全文获取类型
收费全文 | 22044篇 |
免费 | 3550篇 |
国内免费 | 3288篇 |
专业分类
化学 | 10809篇 |
晶体学 | 501篇 |
力学 | 1255篇 |
综合类 | 222篇 |
数学 | 387篇 |
物理学 | 7113篇 |
无线电 | 8595篇 |
出版年
2024年 | 61篇 |
2023年 | 296篇 |
2022年 | 602篇 |
2021年 | 941篇 |
2020年 | 787篇 |
2019年 | 720篇 |
2018年 | 668篇 |
2017年 | 981篇 |
2016年 | 1201篇 |
2015年 | 1151篇 |
2014年 | 1471篇 |
2013年 | 1949篇 |
2012年 | 1715篇 |
2011年 | 1682篇 |
2010年 | 1280篇 |
2009年 | 1427篇 |
2008年 | 1440篇 |
2007年 | 1390篇 |
2006年 | 1199篇 |
2005年 | 1090篇 |
2004年 | 984篇 |
2003年 | 805篇 |
2002年 | 667篇 |
2001年 | 565篇 |
2000年 | 529篇 |
1999年 | 487篇 |
1998年 | 397篇 |
1997年 | 369篇 |
1996年 | 303篇 |
1995年 | 258篇 |
1994年 | 219篇 |
1993年 | 244篇 |
1992年 | 199篇 |
1991年 | 117篇 |
1990年 | 111篇 |
1989年 | 95篇 |
1988年 | 113篇 |
1987年 | 59篇 |
1986年 | 51篇 |
1985年 | 37篇 |
1984年 | 26篇 |
1983年 | 19篇 |
1982年 | 34篇 |
1981年 | 29篇 |
1980年 | 18篇 |
1979年 | 32篇 |
1978年 | 17篇 |
1977年 | 13篇 |
1976年 | 8篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 376 毫秒
91.
92.
93.
X7R弛豫复相铁电陶瓷的研究 总被引:1,自引:0,他引:1
以PZN-BT(低温相)、PMN-PT(高温相)为起始组元,采用混合烧结法制备了具有X7R介电特性电子瓷料,其介电温谱在55~+125℃范围内相当平坦,室温相对介电常数高达3 877,并从离子的键型分析了影响介电温度稳定性的原因,初步探讨了提高介电温度稳定性的机制。 相似文献
94.
95.
在加热妒内,由于高温及重力作用,板坯两端的悬臂将产生下桡变形。悬臂越长,下桡将越大,导致悬臂下桡部分与出口端墙相碰,影响出料而造成事故。本文对加热炉内板坯的下桡变形进行了分析计算。首先,建立了板坯的二维非稳态导热模型并用有限差分法计算了炉内板坯随时间的温度变化;然后建立了板坯悬臂粘塑性变形模型,用有限元法计算了高温条件下由重力引起的悬臂的下桡变形。由此理论模型计算出的板坯抽出温度与悬臂端下桡量与实测结果吻合良好,说明该理论模型及模型计算方法是正确的,可用于对装钢生产进行指导。 相似文献
96.
In this work, the melting behaviors of nonisothermally and isothermally melt‐crystallized poly(L ‐lactic acid) (PLLA) from the melt were investigated with differential scanning calorimetry (DSC) and temperature‐modulated differential scanning calorimetry (TMDSC). The isothermal melt crystallizations of PLLA at a temperature in the range of 100–110 °C for 120 min or at 110 °C for a time in the range of 10–180 min appeared to exhibit double melting peaks in the DSC heating curves of 10 °C/min. TMDSC analysis revealed that the melting–recrystallization mechanism dominated the formation of the double melting peaks in PLLA samples following melt crystallizations at 110 °C for a shorter time (≤30 min) or at a lower temperature (100, 103, or 105 °C) for 120 min, whereas the double lamellar thickness model dominated the formation of the double melting peaks in those PLLA samples crystallized at a higher temperature (108 or 110 °C) for 120 min or at 110 °C for a longer time (≥45 min). © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 466–474, 2007 相似文献
97.
S. Mentus Dijana Jelić Veselinka Grudić 《Journal of Thermal Analysis and Calorimetry》2007,90(2):393-397
Thermal decomposition of lanthanum nitrate to lanthanum oxide was carried out by both temperature programmed heating (TPH)
and citrate-gel combustion. The temperature programmed heating was carried out under flow of oxidizing (air), neutral (nitrogen)
and reducing (25 vol.% hydrogen+argone mixture) gases, and the processes were controlled by simultaneous thermogravimetry
and differential thermal analysis. It was shown that hydrogen atmosphere helps to reduce temperatures of all decomposition
steps. The results of TPH were utilized to check the nature of residues in the products of lanthanum nitrate-to-oxide conversion
performed via citrate-gel combustion technique. 相似文献
98.
Govindarajan Sankar A. Sultan Nasar 《Journal of polymer science. Part A, Polymer chemistry》2007,45(8):1557-1570
A series of substituted N‐methylaniline‐blocked polyisocyanates based on 4,4′‐methylenebis(phenyl isocyanate) and poly(tetrahydrofuran) were prepared and characterized thoroughly with FTIR, 1H NMR, and 13C NMR spectroscopy methods. Compared with unsubstituted N‐methylaniline, a blocking agent with an electron‐releasing substituent at the para position took a shorter time, whereas those with an electron‐releasing substituent at the ortho position or an electron‐withdrawing substituent at the ortho and para positions took longer times for the blocking reaction. The thermal dissociation reactions of blocked polyisocyanates were carried out with an FTIR spectrophotometer attached to hot‐stage accessories under dynamic and isothermal conditions. The dynamic method was used to determine the deblocking temperature, and the isothermal method was used to calculate the deblocking kinetics and activation parameters. The cure times of blocked polyisocyanates with hydroxyl‐terminated polybutadiene were also determined. The deblocking temperatures, the results of cure‐time studies, and the kinetic parameters revealed that the thermal dissociation of the N‐methylaniline‐blocked polyisocyanates was retarded by electron‐donating substituents and facilitated by electron‐withdrawing substituents. The action of N‐methylanilines as blocking agents for isocyanate was explained by the formation of a four‐center, intramolecularly hydrogen‐bonded ring structure during the thermal dissociation of the blocked polyisocyanates. The formation of such a hydrogen‐bonded ring structure was confirmed and supported by variable‐temperature 1H NMR studies and entropy parameters, respectively. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1557–1570, 2007 相似文献
99.
100.