全文获取类型
收费全文 | 32404篇 |
免费 | 3121篇 |
国内免费 | 1500篇 |
专业分类
电工技术 | 2293篇 |
技术理论 | 1篇 |
综合类 | 2023篇 |
化学工业 | 5474篇 |
金属工艺 | 1822篇 |
机械仪表 | 2050篇 |
建筑科学 | 2569篇 |
矿业工程 | 999篇 |
能源动力 | 884篇 |
轻工业 | 2434篇 |
水利工程 | 596篇 |
石油天然气 | 1691篇 |
武器工业 | 276篇 |
无线电 | 3967篇 |
一般工业技术 | 4131篇 |
冶金工业 | 1369篇 |
原子能技术 | 418篇 |
自动化技术 | 4028篇 |
出版年
2024年 | 180篇 |
2023年 | 652篇 |
2022年 | 1290篇 |
2021年 | 1548篇 |
2020年 | 1206篇 |
2019年 | 987篇 |
2018年 | 1091篇 |
2017年 | 1226篇 |
2016年 | 994篇 |
2015年 | 1388篇 |
2014年 | 1746篇 |
2013年 | 1918篇 |
2012年 | 2077篇 |
2011年 | 2154篇 |
2010年 | 1828篇 |
2009年 | 1835篇 |
2008年 | 1806篇 |
2007年 | 1582篇 |
2006年 | 1653篇 |
2005年 | 1343篇 |
2004年 | 883篇 |
2003年 | 892篇 |
2002年 | 835篇 |
2001年 | 736篇 |
2000年 | 748篇 |
1999年 | 796篇 |
1998年 | 648篇 |
1997年 | 555篇 |
1996年 | 481篇 |
1995年 | 439篇 |
1994年 | 323篇 |
1993年 | 250篇 |
1992年 | 229篇 |
1991年 | 145篇 |
1990年 | 127篇 |
1989年 | 116篇 |
1988年 | 83篇 |
1987年 | 52篇 |
1986年 | 30篇 |
1985年 | 35篇 |
1984年 | 18篇 |
1983年 | 21篇 |
1982年 | 23篇 |
1981年 | 7篇 |
1980年 | 12篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1975年 | 3篇 |
1951年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Enhancing matrix crystallization has been demonstrated to be an effective method to simultaneously improve the impact toughness and heat resistance of poly(l-lactide) (PLLA) modified with flexible polymers, such as poly(ε-caprolactone) (PCL). Unfortunately, increasing PLLA matrix crystallinity alone cannot guarantee the enhancement of impact toughness in most cases, so other structural parameters should be considered. In this work, taking PLLA/PCL (80/20) blend as an example, the combined roles of matrix crystallization and impact modifier particle size in the toughening have been investigated. PLLA matrix crystallinity was controlled by adding a highly effective nucleating agent and PCL particle size was tailored by varying processing conditions while maintaining constant interfacial adhesion. It is interesting to find that toughening is efficient only if matrix crystallinity and particle size are well matched. With the significant increase of matrix crystallinity, an evident decrease of optimum particle size for toughening PLLA has been identified for the first time. Therefore, suitable particle size is the precondition for highly crystalline matrix to work effectively in the toughening because only small particles (0.3–0.5 μm) are effective in trigger shear yielding mechanism of the matrix needed for good toughness, whereas relatively large particles (0.7–1.1 μm) are only capable of toughening amorphous matrix effectively by initiating multiple crazing of the matrix. Importantly, our findings can be used to well explain the reason for the different roles of matrix crystallization in the toughening of different PLLA blends reported in the literature. Furthermore, the heat resistance of the blend with a highly crystalline matrix is much better than that of the blend with an amorphous one as expected. This work could not only provide a new insight into the synergistic roles of matrix crystallization and modifier particle size in the toughening of PLLA but also set up a universal framework for designing high-performance PLLA products with both good impact toughness and high heat resistance. 相似文献
942.
943.
Novel sulfonated poly(2‐butoxymethylenenorbornene‐co‐2‐(6‐phenoxy‐hexyloxymethylene)‐5‐norbornene [sP(BN/PhHN)] were prepared successfully through vinyl‐addition type polymerization and then sulfonated with concentrated sulfuric acid (98%) as sulfonating agent in a component solvent. The sP(BN/PhHN)‐40 with the maximal degree of sulfonation of 40% can be obtained by controlling the sulfonating reaction time from 8 to 20 h, and a proton conductivity of 3.35 × 10?3 S/cm was achieved at 70°C. The methanol permeabilities of these membranes were in the range from 0.26 to 6.58 × 10?7 cm2/s, which were remarkably lower than Nafion (2.36 × 10?6 cm2/s). TEM analysis revealed that these side‐chain type membranes have a microphase separated structure composed of hydrophilic side‐chain domains and hydrophobic polynorbornene main chain domains. Sulfonated polynorbornene containing soft spacers displayed better properties, such as lower water uptake, high thermal properties, mechanical properties, and low methanol permeability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013 相似文献
944.
A fluorinated hyperbranched polyimide (HBPI) is synthesized by using a triamine monomer, 1,3,5‐tris(2‐trifluoromethyl‐4‐aminophenoxy)benzene (TFAPOB) (B3), as a “core” molecule, 4,4′‐oxydiphthalic anhydride (ODPA) as a A2 monomer, and 4‐aminophthalonitrile as an end‐capping reagent. After that, a series of novel fluorinated hyperbranched polyimides end‐capped with metallophthalocyanines were prepared by the reactions of dicyanophenyl end‐capped hyperbranched polyimide with excessive amounts of 1,2‐dicyanobenzene and the corresponding metal salt in quinoline. The resulting polyimides containing metallophthalocyanine unites shows optical absorption in the visible region. The absorption bands of the polymers in chloroform solution are in the range of 665–701 nm. These polyimides show glass transition temperatures between 216 and 225°C, and the 5 wt % weight loss temperature of the polymers varied from 440 to 543°C under nitrogen. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013 相似文献
945.
This article presents a novel investigation on the Mo/Mg/Ni/O catalysts (Nmm‐cats) with which the noncharring polyolefins are basically transferred to the charring polymers under forced flaming conditions and the flame retardancy of polyolefins is improved dramatically. The results of model charring experiments show that when appropriate Mo/Mg/Ni molar ratio is adopted and only 3% Nmm‐cats is blended, the Nmm‐cats belong to high‐efficient charring catalysts that can deposit 56 and 64 wt % of volatile products in‐situ from linear low‐density polyethylene (LLDPE) and polypropylene (PP), respectively. The charring properties are characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy; and the char‐forming mechanisms are analyzed by wide‐angle X‐ray diffraction experiment. The improvement in flammability properties for LLDPE and PP is demonstrated by using a cone calorimeter. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013 相似文献
946.
Using a conventional solid‐state reaction Ca5A4(VO4)6 (A2+ = Mg, Zn) ceramics were prepared and their microwave dielectric properties were investigated for the first time. X‐ray diffraction revealed the formation of pure‐phase ceramics with a cubic garnet structure for both samples. Two promising ceramics Ca5Zn4(VO4)6 and Ca5Mg4(VO4)6 sintered at 725°C and 800°C were found to possess good microwave dielectric properties: εr = 11.7 and 9.2, Q × f = 49 400 GHz (at 9.7 GHz) and 53 300 GHz (at 10.6 GHz), and τf = ?83 and ?50 ppm/°C, respectively. 相似文献
947.
0.9(Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4–0.1(Ca0.8Sr0.2)TiO3 (MZTS–CST) ceramics were prepared by a conventional solid‐state route. The MZTS–CST ceramics sintered at 1325°C exhibited εr = 18.2, Q × f = 49 120 GHz (at 8.1 GHz), and τf = 15 ppm/°C. The effects of LiF–Fe2O3–V2O5 (LFV) addition on the sinterability, phase composition, microstructure, and microwave dielectric properties of MZTS–CST were investigated. Eutectic liquid phases 0.12CaF2/0.28MgF2/0.6LiF and MgV2O6 were developed, which lowered the sintering temperature of MZTS–CST ceramics from 1325°C to 950°C. X‐ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS) analysis revealed that MZTS and CST coexisted in the sintered ceramics. Secondary phase Ca5Mg4(VO4)6 as well as residual liquid phase affected the microwave dielectric properties of MZTS–CST composite ceramics. Typically, the MZTS–CST–5.3LFV composite ceramics sintered at 950°C showed excellent microwave dielectric properties: εr = 16.3, Q × f = 30 790 GHz (at 8.3 GHz), and τf = ?10 ppm/°C. 相似文献
948.
Li‐Qian Cheng Ke Wang Fang‐Zhou Yao Fangyuan Zhu Jing‐Feng Li 《Journal of the American Ceramic Society》2013,96(9):2693-2695
Alkaline volatilization during sintering in (K, Na)NbO3 (KNN)‐based lead‐free piezoceramics is a problem that causes non‐stoichiometry and hence affects the phase structures and properties. This work investigated the depth‐dependent distributions of alkaline elements in Li‐modified KNN ceramics using X‐ray photoelectron spectroscopy and Raman spectroscopy. It was found that Li evaporated more severely than Na and K, consequently almost no Li elements remained on the as‐sintered surface, while its content became stable at the depth over 55 μm from the outmost surface. In addition, X‐ray diffraction results suggested the phase structure might show discrepancy at the pellet surface and the interior part due to the textured surface grains. 相似文献
949.
Jingyi Huang Guangqing Liu Yuming Zhou Qingzhao Yao Yong Yang Huchuan Wang Lei Ling Ke Cao Yahui Liu Wendao Wu Wei Sun 《Polymer Engineering and Science》2013,53(6):1306-1313
Allyloxy polyethoxy ether (APEG) and succinic anhydride were used to prepare allyloxy polyethoxy carboxylate (APEL). 8‐hydroxy‐1,3,6‐pyrene trisulfonic acid trisodium salt (PY) was reacted with allyl chloride to produce fluorescent monomer 8‐allyloxy‐1,3,6‐pyrene trisulfonic acid trisodium salt (PA). APEL and PA were copolymerized with maleic anhydride (MA) to synthesize PA tagged no phosphate and nitrogen‐free calcium phosphate inhibitor POLY(MA–APEL–PA). Structures of PA, APEG, APEL, and POLY(MA–APEL–PA) were carried out by FTIR and 1H‐NMR. Different MA : APEL mole ratios were employed for the manufacture of POLY(MA–APEL–PA) to study the effect of mole ratio on performance of POLY(MA–APEL–PA). Relationship between POLY(MA–APEL–PA)'s fluorescent intensity and its dosage was studied. The results indicate that capability of POLY(MA–APEL–PA) is heavily depended on the mole ratio of MA : APEL. Correlation coefficient r of POLY(MA–APEL–PA)'s fluorescent intensity and its dosage is 0.9999, and detection limit of POLY(MA–APEL–PA) is 0.86 mg/L. POLY(MA–APEL–PA) can be used to accurately measure polymer consumption on line besides providing excellent calcium phosphate inhibition. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers 相似文献
950.
A vacuum ultraviolet (VUV) photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) has been developed for real-time, quantitative chemical analysis of organic particles in laboratory environments. A nozzle of ~ 0.12 mm orifice combined with an aerodynamic lens assembly and a three stage differential pumping system is used to sample particles at atmospheric pressure. The particles are vaporized on a thermal heater, and then the nascent vapor is photoionized by light generated with a RF-powered VUV lamp. A 0.41 V/cm electric field is used to drive the ions from the ionization region into the ion extraction region where a positive electric pulse repels the ions into a reflectron mass spectrometer. The mass resolution of the spectrometer is ~ 350 and the detection limit is ~ 400 μ m 3 . The signal intensities observed are linear with the mass concentration of aerosols. Oleic acid particles are well quantified with an uncertainty of 15% in mass concentrations ranging from 3.9 mg/m 3 to 392 mg/m 3 . The VUV-ATOFMS has substantial potential for the use in laboratory investigations on organic aerosol chemistry. 相似文献