dissolution论文范文

Abstract: In the modern construction of concrete plays an important role, but the concrete cracks are more common. This article analyzes the causes of concrete cracks is proposed measures to control and prevent the cracks, and curing of the concrete problems early. Key words: concrete crack; cracks; Cracks; conservation First, micro-cracks in concrete Cracks in reinforced concrete structures is to work with. Rather, the hardening process of concrete in the condensation, there exist micro-cracks, because the concrete in the cement and aggregate changes in temperature and humidity conditions to produce the volume of non-uniform deformation, and they bonded together and can not free-form deformation, so the formation of mutual restraint stress; Once the cement and aggregate constraint between the stress is greater than the bond strength and tensile strength of cement itself, will produce micro-cracks. Second, the causes of concrete cracks Concrete cracks are the result of the development of micro-cracks. Concrete cracks for many reasons, for its part, considered to be bound by the deformation of concrete due to tensile stress than the material's sake. 1. Optional inappropriate material defects and the formation of cracks. Expired cement, aggregate excess mud, with active SiO 2 , high alkali cement, limestone aggregate, cement hydration heat and so on. 2. Construction of mishandling the formation of defects and cracks. Plastic concrete sink, was the top bar of the resistance, the formation of cracks along the reinforcement; concrete vibration is not dense, there cellular, easy to form the starting point for a variety of stress fractures; concrete mixer, transit time is too long, so that water evaporation, causing low slump concrete pouring, making the concrete volume in the mesh irregular cracks; rapid drying of concrete made when the initial curing of concrete contact with the atmosphere in the irregular mesh surface cracks; early form removal, concrete not yet established sufficient strength to impose its own components in the actual gravity load, prone to all kinds of stress cracks. 3. Because of the force components, deformation and crack formation of defects. Center tension; center compression; bending; shear; by punching; beam concrete shrinkage and temperature deformation; plate of concrete shrinkage and temperature deformation; in reinforced concrete, the tensile stress is mainly borne by the reinforced concrete is exposed stress. In plain concrete or reinforced concrete on the edges if the tensile stress within the structure there shall be to rely on concrete to bear. General design requirements in both the tensile stress does not appear or appear only very small tensile stress. However, the maximum temperature of concrete in construction to the operation of the cooling period of steady temperature, often caused by a large concrete internal tensile stress. 4. Because of environmental factors affect the formation of defects and cracks. Mainly temperature and humidity changes, the brittleness of concrete and uneven, as well as unreasonable structure, failure of raw materials (such as alkali-aggregate reaction), template deformation, differential settlement of foundation. Concrete dissolution cycles many times by the freeze, the stress generated in the concrete, and promote the development of existing cracks, loose structure, surface cracks, surface spalling or overall collapse. Third, measures to control and prevent the cracks 1. Of cement, water, aggregate, admixtures, reinforcement materials, the improper selection of the formation of cracks on the entry of raw materials must be in accordance with national standards for strict inspection and acceptance of the approach to prevention, where the unqualified use of defective materials shall be , or reduce the level of test use; of these have occurred due to improper selection of materials defects or cracks in concrete produced must be observed in detail for the long term (due to some problems take a while to find), carefully identify its causes and Quality of the problem, study and formulate their treatment and reinforcement. This is because once occurred due to improper material selection, quality problems, often with the universal reason. 2. As long transport time of concrete mixing, pouring too fast, the vibration is not real, it is improper construction joint, move the template the formation of cracks and other reasons can follow the "concrete order" strict implementation of concrete mixing, transport, pouring, vibration pound set and the old concrete construction joint connection. Templates, and form removal, and conservation requirements to prevent, the occurrence of such cracks in the component have been, but also distinguish the type of component, component of the force characteristics of the site where the cracks and the extent of serious cracks were commonly used in concrete cracks reinforcement measures or by filling concrete, steel anchor reinforcement, and even stick steel reinforcement, prestressed reinforcement remedial measures. 3. Due to dry weather, the initial maintenance is not good, the early cold concrete and large changes in temperature and humidity of the cracks were used to enhance the natural hardening process of concrete Results conservation, conservation of heat storage, the use of air-entraining agent to uniform distribution of air bubbles inside the concrete, measures such as temperature expansion joint repair reserved. Severe cold components, some should be removed, some should be reinforced before being used. 4. Because the components have to withstand loads too wide cracks, reasonably designed to prevent the emergence of these cracks; cracks have been too broad component appears to be reinforced by strengthening measures. 5. The foundation of unequal settlement of large cracks too broad and reasonably in the design of the building when checking in the use phase of the settlement to prevent the emergence of these cracks, these cracks have occurred on the structure, to use ground-based control measures proper handling of the foundation, then building the structure reinforcement measures adopted to solve. 6. On the environmental conditions and changes in the use of the crack occurred, according to different properties to different control measures, such as: (1) the use of temperature and humidity changes during the formation of cracks, usually difficult to eradicate, to adopt the protection of reinforced concrete measures to reduce atmospheric humidity changes of the component is appropriate; (2) The cracks resulting from repeated freezing and thawing, in addition to defects and damage has been formed in part to be reinforced or reinforced, but should add insulation on the cold concrete measures; (3) The corrosive medium in the resulting defects and damage a large area, in addition to corrosion and damage should be removed by the site to be reinforced or reinforced, shall use the acid water glass slag cement concrete or concrete overlay to protect; (4) damage due to earthquake seismic structural measures should be adopted to prevent; have been generated by the earthquake damage is not severe earthquake damaged buildings may refer to repair and reinforcement of the solution to the problem. In addition, temperature control and improved from the constraints of the two aspects. Used to improve the aggregate gradation, with a dry hard concrete, mixed with mixture, add air-entraining agent or a plasticizing agent measures to reduce the amount of cement in concrete; water when mixing concrete or gravel with water cooling to reduce the concrete pouring temperature; hot days when the pouring of concrete pouring to reduce the thickness of heat by pouring level; in the concrete laying pipes, pass into the cold water temperature; set reasonable removal time, the surface heat when temperatures plunged to avoid dramatic concrete surface temperature gradient ; Construction of Concrete Blocks and long-term exposure to surface or thin-walled structure, insulation measures taken in the cold season. Measures to improve the constraints are: a reasonable parting block; to avoid excessive fluctuations basis; reasonable arrangements for the construction process, to avoid the excessive height and long-term exposure to the side. In addition, to improve the performance of concrete and improve the crack resistance, enhance conservation, to prevent surface shrinkage, in particular, to ensure the quality of concrete is very important to prevent fractures, special attention should be avoided through the cracks, appears to restore its structural integrity is difficulties, so the construction should be to prevent the occurrence of cracks in the main cross-cutting. Fourth, early curing of concrete Practice shows that cracks in concrete common, most of the surface cracks of different depth, mainly because of the temperature gradient caused by the sudden drop in temperature in cold areas are also easy to form cracks. Therefore, the concrete surface of the insulation to prevent early cracking is particularly important. From the viewpoint of thermal stress, thermal insulation should meet the following requirements: (1) prevent the concrete and the concrete surface temperature difference between inside and outside the gradient, to prevent surface cracks; (2) to prevent the concrete super cool, should try to try to make concrete the construction period of not less than the minimum temperature on the stability of the temperature of concrete used; (3) to prevent cold and old concrete to reduce the constraints between the new and old concrete. Early curing of concrete, the main purpose is to maintain proper temperature and humidity conditions in order to achieve the effect of two aspects, one of the concrete from adverse temperature and humidity deformation of the invasion, to prevent the harmful shrinkage and shrinkage. On the one hand to smooth the cement hydration in order to meet the design strength and crack resistance. V. Conclusion Cracks in concrete above the relationship between the various effects of the theory and practice discussed, although the academic cracks in the concrete and calculation methods are different theories, but for specific advice for prevention and improvement measures more unified, but in practice of effects are good, concrete construction depends on our seeing much comparison, more analysis after problems, and more sum up, with a variety of preventive treatment measures, concrete cracks are completely avoidable.

译文是;1. 介绍 这个小孢字虫目是一个异常和不同的小组 细胞内的寄生生物。 他们经常讲话如古老 优核质由于‘原始’特点例如prokaryoticlike 70S核糖体和小染色体大小[1， 2]。 方面 of他们多孔的组织也是异常的。 小孢子虫目 的amitochondrial，虽然这也许通过 第二次的损失[3]。 他们有原始Golgi用具 并且核分裂被达到，不用溶解 的核信封[4]。 对数字的序列分析 的microsporidian基因建议他们也许是 对真菌[5]和此的有关系的由特征证实 例如几丁质的孢子墙壁和核内分裂。 They是一个普遍和不同的小组和知道 to传染范围从单细胞生物的主人种类到人。 在主人门的他们的普遍存在的存在予以支持对 这论据他们是古老的并且意味着他们 构成每个主寄生物的研究的优秀小组 发展. microsporidia的成功，这钥匙在 他们的传输战略的多样性和灵活性和 生命周期。 Microsporidia是最小，最简单的真核。 在它的范文内，那里生命周期是二基本形态的 形状、meront或者增生性的阶段和孢子 或者传输阶段[6]。 meront有一简单多孔 的构造. 它的质膜直接与联系 有证据证明，这个主人细胞质和寄生生物 利用主人细胞器作为营养素[4的]来源。 孢子有使能生存的厚实，抗性墙壁 这细胞外环境和复杂挤压 通过注射sporoplasm斡旋入侵的器官直接的到寄主细胞里[6的]细胞质。 大受欢迎的主–主传输为生存是重要的 的强制，细胞内寄生生物。 在之内 那里小孢子虫目是不同的战略的范围为 在主人人口的寄生虫的维护。 这些包括 管理和并且使用的间接生命周期水平 和垂直传播。 小孢子虫目传染宽 主人范围包括的是许多节肢动物种类 characterised在季节性人口崩溃之前。 许多特征 的小孢子虫目的许可证寄生生物生存，当主人连续地不是存在栖所。

林肯是美国著名的总统之一，但是我们对它评价要怎么写呢?下面是我为大家带来林肯的评价英文版 范文 ，相信对你会有帮助的。

对林肯的评价英文版篇一

The president of USA is not only head of the state and the government, but also administrative departments of the supreme leader and army commander, usually called . There are 44 presidents in the history of the United States. Abraham Lincoln is the 16th one who maintained the unity of the nation and abolished the slavery that influenced the nation much. He is the first Republican president and also ranked among the greatest president ranked first. USA once held a poll in 2005--"the greatest Americans", Lincoln was elected America figure of second. He is one of the most popular presidents.

On February 12, 1809, Abraham Lincoln was born in a poor family in Hardin County in Kentucky prefecture. His parents are descendants of British immigrants, taking farming and he was young, he helped the family move firewood, water, do farm work and so 1816, Lincoln's family moved to Indiana Southwest. At his age of 9, his mother died who was just 36 years old. Second years, his father married a kindness and enlightened woman named Sarah Bush, and the family lived happily. Because of poverty, his education degree wasn't high. In order to earn money, he was a ferry worker on the Ohio River and a plantation worker when he was young.

Lincoln is a man who loves reading. At his young age, he read all the writings of Shakespeare and "American history", also read lots of books about history and became a learned and wisdom man through self-study. He ran for state senator that meant he embarked on a political road since 1832. Finally he was elected president in 1860. In the 28 years, he has undergone a lot of difficulties that normal people cannot imagine. Eight elections, eight voted out, twice failed in business, and even once mental collapse, many times, he could give up, but he didn't. Just because of not giving up, he became one of the greatest presidents in the history of teacher of the revolution Marx highly evaluated Lincoln, "he is a man who will not be scared by difficulties, not to confuse for success, he refuse to be cowed or submit to his great goal, but never act rashly and blindly, he steadily forward, and never back;...... In a word, he is a rare character that achieved the great realm and still maintained his excellent quality". Owing to his view of abolishing slavery, Lincoln's election, threatened the benefits of the Southern plantation owners. They certainly don't want a man who advocates the abolition of slavery as their president. Then the Civil War broke out in 1861.

At the beginning of the war, Lincoln tried to seek the policy that compromised with the Southern plantation owners. With the military losing ground, Lincoln realized that to abolish slavery truly, there must have sacrifice, peaceful means can not solve any problems. If they want to win the war, they must transfer the enthusiasm of farmers, abolish the slavery, and liberate black slaves .In 1862 May, Lincoln signed the Homestead Act. This measure eliminated the possibility of southern slave owners to seize the western land fundamentally, but also met the urgent needs of the peasants, greatly stimulated the enthusiasm of farmers to participate in. On January 1, 1863,

"Emancipation Proclamation" was promulgated formally, announced the abolition of slavery in the rebel states today, the liberated slaves can call in the union army. Announced the slaves free, fundamentally undermined the rebels' fighting force, meanwhile, the Yankees had abundant 1864, Lincoln put forward slogan that "the people, by the people". It enhanced troop's morale. In April, 1865, the Yankees achieved victory in the Civil War, meaning the war that lasted four years ended. The Civil War is called the second revolution after the War of Independence. Lincoln became a symbol of black liberation.

Because Lincoln can adapt to the demand of the masses, destroyed slavery in a revolutionary way, and solved the people's demand for land in the Civil War of the most critical juncture, thus he promoted the development of America's capitalism, and made important contributions to safeguard national unity and liberate black slaves. Due to the excellent achievements of Abraham Lincoln, he was elected as America's president again on November 8, 1864. However, Lincoln haven't put his post-war policies into effect, tragedy happened. Because slave owners hatred Lincoln extremely. On April 14, 1865 night at ten fifteen, fifth days after the surrender of the Southern army. Lincoln was attacked by an assassin in Ford theatre in Washington. Lincoln's unfortunate death caused a huge shock at home and abroad. American people conveyed profound condolences to him, about 7,000,000 people stood mourning funeral procession on both sides of the road, and there are 1,500,000 people looked at Lincoln's face with reverence.

Lincoln was buried in his hometown -- Illinois Springfield. His wife was later buried in there, too. So far, motor vehicle license plate of Illinois's self-proclaimed "State of Lincoln". In 1867, to commemorate Lincoln, one of the greatest presidents in American's history, the county town of Lancaster County, renamed the Lincoln, and became the state's capital.

Although not to receive advanced education, Lincoln has excellent eloquence and grace, the direct embodiment is the famous Gettysburg address. The last sentence became the definition of modern democratic government: to enable us to get free living under God in this country, the government of the people, by the people and for the people, shall not perish from the earth. There are four presidential giant statues in the Rushmore National Park, Lincoln's is in the right. Lincoln Memorial in the capital of America-- Washington ,is located on the National Mall, is one of the most famous monuments of the national.

Lincoln is an outstanding statesman, also a distinguished lawyer. In order to promote the development of American social, Abraham Lincoln has made tremendous contributions. He gained American's respect, his popularity even more than Washington in American's mind. Remembered for his integrity, kindness and strong personality, he has been one of the American history's most admired president. Historians regard Washington as the father, while Lincoln as the National salvation. In American, the third Monday of February each year is the "President's Day", to commemorate the two great leaders. In brief, Abraham Lincoln is the influential president in the American history.

对林肯的评价英文版篇二

1 Lincoln Full Review

① Lincoln is a famous American bourgeois revolutionaries, politicians, the United States the famous leader of the Civil War. In 1861, Republican Lincoln became the 16th president of the United States, in April the same year, the outbreak of the Civil War. In the early years of the war, Lincoln led the northern army to brave defeat, the people asked the government to take more forceful measures. In 1862, the Lincoln government promulgated the "Homestead Act", "the liberation of the Black Slave Declaration", abolished the slavery in the South, the Northern Army was a solid source of manpower.

⑤ In 1865, Lincoln led the Northern Civil War to the North victory, the United States eventually be maintained, Lincoln was assassinated by southern fanatics.

⑥ Lincoln government through the Civil War, the abolition of the slavery, clear the obstacles to the development of capitalism, for the future economic development to create conditions for music.

⑦ Lincoln to maintain national unity and the liberation of slavery made a significant contribution to the history of the United States as a famous president.

A Brief Comment on Lincoln

① Lincoln is a famous American bourgeois revolutionaries, politicians, the United States the famous leader of the Civil War. ② Lincoln led the American Civil War victory, promulgated the "Homestead Act", "the liberation of the Black Slave Declaration", the abolition of slavery, to eliminate the obstacles to the development of American capitalism, safeguarding national unity, the United States as the world power laid the foundation .

③ Lincoln to maintain national unity and the liberation of black slavery made a significant contribution by the American people's reverence and become a famous president in American history.

3, learning insights

① study his wholeheartedly love the people, the spirit of the people-oriented quality.

② learn his courage to struggle, to the national, national interests as their responsibility and quality of mind.

③ learn his perseverance, and always give up the pursuit and struggle, and ultimately go beyond the quality of self. ④ We should fully study the great quality of the great man of history, and strive to learn scientific and cultural knowledge.

对林肯的评价英文版篇三

Marx and Lincoln were contemporaries, his evaluation of Lincoln is: "This is a will not be intimidated by the difficulties will not be confused by the success of people; his unyielding towards their great goals, and never act rashly Steady, and never retrogression ... ... In short, this is a reached a great realm and still maintain their excellent quality of the rare character of the outstanding moral person was so modest that even he became martyrdom After the fall, the world only to find that he is a hero.

Abraham Lincoln (February 12, 1809 - April 15, 1809) the 16th president of the United States

Political party: Republican Party

February 12, 1809, Lincoln was born in Kentucky Harding County, a poor peasant family, the parents of British immigrants descendants. In 1816, the Lincoln family moved to the southwest Indiana, land reclamation for a years old, Lincoln's mother died. A year later, the father married a kind and hard-working, treat their ex-wife's children as also loves the stepmother, a family living in harmony and happiness. Because of family poverty, Lincoln is not a high degree of education. In order to maintain the family plan, when young Lincoln worked as a ferry on the Ohio River, plantation workers, shopkeepers and carpenters. At the age of 18, tall Lincoln was hired by a shipowner, with a flat barge with the people down the Ohio River, sailing thousands of miles to Orleans. This is the first time he came to a city of 40,000 people.

The journey, Lincoln saw the black slaves suffered inhuman treatment. In 1830, the Lincoln family moved again and came to Coles County, Illinois, in the middle of the United States, "he said to his partner," until I have a chance to crack down on slavery. " Here, Lincoln began to live independently, and in 1832 enlisted. Lincoln's time is very short, after the demobilization, the local residents elected enthusiastic official activities of Lincoln for the state legislature candidate, but his first campaign did not succeed. Before the age of 25, Lincoln has no fixed occupation, four to make a living.

Adult, he became a local land surveyors, proficient in measurement and calculation, often people are invited to solve the boundary dispute. In the hard work, Lincoln is always a love of reading the youth, his night reading lights always flash to late late. In his youth, Lincoln read all of Shakespeare's books, read American History, and read many books on history and self-study he made himself a learned and intelligent person. In 1834 August, 25-year-old Lincoln was elected to the state legislators began his political career at the same time management of rural postal services, also engaged in land surveying, and with the help of friends to study the law. A few years later, he became a lawyer. In 1839, Lincoln met a member of the daughter of Mary Todd. She was 21 years old, well-educated, beautiful and lively, they married in November 1842, after the birth of four sons.

Accumulated the experience of state legislators, in 1847, Lincoln as the Whig party, participated in the congressional election, won the first time, came to the capital of Washington. Before and after, the debate about slavery, has become a major event in American political this debate, Lincoln gradually became anti-slavery. He believes that slavery should eventually be destroyed, first of all in the capital Washington should abolish slavery. Slavery, who represents the interests of the southern plantation owners, frenziedly opposed Lincoln. In 1850, the United States of slave power surge, Lincoln out of Congress, to continue as a lawyer. In 1854, the southern slave owners to enter the new Kansas into the United States, the use of force to force the implementation of slavery, caused a civil war in Kansas.

This year, the Republican Party was founded, Lincoln joined, and in 1856 participated in the Republican vice presidential candidate campaign, without success. June 16, 1858, in the same Douglas campaign published a "family dispute" the famous speech. "The House of Splits can not last," he said. I believe that our government can not endure half the freedom of half of slavery. I do not expect federal dissolution, I do not expect the house to collapse, but I do expect it to stop splitting. It will either turn into one thing or all else into another. During this period, he and Douglas conducted a debate on the issue of slavery, Lincoln believes that slavery should be abolished, but must be abolished by peaceful means, although his campaign did not succeed, but greatly expanded the Political influence .In 1860, Lincoln became the Republican presidential candidate, in November, the election announced to 2 million votes was elected the 16th president of the United States, but in the slave-controlled southern 10 states, he did not get a vote.

Lincoln was elected president of the news, the rapid expansion of North-South contradictions, adhere to the slavery of the South to Lincoln's election as a disaster, Lincoln in South Carolina before the oath to quit the federal .In order to safeguard national unity, war triggered. As the US general election system enjoys popular support, with the election of the new president, the smooth transition of state power into the United States tradition.

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多孔金属材料的制备工艺及性能分析多领域有着广泛的应用前景。本文概述了多孔金属材料的常用制备方法及其主要性能。关键词:多孔金属材料;制备;性能;应用摘 要 :多孔金属材料是一种性能优异的新型功能材料和结构材料 ,具有独特的结构和性能 ,在很科学家极大的兴趣 ,成为材料类研究的热点方向之1 引言一 ,自 20世纪 90年代以来 ,美国的哈佛大学、英国在传统的金属材料中 ,孔洞 (宏观的或微观的 )的剑桥大学、德国的 Fraunhofer材料研究所、日本的被认为是一种缺陷 ,因为它们往往是裂纹形成和扩东京大学等对多孔金属材料的制备工艺和性能进行展的中心 ,对材料的理化性能及力学性能产生不利了广泛的研究 ,获得了一批研究成果 [2-5]。在我国 ,的影响。但是 ,当材料中的孔洞数量增加到一定程多孔金属材料的基础和应用研究也逐步得到重视和度时 ,材料就会因孔洞的存在而产生一些奇异的功发展。近年来 ,研究队伍不断壮大 ,在制备技术、结能 ,从而形成一类新的材料 ,这就是多孔金属材料。构和物性等方面的基础研究以及在各种民用和国防按照孔之间是否连通 ,可以把多孔金属材料分为闭领域的应用研究均取得了一定的进展 ,已经引起我孔和通孔两类 ,如图 1所示。该类材料具有良好的国政府、中科院和航空航天等部门的高度重视 ,尤其吸能性能、高阻尼性能、吸声性能、电磁屏蔽性能及值得一提的是 ,我国在 2005年立项的国家重大基础良好的导热导电性能 [1] ,因而在一般工业领域 (如研究计划 (973计划 )“超轻多孔材料和结构创新构汽车工业 )、国防科技领域及环境保护领域等有着型的多功能化基础研究 ” ,更是体现了对该类材料广泛的应用前景 ,它的设计、开发和应用引起了中外研究的重要性和迫切性。水化物等,然后将均混的混合物压制成密实块体即到目前为止 ,已开发的制备多孔金属的方法很多 ,涉及到的领域也非常广。根据在制备过程中金属所处的状态 ,可将多孔金属的制备工艺分为以下三类 :液相法、粉末烧结法和沉积法。 2. 1 液相法液相法包括的种类比较多 ,且较易制备大块的多孔金属和产品易商业化 ,成为多孔金属材料制备的主要手段,液相法主要包括以下几种: 2. 1. 1 颗粒渗流法颗粒渗流法[ 6 ]原理是首先将颗粒在模具内压实,烘干形成预制块。然后通过压力将金属液渗入中,并强烈搅拌使空心小球分散,最后得到空心球与金属基体形成的多孔金属材料。空心球铸造法的特点是孔径和孔隙率易于控制,材料综合力学性能好。2. 2 粉末冶金法粉末冶金法主要包括粉末烧结发泡法、烧结-脱溶法、松散粉末烧结法、中空球烧结法等。2. 2. 1 粉末烧结发泡法这种工艺[ 12 ]是首先将金属粉末和相应的发泡剂按一定比例均匀混合,发泡剂可以是金属氢化物、半成品,最后将此半成品加热到接近或高于混合物熔点的温度,使发泡剂分解,金属熔化,从而形成多孔泡沫材料。此种方法易于制作近半成品的零件和到颗粒预制块的间隙中,最后将颗粒溶除即可得到通孔结构的多孔金属材料。2. 1. 2 精密铸造法精密铸造法 [8]是首先用耐火材料浆料填满海绵状泡沫塑料的孔隙 ,待耐火材料固化后 ,加热除去塑料 ,即形成一个多孔预制块体。然后把液态金属液浇入到预制块上 ,加压渗流 ,这一点类似于渗流过程。最后再除去耐火材料 ,就形成与原来海绵状塑料结构相同的多孔金属材料。 2. 1. 3 熔融金属发泡法熔融金属发泡工艺可分为两种 ,发泡剂发泡和通气发泡 [9, 10 ]。前者是在熔融的金属液中加入发泡剂 (如 TiH2 ) ;后者则是在金属液中通入气体 (如惰性气体 )。这两种工艺的共同特点是可制备孔隙率高、尺寸大、闭孔结构的多孔金属 ,但过程控制较为复杂 ,孔结构分布均匀性不高。 2. 1. 4 空心球铸造法空心球铸造法 [11 ]的原理是先采用商用酚醛塑料小球在惰性气体环境中加热直至塑料碳化 ,形成中空的小球。然后将这些中空的小球加入到金属液三明治式的复合材料 ,而且孔隙率较高 ,孔分布均匀。 2. 2. 2 烧结 -脱溶法这种制备工艺 [13 ]首先是将金属粉末和可去除填充颗粒均匀混合 ,其中可去除填充颗粒一般包括两类 ,一类为可溶于水或其它溶剂的盐 (如 NaCl等 ),一类为可分解有机物 (如尿素、碳酸氢氨等 ),均混后把混合物压制成致密的半成品 ,然后在一合适的温度烧结。若填充颗粒为可分解有机物 ,则烧结过程中颗粒会分解气化 ;若填充颗粒为可溶性盐 ,则在烧结后可用溶剂将其溶去便得到多孔金属材料。2. 2. 3 松散粉末烧结法松散粉末烧结 [14 ]是把松散状态的金属粉末不经压实直接进行烧结的方法。此种方法可用于生产多孔金属电极。 2. 2. 4 中空球烧结法通过将金属中空球烧结 ,使之扩散结合而制造多孔材料的方法。此方法制造的多孔材料兼有通孔和闭孔。金属中空球可通过下述方法制备 :在球形树脂上化学沉积或电沉积一层金属 ,然后将树脂除 明显的三阶段特征 ,即初始的弹性段 (Linear Elasticity)、中间的平台段 ( Plateau)和最后的致密段 (Densification)。其中 ,平台段的起始点应力称为泡沫材料的屈服或坍塌强度 ,此强度远小于其基体的屈服强度 [1]。当多孔金属材料受到外加载荷时 ,因屈服强度低很容易发生变形 ,而且变形量大、流动应力低 ,在变形过程中通过孔的变形、坍塌、破裂、胞壁摩擦等形式消耗大量能量而不使应力升的。高 ,从而能有效地吸收冲击能。这种在较低应力水形成金属烟。金属烟在自身重力作用及惰性气流的平下吸收大量冲击能的特征正是冲击缓冲所需要携带下沉积和冷却。因其温度低 ,原子难以迁移和扩散 ,故金属烟微粒只是疏散地堆砌起来 ,形成多孔3. 2 高阻尼性能泡沫结构 [16 ]。 多孔金属材料可看作是由三维网络状金属骨架去 ,或将树脂球和金属粉一同混合 ,随后烧结使金属粉结合 ,同时树脂球挥发 [ 15 ]。 2. 3 沉积法沉积法主要包括金属气相蒸发沉积法、原子溅射沉积法和电化学沉积法三种。 2. 3. 1 金属气相蒸发沉积法在较高惰性气氛中 ,缓慢蒸发金属材料 ,蒸发出来的金属原子在前进过程中与惰性气体发生一系列碰撞作用 ,使之迅速失去动能 ,从而部分凝聚起来 ,与高压惰性气体原子碰撞 2. 3. 2 原子溅射沉积法在惰性气体的压力下,元素原子在飞溅路程中,金属原子一方面捕获气体原子 ,另一方面凝聚成金属液滴 ,然后到达衬底。在衬底上获得均匀包裹气体原子的金属体 ,最后在高于金属熔点的温度下把金属加热足够长的时间使捕获的气体膨胀 ,形成多孔金属材料。这种方法的特点是孔结构非常理想 ,但成本昂贵 ,不易制备大件 [ 17 ]。 2. 3. 3 电化学沉积法这种方法是以聚氨基甲酸乙脂发泡材料为骨架 ,进行电解沉积 ,然后加热去除有机聚合物骨架 ,得到多孔金属材料。这种方法制备的多孔材料不但孔隙率高 ,孔分布均匀 ,且孔互相连通呈三维网状结构 [ 18 ]。 3 多孔金属材料的主要性能多孔金属材料作为一类区别于致密材料的新型材料 ,具有一些其基体或母体所不具备的特殊性能和功能 ,主要表现如下 : 3. 1 吸能性能图 4 多孔金属材料典型的压缩应力 -应变曲线多孔金属材料的应力 -应变 (σ -ε)响应具有与孔洞所组成的两相复合材料。除了孔洞与金属基体之间所形成的界面外 ,材料内部还存在其它大量微观的 (主要是位错 )和宏观的 (较小的孔洞和裂纹 )缺陷 ,其组织状态和缺陷分布极不均匀。因此当外力作用于多孔金属材料上时 ,将在基体中产生不均匀的应变 ,特别是在孔洞 (宏观的或微观的 )或裂纹附近 ,其应变情况更为复杂 ,从而引起缺陷区域原子重排。缺陷区的这种响应是粘滞性的 ,因而引起粘滞性应变 ,造成能量的损耗 ,导致材料的阻尼增加。 3. 3 吸声性能多孔金属材料的高孔隙率结构使其具有良好的吸声性能 [19 ]。一般来讲 ,通孔或半通孔多孔金属的吸声效果比闭孔的好。多孔金属材料的吸声机制主要可归为两种 ,即声波经过多孔金属时流动阻力的升高造成的粘性损失以及声波与孔洞表面热量交换造成的热损失。 3. 4 电磁屏蔽、导热和导电性能多孔金属具有良好的导电性和很高的比表面积 ,因此具备很高的电磁屏蔽性能 ,即良好的吸收和反射电磁波的能力。同时又具有良好的导热性能 [ 20, 21 ]。 3. 5 其它性能质轻 ,易着色 ,易加工 ,耐高温。 4 结语 (1)多孔金属材料具有良好的理化性能和力学性能 ,因而可以作为功能材料和结构材料 ,具有良好的应用前景。多孔金属材料的制备工艺很多 ,因而可以满足多样化的需求 ,可以根据不同的应用需求 采用不同的制备工艺。 and energy absorbing characteristic of foamed aluminum. 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Manufacture, characterization and application 其吸声性能 [J]1铸造 ,1998 (4):1 -4. ofcellularmetalsandmetalfoams[J]. ProgressinMateri2 [20 ]黄福祥 ,金吉琰 ,范嗣元等 .发泡金属的电磁屏蔽性能als Science, 2001 (46) : 559 -632. 研究 [J]1功能材料 , 1996 (27) : 52 -54. [8]F. Frei, V. Gergely, A. Mortensen, . Clyne. The [21]J. Kovacik, F. FoamModulusofE2 effectofpriordeformationon thefoamingbehaviorof“form2 lasticity and Electrical Conductivity According To Percola2 grip”precursormaterial[J ] 1Adv. Eng. Mater, 2002 (4): tionTheory[J]. Scr. Mater, 1998 (39):239-246. 749 -752. [责任编辑 朱联营 ] [9]F. S. Han, Z. G. Zhu , J. C. Gao. Compressive deformation On the Preparation and Properties of the PorousMetallicMaterials HAO Gang -ling1 , HAN Fu -sheng2 , LIWei-dong1, BAIShao-min1,YANGNeng-xun 1 (1. College of Physics and Electronic Information, Yanan University, Yanan, Shaanxi 716000 2. KeyLaboratoryofMaterialsPhysics, InstituteofSolidStatePhysics, Chinese Academy of Sciences, Hefei, Anhui 230031) Abstract: Porousmetallicmaterialswithuniqueexcellentstructuresandpropertiescanbeutilizedasnew function2 aland structuralmaterials, which indicatsthattheporousmetallicmaterialshaveawidelypromisingapplication in manyfields. Thevariouspopularmanufacturingmethodsandthemainpropertiesoftheporousmetallicmaterials, in the present paper, were summarized. Key words: porousmetallic materials; preparation; properties; ppplication