全文获取类型
收费全文 | 198279篇 |
免费 | 2130篇 |
国内免费 | 633篇 |
专业分类
电工技术 | 4063篇 |
综合类 | 125篇 |
化学工业 | 26799篇 |
金属工艺 | 7521篇 |
机械仪表 | 5470篇 |
建筑科学 | 4044篇 |
矿业工程 | 752篇 |
能源动力 | 4671篇 |
轻工业 | 14413篇 |
水利工程 | 1706篇 |
石油天然气 | 2842篇 |
武器工业 | 7篇 |
无线电 | 27325篇 |
一般工业技术 | 37453篇 |
冶金工业 | 45513篇 |
原子能技术 | 3566篇 |
自动化技术 | 14772篇 |
出版年
2021年 | 1253篇 |
2019年 | 1290篇 |
2018年 | 2138篇 |
2017年 | 2113篇 |
2016年 | 2181篇 |
2015年 | 1497篇 |
2014年 | 2626篇 |
2013年 | 7971篇 |
2012年 | 4484篇 |
2011年 | 6135篇 |
2010年 | 4945篇 |
2009年 | 5791篇 |
2008年 | 6164篇 |
2007年 | 6213篇 |
2006年 | 5649篇 |
2005年 | 5314篇 |
2004年 | 5266篇 |
2003年 | 5091篇 |
2002年 | 4903篇 |
2001年 | 5315篇 |
2000年 | 4923篇 |
1999年 | 5449篇 |
1998年 | 15648篇 |
1997年 | 10331篇 |
1996年 | 7947篇 |
1995年 | 5792篇 |
1994年 | 5023篇 |
1993年 | 5032篇 |
1992年 | 3392篇 |
1991年 | 3260篇 |
1990年 | 3220篇 |
1989年 | 3042篇 |
1988年 | 2788篇 |
1987年 | 2240篇 |
1986年 | 2303篇 |
1985年 | 2613篇 |
1984年 | 2315篇 |
1983年 | 2068篇 |
1982年 | 1905篇 |
1981年 | 2054篇 |
1980年 | 1793篇 |
1979年 | 1659篇 |
1978年 | 1658篇 |
1977年 | 2032篇 |
1976年 | 2709篇 |
1975年 | 1423篇 |
1974年 | 1371篇 |
1973年 | 1321篇 |
1972年 | 1126篇 |
1971年 | 957篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
281.
In the power industry, current has to be measured for metering and protection purposes. Conventional sensor setups need two transformers to realize the high dynamic range. By the described mixed analog/digital signal processing algorithm, the inherent low photonic noise of the input signals was maintained up to the full sensor output. The system design is based on a rigorous investigation and optimization of the error sources in the signal processing chain. The results are proved by a demonstrator, achieving a dynamic range from 0.05 to 100 times the rated current and a ratio root mean square error of less than 0.5%. 相似文献
282.
The shape of the impactor with the maximum depth of penetration (DOP) for a given impact velocity is found using a numerical procedure for solving a corresponding non-classical variational problem. It is shown that the optimum shape in a general case is close to a blunt cone. The variation of the optimal shape of the impactor and the dependence of the DOP vs. the initial (impact) velocity and friction coefficient is studied. The analysis is performed also for optimal conical impactors. 相似文献
283.
Materials with inherent microstructures like granular media, foams or spongy bones often show a complex constitutive behaviour
on the macroscale while the microscopic constitutive equations may be formulated in a simple fashion. Applying homogenization
procedures allows to transfer the information from the microlevel to the macrolevel.
In the present contribution the porous structure of hard biological tissues, i.e. of spongy bones, is investigated. On the
macroscale the approach is embedded into an extended continuum mechanical setting in order to capture size effects. The constitutive
equations are formulated on the microscopic level taking into account growth and reorientation of the microstructural elements.
By application of a strain-driven numerical homogenization procedure the macroscopic stress response is obtained. 相似文献
284.
Titanium Dioxide, TiO2, is a photocatalyst with a unique characteristic. A surface coated with TiO2 exhibits an extremely high affinity for water when exposed to UV light and the contact angle decreases nearly to zero. Inversely, the contact angle increases when the surface is shielded from UV. This superhydrophilic nature gives a self-cleaning effect to the coated surface and has already been applied to some construction materials, car coatings and so on. We applied this property to the enhancement of boiling heat transfer. An experiment involving the pool boiling of pure water has been performed to make clear the effect of high wettability on heat transfer characteristics. The heat transfer surface is a vertical copper cylinder of 17 mm in diameter and the measurement has been done at saturated temperature and in a steady state. Both TiO2-coated and non-coated surfaces were used for comparison. In the case of the TiO2-coated surface, it is exposed to UV light for a few hours before experiment and it is found that the maximum heat flux (CHF) is about two times larger than that of the uncoated surface. The temperature at minimum heat flux (MHF) for the superhydrophilic surface is higher by 100 K than that for the normal one. The superhydrophilic surface can be an ideal heat transfer surface. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
285.
U. Schmidhammer V. DeWaele S. Mintova E. Riedle T. Bein 《Advanced functional materials》2005,15(12):1973-1978
The ultrafast deprotonation of 2‐(2′‐hydroxyphenyl)benzothiazole (HBT) hosted in nanometer‐sized FAU and MFI zeolites is reported. Samples are prepared via in‐situ incorporation of HBT in the precursor colloidal solutions resulting in the formation of nanometer‐sized zeolites under hydrothermal treatment. The diameter of the zeolite particles formed in the crystalline suspensions is determined by dynamic light scattering and high‐resolution transmission microscopy to lie in the range 40–100 nm. It is shown that the HBT loading does not influence the degree of the zeolite crystallinity but does change the size and the morphology of the individual zeolite nanoparticles. Colloidal suspensions containing the crystalline nanoparticles are well suited for optical investigations since they are sufficiently transparent and clear. The photochemical properties of the HBT guest in the zeolite‐host systems are studied with femtosecond transient transmission spectroscopy. Depending on the acid–base properties either the enol or the keto tautomer of HBT is found to be hosted in the internal voids of the zeolites; upon UV excitation, the HBT‐keto tautomer is converted to the enol form in both MFI‐ and FAU‐type hosts. The HBT photoconversion takes place via an ultrafast deprotonation within 1.5 ps as detected by femtosecond transient absorption spectroscopy. 相似文献
286.
[110]-surface strained-SOI CMOS devices 总被引:1,自引:0,他引:1
Mizuno T. Sugiyama N. Tezuka T. Moriyama Y. Nakaharai S. Takagi S. 《Electron Devices, IEEE Transactions on》2005,52(3):367-374
We have newly developed [110]-surface strained-silicon-on-insulator (SOI) n- and p-MOSFETs on [110]-surface relaxed-SiGe-on-insulator substrates with the Ge content of 25%, fabricated by applying the Ge condensation technique to SiGe layers grown on [110]-surface SOI wafers. We have demonstrated that the electron and the hole mobility enhancement of [110]-surface strained-SOI devices amounts to 23% and 50%, respectively, against the mobilities of [110]-surface unstrained MOSFETs. As a result, the electron and the hole mobility ratios of [110]-surface strained-SOI MOSFETs to the universal mobility of (100)-surface bulk-MOSFETs increase up to 81% and 203%, respectively. Therefore, the current drive imbalance between n- and p-MOS can be reduced. Moreover, both the electron and the hole mobilities of the [110]-surface strained-SOIs strongly depend on the drain current flow direction, which is qualitatively explained by the anisotropic effective mass characteristics of the carriers on a [110]-surface Si. As a result, the [110]-surface strained-SOI technology with optimization of the current flow directions of n- and p-MOS is promising for realizing higher speed scaled CMOS. 相似文献
287.
Erosion and corrosion of metallic materials in particle containing aqueous solutions The combined effect and mutual influence, respectively, of erosion and corrosion of metallic materials in particle containing flows was investigated. Materials under investigation are carbon steel of different hardness levels, ferritic chromium steel, austenitic stainless steel and brass. Corrosive medium was deionized water, acid (pH 4.5), alkaline (pH 10) and chloride (3% NaCl) solution. Quartz particles were used as abrasive at various particle concentrations, grain sizes and flow velocities. For comparison, tests were performed with pure water and under conditions of idle corrosion. Mechanical or corrosive influences dominate depending on material and test conditions, in some cases, synergistic effects of both mechanisms cause an increased mass loss. 相似文献
288.
289.
Segregation of reactive metals at the bonding interface has been observed in various ceramic and/or metal joints bonded with
reactive metal-bearing braze alloys. When a d.c. of 20 mAcm−2 is applied to the ceramic/braze/ceramic system at a brazing temperature of, say, 1373 K, the electric field assists the segregation
at the braze-ceramic interface on the cathode side and suppresses the segregation at the interface on the anode side. This
may imply that reactive metal atoms in the braze can migrate as a cation. E.m.f. measurement on the ceramic (AIN or ZrB2)-metal foil systems with increasing temperature shows that a negative e.m.f. to the ceramic pole appears from about 900 K
for AIN and from 500 K for ZrB2, as does the thermally stimulated current in polymers. These temperatures coincide well with those where the electrical conductivity
of AIN and ZrB2, respectively, begins to increase with increasing temperature. Therefore, it is considered that the polarization of the ceramics
may take place and assist the migration, and consequently segregation, of reactive metals in braze alloys to the braze-ceramic
interface during brazing. 相似文献
290.
M. Kangas J. Villegas N. Kumar T. Salmi D.Yu. Murzin F. Sandelin E. Harlin 《Catalysis Today》2005,100(3-4):363-366
An investigation of the effect of reaction conditions on product distribution in the skeletal isomerisation reaction of linear butenes has been carried out. The main reaction routes over ferrierite have been identified. Beside the main product isobutene, major by-product formation occurs. The unwanted reactions include dimerisation of butene to form octenes, hydrogen transfer yielding small amounts of saturated C3 and C4 hydrocarbons and disproportionation producing propene and pentenes. The most abundant by-products were pentene and propene, though these were not formed in equimolar amounts as could be expected. Oligomerisation experiments of propene over ferrierite produced large amounts of butene and pentene, revealing the presence of adsorbed nonene. The cracking of this surface species to hexene and propene is the most likely reaction route for the excess propene formation. This additional path to propene formation operates mainly at temperatures above 623 K. 相似文献