Construction and Properties of Ultra-deep Microchannel Based on Lamellar Ni-Ni3Si Alloy

Author:Wei Lu Feng

Supervisor:zhao zhi long

Database:Doctor

Degree Year:2018

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Pages:137

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Metallic micro/nano-structures have always been the focus and frontier in the field of micro/nano research.Their unique micro/nano-structure forms and properties of metal materials endow them with extraordinary catalytic,optical,electrical,magnetic and thermal properties.In the process of micro/nano-structure developing toward smaller size,deeper/longer and more complex structure,the development of depth direction has been relatively slow.The research of ultra-deep micro/nano-structure has been limited by micro/nano processing technology and equipment.Inspired by bionics,for the key scientific problem of unrealizable ultra-deep micro/nano-structure,we propose to use a variety of structural materials as template libraries,and select a regular lamellar Ni-Ni3Si eutectic alloy as template to extract ultra-deep Ni3Si microchannels on the premise of fully mastering the directional growth characteristics of the alloy and the electrochemical characteristics of eutectic two-phase.This ultra-deep micro/nano-structure has good application prospects in phase separation,filtration of bacteria and soot,heat exchange,catalysis of chemical reactors,electrode materials,supercapacitors and permeable carriers.The whole research process involves two aspects.First,regular lamellar Ni-Ni3Si eutectic in-situ composites were fabricated by directional solidification technique;the complete preparation process was obtained;the mechanism of regulation and transformation in solidification microstructure was clarified;the eutectic solidification theory was enriched;the results lay a foundation for the size control of Ni3Si microchannels and are important for the preparation of high temperature and high performance aerospace composites.Second,a novel ultra-deep Ni3Si microchannel structure was extracted by using regular lamellar Ni-Ni3Si eutectic alloy as template;the magnetic,electrochemical and selective deposition characteristics of the structures were investigated,which opened up a new direction for the development and application of ultra-deep Ni3Si microchannel structures.(1)The Bridgman directional solidification technique was employed to prepare the Ni-Ni3Si eutectic alloy.At growth rates of 6?15μm/s,average lamellar spacingλave=18.26V-0.5,the width ofa-Ni phaseλα=6.58 V-0.5,the width ofb-Ni3Si phaseλβ=11.57 V-0.5,the width difference between theα-Ni phase andβ-Ni3Si phases Q=4.99 V-0.5.The transition to cellular solidification occurred at 20μm/s.Given that produced larger lamellar spacing under decreasing growth rate,the lamellar structure underwent zigzag instability,which formed curved lamellae.The diffusion-limited growth of the Ni3Si phase decreased phase width and spacing,consequently causing zigzag instability.(2)Straight lamellae,zigzag pattern,and rod pattern were generated in Ni–21.9 at%Si hypereutectic alloys by the same method.The lamellar instability was still Zigzag instability.The growth rates of straight lamellae were 4–12μm/s.The corresponding formula wasλave=17.18 V-0.5.Compared with lamellar eutectic alloy,hypereutectic alloy produced straight lamellar structure with lower growth rate and narrower range.The shape transition from lamellae to rods in the Ni–Si alloy occurred,in turn,by the broken lamellae or elongated rods,dumbbell-shaped rods,peanut-shaped rods,and circular rods.Moreover,this transition did not occur at the same time.(3)Under the optimum potential of 1.34 VSHE,α-Ni phase could be selectively dissolved in Ni-Ni3Si eutectic alloy,whileβ-Ni3Si phase could be corrosion-free.SiO2 retarded the dissolution of Ni3Si phase and was preferentially formed onβ-Ni3Si,resulting in successful selective dissolution.After selective dissolution for 20 h,surface oxides consisted of 71.3 at.%SiO2 and 15.8 at.%Ni(OH)2.Selective dissolution ofβ-Ni3Si was difficult to achieve.In addition,it was found that the variation of lamellar spacing in Ni-Ni3Si eutectic alloy had relatively little effect on the polarization curve.(4)Selective dissolution of Ni phase in Ni-Ni3Si two-phase alloy at 1.34 VSHE resulted in a variety of Ni3Si microchannel structures.With the increase of dissolution time,the depth of microchannel also increased.Within the range of microchannel depth less than 200μm,the depth of microchannel in each sample was basically the same.But with the increase of microchannel depth,the effect of grain orientation on microchannel depth was obvious,which resulted in the different depth of microchannel for each sample.With the decrease of microchannel width,the difference was increasing,and finally the microchannel structure with a thickness of exceeding 1 mm can be formed.(5)PPMS measurements shown that permeable Ni3Si microchannels have soft magnetic properties and low coercivity at low temperatures.Paramagnetic to ferromagnetic transition occurred in the range of 3-15 K.With the temperature increasing from 5 K to 300 K,the magnetization decreased from 0.378 emu/g to 0.036 emu/g at 3 T.Even if the applied magnetic field reached 3 T,no magnetic saturation was found.The material can be used as a low temperature magnetic material applied in micro/nano devices.(6)CV and EIS measurements of Ni3Si microchannels with different depths shown that Ni3Si microchannels with larger depths have better charge storage performance and lower charge transfer resistance.The charge transfer resistance decreased from 38854Ω·cm2 to404.9Ω·cm2,and the charge transfer resistance decreased with the increase of microchannel depth,which will greatly improve the energy conversion efficiency for Ni3Si microchannel as electrode material.(7)Co can be selectively deposited in Ni3Si microchannels in three different micro/nano structures:reticulated translucent ultra-thin nano-Co sheets,Co stripe with isotropic growth,and scallop-like Co.Three structures nucleate,grow,overflow and expand along the microchannel,during which there is no direct deposition of Co on the Ni3Si phase.It shown that Ni3Si microchannel as a new type of metal template has selective deposition characteristics,which can realize the selective deposition of metal materials in the microchannel,and then can construct a variety of new lamellar micro/nanostructure.