全文获取类型
收费全文 | 93篇 |
免费 | 3篇 |
专业分类
化学 | 43篇 |
晶体学 | 2篇 |
力学 | 1篇 |
数学 | 21篇 |
物理学 | 29篇 |
出版年
2020年 | 1篇 |
2019年 | 1篇 |
2017年 | 1篇 |
2016年 | 5篇 |
2015年 | 2篇 |
2014年 | 1篇 |
2013年 | 5篇 |
2012年 | 3篇 |
2011年 | 9篇 |
2010年 | 5篇 |
2009年 | 3篇 |
2008年 | 2篇 |
2007年 | 2篇 |
2006年 | 3篇 |
2005年 | 3篇 |
2004年 | 6篇 |
2003年 | 2篇 |
2002年 | 7篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 6篇 |
1998年 | 2篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1991年 | 1篇 |
1988年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1974年 | 3篇 |
1972年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1930年 | 1篇 |
排序方式: 共有96条查询结果,搜索用时 906 毫秒
21.
22.
Stephen R. Thomas Ph.D. Jerome L. Ackerman Ph.D. James G. Kereiakes Ph.D. 《Magnetic resonance imaging》1984,2(4):341-348
Many technical and logistical questions must be addressed when planning the installation of an NMR imaging system. These considerations become particularly significant when the facility is being established within an existing medical center complex. This paper presents a report on the practical aspects and experience obtained in siting a 6-coil 0.15 T resistive magnet system. The topics discussed include: floor loading; ferromagnetic environment; the effect of iron on the magnet field strength and homogeneity characteristics; shimming procedures; temperature stability requirements; rf shielding; and effects of the magnetic field on common medical instrumentation and magnetic media. It was found that the field shift as a function of the distance of a steel mass from the center of the magnet exhibited an (1/r)5.2±0.5 to (1/r)4.2±0.3 dependence for axial and radial positions respectively which, as expected, is somewhat weaker than the (1/r)6 dependence expected by point dipole approximations. Field distortions caused by the presence of ferromagnetic material in radial positions may be essentially fully compensated with first order transverse shim coils (most conveniently, the x and y imaging gradient coils could be used). Axially distributed material requires, in addition to first order z-gradient correction, higher order axial shim compensation. The temperature stability of the magnet system over the scan period must be better than 0.2°C to insure that temperature-induced field fluctuations are less than the intrinsic static inhomogeneity: and, ideally, below 0.01°C to reduce these fluctuations to less than those caused by power supply instability. 相似文献
23.
We investigate the number of different ways in which a rectangle containing a set of n noncorectilinear points can be partitioned into smaller rectangles by n (nonintersecting) segments, such that every point lies on a segment. We show that when the relative order of the points forms a separable permutation, the number of rectangulations is exactly the (n+1)st Baxter number. We also show that no matter what the order of the points is, the number of guillotine rectangulations is always the nth Schröder number, and the total number of rectangulations is O(n20/n4). 相似文献
24.
I. C. McNeill L. Ackerman S. N. Gupta 《Journal of polymer science. Part A, Polymer chemistry》1978,16(9):2169-2181
The degradation of cis-1,4-polybutadiene, polystyrene, and blends of PB and PS has been studied by thermogravimetry, thermal volatilization analysis, and differential scanning calorimetry. Volatile products have been investigated and separated by subambient TVA and characterized spectroscopically. In the degradation of the blends, there is no change in the nature of the volatile products of degradation, but the rate of degradation of the PS component is markedly reduced. The PB component is the first to break down, and during the initial period of degradation of the PB, the PS degradation is apparently inhibited. It is suggested that some of the volatile products of decomposition of PB, most notably 4-vinylcyclohexene, may diffuse into the PS phase in the blend and act as radical inhibitors. 相似文献
25.
Laura K. G. Ackerman Lukiana L. Anka-Lufford Marina Naodovic Daniel J. Weix 《Chemical science》2015,6(2):1115-1119
The nickel-catalyzed cross-coupling of aryl halides with alkyl radicals derived from alkyl halides has recently been extended to couplings with carbon radicals generated by a co-catalyst. In this study, a new co-catalyst, cobalt phthalocyanine (Co(Pc)), is introduced and demonstrated to be effective for coupling substrates not prone to homolysis. This is because Co(Pc) reacts with electrophiles by an SN2 mechanism instead of by the electron-transfer or halogen abstraction mechanisms previously explored. Studies demonstrating the orthogonal reactivity of (bpy)Ni and Co(Pc), applying this selectivity to the coupling of benzyl mesylates with aryl halides, and the adaptation of these conditions to the less reactive benzyl phosphate ester and an enantioconvergent reaction are presented. 相似文献
26.
Kristen A. McReynolds Laura K.G. Ackerman William W. Brennessel 《Journal of fluorine chemistry》2010,131(11):1108-5467
New synthetic routes to (NHC)copper-trifluoroacetate and -chlorodifluoroacetate complexes were developed (NHC = N-heterocyclic carbenes) so baseline reactivity patterns could be established for the decarboxylative trifluoromethylation of organic halides. In the presence of aryl halides, loss of CO2 from these new precursors occurred at 160 °C concurrent with the formation of aryl-CF3. 相似文献
27.
28.
Anne K. Hermetet Lily J. Ackerman John K. Swearingen Carmina A. Presto Diantha R. Kelman James M. Giesen Karen I. Goldberg Werner Kaminsky Douglas X. West 《Journal of chemical crystallography》2002,32(1-2):17-25
Reaction of 2-amino-6-picoline with 2-, 3-, and 4-tolyl isothiocyanates produces the three N-2-(6-picolyl)-N-tolylthioureas, 6PicTu2T, 6PicTu3T, and 6PicTu4T. 6PicTu2T is mono-clinic, space group P21/n with a = 8.1700(3) Å, b = 25.5840(6) Å, c = 12.6840(5) Å, = 98.8750(16)°, and V = 2619.5(2) Å3 with Z = 8, for d
calc = 1.305 g/cm3. 6PicTu3T is monoclinic, space group C2/c with a = 23.879(3) Å, b = 6.744(3) Å, c = 17.116(10) Å, = 99.26(4)°, and V = 2720(3) Å3 with Z = 8, for d
calc = 1.257 g/cm3. 6PicTu4T is triclinic, space group P-1 with a = 8.829(6) Å, b = 8.8950(15) Å, c = 10.495(3) Å, = 68.63(3)°, = 72.19(4)°, = 63.06(4)°, and V = 681.2(5) Å3 with Z = 2, for d
calc = 1.255 g/cm3. Intermolecular hydrogen bonding involving the thione sulfur and the NH hydrogen and the planarity of the thiourea are affected by the position of substitution on the aryl ring. 1H NMR studies in CDCl3 show the NH hydrogen resonance considerably downfield from other signals in the spectrum of each thiourea. The enthalpies of fusion of the present thioureas reflect the extent of intermolecular hydrogen bonding and are compared to other heterocyclic thioureas. 相似文献
29.
The functional Ito formula, in the form df() = f( + d ) f(),is formulated and proved in the context of a Lie algebra L associatedwith a quantum (non-commutative) stochastic calculus. Here fis an element of the universal enveloping algebra U of L, andf() + d() f() is given a meaning using the coproductstructure of U even though the individual terms of this expressionhave no meaning. The Ito formula is equivalent to a chaoticexpansion formula for f() which is found explicitly. 1991 MathematicsSubject Classification: primary 81S25; secondary 60H05; tertiary18B25. 相似文献
30.
D J Sloop T S Lin J J Ackerman 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,139(1):60-66
An unusual strategy for performing magnetic resonance experiments is demonstrated. Instead of employing conventional radiofrequency transmitter fields to perturb spin state populations away from equilibrium, as is the basis of most magnetic resonance spectrometers today, technological advances now make possible fast switching of the magnetic field orientation to achieve the same effect. This is demonstrated with an electron spin resonance experiment where the magnetic field is switched 90 degrees nonadiabatically with a dead time of a few tens of nanoseconds and an electron free induction decay observed. 相似文献