米脂县农地承载力系统动力学分析

在综合分析米脂县农地承载力系统结构、关系的基础上,以1998年为起始年建立系统动力学模型,根据米脂县社会经济发展现状及未来规划,设定了3种不同的方案和3种营养标准,模拟了米脂县2008-2028年农地人口承载力趋势,通过各方案比较,得出了较优的方案,最后提出了提高该地区农地承载力的政策建议。

EFFECTS OF AN EXTERNAL MAGNETIC-FIELD ON SHALLOW DONOR LEVELS IN SEMICONDUCTORS

An extension of Faulkner's method for the energy levels of the shallow donor in silicon and germanium at zero field is made in order to investigate the effects of a magnetic field upon the excited states. The effective-mass Hamiltonian matrix elements of an electron bound to a donor center and subjected to a magnetic field B, which involves both the linear and quadratic terms of magnetic field, are expressed analytically and matrices are solved numerically. The photothermal ionization spectroscopy of phosphorus in ultrapure silicon for magnetic fields parallel to the [1,0,0] and [1,1,1] directions and up to 10 T is explained successfully.

CHARACTERISTICS OF THE MAGNETIC DEPOPULATION OF SUBBANDS IN VERY NARROW SYSTEMS

We present a model for electrons confined in narrow conducting channels by a parabolic well under moderate to high magnetic fields which takes into account a cutoff in the filling of the subbands. Such a cutoff gives rise to energy-separated subbands and a two-dimensional (2D) like subband depopulation, resulting in a relation between sublevel index n and inverse magnetic field B-1 such that in the high-field regime it changes over to the well-known 2D form as expected, and in the moderate field regime it shows pronounced deviation from linearity. This agrees well with the experimental results. The linear region of the n-B-1 experimental plot is believed to arise from the two dimensionality of the system. Calculations show that no resolvable 1D sublevel exists in the 0.5-mu-m-wide wire at very small magnetic fields (including zero field), which agrees qualitatively with the experimental results found in other wires that the Hall resistance, R(H), approaches its classical value B/n(e)e in this region and R(H) = 0 at B = 0, where n(e) is the electron concentration. In this model the linear and nonlinear regions in the experimental n-B-1 plot are used to extract the characteristic frequency omega-0, and the effective 2D electron concentration N(e)2D, respectively.

LUMINESCENCE-CENTERS IN POROUS SILICON

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100-150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of Gamma and X band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.

黄土高原主要土壤锌有效性及其影响因素

通过测定黄土高原不同区域、不同类型土壤全锌和有效锌含量,研究了黄土高原土壤锌含量及影响有效锌的因素。结果表明,供试土壤全锌主要分布在50～100mg·kg-1之间,有效锌含量较低,其中20.69%在临界值(0.5mg·kg-1)之下,37.93%为低(0.5～1.0mg·kg-1)水平。用直线、幂函数和指数函数回归方程对影响有效锌的主要土壤因素进行回归拟合。发现土壤有效锌与全锌、全氮、全磷、有机质呈显著正相关,与土壤pH值呈显著负相关;幂函数可以较好地拟合有效锌与土壤全氮、有机质的相关关系,有效锌与全锌、全磷、pH值宜用指数函数表示其相关性。通径分析表明土壤性质对有效锌的直接作用系数大小次序为全氮>全锌>pH>全磷,土壤全氮对锌有效性的直接作用最明显。

青海湖鸟岛地区植物群落物种多样性与土壤环境因子的关系

The study on the relationship between plant species diversity and soil factors in the bird island of Qinghai Lake indicated that this island was a low diversity district,its Shannon-Wienner index and species richness decreased with the increasing soil available K,water soluble salt concentration and pH,and there were significant linear and quadratic correlations between them.Stepwise linear regressions showed that soil available K and water soluble salt were the key factors to estimate Shannon-Wienner index and species richness in this island,respectively,and no correlation was found between species evenness and soil factors.

Mechanical relaxation time of comblike polymer electrolyte

Using poly(styrene-co-maleic anhydride) as the backbone and poly(ethylene glycol) methyl ether as side chains,three kinds of comblike polymers of different side chain length were synthesized. The Li-salt complexes and their firms were prepared. The dynamic mechanical properties were investigated. It was found that the main chain was rigid and the side chain was flexible in this comblike polymer system. Based on the time-temperature equivalence principle, a master curve was constructed. By selecting T-alpha as reference temperature, Arrhenius plots of shift factor and iso-free-volume plots were attained. The values of WLF parameters C-1 and C-2 increase with increasing salt concentration. By reference to T-0 = 50 degrees C, the relation between the average relaxation time 1g tau(c) and Li-salt concentration C is linear. The master curves are displaced progressively to higher frequencies as the M-w of side chains is increased. The relation between the average relaxation time 1g tau(n) and M-w of side chains is also linear. And the master curves are movable with the change of salts. It shows the effect of different kinds of salt on relaxation time.

Microelectrode voltammetric measurement of heterogeneous electron transfer rate constants for reduction of 7,7,8 ',8 '-tetracyanoquinodimethane in polymer electrolytes

This paper presents a microelectrode voltammetric determination of heterogeneous electron transfer rate constants (k(s)) and diffusion coefficients (D) of 7,7,8',8 '-tetracyanoquinodimethane (TCNQ) in polyelectrolytes. The diffusion coefficients are estimated using cyclic voltammetry under linear diffusion conditions, and the heterogeneous electron transfer rate constants are obtained under mixed linear and radial diffusion in the polyelectrolyte. k(s) and D increase with increasing temperature, and the activation barriers of the electrode reaction for reduction of TCNQ are obtained. On the other hand, the dependencies of D and k(s) of TCNQ on the size and charge of the counterion are compared in the polyelectrolyte. (C) 1998 Elsevier Science Ltd. All rights reserved.

Calculation of nonlinear optical properties of KNdP4O12

The second-order nonlinear optical (NLO) tensor coefficients of KNdP4O12 (KNP) are theoretically predicted from its crystal structural data, by using the chemical bond theory of complex crystals and the modified bond charge model. Linear and nonlinear optical contributions of each type of bond to the total linearity (chi) and nonlinearity (d(ij)) of KNP are quantitatively determined. The structure-property relationship of KNP is systematically investigated, from the chemical bond viewpoint. Based on the discussion of its structural modifications, we point out that NLO properties of I(NP can be improved effectively using the doping method. Theoretical predictions show KNP to be a promising: self-frequency-doubling laser material.

Structure-property relationships in Li1-xHxIO3 type complex crystals

Chemical bond parameters and the linear and nonlinear optical (NLO) properties of all constituent chemical bonds in Li1-xHxIOx [x (the amount of hydrogen) = 0.0, 0.28, and 0.34] (LHIO) type complex crystals have been investigated from the chemical bond viewpoint, At the same time, the relationship between the crystal structure and its optical properties has been obtained, based on the calculated results of LiIO3, Li0.72H0.28IO3, and Li0.66H0.34IO3. The nonlinear optical properties of LHIO single crystals are found to be particularly sensitive to the H+ impurity concentration. (C) 1998 Academic Press.

The effect of stoichiometry on nonlinear optical properties of LiNbO3

From the chemical bond viewpoint, second-order nonlinear optical (NLO) tensor coefficients of LiNbO3 have been investigated. The single-bond contributions to the second-order NLO susceptibility and the linear susceptibility were determined. The tensor values thus calculated are in good agreement with experimental data. Based on theoretical results of LiNbO3 with Li/Nb = 1, we also have calculated linear and nonlinear optical properties of nonstoichiometric samples with Li/Nb < 1. In the calculation, we find that the Li-O bond is an important type of chemical bond in these LiNbO3 samples, which have large NLO contributions to the total nonlinearities. The refractive indices and second-order NLO tensor coefficients have been determined as a function of the stoichiometry.

Chemical bond analysis of nonlinearity of urea crystal

A novel and quantitative study on structure-property relationships has been carried out in urea crystal, based on the dielectric theory of complex crystals and the modified Levine bond charge model, mainly from the chemical bond viewpoint. For the first time, it was treated like this, and the bond parameters and linear and nonlinear characteristics of constituent chemical bonds were presented quantitatively. The theoretical result agrees satisfactorily with the experimental datum and can reasonably explain the nonlinear origin of urea crystal, that is, the C-N bond in the conjugated system of bonds O double left arrow C<--N-H. At the same time, the novel method should be a useful tool toward the future development of the search for new nonlinear optical (NLO) materials in the organic crystal field.

Calculation of the nonlinear optical coefficient of the NdAl3(BO3)(4) crystal

For the first time, we present the calculation of the nonlinear optical coefficient of the NdAl3(BO3)(4) (NAB) crystal from a systematic and quantitative standpoint. Based on the dielectric theory of complex crystals and the Levine bond charge model, the method of calculation of the second-order nonlinear optical tensor coefficients of complex crystals has been given systematically. The chemical bond parameters and linear and nonlinear susceptibilities of the NAB crystal have been calculated in detail, and the calculated value of d(11)(NAB) is -5.81 x 10(-9) esu, which agrees with the measured value of 4.06 x 10(-9) esu.

THERMAL-DECOMPOSITION PROCESSES OF SOME ELECTRICALLY CONDUCTING POLYMERS INVESTIGATED BY DIRECT PYROLYSIS MASS-SPECTROMETRY

Thermal decomposition processes of poly(thio-1,4-phenylene) (PPS), polythiophene (PT) and polyaniline (PAn) were investigated by direct pyrolysis EI or CI mass spectrometry (DPMS). They can provide up to heptemer pyrolynates and give some structure properties. The results indicate that the thermal degradation all undergoes in radical decomposition, PPS pyrolyzes into linear and cyclic oligmers, but PT and PAn pyrolyze only into linear oligmers.

The nonlinear effective dielectric response of graded composites

The perturbation method is developed to deal with the effective nonlinear dielectric responses of weakly nonlinear graded composites, which consist of the graded inclusion with a linear dielectric function of spatial variables of inclusion material. For Kerr-like nonlinear graded composites, as an example in two dimensions, we have used the perturbation method to solve the boundary value problems of potentials, and studied the effective responses of nonlinear graded composites, where a cylindrical inclusion with linear dielectric function and nonlinear dielectric constant is randomly embedded in a homogeneous host with linear and nonlinear dielectric constants. For the exponential function and the power-law dielectric profiles of cylindrical inclusions, in the dilute limit, we have derived the formulae of effective nonlinear responses of both graded nonlinear composites.