全文获取类型
收费全文 | 52433篇 |
免费 | 4779篇 |
国内免费 | 2842篇 |
专业分类
化学 | 27593篇 |
晶体学 | 500篇 |
力学 | 1629篇 |
综合类 | 187篇 |
数学 | 3863篇 |
物理学 | 11425篇 |
无线电 | 14857篇 |
出版年
2024年 | 116篇 |
2023年 | 753篇 |
2022年 | 1031篇 |
2021年 | 1511篇 |
2020年 | 1352篇 |
2019年 | 1388篇 |
2018年 | 1162篇 |
2017年 | 1167篇 |
2016年 | 1884篇 |
2015年 | 1737篇 |
2014年 | 2279篇 |
2013年 | 3285篇 |
2012年 | 3761篇 |
2011年 | 4016篇 |
2010年 | 2866篇 |
2009年 | 2745篇 |
2008年 | 3492篇 |
2007年 | 3054篇 |
2006年 | 3017篇 |
2005年 | 2751篇 |
2004年 | 2285篇 |
2003年 | 2056篇 |
2002年 | 2036篇 |
2001年 | 1435篇 |
2000年 | 1293篇 |
1999年 | 1039篇 |
1998年 | 804篇 |
1997年 | 705篇 |
1996年 | 728篇 |
1995年 | 578篇 |
1994年 | 502篇 |
1993年 | 472篇 |
1992年 | 410篇 |
1991年 | 348篇 |
1990年 | 311篇 |
1989年 | 244篇 |
1988年 | 200篇 |
1987年 | 147篇 |
1986年 | 119篇 |
1985年 | 153篇 |
1984年 | 100篇 |
1983年 | 87篇 |
1982年 | 94篇 |
1981年 | 94篇 |
1980年 | 49篇 |
1979年 | 59篇 |
1978年 | 50篇 |
1976年 | 43篇 |
1975年 | 38篇 |
1974年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
单甲基原位改性SiO2疏水减反膜的制备与性能研究 总被引:1,自引:0,他引:1
在碱性条件下通过TEOS和MTES的共水解缩聚反应制备了单甲基原位改性的SiO2溶胶,并使用提拉法在K9玻璃基片上镀制了疏水减反膜。通过透射电镜(TEM)考察了镀膜溶胶的微结构,分别使用红外光谱(FTIR)分析了薄膜的组分,用原子力显微镜(AFM)观察了薄膜的表面形貌和起伏状况,用紫外可见光谱(UV-vis)考察了薄膜的减反射性能,用接触角仪测量了薄膜对水的接触角。并使用“R-on-1”的方式测量了薄膜在Nd:YAG激光(1 064 nm,1 ns)作用下的损伤阈值。结果表明,通过共水解缩聚反应可以把甲基引入镀膜溶胶簇团中,改善了溶胶簇团的网络结构,使薄膜得到相当好的疏水性能和更好的抗激光损伤性能,同时薄膜能保持较好的减反射性能。 相似文献
102.
Homo-apioneplanocin A was efficiently synthesized via stereoselective hydroxymethylation, regio- and chemoselective hydroboration, and chemoselective oxidation as key steps from d-ribose. 相似文献
103.
Vinyl and diene derivatives of thiolactomycin have been prepared via Horner-Wadsworth-Emmons olefination from protected 5-formyl-3,5-dimethylthiotetronic acid. Several 4-position protecting groups and a variety of phosphonates were evaluated, with MOM protection and β-ketophosphonates yielding the highest ratio of the desired product to deformylated product. 相似文献
104.
Woo-Jin Han Eun-Kyoung Kim Yung-Hwan Oh 《Electronics letters》2002,38(6):292-294
A novel frame interpolation technique for two-band linear predictive coding (LPC) vocoders is proposed for maintaining natural speech quality at bit rates below 1 kbit/s. Experimental results show that the speech quality of the proposed vocoder is quite natural at bit rates 880 bit/s and comparable to that of 4.8 kbit/s CELP 相似文献
105.
Smart TDI readout circuit for long-wavelength IR detector 总被引:3,自引:0,他引:3
Byunghyuk Kim Hee Chul Lee 《Electronics letters》2002,38(16):854-855
A smart time delay and integration (TDI) readout circuit is suggested which performs background suppression, cell-to-cell non-uniformity compensation, and dead pixel correction. Using the smart TDI readout circuit, the integration capacitor area occupying almost the whole area of a unit-cell can be reduced to one-fifth and transimpedance gain can increase by five times. From measurement results, it is found that the skimming current error for a few hundred nA background current is < 1.25 nA corresponding to LSB/2 of ADC and the non-uniformity introduced by cell-to-cell background current variation is reduced to 1.02 nA 相似文献
106.
Zhang Li Chun Jin Hai Yan Ye Hong Fei Gao Yu Zhi Ning Bao Jun Mo Bang Xian 《Electron Devices, IEEE Transactions on》2002,49(6):1075-1076
A polysilicon emitter RCA transistor (an ultra-thin interfacial oxide layer exists between polysilicon and silicon emitter) is presented which can operate at 77 K for the first time. An ultra-thin (1.5 nm) interfacial oxide layer is grown deliberately between polysilicon and silicon emitter using RCA oxidation and excellent device stability is obtained after rapid thermal annealing (RTA) treatment in nitrogen atmosphere. The RCA transistor exhibits good electrical performance at very low temperature for an emitter area of 3 × 8 μm2. The maximum toggle frequency of a 1:2 static divider is 1.2 GHz and 732 MHz at 300 K and 77 K, respectively 相似文献
107.
Syntheses, structures, and properties of novel molybdenum and tungsten complexes of fullerenes 总被引:1,自引:0,他引:1
The synthesis and characterization of several fullerene-based organometallic complexes containing Mo and W is reported. 相似文献
108.
The fuzzification of convex sets in median algebras is considered, and some of their properties are investigated. A characterization of finite valued fuzzy convex set is given. 相似文献
109.
Yu-Soo Lee Hyun-Ju Chung Jong-Han Joung Eung-Jo Kim Hee-Je Kim 《Optics & Laser Technology》2004,36(1):461-61
We describe the pulse forming of pulsed CO2 laser using multi-pulse superposition technique. Various pulse shapes, high duty cycle pulse forming network (PFN) are constructed by time sequence. This study shows a technology that makes it possible to make various long pulse shapes by activating SCRs of three PFN modules consecutively at a desirable delay time with the aid of a PIC one-chip microprocessor. The power supply for this experiment consists of three PFN modules. Each PFN module uses a capacitor, a pulse forming inductor, a SCR, a high voltage pulse transformer, and a bridge rectifier on each transformer secondary. The PFN modules operate at low voltage by driving the primary of HV pulse transformer. The secondary of the transformer has a full-wave rectifier, which passes the pulse energy to the load in a continuous sequence.We investigated various long pulse shapes as different trigger time intervals of SCRs among three PFN modules. As a result, we could obtain laser beam with various pulse shapes and durations from about 250 to 1000 μs. 相似文献
110.
A (w,r) cover‐free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super‐simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc. 相似文献