氧化物气凝胶高温红外改性研究

氧化物气凝胶是一类具有超低热导率的新型纳米多孔材料,在航空航天等高温隔热领域具有广阔的应用前景。然而目前常见的氧化硅、氧化铝等氧化物气凝胶受材料/结构单元固有性质的限制,其通常具有红外透明性,而高温下辐射传热占据主导地位,大量红外辐射会透过气凝胶,热导率急剧攀升,导致其高温隔热性能较差,因此需要对其进行高温红外改性,从而满足更高的隔热性能需求。本文综述了近年来添加遮光剂、纤维及调整气凝胶的结构/形貌在提高氧化物气凝胶高温隔热性能方面的研究进展,并对未来研究方向进行展望。

基于水性气相二氧化硅浆料制备柔性二氧化硅气凝胶复合毡

二氧化硅气凝胶由于其低导热率在隔热应用方面具有巨大的潜力.然而,它们通常具有较差的机械性能,需要在保持低热导率的同时增强机械性能.本研究以商业化气相二氧化硅和甲基三甲氧基硅烷为硅源,并以水和乙醇为溶剂形成浆料.基于此,二氧化硅气凝胶块体(SAMs)可通过常压干燥进行制备,且无需额外的表面改性或溶剂置换.制备的SAMs保持了典型的纳米孔结构,具有低密度(0.24 g cm^(-3))、收缩率(4%)和热导率(0.046 W m^(-1)K^(-1)).通过辊压将浆料浸渍到纤维毡中,并通过浆料热固化和常压干燥制备出二氧化硅气凝胶毡(SABs).制备的SABs具有良好的柔韧性和机械性能,便于安装和隔热应用,并显著减少了生产周期和成本.此外,基于SAMs的纳米孔结构和低收缩率,通过调控遮光剂的粒径和质量分数进一步降低了SABs的高温热导率,优化后的SABs在800℃的热导率低至0.054 W m^(-1)K^(-1).

Ironing effect on surface integrity and fatigue behavior during ultrasonic peening drilling of Ti-6Al-4V

Imposing compressive residual stress field around a fastening hole serves as a universal method to enhance the hole fatigue strength in the aircraft assembly filed.Ultrasonic Peening Drilling(UPD)is a recently proposed hybrid hole making process,which can achieve an integration of strengthening and precision-machining with a one-shot-drilling operation.Due to the ironing effect of tool flank surface,UPD introduces large compressive residual stress filed in hole subsurface.In order to reveal the strengthening mechanism of UPD,the influence of ultrasonic vibration and tool dynamic relief angle on ironing coverage rate and its corresponding effect on surface integrity in UPD were analyzed.The experiments were conducted to verify the influence of ironing effect on surface integrity and fatigue behavior of Ti-6Al-4V hole in UPD.The results indicate that the specimen features smaller surface roughness,higher micro-hardness,plastic deformation degree and circumferential compress residual stress under higher ironing coverage rate.The fatigue life increases with the raise of ironing coverage rate,and the fatigue source site in UPD shifts from surface to subsurface comparing with that without vibration assistance.The results demonstrates that a better strengthening effect can be obtained by reasonably controlling the ironing coverage rate in UPD.

Robust,fire-resistant,and thermal-stable SiZrNOC nanofiber membranes with amorphous microstructure for high-temperature thermal superinsulation

Ceramic nanofibers with robust mechanical properties,high-temperature resistance,and superior thermal insulation performance are promising thermal insulators used under extreme conditions.However,developing of ceramic fibers with both low solid thermal conductivity(λs)and low infrared radiation thermal conductivity(λr)is still a great challenge.Herein,according to the Ioffe-Regel limit theory,we report a novel SiZrNOC nanofiber membrane(NFM)with a typically amorphous structure by combining the electrospinning method and high-temperature pyrolysis technique in a NH3 atmosphere.The prepared SiZrNOC NFM has a high tensile strength(1.98±0.09 MPa),excellent thermal stability(1100℃in air),and superior thermal insulation performance.The thermal conductivity of SiZrNOC NFM was 0.112 W·m^(−1)·K^(−1) at 1000℃,which is obviously lower than that of the traditional ceramic fiber membranes(>0.2 W·m^(−1)·K^(−1) at 1000℃).In addition,the prepared SiZrNOC NFM-reinforced SiO2 aerogel composites(SiZrNOCf/SiO2 ACs)exhibited ultralow thermal conductivity of 0.044 W·m^(−1)·K^(−1) at 1000℃,which was the lowest value for SiO2-based aerogel composites ever reported.Such superior thermal insulation performance of SiZrNOC NFMs was mainly due to significant decreasing of solid heat conduction and thermal radiation by the fancy amorphous microstructure and high infrared shielding compositions.This work not only provides a promising high-temperature thermal insulator,but also offers a novel route to develop other high-performance thermal insulating materials.

Microfabrication of functional polyimide films and microstructures for flexible MEMS applications

Polyimides are widely used in the MEMS and flexible electronics fields due to their combined physicochemical properties,including high thermal stability,mechanical strength,and chemical resistance values.In the past decade,rapid progress has been made in the microfabrication of polyimides.However,enabling technologies,such as laser-induced graphene on polyimide,photosensitive polyimide micropatterning,and 3D polyimide microstructure assembly,have not been reviewed from the perspective of polyimide microfabrication.The aims of this review are to systematically discuss polyimide microfabrication techniques,which cover flm formation,material conversion,micropatterning,3D microfabrication,and their applications.With an emphasis on polyimide-based flexible MEMS devices,we discuss the remaining technological challenges in polyimide fabrication and possible technological innovations in this field.

Extrusion 3D printing of carbon nanotube-assembled carbon aerogel nanocomposites with high electrical conductivity

Carbon nanotubes(CNTs)with high aspect ratio and excellent electrical conduction offer huge functional improvements for current carbon aerogels.However,there remains a major challenge for achieving the on-demand shaping of carbon aerogels with tailored micro-nano structural textures and geometric features.Herein,a facile extrusion 3D printing strategy has been proposed for fabricating CNT-assembled carbon(CNT/C)aerogel nanocomposites through the extrusion printing of pseudoplastic carbomer-based inks,in which the stable dispersion of CNT nanofibers has been achieved relying on the high viscosity of carbomer microgels.After extrusion printing,the chemical solidification through polymerizing RF sols enables 3D-printed aerogel nanocomposites to display high shape fidelity in macroscopic geometries.Benefiting from the micro-nano scale assembly of CNT nanofiber networks and carbon nanoparticle networks in composite phases,3D-printed CNT/C aerogels exhibit enhanced mechanical strength(fracture strength,0.79 MPa)and typical porous structure characteristics,including low density(0.220 g cm^(-3)),high surface area(298.4 m^(2)g^(-1)),and concentrated pore diameter distribution(~32.8nm).More importantly,CNT nanofibers provide an efficient electron transport pathway,imparting 3D-printed CNT/C aerogel composites with a high electrical conductivity of 1.49 S cm^(-1).Our work would offer feasible guidelines for the design and fabrication of shape-dominated functional materials by additive manufacturing.

超轻纳米纤维气凝胶的制备及其应用

超轻纳米纤维气凝胶是一种以一维纳米纤维为基本构筑单元的新型气凝胶材料,相比于传统气凝胶,其不仅具有更高的孔隙率和更低的密度,还拥有更优异的机械性能和理化性质,因此该材料的先进制备技术和在新兴领域的创新性应用是近年来超轻气凝胶领域的研究热点。本文结合国内外研究现状,按照材料体系分类系统综述了超轻纤维气凝胶的制备方法、结构特点以及在隔热、吸附、电化学、传感和生物医学等领域的重要应用,提出了现阶段该材料面临的一些挑战,并展望了其在未来的发展方向。

Structural and Mechanical Characterization ofAtrina Pectinata and Freshwater Mussel Shells

The microstructures ofAtrina pectinata and freshwater mussel shells are investigated by optical microscopy and scanning electron microscopy. The mechanical properties of these shells are characterized by nanoindentation and three-point bending tests. Results show that both shells possess a prismatic microstructure mainly composed of columnar crystals and an organic matrix. The fracture toughness of the prismatic structure of Atrina pectinata and freshwater mussel are approximately 1.15 MPa.m1/2 and 0.87 MPa.m1/2, respectively, while the fracture toughness of natural calcite is approximately 0.2 MPa.m1/2. Calculated results from indentations agree with those obtained from the three-point bending tests. The columnar crystal material shows excellent fracture toughness due to grain refinement. In addition, the organic matrix of the prismatic layer can arrest cracks, and thereby improves the fracture toughness.

Investigation on the Lateral Line Systems of Two Cavefish： Sinocyclocheilus Macrophthalmus and S. Microphthalmus （Cypriniformes： Cyprinidae）

Cavefish, with sensitive lateral lines, can swim freely and locate preys in invisible and complex cave environments, though their eyes are greatly degenerated. Investigations on the morphology and distribution characteristics of their lateral line systems would benefit our understanding of the high-sensitivity mechanism of the fish. In this study, the arrangement and morphology of the lateral lines are described for two species ofSinocyclocheilus： S. macrophthalmus and S. microphthalmus, which live in the karst caves in Guangxi, China. The behavior experiments indicate that the lateral line system of the S. macrophthalmus is more sensitive at a low vibration frequency range from 20 Hz to 70 Hz. The cephalic and trunk lateral line systems both contribute to the efficient object-locating capability. For both of the two species of cavefish, the diameter of the lateral canal nearby the neuromasts is narrower than that nearby the canal pores. This variation can increase the normal pressure to the surface of the cupula, and increase the sensitivity of the canal lateral line system.

Development of a Tactile and Slip Sensor with a Biomimetic Structure-enhanced Sensing Mechanism

Tactile and slip sensors have gained tremendous attention for their promising applications in the fields of smart robotics,implantable medical devices and minimally invasive surgery.Inspired by the structure-enhanced sensing mechanisms of human fingertips and tree frog toes,we developed a tactile and slip sensor by combining biomimetic surface microstructures with highly sensitive P(VDF-TrFE)nanofiber sensors on a flexible polyimide substrate.As the surface microstructures could mediate the micro-vibration induced by slip motion,the frequencies of output signals revealed a strong correlation with the periods of microstructures.In addition,we proposed a method to discriminate touch force from slip motion using the criterion of standard deviation of time delay from the output signals of neighboring sensor elements.

Exquisite structure of the lateral line system in eyeless cavefish Sinocyclocheilus tianlinensis contrast to eyed Sinocyclocheilus macrophthalmus(Cypriniformes:Cyprinidae)

In this study,the lateral line systems in Chinese cavefish eyeless Sinocyclocheilus tianlinensis and eyed Sinocyclocheilus macrophthalmus were investigated to reveal their morphological changes to survive in harsh environments.Compared with the eyed cavefish S.macrophthalmus(atypical),the lateral line system in the eyeless cave-fish S.tianlinensis(typical)has certain features to adapt to the dark cave environments:the superficial lateral line system in the eyeless species possesses a higher number of superficial neuromasts and more hair cells within an individual neuromast,and the trunk lateral line canal system in S.tianlinensis exhibits larger canal pores,higher canal diameter and more pronounced constrictions.Fluid–structure interaction analysis suggested that the trunk lateral line canal system in the eyeless S.tianlinensis should be more sensitive than that in the eyed S.macrophthalmus.These morphological features of the lateral line system in the eyeless S.tianlinensis probably enhance the functioning of the lateral line system and compensate for the lack of eyes.The revelation of the form–function relationship in the cavefish lateral line system provides inspiration for the design of sensitive artificial flow sensors.

Mechanically strengthened graphene-Cu composite with reduced thermal expansion towards interconnect applications

High-density integration technologies with copper(Cu)through-silicon via(TSV)have emerged as viable alternatives for achieving the requisite integration densities for the portable electronics and micro-electro-mechanical systems(MEMSs)package.However,significant thermo-mechanical stresses can be introduced in integrated structures during the manufacturing process due to mismatches of thermal expansion and the mechanical properties between Cu and silicon(Si).The high-density integration demands an interconnection material with a strong mechanical strength and small thermal expansion mismatch.In this study,a novel electroplating method is developed for the synthesis of a graphene-copper(G-Cu)composite with electrochemically exfoliated graphenes.The fabrication and evaluation of the G-Cu composite microstructures,including the microcantilevers and micromirrors supported by the composite,are reported.We evaluated not only the micromechanical properties of the G-Cu composite based on in-situ mechanical resonant frequency measurements using a laser Doppler vibrometer but also the coefficients of thermal expansion(CTE)of the composite based on curvature radius measurements at a temperature range of 20–200℃.The Young’s modulus and shear modulus of the composite are approximately 123 and 51 GPa,which are 1.25 times greater and 1.22 times greater,respectively,than those of pure Cu due to the reinforcement of graphene.The G-Cu composite exhibits a 23%lower CTE than Cu without sacrificing electrical conductivity.These results show that the mechanically strengthened G-Cu composite with reduced thermal expansion is an ideal and reliable interconnection material instead of Cu for complex integration structures.

BTO/P(VDF-TrFE) Nanofiber-based Artificial Lateral Line Sensor with Drag Enhancement Structures

A flexible Artificial Lateral Line(ALL)sensor is presented in this paper,featuring a barium titanate/polyvinylidene fluoride?trifluoroethylene[BTO/P(VDF?TrFE)]nanofiber mat,a hydrogel cupula,and a constriction structure in the canal..The excellent piezoelectric performance of the BTO/P(VDF-TrFE)nanofiber,superior to that of a pristine P(VDF-TrFE)nanofiber,helps improve the sensitivity of the ALL sensor.The hydrogel cupula imitating the cupula in a fish lateral line system enhances the ALL sensitivity through a material-induced drag enhancement mechanism.The constriction mimics the diminution structure found in fish canal lateral line systems,endowing the canal ALL sensor with enhanced sensitivity through a structure-induced drag enhancement mechanism.The contributions of the hydrogel cupula and constriction structure in offering an enhanced sensing performance are studied experimentally,in comparison with conventional ALL sensors.The constriction structure in the canal helps modify the frequency response of the canal ALL sensor,i.e.,attenuating low frequencies while amplifying high frequencies.The proposed biomimetic flow sensor is expected to aid in the development of smart skins for underwater robotics applications.

Hydrodynamic Perception Using an Artificial Lateral Line Device with an Optimized Constriction Canal

To perform flow-related behaviors in darkness,blind cavefish have evolved Lateral Line Systems(LLSs)with constriction canals to enhance hydrodynamic sensing capabilities.Mimicking the design principles,we developed a Canal-type Artificial Lateral Line(CALL)device featuring a biomimetic constriction canal.The hydrodynamic characterization results revealed that the sensitivity of the canal LLS increases with the decrease in the width(from 1 mm to 0.6 mm)and length(from 3 mm to 1 mm)of the constriction canal,which is in accordance with the modeling results of canal mechanics.The CALL device was characterized in Kármán vortex streets generated by a cylinder in a laminar flow.The CALL device was able to identify the diameter of the cylinder,with a mean identification error of approximately 2.5%.It also demonstrated the identification ability of wake width using the CALL device,indicating the potential for application in hydrodynamic perception.

Bio-inspired Flexible Airflow Sensor with Self-bended 3D Hair-like Configurations

This paper presents a novel flexible airflow sensor based on four curved microcantilevers arranged in a cross-form configuration.A self-bending method based on MEMS technology has been used to fabricate the curved microcantilevers structure,and this method can transfer a 2D plane structure into a 3D structure with good consistency in the morphology.The curved microcantilever consists of a polyimide(PI)top layer,silicon(Si)bottom layer,and platinum(Pt)piezoresistor at the root of the cantilever.The difference in the in-plane residual stress between the PI and Si layers bent the microcantilever upward.The curved-up microcantilever transfers the fluidic momentum that acts on it to drag force,which deflects the curved-up microcantilever and changes the resistance of the piezoresistor.To realize temperature compensation and decrease the noise,a reference resistor and an ambient temperature detector were integrated for the Wheatstone half-bridge measurement and temperature monitoring,respectively.The cross-form configuration of the curved-up cantilevers has high sensitivity advantages and possesses direction-sensing ability.Experimental results show that the sensitivity of the sensors increased as a function of the airflow velocity,and the sensors exhibited a maximum resolution of 4 mm⋅s^(−1) and a maximum sensitivity of 60.35 mV⋅(ms^(−1))^(−1) when the airflow velocity was larger than 38.5 m⋅s^(−1).

Enhanced flow sensing with interfacial microstructures

Biological flow receptors show astonishing performance and are used as models for the design of novel flow sensors.However,the functional importance of interfacial microstructures is seldom discussed in previous review papers.Herein,this review summarises the underlying biomechanical principles in the biological flow receptors and describes the recent progress in bio-inspired flow sensors,in which the underlying sensing-enhancement mechanisms are emphasised.