were compared to experimental data including concentration of inorganic nitrogen in soil, crop yield, and nitrogen accumulated in the aboveground part of the crop. Based on this validation it is concluded that the overall performance of the model is satisfactory although some minor adjustments of the model may prove to be necessary. 相似文献
50.