Materials 2011, 4, 816-824; doi:10.3390/ma4040816
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`OPEN ACCESS
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`
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`materials
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`ISSN 1996-1944
`www.mdpi.com/journal/materials
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`Review
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`Progress of Application Researches of Porous Fiber Metals
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`Zhengping Xi 1, Jilei Zhu 1,2,*, Huiping Tang 1, Qingbo Ao 1, Hao Zhi 1, Jianyong Wang 1 and
`Cheng Li 1
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`1 State Key Laboratory of Porous Metal Materials, Northwest Institute for Non-ferrous Metal
`Research, Xi’an 710016, China; E-Mails: xizp@c-nin.com (Z.X.); hptang@c-nin.com (H.T.);
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`aqbpshi@yahoo.cn (Q.A.); zhihao1983@163.com (H.Z.); wangjy73@163.com (J.W.);
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`etoleecheng@yahoo.com.cn (C.L.)
`2 State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an
`710049, China
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`* Author to whom correspondence should be addressed; E-Mail: zhu_jl@126.com;
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`Tel.: +86-29-8623-1095; Fax: +86-29-8626-4926.
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`Received: 1 March 2011; in revised form: 1 April 2011 / Accepted: 12 April 2011 /
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`Published: 19 April 2011
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`Abstract: Metal fiber porous materials with intrinsic properties of metal and functional
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`properties of porous materials have received a great deal of attention in the fundamental
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`research and industry applications. With developments of the preparation technologies and
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`industrial requirements, porous fiber metals with excellent properties are developed and
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`applied in many industry areas, e.g., sound absorption, heat transfer, energy absorption and
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`lightweight structures. The applied research progress of the metal fiber porous materials in
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`such application areas based on the recent work in our group was reviewed in this paper.
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`Keywords: metal
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`fiber; porous material;
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`sound
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`absorption; heat
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`transfer;
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`energy absorption
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`1. Introduction
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`Metal fiber porous materials has received much attention due to their unique controllable porous
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`structure and special physical properties, such as lower density, larger specific surface area, higher
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`mechanical strength and excellent permeability, and show good potential for applications in the
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`industries, since they combine the properties of metal and its internal porous structure. With the
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`Materials 2011, 4
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`increasing requirements of industry, metal fiber has shown its superiority in a very wide application
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`area, e.g., textile, filtering, sound absorption, heat transfer, battery electrodes and fiber reinforced
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`composites. In the metal fiber family, many types have been developed, e.g., stainless steel fiber,
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`nickel fiber, aluminum and its alloy fiber, and iron inter metallic fiber, etc. The metal fibers can be
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`prepared by four methods: high-temperature spraying molten metal (melt spinning method), cutting
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`method, drawing method, and the chemical method. Normally, the diameter of the fiber obtained by
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`these four techniques is in the range of 1~100 μm.
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`As a new generation of the metal porous material, metal fiber porous material contains a large
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`number of irregular pores constructed by the fiber and pore geometries. Metal fibers cross each other
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`and are packed together like a bird’s nest. In the traditional application areas, such as filtration and
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`separation, metal fiber porous material becomes the most popular product because of its good
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`properties, e.g., permeability, high strength, corrosion resistance, high temperature resistance, foldable,
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`renewable, and long service life, etc. The pore structures can be controlled for high filter accuracy and
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`greater dirt holding capacity than mesh and sintered powder filter (see Table 1) [1].
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`Table 1. Performance comparison between the sintered metal fiber felt and other
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`filter materials.
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`Compared with mesh
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`Dirt-holding Capacity
`Air Permeability
`Filtration efficiency
`Porosity
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`>3~4 times
`>2~3 times
`>3~15 times
`>4~20 times
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`Compared with sintered powder
`filter
`>1.5~5 times
`>21~600 times
`>2~5 times
`>2~10 times
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`Due to its high efficiency in applications, metal fiber porous material can be applied under special
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`conditions with high temperature, high pressure and corrosive environment. Metal fiber porous
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`materials used for filtration are usually prepared to be about a 1–2 mm thickness felt made of stainless
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`steel fibers and FeCrAl fibers. They are widely used in polymers filtration, food and beverage
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`filtration, hot gas filtration, automobile airbags, and so on. For instance, gradient filter materials made
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`from metal fiber possess a higher filtering accuracy than other filter materials used in juice filtration;
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`used as filter parts for vehicle safety air bags, it can control the gas expansion velocity after impact,
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`filter the particles in the gas at high temperature and cool down the hot gas to protect the body in a
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`form of airbag [2].
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`The porosity of metal fiber porous material can be as high as 98% while the pore size is smaller
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`than 10 µm, and the three-dimensional pore space is constructed based on the inter-connections. Fiber
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`porous metals with high porosity and small pore size are features of interest due to their typical
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`integration structure and the functional material. This kind of metal porous material attracts engineers’
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`attention and stimulates researches of materials science. The special and designable properties of the
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`pore structure for the metal fiber porous materials provide a wide application in many industrial areas.
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`The research progress regarding the porous metal fiber material in the application of sound absorption,
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`enhanced phase change heat transfer, energy absorption is reviewed in this paper based on research
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`work of authors’ group.
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`2. Progress of Application Researches of Sound Absorption
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`Compared with mineral wool, wood fiber board and polymer foam, the metal fiber porous material
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`shows a great potential in the properties of sound absorption, high strength, high temperature
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`resistance, anticorrosion, air impact resistance, weather resistance and designable structure [3]. The
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`metal fiber porous material is becoming irreplaceable for noise-control in harsh environments. For
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`instance, fiber porous sound-absorbed materials, prepared by 1Cr18Ni9 stainless steel fibers of
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`diameter 8~100 μm, were applied by Boeing as silencer in intake, exhaust port auxiliary unit of
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`Aero-engine and sound absorption liner in the engine. Absorption coefficient of this kind of fiber
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`porous material is close to the ultra-fine glass wool with 750 Hz, higher than the ultra-fine glass wool
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`above 750 Hz. Aluminum fiber absorbing materials have been used in noise control of concert halls,
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`exhibition halls, classrooms, and highways, subways, tunnels and the other humid underground areas.
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`Recently, metal fiber materials have been applied as silencers in cars, such as Audi and Santana [4].
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`Generally, the sound absorption properties of porous material are good in high-frequency but poor
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`in low frequency based on the sound adsorbing principle. By designing and optimizing the pore
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`structure, the sound absorbing performance of the fiber porous material is significantly improved in
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`low frequency and the bandwidth of the sound adsorption. Wang X.L. reported on a semi-empirical
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`and nonlinear flow resistance model for metal fiber materials [5,6]. This model combined with the
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`acoustic model of Umnova-Attenborough can predict the acoustic performances of the metal fiber
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`porous materials. Zhang Bo proposed a relatively simple extended model of sound absorption
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`properties for the metal porous materials [7].
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`Tang H.P characterized the sound absorption properties of the metal fiber porous materials with
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`different structures to optimize pore structure, and investigate the effects of the porosity, thickness of
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`material and airspace on sound absorption performances [8-10]. The metal fiber porous materials
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`prepared show excellent sound performances. The acoustic absorption coefficient of FeCrAl fiber
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`porous materials of 91% porosity and 10 mm thickness is more than 95% in the frequency range from
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`4,500 to 6,400 Hz (shown in Figure 1). It was found that the porosity and the diameter play a
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`significant role in sound absorption. The sound absorption shows a good performance in the medium
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`and low frequency range with a smaller porosity, greater thickness and larger diameter, but it performs
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`poorly in the high frequency region. Nevertheless, the FeCrAl fiber porous material maintains a stable
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`absorption property in a wide range of sound pressures varying from 100 dB to 140 dB, as shown in
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`Figure 1(b). Effect of high intensity sound on sound adsorption performance of gradient structure was
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`similar to ordinary sound pressure from 20 dB to high sound intensity conditions of 100 dB. Other
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`structures do not have this characteristic, such as perforation plates.
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`The metal fiber porous material with a gradient pore structure was prepared to improve sound
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`absorption properties. As shown in Figure 2, the optimized gradient fiber porous materials have
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`excellent sound absorption properties, even at high temperature and in high intensity conditions. The
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`sound absorption performance at low frequency has been significantly improved while the sound
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`absorption coefficient at high frequency remains at a high position (Absorption-frequency curve is
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`flat). The absorption properties changes as the porosity varies from the highest to the lowest in
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`gradient porous structures. Therefore, it can be concluded that the gradient structure has an important
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`effect on the absorption properties. It was found that the gradient porous structure with a higher
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`porosity and greater thickness presents better sound absorption.
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`Figure 1. Sound absorption properties of FeCrAl fiber porous material: (a) the influence of
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`porosity; (b) stable properties at different SPL.
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`(a)
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`(b)
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`Figure 2. Sound absorption properties of gradient under different conditions: (a) normal
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`conditions; (b) at high temperature; (c) at high intensity pressure.
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`(a)
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`(b)
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`(c)
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`3. Progress of Application Researches of Phase Change Heat Transfer
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`Phase change heat transfer as a kind of advanced heat transfer has been used in many fields. Metal
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`fiber porous material is a perfect working media where phase change heat transfer occurs [11], which
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`plays a significant role in numerous industrial applications because of high boiling heat transfer, low
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`boiling temperature difference, good blockage resistance, long life, etc.
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`Pore structure is the key factor to the heat transfer properties of fiber porous material. In the early
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`1980s, AG Kostornov and LG Galstyanfrom made systematic studies on the influence factors of heat
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`transfer coefficient of metal fibers [12], and indicated that the thermal conductivity of the material
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`drops with an increase in fiber diameter, a significant decrease in thermal conductivity is promoted by
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`another factor having a detrimental effect on the quality of the contacts between the fibers, a drop in
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`sintering temperature. L. Tadrist analyzed in detail the effects of the fluid flow rate of liquid in porous
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`surface and aspect ratio of metal fiber on heat transfer performance under forced convection without
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`phase change [13]. The studies from South China University of Technology show that porosity is one
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`of most important structural parameters influencing heat transfer performance, especially in the low
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`filling rate [14].
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`Authors of this paper prepared metal fiber porous surface materials on the surface of tube and plate
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`by sintering method [15,16]. The metal fiber porous surface materials with interpenetrating pore
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`structures composited of randomly stacked fibers have large numbers of form metallurgical bonding
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`from fiber to fiber and fibers to substrate (as shown in Figure 3). Such a pore structure provides
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`numerous bubble nucleations, and ensures rapid heat transfer from substrate to porous surface, which
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`contributes to a good heat transfer performance. The pool boiling heat transfer tests indicate that the
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`porosity, thickness and fiber diameter of materials have great effects on pool boiling heat transfer
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`performance [17,18]. In order to obtain optimum heat transfer enhancement effect, it is necessary to
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`chose suitable fiber diameter, thickness and porosity. The heat transfer coefficient of porous copper
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`fibers with optimized pore structure is about six times higher than that of smooth surface as shown in
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`Figure 4 [9].
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`Figure 3. SEM photos of sintered porous materials with stainless steel fibers (a); and
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`copper fibers (b).
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`a
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`b
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`Figure 4. The boiling curves of metal fiber porous surface.
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`4. Progress of Application Researches of Energy Absorption and Ultra-Light Structure
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`With developments in aerospace, high-speed rail trains and the car industry, the sandwich structure
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`with ultra-light material has gradually become a research focus. Sandwich structure combines many
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`properties: ultra-light weight, high specific strength and specific stiffness, high energy absorption,
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`sound absorption, and thermal insulation, which make it a perfect application in high energy
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`consumption equipment (automotive, high-speed train, aerospace, ships, etc.) [19]. The core materials
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`of the sandwich structure are mainly fiber material, grid material, aluminum honeycomb and
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`aluminum foam. Theoretical and experimental studies show that porous metals have outstanding
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`energy absorption properties. Compared with aluminum foams, metal fiber porous materials have
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`excellent mechanical properties, such as higher compressive strength and excellent energy absorption
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`capacity [20]. At present, there is an increasing interest being shown in the energy absorption capacity
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`of porous metals.
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`Volvo Car Corporation is the first institute to research and apply metal fiber porous sandwich
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`structure, which developed the ultra-light stainless steel sheet used sandwich structure (HSSA, Hybrid
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`Stainless Steel Assembly) [21]. The HSSA structure is composed of stainless steel fibers (fiber length
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`is 1 mm, diameter less than 20 μm) bonded with thin stainless steel faceplates, the HSSA is lighter and
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`has higher rigidity than aluminum, and has characteristics of sound insulation and shock absorption.
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`The weight of the car manufactured with HSSA is about 50%–70% lighter than traditional cars.
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`Further research indicated that energy absorption capacity of HSSA is 50%–60% higher than solid
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`metal plates [22-24]. Researches performed by Qiao J.C. show that metal fiber porous material has a
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`strong energy absorption capacity [25]. The energy absorption capacity of the sintered metal fiber
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`porous material is about 10 times that of aluminum foam at the same porosity. Pore structures of metal
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`fiber porous material is the key factor that affects its mechanics and energy absorption properties. The
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`mechanical properties of metal fiber porous material are dependent on the combination between the
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`fibers and the number of the metallurgy node. The higher the sintering bonding per unit volume and
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`the bonding intensity are, the better the mechanical properties of materials.
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`In the preparation of metal fiber sandwich material research, Massachusetts Institute of Technology
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`and Cambridge have developed two kinds of sandwich preparation methods: CAMBOSS (Cambridge
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`Bonded Steel Sheets) and CAMBRASS (Cambridge Brazed Steel Sheets). The difference between
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`CAMBOSS and CAMBRASS is the combination method of plates and metal fiber material, the former
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`uses bonding method and the latter uses brazing method [26]. Fraunhofer-Gesellschaft in Germany
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`prepared the aluminum fibers sandwiches by low temperature transient liquid phase sintering [27].
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`Cheng Li and Wang J.Y proposed and implemented porous metal fiber sandwich materials with the
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`erect fiber core body by sintering process [28]. According to dynamic mechanical analysis, this type of
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`fiber sandwich material is a kind of sensitive material, and its pore structure and mechanical properties
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`of such erect fiber core body is anisotropic. As shown in Figure 5, the compression properties and
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`energy absorption efficiency are anisotropy. There are more apparent yield wave peaks, yield wave
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`valley and longer plastic deformation platform in the longitudinal direction, and both the Young’s
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`module and yield strength of the porous metal fibers compressed in longitudinal direction are higher
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`than compressed in transverse direction. The energy absorption efficiency compressed in longitudinal
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`direction is about 25% higher than that compressed in transverse direction obviously.
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`Figure 5. Compression anisotropy of porous metal fibers with a porosity of 80%:
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`(a) Strain-stress curve; (b) Energy absorption efficiency.
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`5. Recommendations
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`With the progress and development of industry and technology, some new properties and
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`applications for metal fiber porous materials are being developed. Currently, a small portion of
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`performance and application have captured researcher’s interest, yet much more potentially could be
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`developed or is still restricted at the experimental research stage. A lot of work is still required,
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`focusing on preparation and application research.
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`Acknowledgements
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`This work is supported by National “973” Program (China) (Grant No. 2006CB601201, and
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`2011CB610302). The authors wish to express their thanks to Changqing Chen and Tiannin Chen from
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`Xi’an Jiaotong University, Jinliang Xu from Guangzhou Institute of Energy Conversion, Xiaolin Wang
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`from Institute of Acoustics (IACAS) for the help and valuable discussion during the research of metal
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`fiber porous materials properties and application.
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`© 2011 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
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`distributed under
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`the
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`terms and conditions of
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`the Creative Commons Attribution
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`license
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`(http://creativecommons.org/licenses/by/3.0/).
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