全文获取类型
收费全文 | 5092篇 |
免费 | 95篇 |
国内免费 | 30篇 |
专业分类
化学 | 3260篇 |
晶体学 | 46篇 |
力学 | 184篇 |
数学 | 479篇 |
物理学 | 1248篇 |
出版年
2020年 | 53篇 |
2019年 | 42篇 |
2018年 | 31篇 |
2016年 | 83篇 |
2015年 | 66篇 |
2014年 | 88篇 |
2013年 | 205篇 |
2012年 | 184篇 |
2011年 | 228篇 |
2010年 | 121篇 |
2009年 | 106篇 |
2008年 | 222篇 |
2007年 | 232篇 |
2006年 | 213篇 |
2005年 | 218篇 |
2004年 | 185篇 |
2003年 | 204篇 |
2002年 | 177篇 |
2001年 | 129篇 |
2000年 | 141篇 |
1999年 | 76篇 |
1998年 | 65篇 |
1997年 | 65篇 |
1996年 | 101篇 |
1995年 | 67篇 |
1994年 | 65篇 |
1993年 | 82篇 |
1992年 | 80篇 |
1991年 | 74篇 |
1990年 | 69篇 |
1989年 | 58篇 |
1988年 | 84篇 |
1987年 | 70篇 |
1986年 | 67篇 |
1985年 | 83篇 |
1984年 | 81篇 |
1983年 | 46篇 |
1982年 | 68篇 |
1981年 | 68篇 |
1980年 | 59篇 |
1979年 | 78篇 |
1978年 | 54篇 |
1977年 | 84篇 |
1976年 | 68篇 |
1975年 | 64篇 |
1974年 | 69篇 |
1973年 | 67篇 |
1972年 | 33篇 |
1971年 | 34篇 |
1968年 | 27篇 |
排序方式: 共有5217条查询结果,搜索用时 312 毫秒
61.
62.
63.
The new isocyanato carborane anion, [7-OCN-7-CB10H12]− was prepared by the reaction of 7-H3N-7-CB10H12 with triphosgene in the presence of triethylamine. The structure of this compound was established by 1H, 11B and 13C NMR as well as IR spectroscopy. The reactivigty of this compound with a series of organic amines was investigated. The resulting ureas were obtained in good to excellent yields. This approach is suitable for development of compounds for use in tumor selective Boron Neutron Capture Therapy (BNCT). 相似文献
64.
Treatment of [Ti(O-i-Pr)(2)Cl(2)] with NaL(OEt) (L(OEt)(-) = [CpCo[P(O)(OEt)(2)](3)](-), Cp = eta(5)-C(5)H(5)) afforded [L(OEt)Ti(O-i-Pr)(2)Cl] that reacted with HCl in ether to give [L(OEt)TiCl(3)] (1). The average Ti-O and Ti-Cl distances in 1 are 1.975 and 2.293 A, respectively. Reaction of titanyl sulfate with NaL(OEt) in water followed by addition of HBF(4) afforded [L(OEt)TiF(3)] (2), the Ti-O and Ti-F distances of which are 2.020(2) and 1.792(2) A, respectively. The Zr(IV) analogue [L(OEt)ZrF(3)] (3) was prepared similarly from zirconyl nitrate, NaL(Oet), and HBF(4) in water. The Zr-O and average Zr-F distances in 3 are 2.139(2) and 1.938(2) A, respectively. Treatment of 1 with tetrachlorocatechol (H(2)Cl(4)cat) afforded [L(OEt)Ti(Cl(4)cat)Cl] (4). The average Ti-O(P), Ti-O(C), and Ti-Cl distances in 4 are 1.972, 1.926, and 2.334 A, respectively. Hydrolysis of 4 in the presence of Et(3)N yielded the mu-oxo dimer [(L(OEt))(2)Ti(2)(Cl(4)cat)(2)(mu-O)] (5). The average Ti-O(P), Ti-O(C), and Ti-O(Ti) distances in 5 are 2.027, 1.926, and 1.7977(9) A. Treatment of 1 with 1,1'-binaphthol (BINOLH(2)) in the presence of Et(3)N afforded [(L(OEt))(2)Ti(2)(mu-O)(2)(mu-BINOL)] x 2BINOLH(2) (6.2BINOLH(2)). Complex 1 is capable of catalyzing ring opening of epoxides with Me(3)SiN(3) under solvent-free conditions presumably via a Ti-azide intermediate. 相似文献
65.
Loredana Protesescu Joaquín Calbo Kristopher Williams William Tisdale Aron Walsh Mircea Dinc 《Chemical science》2021,12(17):6129
The development of synthetic routes to access stable, ultra-small (i.e. <5 nm) lead halide perovskite (LHP) quantum dots (QDs) is of fundamental and technological interest. The considerable challenges include the high solubility of the ionic LHPs in polar solvents and aggregation to form larger particles. Here, we demonstrate a simple and effective host–guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites. Cr3O(OH)(H2O)2(terephthalate)3 (Cr-MIL-101), made of large mesopore-sized pseudo-spherical cages, allows fast and efficient diffusion of perovskite precursors within its pores, and promotes the formation of stable, ∼3 nm-wide lead bromide perovskite QDs. CsPbBr3, MAPbBr3 (MA+ = methylammonium), and (FA)PbBr3 (FA+ = formamidinium) QDs exhibit significantly blue-shifted emission maxima at 440 nm, 446 nm, and 450 nm, respectively, as expected for strongly confined perovskite QDs. Optical characterization and composite modelling confirm that the APbBr3 (A = Cs, MA, FA) QDs owe their stability within the MIL-101 nanocrystals to both short- and long-range interfacial interactions with the MOF pore walls.We demonstrate a simple and effective host–guest strategy for preparing ultra-small lead bromide perovskite QDs through the use of nano-sized MOFs that function as nucleating and host sites. 相似文献
66.
Summary The effect of a series of polyacrylic acids, ranging in molecular weight from 1.67×104 to 2.36×106, on the stability of positively charged silver iodide particles has been examined. Flocculation of the sol occurred at a well defined concentration of polyacrylic acid,c
f
, and a further increase in concentration of the polyelectrolyte caused restabilization of the sol. Over the range examinedc
f
appeared to be related to the viscosity average molecular weight of the acid,M
v
, by an equation of the form,c
f
=a ·M
v
– b wherea andb are constants. 相似文献
67.
K. Seshadri F. A. Williams 《Journal of polymer science. Part A, Polymer chemistry》1978,16(7):1755-1778
Since poly(methyl methacrylate) is known to depolymerize largely to its monomer when heated, the chemical kinetics in the gaseous diffusion flame produced by this polymer in a fire may coincide with that of burning liquid methyl methacrylate. To test this hypothesis, flat diffusion flames were probed and extinguished adjacent to surfaces of each of these fuels. Profiles of temperature and of concentrations of stable chemical species are reported, as are gas velocities of approach flow required to produce extinction for various oxygen/nitrogen ratios of the stream. Results revealed structural differences attributable to differing thermal properties of the fuels. Many fuel species were observed in the gas phase, their profiles being partially rationalized on the basis of a suggested decomposition mechanism for gaseous methyl methacrylate. Overall kinetic parameters for gasphase combustion, obtained by use of extinction results in a previously developed theory, are nearly the same for the polymer and monomer but appear to differ by amounts exceeding experimental uncertainties. It is suggested that this may be traced to small differences in fuel species leaving the condensed phase which, for the polymer, is covered by a thin, two-phase region. 相似文献
68.
Reaction of [Nb(NMe(2))(5)] with 10 equiv of 2,6-Me(2)C(6)H(3)SH in toluene results in the formation of red crystals of [Nb(SC(6)H(3)Me(2)-2,6)(5)]. Crystal structure analysis of [Nb(SC(6)H(3)Me(2)-2,6)(5)] showed that the niobium center adopts a distorted trigonal bipyramidal geometry. Niobium disulfide, NbS(2), has been successfully prepared via a thio "sol-gel" process using [Nb(SC(6)H(3)Me(2)-2,6)(5)] as the metal source. In contrast, vapor phase thin-film studies revealed that [Nb(SC(6)H(3)Me(2)-2,6)(5)] functions as a single-source precursor to NbS films. 相似文献
69.
Kadhim H. Al-Obaidi Robert D. Gillard Leon A. P. Kane-Maguire Peter A. Williams 《Transition Metal Chemistry》1977,2(1):64-66
Summary Bis-(5-nitro-1,10-phenanthroline)platinum(II) dichloride and diperchlorate have been prepared. The reaction between the parent cation and hydroxide ion has been studied using 1 FI n.m.r. spectroscopy and found to involve attack at the ligand. The bis-(2,2-bipyridyl)platinum(II) ion has been shown to be highly reactive towards methoxide ion. The dissociation of a 2,2-bipyridyl ligand is preceded by attack at the ligand.Part XI: R. D. Gillard, t.. A. P. Kane-Maguire and P. A. Williams,Transition Met. Chem., 2, 47 (1977).On leave from the University of Baghdad, Iraq. 相似文献
70.
D. Cameron P. H. Hemberger J. E. Alarid C. P. Leibman J. D. Williams 《Journal of the American Society for Mass Spectrometry》1993,4(10):774-781
The feasibility of using an ion trap detector for real-time monitoring of volatile organic compounds in atmospheric samples is demonstrated. Detection limits in the low part-per-million to low part-per-billion ranges are achieved with an operating dynamic range of at least 5 orders of magnitude. Sample introduction through semipermeable membranes and molecular leak valves was evaluated. The membrane introduction method provided detection limit enhancements of 10–300 times over the molecular leak method. The effects of partial pressure from air and helium buffer gas were studied, and it was determined that, for optimum amounts of air, the helium buffer gas had no apparent effect on sensitivity or resolution. Air acts as the buffer gas to enhance ion signals by collisionally damping ion motion. The enhancement of molecular ion abundance at high partial pressures of air is postulated to be due to charge exchange and possibly collisional stabilization reactions. 相似文献