浑善达克沙地榆树树干液流动态研究

本文应用Granier热扩散技术对内蒙古浑善达克沙地榆树疏林林分进行树干液流通量研究，分别选择胸径大小占林分80%以上林木的三种胸径级（15-25cm，25-35cm，35-45cm）榆树样树各三株，应用Granier探针进行每隔15min的树干液流测定，同步测定林分环境因子，包括空气温度、相对湿度、土壤温度、土壤湿度、光合有效辐射、风速等10余个环境因子，在生长季节进行了5个月的测定，详细研究和分析了榆树树杆液流特征，其中包括昼夜变化趋势和规律、不同天气状况下的液流密度变化特点以及在生长季不同时期的液流密度变化规律，阐述了不同胸径级榆树树干液流变化特征。同时利用连续获得的树干液流通量与环境因子数据，分析了树干液流通量与环境因子的关系，阐明不同时期影响树干液流通量变化的主导因素，以及树干液流密度对环境因子的响应规律。并通过钻取样地内的树木生长年轮芯测定了榆树不同径级的边材面积，建立边材和胸径的关系方程，估算了不同胸径林分的边材面积以及冠层蒸腾的季节动态。 在晴朗的白天，不同胸径级榆树的树干液流密度变化曲线均呈单峰曲线，只是峰值持续的时间不同。不同胸径级榆树树干液流密度变化具有明显的昼夜节律性。在生长季结束后液流并不是马上停止，而是有微弱的波动。阴雨天液流密度变化曲线均不规则且液流密度值较小，甚至出现多峰曲线，而且峰值明显;液流密度的变化出现较大的波动性，夜间却有液流发生，而且值较晴天的夜间液流密度稍大。 林分不同胸径级榆树树干大径级木、中径级木和小径级木液流通量特征表现为: 榆树大径级木液流密度的峰值大于中等木，中等木峰值大于小径级木，差异明显;大径级木由于年龄较大，液流密度曲线变化平缓，没有明显的峰值，呈弧形曲线; 而且不同年龄的树木反映差别比较大，但是总体趋势还是受胸径的影响。 随季节推移生长季各月份液流密度均值基本逐渐减小，进入生长季末期(10月)基本处于微弱波动状态。液流通量月平均值最大值一般都出现在6-8月，大径级木、中径级木和小径级木各月液流通量变化差异较大。日液流通量比较分析得出，大径级木日液流通量最大值分别为394.53，中径级木为66.04，小径级木为63.56 kg∙ d-1。综合分析对不同胸径树木影响较大的环境因子为VPD，PAR，Ta; 但是在不同生长季节，对不同类型树木起主要作用的环境因子也不尽相同，但是PAR与VPD是其中基本主要起作用的两个因子。 通过建立边材与胸径的关系方程，得出边材和胸径关系(R2=0.99)。通过经验公式计算得出，林分生长期的蒸腾耗水量相当于2.24mm/day, 低于当地同期降雨量2.78mm/day.

青藏高原几种植物的光合特性研究

通过气体交换、荧光猝灭动力学以及反射光谱等技术研究了两个青稞（Hordeum vulgare L.）品种的光合特性及激发能分配。结果表明，青稞的光饱和点1000 μmol m-2 s-1左右。在0～500 μmol m-2 s-1的光强范围里，青稞叶片的光呼吸(Pr)随着光强升高而增加;光强超过500 μmol m-2 s-1以后，光呼吸变化不明显。光呼吸占总光合的比例(Pr/Pm)随光强增强下降。随着光强增强，PSⅡ有效光化学量子效率（Fv′/Fm′），PSⅡ反应中心的实际光化学量子效率（ΦPSⅡ），光化学猝灭系数（qP）不断降低而青稞叶片的非光化学猝灭(NPQ)不断升高，说明越来越多的光能以热耗散的形式耗散掉。光谱分析表明△PRI 随着青稞叶片暴露于光下的时间迅速增大。因此，我们认为光呼吸不是青稞主要的光破坏防御机制，依赖叶黄素循环的热耗散可能是田间青稞耗散过剩光能的主要途径。 通过气体交换、荧光猝灭动力学等技术研究了四种乔木在拉萨和那曲的光合特性及激发能分配。结果表明，四种乔木藏川杨（Populus szechuanica var. tibetica schneid.），银白杨（Populus alba L.），左旋柳(Salix paraplesia var. subintegra C. Wang et P. Y. Pu)，墨竹柳(Salix maizhokunggarensis N. Chao)在拉萨市的光合速率（Pn），叶片气孔导度（Gs），蒸腾速率（Tr）均显著高于那曲。藏川杨和墨竹柳的光下实际光化学效率（ΦPSⅡ）在拉萨显著高于那曲，银白杨和左旋柳的光下实际光化学效率在拉萨和那曲没有显著差异。四种乔木开放反应中心激发能捕获效率（Fv′/Fm′）和天线热耗散（1－Fv′/Fm′）在拉萨和那曲的差异不显著。测量光合时的气温（Tair）拉萨显著高于那曲，除墨竹柳外叶温（Tleaf）也显著高于那曲，墨竹柳的上述两参数在两地间无显著差异。除藏川杨外其余三种乔木在拉萨的胞间二氧化碳浓度（Ci）显著高于那曲，气孔限制值(Ls)显著低于那曲，藏川杨的上述两指标在两地间无显著差异。除墨竹柳外，其余三种乔木在两地的光合（Pn）与叶温（Tleaf）成显著正相关。对银白杨和左旋柳来说，低叶温通过降低气孔导度（Gs）从而降低胞间二氧化碳浓度（Ci）是造成那曲光合低的主要因素之一。对于墨竹柳来说，可能有其他非温度的环境条件影响其气孔导度进而造成气孔限制。此外，叶温可能主要通过非气孔限制来影响藏川杨的光合速率。因此，我们认为在西藏地区不同乔木对海拔高度的响应机制可能不同，但具体机制还需要进一步研究。

黄土高原半干旱区侧柏(Platycladus orientalis)树干液流动态

应用热扩散式树干茎流计(TDP)于2008年4～10月对黄土高原安塞县侧柏人工林树干液流速率进行了连续测定,并对周围气象、土壤水分等多个环境因子进行了同步测定。结果表明:侧柏在不同月份晴天树干液流速率变化具有明显的昼夜节律性,呈单峰曲线;且各月液流速率日均值受土壤供水水平限制总体上呈下降趋势,即4月份最大,为0.00135cm.s-1;10月份最小为0.00011cm.s-1;树干液流速率与光合有效辐射、大气温度、水汽压差呈极显著正相关,与相对湿度呈负相关,其相关程度:光合有效辐射>水汽压差>大气温度>相对湿度,并可用线性表达式来估算;侧柏边材面积和地径呈幂指数关系,并以此结合密度估算出样地侧柏人工林的边材面积为4.65m2,最终估算出侧柏人工林生长季总耗水量为1159.6t.hm-2。

乔木蒸腾耗水量研究方法述评与展望

参阅了大量国内外有关乔木蒸腾研究方法文献,认为乔木蒸腾量研究方法主要有二大类,即组织器官测定、单木测定;分类对典型研究方法(快速称重法、气孔计法、整株容器称重法、同位素示踪法、热脉冲法、树干热平衡法、热扩散探针法)进行了述评,对比分析了各种方法间的优缺点及其适用范围;展望了乔木蒸腾耗水作用研究方法的应用前景,认为热技术法是未来几年内的主要测定方法。

黄土高原半干旱区人工林刺槐展叶期树干液流动态分析

应用热扩散式树干茎流计(TDP)于2008年4月26日至5月31日,在黄土高原半干旱区安塞县对人工林刺槐展叶期树干液流及其气象、土壤水分等6个指标进行连续测定。结果表明:刺槐展叶期可分为芽期、展叶初期、中期和全叶期。在芽期,刺槐树干液流速率日变化无明显昼夜波动;在展叶初期至全叶期日变化呈现出从微弱波动逐渐增大到趋于平稳的剧烈波动;在展叶中期以后液流速率表现为上升快、下降缓慢的单峰曲线;在全叶期平均峰值约为0.0027cm.s-1;树干液流速率与光合有效辐射强度、大气温度、水蒸气压亏缺和风速呈极显著正相关,与相对湿度呈负相关,其相关程度依次为光合有效辐射强度>大气温度>水蒸气压亏缺>相对湿度>风速,且可用光合有效辐射强度和大气温度线性表达式来估测;土壤水分在展叶期呈逐渐减少趋势,但对树干液流的胁迫不显著;在展叶期刺槐单株日蒸腾耗水量随直径的增大而增大并与胸径呈良好的线性关系,可用来估算展叶期刺槐人工林蒸腾耗水量。

Response of chlorophyll fluorescence to dynamic light in three alpine species differing in plant architecture

A study was carried out to examine the effect of dynamic photosynthetically active photon flux density (PPFD) on photoinhibition and energy use in three herbaceous species, prostrate Saussurea superba, erect-leaved S. katochaete, and half-erect-leaved Gentiana straminea, from the Qinghai-Tibet Plateau. Chlorophyll fluorescence response was measured under each of three sets of high-low PPFD combinations: 1700-0, 1400-300, and 1200-500 mu mol m(-2) s(-1), illuminating in four dynamic frequencies: 1, 5, 15, and 60 cycles per 2 h. The total light exposure time was 2h and the integrated PPFD was the same in all treatments. The highest frequency of PPFD fluctuation resulted in the lowest photochemical activity, the highest level of non-photochemical quenching, and the greatest decrease of F-v/F-m (maximal photochemical efficiency of PSII). The 5 and 15 cycles per 2h treatments resulted in higher photochemical activity than the 1 cycle per 2h treatment. The 1700-0 PPFD combination led to the lowest photochemical activity and more serious photoinhibition in all species. S. superba usually exhibited the highest photochemical activity and CO2 uptake rate, the lowest reduction of F-v/F-m,F- and the smallest fraction of energy in thermal dissipation. With similar fractions of thermal dissipation, S. katochaete had relatively less photoinhibition than G. straminea owing to effective F-o quenching. The results suggest that high frequency of fluctuating PPFD generally results in photoinhibition, which is more serious under periods of irradiation with high light intensity. (c) 2005 Elsevier B.V. All rights reserved.

Quantum dissipation and broadening mechanisms due to electron-phonon interactions in self-formed InGaN quantum dots

Quantum dissipation and broadening mechanisms in Si-doped InGaN quantum dots are studied via the photoluminescence technique. It is found that the dissipative thermal bath that embeds the quantum dots plays an important role in the photon emission processes. Observed spontaneous emission spectra are modeled with the multimode Brownian oscillator model achieving an excellent agreement between experiment and theory for a wide temperature range. The dimensionless Huang-Rhys factor characterizing the strength of electron-LO-phonon coupling and damping constant accounting for the LO-phonon-bath interaction strength are found to be similar to 0.2 and 200 cm(-1), respectively, for the InGaN QDs. (c) 2006 American Institute of Physics.

Thermal induced facet destructive feature of quantum cascade lasers

We present a study on the facet damage profile of quantum cascade lasers (QCLs). Conspicuous cascade half-loop damage strips on front facet are observed when QCLs catastrophically failed. Due to the large difference on thermal conductivities between active region and the substrate, dominant heat is compulsively driven to the substrate. Abundant heat accumulation and dissipation on substrate build large temperature gradient and thermal lattice mismatch. Thermal-induced stress due to sequential mismatch leads to the occurrence of the multistep damages on front facet. Good agreement is achieved between the observed locations of damaged strips and the calculated results.

Experimental study on energy dissipation in a 1 kW arcjet thruster

As a simple and reliable propulsion system, arcjet thrusters have been used in multiple satellite missions. In order to improve the efficiency of arcjet thrusters, energy dissipation study was carried out in a 1 kW arcjet thruster with pure N2, H2-N2 and H2 as the propellant. Using a 698 nm interference filter, thermal radiation was isolated from arc and plume emissions and the internal nozzle temperature was obtained by converting the thermal radiation signals. Results show that the addition of hydrogen leads to higher nozzle temperature, which is the determining factor for the mode of arc root attachment. At lower nozzle temperatures, constricted type attachment with unstable motions of the arc root was observed, while a fully diffused and stable arc root was observed at elevated nozzle temperatures. Output energy distribution analysis shows that losses from frozen flow and exhaust thermal losses are the main parts in limiting the efficiency of arcjet thrusters.

Experimental study on energy dissipation in a 1 kW arcjet thruster

As a simple and reliable propulsion system, arcjet thrusters have been used in multiple satellite missions. In order to improve the efficiency of arcjet thrusters, energy dissipation study was carried out in a 1 kW arcjet thruster with pure N2, H2-N2 and H2 as the propellant. Using a 698 nm interference filter, thermal radiation was isolated from arc and plume emissions and the internal nozzle temperature was obtained by converting the thermal radiation signals. Results show that the addition of hydrogen leads to higher nozzle temperature, which is the determining factor for the mode of arc root attachment. At lower nozzle temperatures, constricted type attachment with unstable motions of the arc root was observed, while a fully diffused and stable arc root was observed at elevated nozzle temperatures. Output energy distribution analysis shows that losses from frozen flow and exhaust thermal losses are the main parts in limiting the efficiency of arcjet thrusters.

Melting of Au and Al in nanometer Fe/Au and Fe/Al multilayers under swift heavy ions: A thermal spike study

Knowing that Fe is sensitive to swift heavy ion irradiations whereas Au and Al are not, the behavior of nanometric metallic multilayer systems, like [Fe(3 nm)/Au(x)](y) and [Fe(3 nm)/Al(x)](y) with x ranging between 1 and 10 mn, were studied within the inelastic thermal spike model. In addition to the usual cylindrical geometry of energy dissipation perpendicular to the ion projectile direction, the heat transport along the ion path was implemented in the electronic and atomic sub-systems. The simulations were performed using three different values of linear energy transfer corresponding to 3 MeV/u of Pb-208, Xe-132 and Kr-84 ions. For the Fe/Au system, evidence of appearance of a molten phase was found in the entire Au layer, provided the Au thickness is less than 7 nm and 3 nm for Pb and Xe ions, respectively. For the Fe/Al(x) system irradiated with Pb ions, the Al layers with a thickness less than 4 nm melt along the entire ion track. Surprisingly, the Fe layer does not melt if the Al thickness is larger than 2 nm, although the deposited energy surpasses the electronic stopping power threshold of track formation in Fe. For Kr ions melting does not occur in any of the multilayer systems.

圆柱受限空间内喷雾燃烧振荡特性研究=Investigation of the Characteristics of Thermal Instability in A Confined Combustor

本文研究了圆柱受限空间内的喷雾火焰嫩烧压力振荡的特性。为了更清楚地了解火焰的构造, 首先测量了火焰的温度场，在较大的一次风和二次风变化范围内, 测量了压力的振荡特性。结果表明，火焰的稳定是由回流区完成的, 在较小的一次风燃料当量比和中等的二次风量时, 振荡最强, 达到100Pa左右的量级,其频率为200-230Hz左右,分析表明燃烧室中的振荡是轴向驻波振荡。

Anomalous Thermal Stability of Nd-Fe-Co-Al Bulk Metallic Glass

The thermal stability of Nd60Fe20Co10Al10 bulk metallic glass (BMG) has been studied by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), isochronal dilatation and compression tests. The results show that the glass transition of the BMG takes place quite gradually between about 460 and 650 K at a heating rate of 0.17 K/s. Several transformation processes are observed during continuous heating with the first crystallization process beginning at about 460 K, while massive crystallization takes place near the solidus temperature of the alloy. The positive heat of mixing between the two major constituents, Nd and Fe, and, consequently, a highly inhomogeneous composition of the attained amorphous phase are responsible for the anomalous thermal stability in this system. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.