全文获取类型
收费全文 | 5104篇 |
免费 | 744篇 |
国内免费 | 308篇 |
专业分类
化学 | 2547篇 |
晶体学 | 115篇 |
力学 | 188篇 |
综合类 | 26篇 |
数学 | 67篇 |
物理学 | 3213篇 |
出版年
2024年 | 9篇 |
2023年 | 16篇 |
2022年 | 60篇 |
2021年 | 79篇 |
2020年 | 81篇 |
2019年 | 73篇 |
2018年 | 96篇 |
2017年 | 138篇 |
2016年 | 178篇 |
2015年 | 146篇 |
2014年 | 261篇 |
2013年 | 381篇 |
2012年 | 313篇 |
2011年 | 361篇 |
2010年 | 242篇 |
2009年 | 270篇 |
2008年 | 275篇 |
2007年 | 289篇 |
2006年 | 270篇 |
2005年 | 285篇 |
2004年 | 248篇 |
2003年 | 237篇 |
2002年 | 239篇 |
2001年 | 172篇 |
2000年 | 162篇 |
1999年 | 131篇 |
1998年 | 141篇 |
1997年 | 123篇 |
1996年 | 118篇 |
1995年 | 77篇 |
1994年 | 92篇 |
1993年 | 85篇 |
1992年 | 66篇 |
1991年 | 46篇 |
1990年 | 47篇 |
1989年 | 23篇 |
1988年 | 30篇 |
1987年 | 31篇 |
1986年 | 9篇 |
1985年 | 14篇 |
1984年 | 23篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 38篇 |
1980年 | 31篇 |
1979年 | 57篇 |
1978年 | 31篇 |
1973年 | 3篇 |
1972年 | 2篇 |
1957年 | 1篇 |
排序方式: 共有6156条查询结果,搜索用时 687 毫秒
251.
252.
Nelaka Govinna Ilin Sadeghi Ayse Asatekin Peggy Cebe 《Journal of Polymer Science.Polymer Physics》2019,57(6):312-322
We report the structure and thermal properties of blends comprising poly(vinylidene fluoride) (PVDF) and a random fluorinated copolymer (FCP) of poly(methyl methacrylate)‐random‐1H,1H,2H,2H‐perfluorodecyl methacrylate, promising membrane materials for oil–water separation. The roles of processing method and copolymer content on structure and properties were studied for fibrous membranes and films with varying compositions. Bead‐free, nonwoven fibrous membranes were obtained by electrospinning. Fiber diameters ranged from 0.4 to 1.9 μm, and thinner fibers were obtained for PVDF content >80%. As copolymer content increased, degree of crystallinity and onset of degradation for each blend decreased. Processing conditions have a greater impact on the crystallographic phase of PVDF than copolymer content. Fibers have polar beta phase; solution‐cast films contain gamma and beta phase; and melt crystallized films form alpha phase. Kwei's model was used to model the glass transition temperatures of the blends. Addition of FCP increases hydrophobicity of the electrospun membranes. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 312–322 相似文献
253.
Xin Dong Juan Yang Xiao-Ting Zheng Yan Chen Li-Hong Ye Bin Wang Hui Zheng Jun Cao Qiu-Yan Wang Yu-Han Hu 《Electrophoresis》2020,41(1-2):123-130
A novel micro matrix solid phase dispersion method was successfully used for the extraction of quaternary alkaloids in Phellodendri chinensis cortex. The elution of target compounds was accomplished with sodium hexanesulfonate as the eluent solvent. A neutral ion pair was formed between ion-pairing reagent and positively charged alkaloids in this process, which was beneficial for selectively extraction of polar alkaloids. Several parameters were optimized and the optimal conditions were listed as follows: silica gel as the sorbent, silica to sample mass ratio of 1:1, the grinding time of 1 min. The exhaustive elution of targets was achieved by 200 µL methanol/water (9:1) containing 150 mM sodium hexane sulfonate at pH 4.5. The method validation covered linearity, recovery, precision of intraday and interday, limits of detection, limits of quantitation, and repeatability. This established method was rapid, simple, environmentally friendly, and highly sensitive. 相似文献
254.
Tingke Rao Prof. Maria Luisa Saladino Prof. Yuanxing Fang Prof. Xinchen Wang Dr. Cristina Giordano 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(72):16676-16682
A versatile synthetic strategy for the preparation of multimetallic oxynitrides has been designed and here exemplarily discussed considering the preparation of nanoscaled zinc–gallium oxynitrides and zinc–gallium–indium oxynitrides, two important photocatalysts of new generation, which proved to be active in key energy related processes from pollutant decomposition to overall water splitting. The synthesis presented here allows the preparation of small nanoparticles (less than 20 nm in average diameter), well-defined in size and shape, yet highly crystalline and with the highest surface area reported so far (up to 80 m2 g−1). X-ray diffraction studies show that the final material is not a mixture of single oxides but a distinctive compound. The photocatalytic properties of the oxynitrides have been tested towards the decomposition of an organic dye (as a model reaction for the decomposition of air pollutants), showing better photocatalytic performances than the corresponding pure phases (reaction constant 0.22 h−1), whereas almost no reaction was observed in absence of catalyst or in the dark. The photocatalysts have been also tested for H2 evolution (semi-reaction of the water splitting process) with results comparable to the best literature values but leaving room for further improvement. 相似文献
255.
Small molecule‐mediated glass transition of acrylic copolymers: Effect of hydrogen bonding strength on glass transition temperature
下载免费PDF全文
![点击此处可从《Journal of Polymer Science.Polymer Physics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Chongyang Liu Xiaotong Yin Yu Lin Aiguo Guan Guozhang Wu 《Journal of Polymer Science.Polymer Physics》2015,53(6):400-408
Poly(styrene‐co‐ethyl acrylate) [P(St‐co‐EA)] with different ratios of St/EA was mixed with the small molecule 4,4′‐thio‐bis(6‐tert‐butyl‐m‐methyl phenol) (AO300) to investigate the influence of hydrogen bonding strength on the glass transition behavior. The glass transition temperature (Tg) linearly increased after adding AO300, and the slope value decreased with increased St/EA ratio. All lines could be extended to 62 °C, demonstrating that Tg of the small molecule in situ detected by the polymer chain was much higher than that by small molecule itself (29 °C). Fourier transform infrared spectroscopy analysis showed that the small molecules began to be self‐associated at a concentration where the hydrogen bonded carbonyl ratio of the bulk polymer was approximately 0.5 and irrespective of the St/EA ratio. Above the critical loading, the mixture's Tg negatively deviated from the linearly extended lines because of self‐association of the small molecules. The apparent Tg of AO300 was found to strongly depend on intermolecular hydrogen bonding number and strength. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 400–408 相似文献
256.
Manoj K. Kolel‐Veetil Dawn D. Dominguez Christopher A. Klug Kenan P. Fears Syed B. Qadri Daniel Fragiadakis Teddy M. Keller 《Journal of polymer science. Part A, Polymer chemistry》2013,51(12):2638-2650
Structural isomers of thermo‐oxidatively stable poly(carborane‐siloxane‐arylacetylene) (PCSAA), namely, m‐PCSAA and p‐PCSAA, were synthesized by the reaction of the dimagnesium salts of m‐diethynylbenzene or p‐diethynylbenzene with 1,7‐bis(chlorotetramethyldisiloxyl)‐m‐carborane. The developed polymers have exceptional thermo‐oxidative properties similar to their diacetylene counterpart poly(carborane‐siloxane‐acetylene), PCSA. Thermal treatment of either of the PCSAAs results in a fully crosslinked thermoset by 500 °C resulting from the cycloaddition reactions involving the acetylene and aryl functionalities and subsequent formation of bridging disilylmethylene entities as discerned from Fourier transform infrared, 13C and 29Si solid‐state NMR, and XPS studies. X‐ray diffraction analysis revealed that the thermosets obtained from p‐PCSAA possess enhanced crystallinity when compared to that obtained from m‐PCSAA possibly due to more efficient packing interactions of the p‐diethynylbenzene groups during thermoset formation. The presence of the aryl groups in the backbone of the PCSAAs' chains appeared to have enhanced the storage and bulk moduli of their thermosets when compared to the thermoset of PCSA. Dielectric studies of m‐PCSAA and p‐PCSAA revealed segmental relaxation peaks, α, above their glass transition temperatures with p‐PCSAA exhibiting a broader peak with a slower relaxation rate than m‐PCSAA. © 2013 Wiley Periodicals, Inc.? J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2638–2650 相似文献
257.
A new AB type of monomer 4′-fluoro-3,5-dimethyl-3′-trifluoromethyl-biphenyl-4-ol has been synthesized that leads to a new poly(arylene ether) by self polycondensation reaction. The monomer and the polymer have been well characterized by elemental analyses, FT-IR and NMR spectroscopy. Both FT-IR and NMR spectra of the polymers did not show any terminal phenoxy group indicating high conversion. The polymer showed glass transition at 278°C and very good thermal stability in synthetic air. GPC results indicate high molar mass development; Mw = 53200 and MWD = 2.29. 相似文献
258.
Brian Dillman Julie L. P. Jessop 《Journal of polymer science. Part A, Polymer chemistry》2013,51(9):2058-2067
The effects of chain transfer agents (CTA) on cationic ring‐opening polymerization of 3,4‐epoxycyclohexylmethyl‐3′,4′‐epoxycyclohexane carboxylate (EEC) were explored. EEC was polymerized in the presence of various CTAs, and epoxide conversions monitored via Raman spectroscopy. Polymer films were prepared and analyzed by dynamic mechanical analysis. Many of the organic alcohols studied greatly enhanced epoxide polymerization rates and conversion levels. The gel fraction of polymer specimens decreased rapidly with increasing amounts of octanol (gel fraction >90% up to 0.3 equiv OH) but remained high with increasing amounts of 1,2‐propanediol (gel fraction >90% up to 0.6 equiv OH). Increasing the size of primary alcohols had little effect on the polymerization rates and conversions. The polymerization rate decreased with increasing alcohol substitution (1°>2°>3°). Acidic alcohols had very low impact on conversion and polymerization rates relative to the neat epoxy resin. The glass transition temperature was inversely related to the size and amount of CTA. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013 相似文献
259.
The purpose of this study was to measure the radiation exposure to operator and patient during cardiac electrophysiology study, radiofrequency catheter ablation and cardiac device implantation procedures and to calculate the allowable number of cases per year. We carried out 9 electrophysiology studies, 40 radiofrequency catheter ablation and 11 cardiac device implantation procedures. To measure occupational radiation dose and dose–area product (DAP), 13 photoluminescence glass dosimeters were placed at eyes (inside and outside lead glass), thyroids (inside and outside thyroid collar), chest (inside and outside lead apron), wrists, genital of the operator (inside lead apron), and 6 of photoluminescence glass dosimeters were placed at eyes, thyroids, chest and genital of the patient. Exposure time and DAP values were 11.7?±?11.8?min and 23.2?±?26.2?Gy?cm2 for electrophysiology study; 36.5?±?42.1?min and 822.4?±?125.5?Gy?cm2 for radiofrequency catheter ablation; 16.2?±?9.3?min and 27.8?±?16.5?Gy?cm2 for cardiac device implantation procedure, prospectively. 4591 electrophysiology studies can be conducted within the occupational exposure limit for the eyes (150?mSv), and 658-electrophysiology studies with radiofrequency catheter ablation can be carried out within the occupational exposure limit for the hands (500?mSv). 1654 cardiac device implantation procedure can be conducted within the occupational exposure limit for the eyes (150?mSv). The amounts of the operator and patient's radiation exposure were comparatively small. So, electrophysiology study, radio frequency catheter ablation and cardiac device implantation procedures are safe when performed with modern equipment and optimized protective radiation protect equipment. 相似文献
260.
《Analytical letters》2012,45(15):3153-3163
ABSTRACT Thermal lens spectrometry has been used for the determination of iron in calf serum. The method is based on dissociation of Fe3+ from proteins, reduction of Fe3+ to Fe2+ and formation of a coloured complex between Fe2+ and bathophenanthroline. Contrary to the spectrophotometric method, it is shown that thermal lens spectrometry is less sensitive to scattering caused by the presence of small particles remaining in the test solution after the deproteinization step. The background signal is very small and the response is only slightly dependent on the amount of scattering particles. The method is reliable, sensitive and reproducible. The limit of detection for iron is 4 ppb and the relative standard deviation is around 2%. It is expected that the volume of serum sample necessary for an analysis can be reduced to less than 100 μl. 相似文献