黄土塬区三种豆科牧草的土壤养分剖面分布特征与平衡

为了解豆科牧草短期种植对土壤养分环境影响的进程和规律,通过田间试验对沙打旺、苜蓿和胡枝子等3种豆科牧草以不同密度单播、混播对土壤有机质、全氮、全磷剖面分布和平衡输出的影响进行了定性和定量分析。所有处理土壤全氮和有机质含量在土壤剖面2m深度范围内均呈典型的"S"形分布,全磷呈抛物线形分布。单播牧草固氮能力与播种密度呈正相关;苜蓿固氮能力最强,高密度下表观生物固氮量达507.5kg·hm-2。沙打旺生长1年可使土壤有机质平均净增3.51%。沙打旺和苜蓿全磷平均输出比率分别为43.14%和40.24%,显著高于胡枝子(23.74%);胡枝子与沙打旺、苜蓿的两两混播处理和3种牧草混播处理平均全磷输出比率分别为20.73%、26.33%、25.83%。试验结果表明,3种豆科牧草均可显著提高土壤有机质累积,沙打旺和苜蓿对土壤全氮和全磷的消耗显著大于胡枝子,但前两者的固氮能力也强于后者。以适当密度进行的不同牧草混播处理由于种间良性竞争和共生协调作用可优化混播群体对土壤养分的消耗利用。

由二酐的异构体合成的聚酰亚胺

聚酰亚胺是一类综合性能优异的重要耐热高分子材料，它在航空，航天工业，微电子工业等诸多领域获得了广泛的应用。异构的聚酰亚胺是近几年新兴起的结构材料之一，与传统结构的聚酰亚胺比较，具有更高的玻璃化温度，良好的溶解性能，低的熔融粘度，宽的加工窗口，和相当的机械性能和热稳定性，是一种较有发展前景的结构材料；同时异构的二酐单体又可以用来合成芳香环状聚酰亚胺，对这些十分稳定又具有很好溶解性能的环状齐聚体的研究还刚刚开始。为此，本论文主要就以下方面开展研究工作：1，以环己烯为原料，经六步，高收率地合成了PMDA的异构体，MPDA。发现MPDA在室温下同甲醇和苯胺的开环选择反应分别得到了不对称的酷化产物和对称的酞化产物，这种开环选择的差别可归结于在酞化反应过程中形成了环状氢键中间体。通过MPDA和苯胺的亚胺化反应研究，发现在DMAc中热环化得到单亚胺化合物和双亚胺化合物的混合物，而在对氯酚中热环化或者在DMAc中化学环化均只得到唯一双亚胺化合物。单晶X-Ray衍射证实了单亚胺化合物的结构。发现在DMAc中热环化时单亚胺化合物和双亚胺化合物可以相互转化，而在氮气或者真空高温条件下单亚胺化合物不能转化成双亚胺化合物，从而提出了反应机制，即少量DMAc中的水分可以影响生成单亚胺化合物。发现MPDA和ODA或者MDA聚合时有很大的生成环状聚酰亚胺的倾向，它们的环状性质由IR，MALDI-TOF-MS，和NMR所证实。发现环状聚酰胺酸放置时，不同的环之间可以发生转换，从而可以提高受热力学控制的环的收率。用溶剂萃取和重结晶的方法分离得到了环状聚酰亚胺二聚体，用单晶X-Ray衍射的方法表征了二聚体包容不同溶剂时的分子结构，从分子水平的角度研究了环状聚酞亚胺的结构和性能的关系。除了MPDA得到的聚酞亚胺有较高的g值外，异构体聚酰亚胺的其它性能如力学性能、热稳定性能、溶解度性能、形态结构性能等差别不大。2．以2，3-二甲基苯胺为原料，经四步，高收率地得到了3,3'-BTDA。发现螺双内酚在乙酸配中回流可以直接有效地转化成二配3,3'-BTDA，并提出了转化机制。单晶X-Ray衍射表明3,3'-BTDA和3,4'-BTDA都有非共平面的分子结构。通过研究异构的BTDA得到的聚酞亚胺，发现并表征了3,3'-BTDA和ODA聚合时有很大的生成环状聚酞亚胺的倾向。异构化BTDA得到的聚酞亚胺的Tg值规律：3,3'-BTDA＞3,4'-BTDA＞4,4'-BTDA；溶解性能规律：3,3'-BTDA＞3,4'-BTDA＞4,4'-BTDAA；融体粘度规律：3,3'-BTDA＞4,4'-BTDAA3,4'-BTDA；热机械性能规律：4,4'-BTDA澎3,4'-BTDA＞3,3'-BTDA。可以看出3,4'-BTDAA得到的聚酰亚胺既有良好的加工性能，又能保持材料的热机械性能。由DSC测得的异构模型化合物的Tm，△Hm，和△Sm数据可以有效地解释异构聚酰亚胺的Tg规律。3．合成了cis-1,2,3,4-CHDA，发现在一定条件下可以转化成它的异构体trans-1,2,3,4-CHDA，1HNMR直接跟踪热异构化反应的进程，并提出了转化机制。单晶X-Ray衍射确定了两个二配异构体的绝对构型。发现在用trans-1,2,3,4-CHDA制备聚酞亚胺时无论用两步法或者一步法，均能很容易地得到高分子量的聚合物。而它的异构体cig-1,2,3,4-CHDA在制备聚酞亚胺时，由于形成环状聚酞亚胺，用传统的方法得不到高分子量的聚合物，增加单体的浓度可以有效制得高分子量的聚合物。发现顺式聚合物的溶解性能比反式异构体的好，Tg值比相应反式异构体的高，其它如热氧化稳定性，机械性能，光学性能，异构体间没有明显的差别。但是，两个异构体都可以制得坚韧的透明薄膜。

γ-辐照尼龙－610的热学性能

尼龙－610除具备一般聚酰胺的优良特点外，与尼龙－6、尼－66等大批量生产的工程塑料相比，还有其独特的优良特性，即尼龙－610吸水性较小，尺寸比较稳定。因而它广泛应用于制造日用、机械、电器及航空等工业制品。尽管尼龙－610 性能优良，但对一些使用温度和尺寸精度要求高的产品而言，还不够理想。如果将尼龙－610经γ－射线辐照，分子间形成空间网络，可能使产品的尺寸更稳定，使用温度更高，使用领域更为扩展。作为高分子材料，尼龙－610的结构和性能方面的研究很少进行，经γ－射线辐照交联后产物的结构与性能的研究则更为少见。这在一定程度上限制了尼龙－610的加工和使用。本文用DSC示差扫描量热仪，广角X－射线衍射仪等手段研究尼龙－610本身的热性能和结构特性，以及辐照后尼龙－610的热性能。采用对尼龙－610样品进行部分湿度扫描、变更升、降温速度及变更等温结晶温度的方法研究了尼龙－610的熔融、结晶行为。发现在等速升温过程中，均有多重熔融峰出现，它们对升温速率、降温速率及等温结晶的湿度分另有没程度的依赖性。高温熔融峰几科不受升温速度、降温速度及等温结晶温度影响，它主要是在扫描过程中试样原有结晶不完善部分在低温下熔化和再结晶或重排而形成的介稳态α－型晶体的熔化，或在玻璃化变温度Tg以上冷结晶形成的介稳态结晶的重组和重排而形成晶体的熔化。低温熔融峰受升温速率、降温速度及等温结晶的温度影响很大，是由热历史造成的结晶的熔融。多重熔融峰不是来自于不同晶型，实验发现尼龙－610在等速降温和等温结晶过程中产生的结晶均为α－型三斜晶系。采用Tm与Tc作图外推的方法，估得尼龙－610样品的平衡熔融温度为。Tm°= 518.1 K。此值与Van krevelen的报导值516K相近。利用聚合物熔化热和与其对应比容Vsp外推的方法，求得尼龙－610的平衡熔融热为：△Hm°= 51.8 cal/gram。用DSC－RC示差扫描量热 仪研究了室温、真空气氛下，经60 ℃ γ射线辐照的尼龙－610的热行为及辐效应。发现随辐照剂量增加，熔融峰湿度向低温方向移动，从熔体等速降温时，只有单一结晶峰出现，随辐照剂量增加，结晶峰变宽，结晶温度下降，且熔融热与结晶热也随剂量增加而减小。尼龙－610样品在400－700Mrad之间的剂量辐照时，第二次等速升温的熔融谱图上，有冷结晶峰出现，此冷结晶峰随热历史及辐照剂量变化。它主要是尚未形成完整间网络和技化的尼龙－610大分子的贡献。辐照尼龙－610也符合charlesby-Pinner无规交联议程。用Charlesby-pinner方程算得尼龙－610样品的辐照交联G（CL）＝ 0.57。选择适当的等温结晶温度，用DSC－RC示差扫描是热仪研究了辐照尼龙－610榈的等温结晶动力学。动力学计算由3600TADS计算机所带部分面积程序给出。实验出现，随辐照剂量增加，尼龙－610的Avrami指数n减小。结晶表面自由能σ＿e随辐照剂量加而增大，在同一结晶条件下，辐照剂量愈大，球晶数目愈多，结晶愈不完整，说明尼龙－610经γ－射线辐照交联后可能由均相三维无热成核变为异相单维无热成核。且化学交联点，可能充当结晶的成核中心。

熔融氯化物电解制备NdFe、DyFe和DyAl合金的研究

本文对电解熔融氯化物制备稀土金属钕及钕铁、镝铁合金的电极过程及镝在镝铝合金中的扩散系数，用氯化物熔体电解制备钕铁合金的电解工艺，进行了系统研究。利用线性扫描伏安法（LSV）和计时电位法研究了Nd~(3+)，Dy~(3+)在等摩尔Kcl-NaCl熔体中，在Mo, Fe, Pt三种不同电极上的电极过程；还研究了在可耗铁电极上，电解制备NdFe合金的电解工艺；测定了Dy~(3+)在该熔体中的扩散系数及Dy原子在Al-Dy合金中的扩散系数。研究结果表明，Nd~(3+)在Mo电极上的反应接近于扩散控制的反应。为不可逆过程。反应是一步完成的，转移系数α为0.8。计时电位法证明，Nd同熔体具有较强烈地作用。Nd~(3+)在Fe电极上的反应二步进行。首先是Nd的析出，立即与Fe合金化生成NdFe合金。经X-射线衍射分析证明，合金组成为Nd_2Fe。由于合金化的结果，使Nd~(3+)的析出电位向正方向飘移。第二步反应是由于有大量的Nd析出，Nd向Fe电极内部的扩散已不足以使析出的Nd与Fe合金化，造成纯Nd的析出。由于Nd沉积在电极表面上，活度逐渐趋向于1，故Nd~(3+)的析出电位又向负方向飘移。由于合金化，反应的全过程为不可逆的，αn为0.55～0.65。NdFe合金电解工艺的研究表明，利用体系NdCl_3-Kcl·NaCl-LiF是合适的。LiF的加入有助于溶解电解质中的水不溶物，降低熔体的粘度。浓度和温度对合金化影响较大。深度过高，则有大量纯Nd析出。由于Nd同熔体作用强烈，对电解不利。浓度太低，则金属Na易析出导致电流效率降低。温度高，有利于合金化；但合金中Nd的含量过高且电解质挥发严重；并且Nd同熔体的作用更强烈了。控制电解温度在750～800 ℃，有利于形成合金。我们得到最佳电解工艺条件为：当温度在750～760 ℃时；电流密度在10～13安培/厘米~2；NdCl_3浓度为30%(wt)时，电流效率达最大值为56%。利用LSV和计时电位法对Dy~(3+)在Mo电极上的电极过程进行研究的结果表明，Dy~(3+)在Mo电极上的反应为一步生成金属的反应。反应是扩散控制的可逆过程。反应式为Dy~(3+) + 3e = Dy。利用扫描电镜进一步证实了这个结果。计时电位法测得了Dy~(3+)在该熔体中的扩散系数D与温度下的关系为LgD = 1.65 - 7376/T ± 0.34。同Nd比较，Dy同熔体的作用不很强烈。Dy~(3+)在Mo电极上的电极过程很好地符合可逆反应的规律。LSV方法研究Dy~(3+)在Pt，Fe电极上的电极过程表明，Dy~(3+)在Pt电极上的反应分二步进行。第一步反应为扩散控制的可逆反应。用卷积积分计算出第一步反应的转移电子数为1。与利用扩散方程计算的结果一致。所以，Dy~(3+)在Pt电极上的反应为，Dy~(3+) + e = Dy~(2+) ，Dy~(2+) + 2e = Dy。Dy~(3+)在Fe电极上的反应为不可逆过程。反应为一步还原为金属的反应。阳极溶解伏安曲线表明，Dy在低于800 ℃不与Fe形成合金。在Fe为电极制备DyFe合金，电解温度选择在900 ℃左右为宜。利用阳极计时电位法测定了Dy在Al-Dy合金中的扩散系数。结果表明，Dy在合金内部向合金表面的扩散为阳极溶解反应的控制步骤。测得扩散系数同温度的关系为LgD = 5.81 - 10662/T ± 0.51。(700～850 ℃)扩散活化能为204.1 kJ/mole。较高的活化能说明Dy在Al-Dy合金中的扩散相当困难。

三氧化硫直接磺化DDB宏观动力学

精确定量地设计一个高效的直接磺化反应器需要基础反应动力学和发生于界面处的热量和质量传递过程的知识，然而，到目前为止还没有人对三氧化硫直接磺化DDB的动力学进行系统的研究。本文在气液搅拌式反应器中研究了该反应的动力学特性并且通过在420nm处吸光度的测定探讨了反应条件对产品着色速率的影响。结果表明：该反应相对于DDB和SO_3的反应级数分别为1和2，反应活化能E = 70.6 kJ/mole，指前因子A = 6.97 * 10~(13)，反应温度和气相SO_3的浓度是影响产品着色的重要因素。

Intersubband absorption from In0.26Ga0.74As/GaAs quantum dot superlattice

We have grown a high-quality 20 period InGaAs/GaAs quantum dot superlattice with a standard structure typically used for quantum well infrared photodetector. Normal incident absorption was observed around 13-15 mu m. Potential applications for this work include high-performance quantum dot infrared detectors.

Characteristics of InAs epilayers for Hall effect devices grown on GaAs substrates by MBE

InAs thin films with good characteristics were grown on GaAs (0 0 1) substrates by molecular beam epitaxy. Cross-sectional transmission electron microscopy indicated that most of the threading dislocations formed by the interaction of misfit dislocations are annihilated above a small thickness. The high electron mobility and small temperature dependence of InAs epilayers are useful for magnetic sensors which is demonstrated by the properties of Hall effect devices.

Luminescence properties of ZnSe films grown by hot wall epitaxy

The photoluminescence (PL) properties of ZnSe films grown by hot wall epitaxy are reported. The PL spectra show clear neutral donor-bound exciton peak; donor acceptor pair (DAP) peak, conduction band to acceptor (CA) peak, and their phonon replicas until fourth order. The conduction band to acceptor peak and it's phonon replicas exist until room temperature. From the ratio of PL intensities of DAP and CA peaks and their replicas, we obtain the Huang-Rhys factor S = 0.58, in agreement with other experiments for acceptor-bound exciton transitions. From the temperature dependence of PL intensities we derive the activation energy of thermal quenching process for the DAP transitions as about 7 meV.

Self-formed InGaAs/GaAs quantum dot superlattice and direct observation on strain distribution in the capped superlattice

We have demonstrated a 20 period dislocation-free InGaAs/GaAs quantum dot superlattice which is self-formed by the strain from the superlattice taken as a whole rather than by the strain from the strained single layer. The island formation does not take place while growing the corresponding strained single layer. From the variation of the average dot height in each layer, the strain distribution and relaxation process in the capped superlattice have been examined. It is found that the strain is not uniformly distributed and the greatest strains occur at two interfaces between the superlattice and the substrate and the cap layer in the capped superlattice. (C) 1997 American Institute of Physics.

Deep centers in AlGaAs/GaAs GRIN-SCH SQW laser structures grown by MBE and MOCVD

The deep centers in AlGaAs/GaAs graded index-separate confinement heterostructure single quantum well (GRIN-SCHSQW) laser structures grown by MBE and MOCVD have been investigated using deep level transient spectroscopy (DLTS) technique, The majority and minority carrier DLTS spectra show that the deep (hole and electron) traps (Hi and E3), having large capture cross sections and concentrations, are observed in the graded n-AlxGa1-xAs layer of laser structures in addition to the well-known DX centers. For laser structures grown by MBE, the deep hole trap H1 and the deep electron trap E3 may be spatially localized in the interface regions of discontinuous variation Al mole fraction of the n-AlxGa1-xAs layer with x = 0.20-0.43. For laser structures grown by MOCVD, the deep electron trap E3 may be spatially localized in the n-AlxGa1-xAs layer with x = 0.18-0.30, and the DX center may be spatially localized in the interface regions of discontinuous variation Al mole fraction of the AlxGa1-xAs layer with x = 0.22-0.30.

RF-MBE Grown AlGaN/GaN HEMT Structure with High Al Content

A Si doped AlGaN/GaN HEMT structure with high Al content (x= 44%) in the barrier layer is grown on sapphire substrate by RF-MBE. The structural and electrical properties of the heterostructure are investigated by the triple axis X-ray diffraction and Van der Pauw-Hall measurement, respectively. The observed prominent Bragg peaks of the GaN and AlGaN and the Hall results show that the structure is of high quality with smooth interface.fabricated and characterized. Better DC characteristics, maximum drain current of 1.0A/mm and extrinsic transconductance of 218mS/mm are obtained when compared with HEMTs fabricated using structures with lower Al mole fraction in the AlGaN barrier layer. The results suggest that the high Al content in the AlGaN barrier layer is promising in improving material electrical properties and device performance.

Observation of the resonant Raman behavior of individual single-walled carbon nanotubes

The different resonant Raman scattering process of single-walled carbon nanotubes (SWNTs) has been found between the Stokes and anti-Stokes sides of the radial breathing modes (RBMs), and this provides strong evidence that Raman spectra of some special diametric SWNTs are in resonance with their electronic transitions between the singularities in the one-dimensional electronic density of states in the valence and conduction bands, and other SWNTs axe beyond the resonant condition. Because of the coexistence of resonant and non-resonant Raman scattering processes for different diametric SWNTs, the relative intensity of each RBM does not reflect the proportion of a particular SWNT.

Dislocations in InAs epilayers grown by MBE on GaAs substrates under various conditions

Using Transmission Electron Microscopy, we studied the misfit and threading dislocations in InAs epilayers. All the samples, with thickness around 0.5 mu m, were grown on GaAs(001) substrates by molecular beam epitaxy under As-rich or in-rich conditions. The As-rich growth undergoes 2D-3D mode transition process, which was inhibited under In-rich surface. High step formation energy under As-deficient reconstruction inhibits the formation of 3D islands and leads to 2D growth. The mechanism of misfit dislocations formation was different under different growth condition which caused the variation of threading dislocation density in the epilayers.

Growth of Fe doped semi-insulating InP by LP-MOCVD

The semi-insulating InP has been grown using ferrocene as a dopant source by low pressure MOCVD. Fe doped semiinsulating InP material whose resistivity is equal to 2.0x10(8)Omega*cm and the breakdown field is Beater than 4.0x10(4)Vcm(-1) has been achieved. It is found that the magnitude of resistivity increases with growing pressure enhancement under keeping TMIn, PH3, ferrocene (Fe(C5H5)(2)) flow constant at 620 degrees C growth temperature. Moreover, the experimental results which resistivity varies with ferrocene mole fraction are given. It is estimated that active Fe doping efficiency; eta, is equal to 8.7x10(-4) at 20mbar growth pressure and 620 degrees C growth temperature by the comparison of calculated and experimental results.

Modification of emission wavelength of self-assembled In(Ga)As/GaAs quantum dots covered by InxGa1-xAs(0 <= x <= 0.3) layer

Red shifts of emission wavelength of self-organized In(Cla)As/GaAs quantum dots (QDs) covered by 3 nm thick InxGa1-xAs layer with three different In mole fractions (x = 0.1, 0.2 and 0.3, respectively) have been observed. Transmission electron microscopy images demonstrate that the stress along growth direction in the InAs dots was reduced due to introducing the InxGa1-xAs (x = 0.1, 0.2 and 0.3) covering layer instead of GaAs layer. Atomic force microscopy pictures show a smoother surface of InAs islands covered by an In0.2Ga0.8As layer. It is explained by the calculations that the redshifts of the photoluminescence (PL) spectra from the QDs covered by the InxGa1-xAs (x greater than or equal to 0.1) layers were mainly due to the reducing of the strain other than the InAs/GaAs intermixing in the InAs QDs. The temperature dependent PL spectra further confirm that the InGaAs covering layer can effectively suppress the temperature sensitivity of PL emissions. 1.3 mum emission wavelength with a very narrow linewidth of 19.2 mcV at room temperature has been obtained successfully from In,In0.5Ga0.5As/GaAs self-assembled QDs covered by a 3-nm In0.2Ga0.2As strain reducing layer. (C) 2001 Elsevier Science B.V. All rights reserved.