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461.
A compact open-path optical ammonia detector is developed. A tunable external-cavity diode laser operating at 1.5 μm is used to probe absorptions of ammonia via the cavity-enhanced absorption (CEA) technique. The detector is tested in a climate chamber. The sensitivity and linearity of this system are studied for ammonia and water at atmospheric pressure. A cluster of closely spaced rovibrational overtone and combination band transitions, observed as one broad absorption feature, is used for the detection of ammonia. On these molecular transitions a detection limit of 100 ppb (1 s) is determined. The ammonia measurements are calibrated independently with a chemiluminescence monitor. Compared to other optical open-path detection methods in the 1–2 μm region, the present result shows an improved sensitivity for contactless ammonia detection by over one order of magnitude. Using the same set-up, a detection limit of 100 ppm (1 s) is determined for the detection of water at atmospheric pressure. Received: 19 January 2000 / Revised version: 6 March 2000 / Published online: 7 June 2000  相似文献   
462.
463.
C(3)-symmetrical disks 1, preorganized by acylated 2,2'-bipyridine-3,3'-diamine moieties and decorated with nine identical chiral, lipophilic tails, aggregate into a dynamic helix in apolar solvents. The aggregates, previously shown to be governed by the "sergeants-and-soldiers" principle when mixed with achiral analogues, are now also revealed to obey the "majority-rules" effect, a phenomenon not earlier observed in nonpolymers. Our experimental circular dichroism data can be accurately described with a recently developed theory. A fit of the theory to the experimental results shows that the mismatch penalty, i.e., the free energy of a monomer present in a helix of its nonpreferred screw sense (0.94 kJ/mol), is about 8 times lower than the penalty for a helix reversal (7.8 kJ/mol). This corresponds well to our vision of the supramolecular architecture of the disks.  相似文献   
464.
In this work, the C(2)F(4)(X(1)A(g)) + O((3)P) reaction was investigated experimentally using molecular beam-threshold ionization mass spectrometry (MB-TIMS). The major primary products were observed to be CF(2)O (+ CF(2)) and CF(3) (+ CFO), with measured approximate yields of % versus %, respectively, neglecting minor products. Furthermore, the lowest-lying triplet and singlet potential energy surfaces for this reaction were constructed theoretically using B3LYP, G2M(UCC, MP2), CBS-QB3, and G3 methods in combination with various basis sets such as 6-31G(d), 6-311+G(3df), and cc-pVDZ. The primary product distribution for the multiwell multichannel reaction was then determined by RRKM statistical rate theory and weak-collision master equation analysis. It was found that the observed production of CF(3) (+ CFO) can only occur on the singlet surface, in parallel with formation of ca. 5 times more CF(2)O(X) + CF(2)(X(1)A(1)). This requires fast intersystem crossing (ISC) from the triplet to the singlet surface at a rate of ca. 4 x 10(12) s(-1). The theoretical calculations combined with the experimental results thus indicate that the yield of triplet CF(2)(?(3)B(1)) + CF(2)O formed on the triplet surface prior to ISC is < or =35%, whereas singlet CF(2)(X(1)A(1)) + CF(2)O is produced with yield > or =60%, after ISC. In addition, the thermal rate coefficients k(O + C(2)F(4)) in the T = 150-1500 K range were computed using multistate transition state theory and can be expressed as k(T) = 1.67 x 10(-16) x T(1.48) cm(3) molecule(-1) s(-1); they are in agreement with the available experimental results in the T = 298-500 K range.  相似文献   
465.
466.
Phase solubility analysis is used to investigate the complex formation of alfaxalone with various cyclodextrins(2-hydroxypropyl--cyclodextrine [HPBCD],-cyclodextrin [BCD] and2-hydroxypropyl--cyclodextrin [HPGCD]).The complexationwith HPBCD was studied in more detail by looking at the effect of temperature on the stability constants using phase solubility analysis. HPLC-analysis was used to measure the dissolved amount of alfaxalone.The solubility of alfaxalone increases linearly with increasing concentration of cyclodextrin, suggesting the formation of a 1 : 1 complex. For the parent BCD the complex starts precipitating out of solution when the solubilizer concentration exceeds 0.25% making the unsubstituted BCD less useful for the preparation of solutions of alfaxalone. Substituted cyclodextrins do not form insoluble complexes with alfaxalone. The complexation constant for BCD and HPBCD are comparable in magnitude, but for HPGCD, the constant is substantially lower.The effect of temperature on thecomplexation constant was also studied at elevated temperature. Increasing the temperature results in an increased S0 (solubility without HPBCD) and a decrease in the value of the complexation constant. The net effect results in minor changes of the solubility of alfaxalone as a function of temperature. Based on regression analysis, the change in enthalpy for complex formation between alfaxalone and HPBCD is calculated as -4610 cal/mol.The results indicate that substitutedcyclodextrins are useful in the preparation of solutions of alfaxalone. Since 1 : 1 complexes are formed there is no theoretical danger for precipitation on dilution, e.g., after injection.  相似文献   
467.
We present a novel solid solution/dispersion technology with glass thermoplastic properties that provide good dissolution rates and oral bioavailabilities for poorly water-soluble weak bases. In this process, a thermoplastic gum was prepared by mixing a polyhydroxy acid such as citric acid or tartaric acid with a weakly basic drug, hydroxypropyl--cyclodextrin (HP--CD) and a cellulose polymer such as hydroxypropylmethylcellulose (HPMC) in a protic solvent. Removal of the solvent gave a material which could be loaded into hard gelatin capsules. Several model compounds were processed in this manner including methylene blue and itraconazole. The resulting data indicated that dissolution properties of GTS's based on methylene blue was pH independent and rapid with 80% dissolved within 30 min. Three GTS formulations of itraconazole containing 100 mg of the drug and 500 mg of citric acid as well as various concentrations of HP--CD and HMPC were found to dissolve rapidly (100% in 45 min). One of these formulations was selected for human pharmacokinetic evaluation and demonstrated significant oral bioavailability relative to unmanipulated drug.The studies suggest that the components of the GTS provide for solubilization through complexation and reduced pH and that the cellulose polymer acts to inhibit recrystallization of the supersaturated solution formed. The rationaldevelopment of the GTS dosage form can be useful for generating acceptable formulations for poorly water-soluble drug candidates.  相似文献   
468.
Let be a distance-regular graph with adjacency matrix A. Let I be the identity matrix and J the all-1 matrix. Let p be a prime. We study the p-rank of the matrices A + bJcI for integral b, c and the p-rank of corresponding matrices of graphs cospectral with .Using the minimal polynomial of A and the theory of Smith normal forms we first determine which p-ranks of A follow directly from the spectrum and which, in general, do not. For the p-ranks that are not determined by the spectrum (the so-called relevant p-ranks) of A the actual structure of the graph can play a rôle, which means that these p-ranks can be used to distinguish between cospectral graphs.We study the relevant p-ranks for some classes of distance-regular graphs and try to characterize distance-regular graphs by their spectrum and some relevant p-rank.  相似文献   
469.
Finite element solution methods for the incompressible Navier-Stokes equations in primitive variables form are presented. To provide the necessary coupling and enhance stability, a dissipation in the form of a pressure Laplacian is introduced into the continuity equation. The recasting of the problem in terms of pressure and an auxiliary velocity demonstrates how the error introduced by the pressure dissipation can be totally eliminated while retaining its stabilizing properties. The method can also be formally interpreted as a Helmholtz decomposition of the velocity vector. The governing equations are discretized by a Galerkin weighted residual method and, because of the modification to the continuity equation, equal interpolations for all the unknowns are permitted. Newton linearization is used and at each iteration the linear algebraic system is solved by a direct solver. Convergence of the algorithm is shown to be very rapid. Results are presented for two-dimensional flows in various geometries.  相似文献   
470.
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