全文获取类型
收费全文 | 1719篇 |
免费 | 59篇 |
国内免费 | 6篇 |
专业分类
电工技术 | 67篇 |
综合类 | 3篇 |
化学工业 | 442篇 |
金属工艺 | 54篇 |
机械仪表 | 38篇 |
建筑科学 | 40篇 |
矿业工程 | 1篇 |
能源动力 | 63篇 |
轻工业 | 154篇 |
水利工程 | 15篇 |
石油天然气 | 2篇 |
无线电 | 166篇 |
一般工业技术 | 342篇 |
冶金工业 | 216篇 |
原子能技术 | 34篇 |
自动化技术 | 147篇 |
出版年
2023年 | 7篇 |
2022年 | 20篇 |
2021年 | 35篇 |
2020年 | 15篇 |
2019年 | 19篇 |
2018年 | 40篇 |
2017年 | 32篇 |
2016年 | 57篇 |
2015年 | 27篇 |
2014年 | 48篇 |
2013年 | 86篇 |
2012年 | 83篇 |
2011年 | 103篇 |
2010年 | 85篇 |
2009年 | 87篇 |
2008年 | 97篇 |
2007年 | 73篇 |
2006年 | 59篇 |
2005年 | 52篇 |
2004年 | 42篇 |
2003年 | 49篇 |
2002年 | 52篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 40篇 |
1998年 | 107篇 |
1997年 | 71篇 |
1996年 | 43篇 |
1995年 | 35篇 |
1994年 | 27篇 |
1993年 | 20篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 19篇 |
1988年 | 13篇 |
1987年 | 12篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1977年 | 4篇 |
1976年 | 10篇 |
1972年 | 6篇 |
1969年 | 5篇 |
1968年 | 3篇 |
1967年 | 3篇 |
排序方式: 共有1784条查询结果,搜索用时 234 毫秒
991.
Kushibiki J Ohashi Y Ono Y Sasamata T 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》2002,49(7):905-914
This paper describes the first demonstration for feeding back the results obtained by the line-focus-beam ultrasonic material characterization (LFB-UMC) system to the crystal growth conditions for optical-grade LiTaO3 crystals and for achieving much improved homogeneity of chemical composition. We evaluated a commercially available optical-grade LiTaO3 single crystal with a nominally congruent composition in detail, by measuring distributions of the velocities of leaky surface acoustic waves (LSAW) along the Y-axis direction for a Z-cut specimen plate prepared from the crystal grown in the Y-axis direction. We detected an increment of 0.66 m/s in LSAW velocity along the pulling axis direction corresponding to 0.024 mol% in Li2O content, and the compositional gradient was +0.346×10-3 (Li2O-mol%)/mm. By experimentally obtaining the starting material composition dependence of the gradients, we developed a method of estimating the proper composition ratio that would lead to a more homogeneous crystal. We grew a new crystal with a Li2O content of 48.47 mol%, resulting in a very small compositional gradient of +0.046×10-3 (Li 2O-mol%)/mm and a compositional homogeneity of less than 0.012 Li2O-mol% in a Z-cut area of 50 mm×50 mm used for device substrates 相似文献
992.
Nakaishi T Iio K Yamamoto K Urabe I Yomo T 《Journal of Bioscience and Bioengineering》2002,93(3):322-327
The kinetic properties of Qbeta replicase, an RNA-dependent RNA polymerase, were investigated experimentally. The reaction at the A-incorporation site was inhibited by UTP and CTP with inhibition constants of 3.2 and 2.7 mM, respectively, while the reactions at the U-, G-, and C-incorporation sites were inhibited by ATP with inhibition constants of 1.09, 1.25, and 1.48 mM, respectively. When nucleotide concentrations were low, C was incorporated at the fastest rate and G at the slowest. Accordingly, the G-incorporation step largely limits the overall reaction rate. From the obtained kinetic parameters, calculations showed that the optimum ratio of the concentrations of the four nucleotides could be achieved by increasing the ratio of GTP concentration with a concomitant decrease in the ratios of CTP and ATP concentrations. Consequently, a 60 to 140% increase in the reaction rate is expected as compared to the rate with equimolar ratio of the four nucleotides. 相似文献
993.
Polarization-multiplexed phase-only diffractive optical elements with subwavelength structures are proposed and fabricated. The differences among the phase modulations result from the differences among the effective indices exhibited in the subwavelength structures with various filling factors and surface profiles, and the phase retardations are obtained by the relief depth of the structures. The polarization-selective property is achieved by the polarization dependence of the effective indices exhibited in the one-dimensional subwavelength structures and the polarization independence exhibited in the two-dimensional structures. Additionally, the polarization contrast of our polarization-multiplexed elements, defined as the cross talk between the two polarization incidences, is independent of the relief depth. The principle of the polarization multiplexing by use of the subwavelength structures is described, and the fabrication results for the polarization-multiplexed computer-generated holograms are demonstrated. 相似文献
994.
The effects of the physicochemical characteristics of a silica-chitosan complex membrane (SiCM) on the expression of tissue plasminogen activator (tPA) by contacting cells were investigated with the aim of improving the biocompatibility of the novel implant biomaterial. Expression of tPA is considered to be effective in wound healing by preventing thrombus formation, which causes inflammatory responses and rejection of implant materials. Inducing the epithelial cells surrounding implant materials to secrete tPA, which serves as an early signaling system to proliferate cells underlying connective tissues, would be further effective in accelerating wound healing. An epithelial 293 cell line derived from human embryonic kidney and a fibroblast IMR-90 cell line from human lung possessing the ability to secrete tPA were cultured on SiCMs, whose composition was stepwise controlled by adjusting the mixing ratio between silica and chitosan to give silica contents of 20, 33, 43, and 50wt%. Both strains showed strong adhesion on chitosan (0%-SiCM) and 50%-SiCM. The cell proliferation rates were also accelerated in a manner that was dependent on the increase in the adhesion strength of the cells cultured on the SiCMs. Furthermore, the tPA activity in the culture medium increased in accordance with the cell density, while the cellular specific activity of IMR-90 cells to secrete tPA was synergistically enhanced by strong adhesion and a high cell density on the surface of chitosan and 50%-SiCM. Analysis of the physico-chemical effects of the SiCMs revealed that the cells were dominantly affected by the surface hydrophobicity rather than by the zeta potential, as well as by the mixing ratio between chitosan and silica. The wet contact angles of 50%-SiCM and chitosan, which were 68 degrees and 65 degrees , respectively, were found to be suitable for adhesion and growth of both the epithelial 293 cells and fibroblast IMR-90 cells. A hydrophobic surface at 65 degrees -68 degrees was also effective for the production of tPA by IMR-90 cells, whereas the tPA activity of 293 cells reached its highest level on the SiCM with a wet contact angle of 63 degrees . These results suggest that a suitable adhesion strength is a significant factor in the expression of tPA by cells contacting an implant biomaterial. 相似文献
995.
996.
A waveguide dispersion measurement technique using a simple fitting function for wavelength dependence of the root-mean-square width in the far field and its theoretical measurement error are discussed. Results indicate that waveguide dispersion can be evaluated with theoretical accuracy of 0.5 ps/km/nm using this technique. The waveguide dispersions for conventional and dispersion-shifted fibers have been measured using the far-field pattern method. Good agreement between measured and theoretical values of waveguide dispersion was obtained. Material dispersion and dispersion sensitivity evaluation methods that are applications of this technique are also described 相似文献
997.
Kazuo Yamaura Ittsusei Mizutani Kumiko Monma Tetsuya Tanigami Shuji Matsuzawa 《应用聚合物科学杂志》1986,31(2):521-533
The direct wet-spinning of syndiotacticity-rich poly(vinyl alcohol) from ammonolysis solutions of poly(vinyl trifluoroacetate) was carried out. The high draft in spinning could not be accomplished at lower and higher polymer concentration, but it was accomplished at a certain middle polymer concentration. The drawn fibers with high draft have not always high strength. The fibers with high strength were obtained from fibers wound at moderate draft in spinning and then drawn at higher times without solvent. The strength was independent on the degree of polymerization (in a range from DP = 1870 to DP = 5580), but it became higher in the case of thinner fibers obtained from the solutions of lower polymer concentration. Here, the highest Young's modulus and strength at break were 13.8 GPa and 12 gr/denier, respectively. 相似文献
998.
Limits on the accuracy of 3-D thickness measurement in magnetic resonance images--effects of voxel anisotropy 总被引:3,自引:0,他引:3
Sato Y Tanaka H Nishii T Nakanishi K Sugano N Kubota T Nakamura H Yoshikawa H Ochi T Tamura S 《IEEE transactions on medical imaging》2003,22(9):1076-1088
Measuring the thickness of sheet-like thin anatomical structures, such as articular cartilage and brain cortex, in three-dimensional (3-D) magnetic resonance (MR) images is an important diagnostic procedure. This paper investigates the fundamental limits on the accuracy of thickness determination in MR images. We defined thickness here as the distance between the two sides of boundaries measured at the subvoxel resolution, which are the zero-crossings of the second directional derivatives combined with Gaussian blurring along the normal directions of the sheet surface. Based on MR imaging and computer postprocessing parameters, characteristics for the accuracy of thickness determination were derived by a theoretical simulation. We especially focused on the effects of voxel anisotropy in MR imaging with variable orientation of sheet-like structure. Improved and stable accuracy features were observed when the standard deviation of Gaussian blurring combined with thickness determination processes was around square root of 2/2 times as large as the pixel size. The relation between voxel anisotropy in MR imaging and the range of sheet normal orientation within which acceptable accuracy is attainable was also clarified, based on the dependences of voxel anisotropy and the sheet normal orientation obtained by numerical simulations. Finally, in vitro experiments were conducted using an acrylic plate phantom and a resected femoral head to validate the results of theoretical simulation. The simulated thickness was demonstrated to be well-correlated with the actual in vitro thickness. 相似文献
999.
Optical nonreciprocal devices with a silicon guiding layer fabricated by wafer bonding are proposed. The optical nonreciprocal devices are composed of a magneto-optic waveguide with a magnetic garnet/Si/SiO2 structure. Nonreciprocal characteristics are obtained by an evanescent field penetrating into the upper magnetic garnet cladding layer. Several kinds of the optical nonreciprocal device are investigated with the magneto-optic waveguide and designed at a wavelength of 1.55 microm. As a preliminary experiment, wafer bonding between Gd3Ga5O12 and Si was studied. Wafer bonding was successfully achieved with heat treatment at 220 degrees C in H2 ambient. 相似文献
1000.
Kawachi T Kado M Tanaka M Hasegawa N Nagashima K Sukegawa K Lu P Takahashi K Namba S Koike M Nagashima A Kato Y 《Applied optics》2003,42(12):2198-2205
A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated. 相似文献