全文获取类型
收费全文 | 23456篇 |
免费 | 2256篇 |
国内免费 | 4389篇 |
专业分类
化学 | 6324篇 |
晶体学 | 803篇 |
力学 | 1250篇 |
综合类 | 247篇 |
数学 | 117篇 |
物理学 | 3132篇 |
无线电 | 18228篇 |
出版年
2024年 | 152篇 |
2023年 | 532篇 |
2022年 | 610篇 |
2021年 | 642篇 |
2020年 | 475篇 |
2019年 | 630篇 |
2018年 | 371篇 |
2017年 | 741篇 |
2016年 | 802篇 |
2015年 | 779篇 |
2014年 | 1810篇 |
2013年 | 1372篇 |
2012年 | 1516篇 |
2011年 | 1594篇 |
2010年 | 1562篇 |
2009年 | 1805篇 |
2008年 | 2311篇 |
2007年 | 1394篇 |
2006年 | 1215篇 |
2005年 | 1807篇 |
2004年 | 2099篇 |
2003年 | 1364篇 |
2002年 | 903篇 |
2001年 | 771篇 |
2000年 | 497篇 |
1999年 | 327篇 |
1998年 | 249篇 |
1997年 | 214篇 |
1996年 | 258篇 |
1995年 | 259篇 |
1994年 | 225篇 |
1993年 | 140篇 |
1992年 | 153篇 |
1991年 | 146篇 |
1990年 | 142篇 |
1989年 | 122篇 |
1988年 | 24篇 |
1987年 | 17篇 |
1986年 | 18篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 12篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1973年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
971.
在乙醇-水混合体系中,以氨水为沉淀剂共沉淀合成了甲氨蝶呤/层状双金属氢氧化物(MTX/LDH)纳米复合物,采用控制水热处理时间的方式来调控其性能。利用X-射线衍射(XRD)、透射电镜(TEM)和红外光谱(FT-IR)等表征手段,对其结构及形貌进行了表征。研究表明:MTX分子以单层倾斜或垂直方式插入LDH层间,随着水热处理时间的不同,MTX在层间的倾斜角度发生了变化;水热处理时间对产物的结晶度、粒径和层间排列方式都有影响,当水热处理时间为12h时,得到的MTX/LDH纳米复合物的结晶度最高,单分散性最好。在磷酸缓冲液中考察了MTX/LDH纳米复合物的缓释性能,结果表明样品均呈现出良好的缓释性能,释放速率先快后慢。重点考察了这几种MTX/LDH纳米复合物作用于肺癌细胞A549的细胞生物实验,研究表明这几种复合物对肺癌细胞A549都具有良好的抑制作用,其效果与纳米复合物的单分散性和粒径有密切的关系,单分散性越好,粒径分布越均匀,对A549癌细胞的抑制效果越好。 相似文献
972.
通过低温溶剂热的方法成功制备出了LiCr0.2Ni0.4Mn1.4O4尖晶石正极材料。通过此法,溶液的饱和蒸汽压急剧降低且在室温(25℃)下即可沸腾。所有的金属离子可在随后的热聚合过程中均匀分散且煅烧后所得材料无杂质相生成。采用了热重分析,X射线衍射,扫描电镜、循环伏安,交流阻抗等测试手段对材料进行了表征。结果表明:此法所得材料含有Mn3+,为Fd3m晶型,且其形貌规则、粒度分布均一。1C和10C下放电容量为140.5和121.0 mAh·g-1,10C下100次循环容量保持率高达96.9%。其优异的电化学性能可归因于均相的前驱体制备过程,高结晶度且无杂相生成,以及较高的锂离子扩散系数诸因素的共同作用。 相似文献
973.
以Co(NO3)2·6H2O、Na2WO4·2H2O为主要原料,去离子水为溶剂,利用水热法在不同条件下制备了一系列的纳米CoWO4,用XRD、TEM和比表面分析仪对产品的物相、形貌和比表面积进行了表征。较系统地探讨了水热条件(反应混合物pH值、反应时间、反应温度等)对产物物相和形貌的影响,并研究了不同形貌产品对甲醛、乙醇、氨气、苯和丙酮等的敏感性能。结果表明:水热条件对产品的物相和形貌有影响,在不同水热条件下,可成功制备CoWO4纳米颗粒、纳米立方体及纳米棒;以纳米颗粒、纳米立方体及纳米棒样品制成的气敏元件对被试气体有不同程度的响应,其中以纳米颗粒为基的元件在210℃对1 000μL·L-1NH3灵敏度为3.3。 相似文献
974.
A well-ordered and spherical LiNi0.6Co0.2Mn0.2O2 cathode material was successfully synthesized from Ni and Mn concentration-gradient precursors via co-precipitation. The crystal structure, morphology and electrochemical properties of LiNi0.6Co0.2Mn0.2O2 were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, and charge-discharge tests. The material delivered an initial discharge capacity of 174.3 mAh/g at 180 mA/g (1 C rate) between 2.8 and 4.3 V and more than 93.1% of that was retained after 100 cycles. In addition, it also exhibited excellent rate capability, high cut-off voltage and temperature performance. 相似文献
975.
Min-Jie Shi Sheng-Zhong Kou Bao-Shou Shen Jun-Wei Lang Zhi Yang Xing-Bin Yan 《中国化学快报》2014,25(6):859-864
Ionic liquid gel polymers have widely been used as the electrolytes in all-solid-state supercapacitors, but they suffer from low ionic conductivity and poor electrochemical performance. Arc discharge is a fast, low-cost and scalable method to prepare multi-layered graphene nanosheets, and as-made graphene nanosheets (denoted as ad-GNSs) with few defects, high electrical conductivity and high thermal stability should be favorable conductive additive materials. Here, a novel ionic liquid gel polymer electrolyte based on an ionic liquid (EM1MNTF2) and an copolymer (P(VDF-HFP)) was modified by the addition of ad-GNSs as an ionic conducting promoter. This modified gel electrolyte shows excellent thermal stability up to 400 ℃ and a wide electrochemical window of 3 V. An all-solid-state supercapacitor based on commercial activated carbon was fabricated using this modified ionic liquid gel polymer electrolyte, which shows obviously improved electrochemical behaviors compared with those of the corresponding all-solid-state supercapacitor using pure ionic liquid gel polymer electrolyte. Specially, smaller internal resistance, higher specific capacitance, better rate performance and cycling stability are achieved. These results indicate that the ionic liquid gel polymers modified by ad-GNSs would be promising and suitable gel electrolytes for high performance all-solid-state electrochemical devices. 相似文献
976.
A phthalazinone monomer with an allyl group, i.e. 4-(3-allyl-4-hydroxyphenyl)phthalazin-1(2H)-one, was synthesized and then copolymerized with 4-(4-hydroxylphenyl)(2H)-phthalazin-1-one and 2,6- diflurobenzonitrile by means of aromatic nucleophilic polycondensation to provide a series of crosslinkable poly(aryl ether nitrile)s. The virgin copolymers exhibited good solubility in polar organic solvents with relative high molecular weights (Mw: 45,130-58,403, inherent viscosities: 0.58-0.75 dL/ g). After cross-linking, the thermal stability and solvent resistance of the polymer increased. 相似文献
977.
Guan Wu ;Ran Ran ;Bote Zhao ;Yujing Sha ;Chao Su ;Yingke Zhou ;Zongping Shao 《天然气化学杂志》2014,(3):363-375
Amorphous carbon and graphene co-modified LiFePO_4 nanocomposite has been synthesized via a facile polyol process in connection with a following thermal treatment.Various characterization techniques,including XRD.Mossbauer spectra,Raman spectra,SEM,TEM,BET,O_2-TPO,galvano charge-discharge,CV and EIS were applied to investigate the phase composition,carbon content,morphological structure and electrochemical performance of the synthesized samples.The effect of introducing way of carbon sources on the properties and performance of LiFePO_4/C/graphene composite was paid special attention.Under optimized synthetic conditions,highly crystalized olivine-type LiFePO_4was successfully obtained with electron conductive Fe_2P and FeP as the main impurity phases.SEM and TEM analyses demonstrated the graphene sheets were randomly distributed inside the sample to create an open structured LiFePO_4 with respect to graphene,while the glucosederived carbon mainly coated over LiFeP04 particles which effectively connected the graphene sheets and LiFePO_4 particles to result in a more efficient charge transfer process.As a result,favorable electrochemical performance was achieved.The performance of the amorphous carbon-graphene co-modified LiFePO_4 was further progressively improved upon cycling in the first 200 cycles to reach a reversible specificcapacity as high as 97 mAh·g~(-1) at 10 C rate. 相似文献
978.
Changchang Xu Li Li Fangyuan Qiu Cuihua An Yanan Xu Ying Wang Yijing Wang Lifang Jiao Huatang Yuan 《天然气化学杂志》2014,(3):397-402
Assisted by graphene oxide(GO),nano-sized LiMn0.6Fe0.4PO4 with excellent electrochemical performance was prepared by a facile hydrothermal method as cathode material for lithium ion battery.SEM and TEM images indicate that the particle size of LiMn0.6Fe0.4PO4(S2)was about 80 nm in diameter.The discharge capacity of LiMn0.6Fe0.4PO4 nanoparticles was 140.3 mAh-g^1 in the first cycle.It showed that graphene oxide was able to restrict the growth of LiMn0.6Fe0.4PO4 and it in situ reduction of GO could improve the electrical conductivity of LiMn0.6Fe0.4PO4 material. 相似文献
979.
In order to develop the conversion of heavy oil with a high yield of propylene in the catalytic cracking process, ZSM-5 zeolite was modified by tungsten and phosphorus, which was proved to be an effective method. Characterization results show that the improvement of catalytic performance could be correlated to the interaction of phosphorus and tungsten species on ZSM-5. P inhibited the aggregation of tungsten species on ZSM-5 and was conductive to convert the tungsten species with octahedral coordination into tetrahedral coordination. And this ultimately led to that more acid sites were reserved after hydrothermal treatment in the tungsten and phosphorus co-modified ZSM-5 catalyst. Phosphorus species played an important role to restrain the dehydrogenation activity of tungsten. In addition, a model reflecting the interaction between tungsten species and ZSM-5 framework was proposed. 相似文献
980.
The cell performance and temperature gradient of a tubular solid oxide fuel cell with indirect internal reformer(IIR-SOFC) fuelled by natural gas, containing a typical catalytic packed-bed reformer, a catalytic coated wall reformer, a catalytic annular reformer, and a novel catalytic annular-coated wall reformer were investigated with an aim to determine the most efficient internal reformer system. Among the four reformer designs, IIR-SOFC containing an annular-coated wall reformer exhibited the highest performance in terms of cell power density(0.67 W cm-2)and electrical efficiency(68%) with an acceptable temperature gradient and a moderate pressure drop across the reformer(3.53×10-5kPa).IIR-SOFC with an annular-coated wall reformer was then studied over a range of operating conditions: inlet fuel temperature, operating pressure, steam to carbon(S : C) ratio, gas flow pattern(co-flow and counter-flow pattern), and natural gas compositions. The simulation results showed that the temperature gradient across the reformer could not be decreased using a lower fuel inlet temperature(1223 K–1173 K)and both the power density and electrical efficiency of the cell also decreased by lowering fuel inlet temperature. Operating in higher pressure mode(1-10 bar) improved the temperature gradient and cell performance. Increasing the S : C ratio from 2 : 1 to 4 : 1 could decrease the temperature drop across the reformer but also decrease the cell performance. The average temperature gradient was higher and smoother in IIR-SOFC under a co-flow pattern than that under a counter-flow pattern, leading to lower overpotential and higher cell performance. Natural gas compositions significantly affected the cell performance and temperature gradient. Natural gas containing lower methane content provided smoother temperature gradient in the system but showed lower power density and electrical efficiency. 相似文献