全文获取类型
收费全文 | 23903篇 |
免费 | 3774篇 |
国内免费 | 2101篇 |
专业分类
电工技术 | 2648篇 |
技术理论 | 2篇 |
综合类 | 2069篇 |
化学工业 | 4247篇 |
金属工艺 | 723篇 |
机械仪表 | 1281篇 |
建筑科学 | 824篇 |
矿业工程 | 529篇 |
能源动力 | 412篇 |
轻工业 | 706篇 |
水利工程 | 612篇 |
石油天然气 | 386篇 |
武器工业 | 188篇 |
无线电 | 5901篇 |
一般工业技术 | 2916篇 |
冶金工业 | 413篇 |
原子能技术 | 422篇 |
自动化技术 | 5499篇 |
出版年
2024年 | 141篇 |
2023年 | 625篇 |
2022年 | 807篇 |
2021年 | 992篇 |
2020年 | 997篇 |
2019年 | 952篇 |
2018年 | 962篇 |
2017年 | 1138篇 |
2016年 | 1178篇 |
2015年 | 1254篇 |
2014年 | 1587篇 |
2013年 | 1644篇 |
2012年 | 1920篇 |
2011年 | 2013篇 |
2010年 | 1306篇 |
2009年 | 1333篇 |
2008年 | 1272篇 |
2007年 | 1430篇 |
2006年 | 1345篇 |
2005年 | 1109篇 |
2004年 | 989篇 |
2003年 | 865篇 |
2002年 | 597篇 |
2001年 | 520篇 |
2000年 | 461篇 |
1999年 | 347篇 |
1998年 | 313篇 |
1997年 | 233篇 |
1996年 | 251篇 |
1995年 | 185篇 |
1994年 | 178篇 |
1993年 | 152篇 |
1992年 | 132篇 |
1991年 | 103篇 |
1990年 | 69篇 |
1989年 | 80篇 |
1988年 | 54篇 |
1987年 | 31篇 |
1986年 | 63篇 |
1985年 | 35篇 |
1984年 | 24篇 |
1983年 | 25篇 |
1982年 | 20篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 3篇 |
1975年 | 5篇 |
1974年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
11.
13.
《Ceramics International》2019,45(14):16940-16947
Coordination chemistry, bond state and vibrational spectrum of co-substituted microwave dielectric NdNb1-x(Zr0.5W0.5)xO4 ceramics (x = 0.01∼0.05) were investigated. Raman spectra and XRD refinement showed a solid solution was formed. The compressed and elongated chemical bonds are responsible for the variations of crystal parameters and cell volume. Calculated chemical bond parameters indicated bond covalency, lattice energy and Nb-site bond energy act on the fluctuations of the permittivity, quality factor and temperature coefficient, respectively. Meanwhile, the infrared vibrational spectrum is fitted to quantify the contributions of observed IR mode to the intrinsic loss. Compact ceramic possesses excellent properties: εr ∼ 19.2, Q × f ∼ 55282 GHz and τf ∼ -11.36 ppm/°C with x = 0.04, at 1250°C. 相似文献
14.
How to improve the sensitivity of the temperature-sensing luminescent materials is one of the most important objects currently. In this work, to obtain high sensitivity and learn the corresponding mechanism, the rare earth (RE) ions doped Y4.67Si3O13 (YS) phosphors were developed by solid-state reaction. The phase purity, structure, morphology and luminescence characteristics were evaluated by XRD, TEM, emission spectra, etc. The change of the optical bandgaps between the host and RE-doped phosphors was found, agreeing with the calculation results based on density-functional theory. The temperature-dependence of the upconversion (UC) luminescence revealed that a linear relationship exists between the fluorescence intensity ratio of Ho3+ and temperature. The theoretical resolution was evaluated. High absolute (0.083 K−1) and relative (3.53% K−1 at 293 K) sensitivities have been gained in the YS:1%Ho3+, 10%Yb3+. The effect of the Yb3+ doping concentration and pump power on the sensitivities was discussed. The pump-power–dependence of the UC luminescence indicated the main mechanism for high sensitivities in the YS:1%Ho3+, 10%Yb3+. Moreover, the decay-lifetime based temperature sensing was also evaluated. The above results imply that the present phosphors could be promising candidates for temperature sensors, and the proposed strategies are instructive in exploring other new temperature sensing luminescent materials. 相似文献
15.
Crystallographic characteristics and microwave dielectric properties of Ni-modified MgTa2O6 ceramics
Liang Shi Xueying Wang Rui Peng Gang Wang Cheng Liu Xiaolei Shi Dainan Zhang Huaiwu Zhang 《Ceramics International》2021,47(16):22514-22521
Ni2+ modified MgTa2O6 ceramics with a trirutile phase and space group P42/mnm were obtained. The correlations between crystallographic characteristics and microwave dielectric performance of MgTa2O6 ceramics were systematically studied based on the chemistry bond theory (PVL theory) for the first time. The results indicate that the introduction of Ni2+ causes a change in polarizability and the Mg–O bond ionicity, which contributes to the variation of dielectric constant. Moreover, the lattice energy, and packing fraction, full width at half maximum of the Raman peak of Ta–O bond, as the quantitative characterization of crystallographic parameters, regulate the dielectric loss of MgTa2O6 ceramics in GHz frequency band. In addition, the study of sintering behavior shows that the densification and micromorphology are the crucial factors affecting the microwave dielectric performance. Typically, Ni2+ doping on the A-site of MgTa2O6 can effectively promote the Q × f values to 173,000 GHz (at 7.43 GHz), which ensures its applicability in 5G communication technology. 相似文献
16.
Madhab Chandra Tripathy Debasmita Mondal Karabi Biswas Siddhartha Sen 《International Journal of Circuit Theory and Applications》2015,43(6):776-792
The present work reports the realization of an analog fractional‐order phase‐locked loop (FPLL) using a fractional capacitor. The expressions for bandwidth, capture range, and lock range of the FPLL have been derived analytically and then compared with the experimental observations using LM565 IC. It has been observed that bandwidth and capture range can be extended by using FPLL. It has also been found that FPLL can provide faster response and lower phase error at the time of switching compared to its integer‐order counterpart. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
17.
ABSTRACT The thermoplastic and low dielectric constants polyimides were introduced. The polyimides were prepared by pyromellitic dianhydride (PMDA) or 4,4?-(4,4?-Isopropylidenediphenoxy)diphthalic anhydride (BPADA) as anhydride monomer and 4,4?-oxydianiline (ODA) or 2,2-bis(4-(4-aminephenoxy)phenyl)propane (BAPP) as amine monomer. The polyimides were well characterized by FT-IR, thermogravimetric analysis, dynamic thermomechanical analysis, dielectric measurement, and tensile test. The dielectric constants were 2.32–2.95 compared with 3.10 of ODA-PMDA polyimide, while partly polyimides were thermoplastic. The results indicated anhydride monomers, containing lateral methyl groups, made polyimides become thermoplastic. The results of molecular simulations via Materials Studio also proved this conclusion. 相似文献
18.
Chitosan microspheres containing bromocresol green, cresol red, and phenolphthalein for corrosion detection, through pH change, are synthesized in order to be used in protective coatings for aluminium alloys. Microspheres containing corrosion detection species are characterized morphologically (SEM) and physico‐chemically (FTIR, TGA). Release studies (UV–vis) are performed in corrosion‐promoting conditions (pH, NaCl), and detection studies by immersion in media associated with corrosion activity while microspheres' sensing activity is evaluated visually. Electrochemical characterization of AA2024 substrates in the presence of chitosan spheres is performed to understand material performance, and a color change is observed as a result of local pH increase in cathodic areas when corrosion takes place. These findings can be correlated with the results from release studies and seem a promising approach for corrosion sensing purposes, not only because pH increase is possible to detect due to corrosion, but also because chitosan is considered an environmentally friendly material. 相似文献
19.
20.
《Journal of the European Ceramic Society》2020,40(8):3035-3041
BaCu2-xCoxSi2O7 solid solutions with orthorhombic structure (Pnma) were prepared by solid-state reaction method. The phase synthesis process, structural evolution and microwave dielectric properties of BaCu2-xCoxSi2O7 ceramics were investigated. Single BaCu2Si2O7 phase was obtained when calcined at 950 °C for 3 h and was decomposed into BaCuSi2O6 phase when calcined at 1075 °C for 3 h. The sintering process was effectively promoted when Cu2+ was replaced by Co2+ and the maximum solubility of BaCu2-xCoxSi2O7 was located between 0.15 and 0.20. P-V-L complex chemical bond theory and Raman spectra were used to explain the structure-property correlations of BaCu2-xCoxSi2O7 ceramics. The corrected dielectric constant (εr-corr) of BaCu2-xCoxSi2O7 ceramics decreased monotonously with the susceptibility (Σχμ) and ionic polarizability of primitive unit cell. The quality factor (Q × f) increased with bond strength and lattice energy (Ucal), especially the lattice energy of the Si-O bond. The temperature coefficient of resonant frequency (τf) was determined by the susceptibility and lattice energy of the Cu/Co-O bond. The following optimum microwave dielectric properties were obtained at x = 0.15 when sintered at 1000 °C for 3 h: εr = 8.45, Q×f =58958 GHz and τf = -34.4 ppm/°C. 相似文献