碳化硅掺杂改善二元糖醇纳米胶囊的性能

甘露醇和肌醇作为具有较高潜热的糖醇相变材料,在中温储热方面具有广阔的应用前景。二元糖醇的相变温度可以通过改变糖醇各组分的比例来调节,因而其应用温度范围与单糖醇相比较宽。糖醇在热循环中存在泄漏、氧化和过冷等问题,对糖醇进行微纳胶囊化和添加成核剂是有效解决措施。采用溶胶-凝胶法制备糖醇胶囊制备了碳化硅掺杂甘露醇和肌醇二元糖醇纳米胶囊,采用差示扫描量热仪、扫描电子显微镜、热重分析仪和热循环试验研究了纳米胶囊的相变特性和热稳定性,探讨了碳化硅的影响。结果表明,纳米胶囊具有较高的相变焓值、包封率和储能效率,热分解温度相比二元糖醇提高了,经过热循环后胶囊的过冷度相对于二元糖醇降低了32.2℃。     

纳米二氧化钛-赤藻糖醇储能体系实验研究

针对移动供热的温度范围,相变温度为119℃的赤藻糖醇具有很大的应用潜力,但其存在导热系数低、放热不稳定、容易过冷等不足。本文通过添加纳米材料作为成核剂改善赤藻糖醇性能,制备了不同纳米材料的相变复合材料。对样品的融化-凝固进行观察记录,绘制时间温度曲线并对纳米氧化钛在质量分数不同的情况下进行体积膨胀率和密度、过冷度、相变潜热、导热系数测定,测试表明,相对于纯赤藻糖醇,添加纳米级物质作为成核剂可以减小相变材料的过冷度,并且在一定程度上可以提高相变材料的导热系数;纳米二氧化钛对样本的密度数值上有0.02~0.08 g/cm~3的波动,膨胀率呈减小的趋势;添加0.1%的纳米二氧化钛-赤藻糖醇过冷度下降37.91%,潜热值下降2.25%,固态导热系数增大3.67倍。     

冷冻产品用无机盐相变蓄冷材料的制备及性能研究

研制了一种适用于冷冻产品贮藏运输的高效低温无机盐相变蓄冷材料。将NH4Br、KCl、CaCl_2等无机盐与NH_4Cl复配,利用差式扫描量热仪(DSC)筛选了NH_4Br-NH_4Cl无机盐共混物作为主储能剂,通过添加不同成核剂改善过冷性能,添加不同浓度的黄源胶改善相分离。结果表明:以30%(wt,质量分数,下同)NH_4Br-NH_4Cl无机盐共混物作为主储能剂,1%SrCl_2作为成核剂,5%黄原胶作为增稠剂可获得相变温度为-18~-22℃、相变潜热为288.7J/g、过冷度小、无相分离的相变蓄冷材料。NH_4Br-NH_4Cl无机盐共混物作为主储能剂能够达到冷冻产品温度且对相变潜热影响较小,SrCl_2作为成核剂基本无过冷,黄原胶作为增稠剂解决相分离具有显著效果。     

相变材料的过冷现象及其抑制方法的研究进展

过冷是材料液-固相变过程中为提供离子扩散、晶体生长及晶面扩大所需能量而产生的一种亚稳态。过冷是结晶过程的推动力,但大的过冷度导致相变材料结晶温度降低、结晶时间延迟,使得储存的潜热不能及时释放,储-放热温度不匹配,降低了热能利用效率。过冷度大已成为限制相变储热技术规模化应用的重要影响因素之一。大量实验结果表明:过冷现象与熔体晶核的生成与长大速率、环境温度、接触面的粗糙程度、熔体温度等因素有关,但其产生的内在机制尚不明确,影响规律和调控手段仍需借助实验探索。目前,主要采用外加添加剂或在胶囊化、流体化过程中加入添加剂的添加晶种法,以及通过搅拌、超声振荡和鼓泡等外部刺激的动力学成核法诱发过冷液体结晶。利用外加成核剂、纳米颗粒以及部分未熔化母相晶体作为晶核诱发非均匀成核是抑制过冷现象最常用、最有效的方法。为提高添加剂的分散性和抑制水合无机盐类相变材料的相分离现象,在添加成核剂的同时往往需要加入一定量的增稠剂,但成核剂和增稠剂的添加量需要优化。胶囊化可以改变相变材料的结晶特性,目前的研究一致认为在胶囊化前添加成核剂有利于改善相变材料的过冷度。动力学成核常采用的方法是超声振荡法,其通过空化作用使晶体持续破碎并与熔体混合而提高晶核的分散性和加速结晶过程。在超声的同时添加纳米颗粒也有利于抑制过冷度。本文简述了过冷现象和典型的过冷曲线,分析了影响过冷度的因素,并着重介绍了外加添加剂法、胶囊化法、功能流体法和超声振荡法等抑制过冷度的方法。     

膨胀石墨/八水氢氧化钡复合定型相变材料的制备与储热性能研究

通过在八水氢氧化钡中添加成核剂磷酸二氢钾和强化传热材料膨胀石墨,利用真空吸附法制备了膨胀石墨/八水氢氧化钡复合定型相变材料。采用Leica DM6000M显微镜,"步冷曲线法"和量热法对其热性能进行了表征和测试,结果发现,添加0.5%~2%的磷酸氢二钾可使八水氢氧化钡的过冷度由11.4℃降低到0.5℃以内,1%~3%的膨胀石墨可使过冷度降低到4℃。复合定型相变材料的较优组成为:93%Ba(OH)2·8H2O+6%EG+1%KH2PO4时,其相变潜热为248.3kJ/kg,与纯物质相比可缩短储热时间45.1%,缩短放热时间54.5%,且相变中无液态产生,是较好的"固-固"定型复合相变材料。     

相变储热材料CH3COONa·3H2O的研究及发展趋势

简单介绍了无机水合盐CH3COONa·3H2O用作相变储热材料时的优缺点,针对其存在过冷和相分层问题,目前研究所得的解决方法是添加增稠剂和成核剂.综述了分别添加增稠剂、成核剂及同时添加时对CH3COONa·3H2O相分层和过冷抑制的研究进展,分析了不同的添加剂和添加量对抑制作用的不同以及温度影响添加剂抑制作用的原因,最后指出了CH3COONa·3H2O今后主要的研究方向.     

融化和凝固温度对三水醋酸钠相变储热性能的影响

为了研究储热材料三水醋酸钠(含成核剂十二水磷酸氢二钠和增稠剂羧甲基纤维素)在实际贮能工程中应用的融化和凝固温度范围,通过不同水浴温度下的融化实验和不同冷却条件下的凝固实验,得到了储热材料的相变性能参数.实验结果表明,储热材料的过冷度随着加热水浴温度的升高有增大的趋势,水浴温度在83℃以内时其相变性能较稳定,超过这个温度,其储热性能就无法保证;冷却温度降低,其过冷度有增大的趋势,但只要是在常温条件下的水浴或空气浴中冷却,即使冷却温度变化较大,储热材料的相变性能仍比较稳定.     

三水醋酸钠固液相变性能优化实验研究

三水醋酸钠（SAT）在凝固过程中存在着严重的分层和过冷现象,可以利用羧甲基纤维素（CMC）作为增稠剂并添加成核剂来解决过冷问题。本文通过5种成核剂实验得到了效果最好的成核剂配方,其质量配比为SAT∶十二水磷酸氢二纳∶CMC=100∶8∶4。实验过程中还发现加热温度对药品的性质有较大影响,温度过高会导致过冷度变大甚至直接不相变,通过实验得到SAT作为相变材料时加热温度的上限为82℃。     

赤藓糖醇基复合相变材料的研究进展

相变储热材料通过其相变潜热实现能量的吸收、储存与释放,可以合理有效地利用现有能源、优化使用可再生资源和提高能源利用率.赤藓糖醇的相变温度约118℃,潜热约314 J/g,储能密度大、无腐蚀性,在中温储能领域有广阔的应用前景,现已被广泛应用于太阳能蓄热、工业余废热回收、清洁供暖等领域.然而,赤藓糖醇过冷严重、导热性能相对较差,使得热能无法及时地释放,造成热能利用效率不高,极大地限制了其在储能领域的应用.复合材料制备技术的发展为改善赤藓糖醇的过冷与导热性能提供了一种新的方法,在保留赤藓糖醇优异性能的同时,弥补了它的高过冷和低导热等缺陷.目前,制备赤藓糖醇复合相变材料已成为改善赤藓糖醇性能的主要方法,并取得了显著的成果.引入成核剂可以降低体系的形核势垒,促进成核,从而抑制过冷.纳米金属及其氧化物、膨胀石墨、石墨泡沫等材料的合理引入明显降低了赤藓糖醇的过冷度,最高可降低93％.将赤藓糖醇长时间控制在过冷的亚稳态,是利用其高过冷特性进行跨季节储能应用的关键,但该技术仍处于研究阶段.通过添加高导热材料来增加赤藓糖醇的当量热导率和增大相变蓄热器换热面积是赤藓糖醇热强化的主要措施,当量热导率可高达30 W/(m·K),且热利用率显著提高.相平衡理论为调节赤藓糖醇的相变温度提供了思路,选择合适的有机相变材料可以制备出相变温度在70～120℃范围内可调、潜热大于200 J/g的共晶相变材料.本文简述了赤藓糖醇基复合相变材料的常用制备技术,分别对改善赤藓糖醇过冷性能和导热性能的手段以及如何利用赤藓糖醇的高过冷进行综述,归纳了调控赤藓糖醇相变温度的方法,分析了赤藓糖醇基复合相变材料在实际应用中的优势.     

Na_2SO_4·10H_2O/EG复合相变材料的制备与性能分析

以十水硫酸钠为相变材料,采用真空吸附法制备十水合硫酸钠/膨胀石墨复合相变储能材料(Na_2SO_4·10H_2O/EG),对其融化-凝固、相分离、过冷、潜热等热物性进行测试分析。结果表明:在Na_2SO_4·10H_2O中添加2%(质量分数,下同)硼砂和8%EG后,可得到理想的Na_2SO_4·10H_2O/EG固-固复合相变材料。此时,Na_2SO_4·10H_2O相分离得到消除,过冷度由13.6℃降低到0.6℃以下,相变潜热和体储能密度分别为225.77kJ·kg~(-1)和218.09MJ·m~(-3)。此外,导热率也得到提高,相比于只添加成核剂硼砂的Na_2SO_4·10H_2O PCM,储热时间缩短52.6%,放热时间缩短55.1%,经过500次急剧升温-降温循环后也未出现性能衰减,储/放热性能较好。     

Molecular autonomous agents

I consider an autonomous agent to be a physical system able to act on its own behalf, such as a bacterium swimming up a glucose gradient. I tentatively define an autonomous agent to be a system capable of self-reproduction and at least capable of performing one thermodynamic work cycle. I give a hypothetical chemical example. I then explore the increasingly odd implications of this definition.     

Contrast agents for MRI

Contrast agents are divided into two categories. The first one is paramagnetic compounds, including lanthanides like gadolinium, which mainly reduce the longitudinal (T₁) relaxation property and result in a brighter signal. The second class consists of super-paramagnetic magnetic nanoparticles (SPMNPs) such as iron oxides, which have a strong effect on the transversal (T₂) relaxation properties. SPMNPs have the potential to be utilized as excellent probes for magnetic resonance imaging (MRI). For instance, clinically benign iron oxide and engineered ferrite nanoparticles provide a good MRI probing capability for clinical applications. Furthermore, the limited magnetic property and inability to escape from the reticuloendothelial system (RES) of the used nanoparticles impede their further advancement. Therefore, it is necessary to develop the engineered magnetic nanoparticle probes for the next-generation molecular MRI. Considering the importance of MRI in diagnosing diseases, this paper presents an overview of recent scientific achievements in the development of new synthetic SPMNP probes whereby the sensitive and target-specific observation of biological events at the molecular and cellular levels is feasible.     

High-frequency dynamics of ultrasound contrast agents

Ultrasound contrast agents enhance echoes from the microvasculature and enable the visualization of flow in smaller vessels. Here, we optically and acoustically investigate microbubble oscillation and echoes following insonation with a 10 MHz center frequency pulse. A high-speed camera system with a temporal resolution of 10 ns, which provides two-dimensional (2-D) frame images and streak images, is used in optical experiments. Two confocally aligned transducers, transmitting at 10 MHz and receiving at 5 MHz, are used in acoustical experiments in order to detect subharmonic components. Results of a numerical evaluation of the modified Rayleigh-Plesset equation are used to predict the dynamics of a microbubble and are compared to results of in vitro experiments. From the optical observations of a single microbubble, nonlinear oscillation, destruction, and radiation force are observed. The maximum bubble expansion, resulting from insonation with a 20-cycle, 10-MHz linear chirp with a peak negative pressure of 3.5 MPa, has been evaluated. For an initial diameter ranging from 1.5 to 5 microm, a maximum diameter less than 8 microm is produced during insonation. Optical and acoustical experiments provide insight into the mechanisms of destruction, including fragmentation and active diffusion. High-frequency pulse transmission may provide the opportunity to detect contrast echoes resulting from a single pulse, may be robust in the presence of tissue motion, and may provide the opportunity to incorporate high-frequency ultrasound into destruction-replenishment techniques.     

Acoustic signatures of submicron contrast agents

Previous studies have revealed that hard-shelled submicron contrast agents exhibit large relative expansions and strong acoustical echoes that can be observed experimentally, and predicted by theoretical simulations. In this paper, we study harmonic imaging and pulse-pair imaging techniques designed to assist in the differentiation of these contrast agents from tissue. For harmonic imaging, we apply a high-sensitivity, narrowband strategy that differentiates the microbubble from tissue based on the generation of strong harmonic echoes. For pulse-pair imaging, we apply high spatial resolution, wideband strategies using phase inversion, which relies on the frequency differences observed in response to phase-inverted pulses, and signal subtraction, which takes advantage of the amplitude differences in response to identical pulses. The bubble-to-phantom signal amplitude ratio in the absence of motion approaches 20 dB using phase inversion and 30 dB using signal subtraction; both techniques are robust for up to 50 microm of simulated motion. With the experience gained in these studies, we hope to advance the development of multi-pulse or shaped-pulse techniques that are optimized for specific clinical applications.     

Transfer characteristics of multicomponent cooling agents

A method for calculating and predicting the coefficients of thermal conductivity and viscosity of pure substances and two- or multicomponent mixtures has been developed on the basis of the single-parameter law of the corresponding states.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 4, pp. 620–624, October, 1988.     

Raft-forming agents: antireflux formulations

Gastroesophageal reflux disease (GERD) is caused by excessive reflux of gastric content and duodenal bile into the esophagus, and impaired clearance of refluxate from the esophagus. In this perspective, raft-forming antireflux formulations offer better alternatives to the conventional therapies for treatment of uncomplicated GERD. In addition to the alginate-based systems, various natural polysaccharides have generated interest as raft-forming agents because of their bioadhesive/mucoprotective nature. Inclination of current therapy is towards natural products for healing of the disease, which also underlines the market potential of this class, demanding for thorough investigation and development of evaluation methods with better in vitro-in vivo correlation.     

Sequential resource allocation utilizing agents

Many manufacturing systems allocate resources, such as machines, sequentially. Sequential allocation of resources can be viewed as a digraph where each vertex represents resources forming nodes in a distributed system and the arcs represent the allocation. The allocation of such resources can be considered to be a distributed problem. Agents are a distributed artificial intelligence paradigm applicable to distributed problems and, therefore, have a potential to be applicable to sequential resource allocation. This paper presents a method of sequential resource allocation utilizing agents, and an AGV system is presented as an example application area. This system was utilized in experiments to test the agent application. Results and an analysis are also presented.     

State-owned enterprises as knowledge-explorer agents

ABSTRACT

This paper deals with the role of the State Owned enterprises (SOEs) in innovation processes. Only a few studies focus on the contribution SOEs as companies might give to produce new knowledge and technological innovation. We argue, however, that SOEs are a pillar of the innovation process and we explore conditions that can make SOEs very effective. Through two in-depth case studies in two different industries (STMicroelectronics in the semiconductor and Thales Alenia Space in the space industry) we illustrate how SOEs can contribute to innovation by exploring new opportunities and recombining different sources of knowledge. We highlight the conditions that can make exploration and recombination possible. We also highlight the ability of the two companies to change their boundaries through a continuous wave of agreements, mergers and acquisitions. This way, they were able to expand beyond their domains in a way that matched the evolution of their original industries.