Micro-sized Structures Research Articles

Critical heat flux enhancement by single-layered metal wire mesh with micro and nano-sized pore structures

This study reports the results of critical heat flux (CHF) enhancements by using single-layered metal wire mesh with micro/nano-sized pore structures with no modification of the original heating surface. Pool boiling tests with the wire mesh showed a significant CHF increment of up to 84% compared with that recorded on a bare surface. And, we investigated the dynamic behavior of the liquid and vapor phases on the boiling surfaces using an IR thermometry technique. From the quantitative measurements of liquid–vapor-solid phase distribution on the heating surface, it was revealed that the presence of the wire mesh and its hydrophilic nature play a role in sustaining a liquid phase inside the heating area that prevents the excessive expansion of the dry spot, and strongly disseminates thermal energy generated by the heater. In other words, the thermal energy generated inside the heater is dispersed more uniformly under the existence of the wire mesh. Using this type of mesh with the micro-sized pore structures for CHF enhancement intrinsically can mitigate changes in thermos-physical and chemical properties, and losses in structural durability for original heating surface. We expect that the meshes can be exploited in applications to several thermal systems.

Raman Spectroscopy Studies of Large Area Graphene and Graphene Molecules

In this study, we present a detailed Raman analysis of large-area graphene, nano-graphene sheets (>25 nm) and graphene molecules with synthetically controlled lateral dimensions between 0.76 to 1.39 nm. The integrated area ratios of D and G bands increase with the size of the molecules, when excited using two different wavelengths (532 nm and 405 nm). The D and G band positions were also found to depend on the size of the graphene molecules. The intensities of the higher-order bands (e.g., 2D, D+G, and 2G), increase for both wavelengths, as the size of the graphene molecules increases. For complete spectroscopic studies of nano- and micro-size graphitic structures, we also probed nano-graphene sheets (>25 nm) and large area graphene using the same excitation wavelengths. The reaction/synthesis time was varied to control size and crystallinity of the graphene molecules, and their corresponding spectral features including integrated area band ratios and the band positions, were thoroughly investigated. These results provide a potential route for using Raman spectroscopy as a diagnostic tool for structural characterization and understanding the reaction dynamics of graphene molecules smaller than 2 nm. Lastly, this work presents a complete representation of the spectroscopic behavior nano- and micro-sized structures (ranging from 0.76 nm to 1cm), which is currently missing from the studies of the present graphitic materials generated via ion-bombardment and other techniques.

Creation of nano eye-drops and effective drug delivery to the interior of the eye

Nano eye-drops are a new type of ophthalmic treatment with increased potency and reduced side effects. Compounds in conventional eye-drops barely penetrate into the eye because the cornea, located at the surface of eye, has a strong barrier function for preventing invasion of hydrophilic or large-sized materials from the outside. In this work, we describe the utility of nano eye-drops utilising brinzolamide, a commercially available glaucoma treatment drug, as a target compound. Fabrication of the nanoparticles of brinzolamide prodrug increases the eye penetration rate and results in high drug efficacy, compared with that of commercially available brinzolamide eye-drops formulated as micro-sized structures. In addition, the resulting nano eye-drops were not toxic to the corneal epithelium after repeated administration for 1 week. The nano eye-drops may have applications as a next-generation ophthalmic treatment.

Effective Light Trapping in Thin Film Silicon Solar Cells with Nano- and Microscale Structures on Glass Substrate.

For thin film silicon-based solar cells, effective light trapping at a broad range of wavelengths (400-1100 nm) is necessary. Normally, etching is only carried out with TCOs, such as SnO2:F and impurity doped ZnO, to form nano-sized craters in the surface morphology to confer a light trapping effect. However, in this study, prior to ZnO:Al etching, periodic structures on the glass substrates were made by photolithography and wet etching to increase the light scattering and internal reflection. The use of periodic structures on the glass substrate resulted in higher haze ratios in the range from 550 nm to 1100 nm, which is the optical absorption wavelength region for thin film silicon solar cells, than obtained by simple ZnO:Al etching. The periodically textured glass with micro-sized structures compensates for the low haze ratio at the middle and long wavelengths of wet etched ZnO:Al. ZnO:Al was deposited on the periodically textured glass, after which the ZnO:Al surface was also etched randomly using a mixed acid solution to form nano-sized craters. The thin film silicon solar cells with 350-nm-thick amorphous silicon absorber layer deposited on the periodic structured glass and etched ZnO:Al generated up to 10.68% more photocurrent, with 11.2% increase of the conversion efficiency compared to the cell deposited on flat glass and etched ZnO:Al.

Hierarchical micro/nanostructures generated by femtosecond laser beams modified through a liquid-crystal spatial light modulator

The speckle effect, generated by a liquid-crystal spatial light modulator (LCSLM), is studied using a femtosecond laser machining system. The speckle effect of the patterned beams produces hierarchical micro/nanostructures on the surface of polished stainless steel (SS 316L). The results show that the hierarchical micro/nanostructures are composed of micro-sized valley structures created by the LCSLM and nano-sized periodic structures induced by the femtosecond laser. By varying the azimuth of the pattern beams in the focal plane, the directions of the inner valley structures can be altered. As a result of these patterned beams with the speckle effect, the fabrication of hierarchical micro/nanostructures structures inside various shapes becomes a simple one-step procedure instead of a complex scanning process. These hierarchical micro/nanostructures can provide advantages for anti-friction, hydrophobic, and hydrophilic applications in surface treatment.

Replication of microstructured tools for electrochemical machining applications

Electrochemical machining (ECM) is a powerful method to machine metals independently of their mechanical properties. Micro-sized structures can be formed with high precision. For this purpose, it is necessary to manufacture tools with adequately fine structures. Conventional ECM tools were manufactured by micro-drilling, -milling, -turning, etc. Depending on the complexity of the structures, these techniques are very elaborate and expensive. Recently, a new procedure, which combines photolithography and electroforming, the so-called PhoGaTool process (photolithographic electroforming of ECM tools), was published. In the present work, we demonstrate how this method can also be used for copying ECM tools that were conventionally manufactured. A copy of an industrial PECM tool (precise electrochemical machining) was manufactured and used for the structural characterization and determination of the replication accuracy. The electroforming parameters like deposition conditions, bath composition as well as physical and chemical parameters were optimized for model systems. The inclusion of bath additives in the metal matrix during the deposition leads to micro-stresses, hence in this contribution, an alternative additive-free electrolyte was used. The accuracy of the process was investigated by means of confocal laser scanning microscopy. Average deviations of the structure depth and the lateral structure dimensions are in the range of 5 %.

Fabrication of cyclodextrins-procainamide supramolecular self-assembly: Shape-shifting of nanosheet into microtubular structure

Encapsulation behavior of α- and β-cyclodextrins (α-CD, β-CD) with procainamide hydrochloride (PCA) has been investigated by absorption, fluorescence, time-resolved fluorescence, proton nuclear magnetic resonance spectroscopy, scanning electron microscope, Fourier transform-infrared spectroscopy, differential scanning calorimetry, and powder X-ray diffraction techniques. Spectral results revealed that PCA forms 1:2 drug–CD2 inclusion complexes with CDs. Novel supramolecular self-assemblies have been fabricated by inclusion complexation of PCA with α-CD/β-CD and characterized by transmission electron microscope and micro-Raman imaging. The obtained results from transmission electron microscope indicated that PCA/α-CD complex could form nano-sized particles. However, when the macrocyclic ring with six glucose units was switched into seven glucose units, the resultant PCA/β-CD complex could be self-assembled to micro-sized tubular structures. Shape-shifting of 2D nanosheet into 1D microtube by simple rolling mechanism was analyzed. Thermodynamic parameters of inclusion process were determined by Parameter Method 3 calculations.

Structural morphology and in vitro toxicity studies of nano- and micro-sized zinc oxide structures

The structural morphology and in vitro toxicity of nano- and micro-sized ZnO structures were investigated. All the ZnO samples were characterized to determine their morphologies, particle sizes, structures and optical bandgaps. Transmission electron microscopy and field-emission scanning electron microscopy results revealed that the morphologies of nano-sized ZnO-N1 and ZnO-N2 samples consisted of spherical and irregularly-shaped particles, respectively. The corresponding particle sizes were 20–40nm and 50–80nm, respectively. The morphologies of micro-sized ZnO-M1 and ZnO-M2 samples were found to be mostly rod-structures and plate-structures, respectively. Rod diameters were 40–100nm whereas plate widths were 50–150nm. ZnO-N1 had the highest toxicity towards the cells, causing the cell viability to be less than 70% for all concentration ranges. This phenomenon was due to the small particle size and release of zinc ions. Micro-sized ZnO-M1 and ZnO-M2 samples had toxicity limits of 0.3mM at most.

A Simple Process to Fabricate Micro Flux Sources with High Magnetic Field Gradient

In this paper, we present a fabrication process to produce the micro-sized magnetic structures based on the hard magnetic powders. Under the magnetic field originated from a micro patterned hard magnetic film, these magnetic powders are magnetically aligned to form arrays of the micro magnets on a polydimethysiloxane (PDMS) substrate. The high magnetic field gradient and stable magnetic flux can be obtained at certain micro-sized area on the surface of the micro magnets. The fabricated structures have been used for trapping iron oxide particles. Generally this fabrication process is simple, low cost and the micro magnets can be used for further applications in biology, medicine and beyond.

Nanostructures formed by cyclodextrin covered procainamide through supramolecular self assembly – Spectral and molecular modeling study

Inclusion complexation behavior of procainamide (PCA) with two cyclodextrins (α-CD and β-CD) were analyzed by absorption, fluorescence, scanning electron microscope (SEM), transmission electron microscope (TEM), Raman image, FT-IR, differential scanning colorimeter (DSC), Powder X ray diffraction (XRD) and 1H NMR. Blue shift was observed in β-CD whereas no significant spectral shift observed in α-CD. The inclusion complex formation results suggest that water molecules also present in the inside of the CD cavity. The present study revealed that the phenyl ring of the PCA drug is entrapped in the CD cavity. Cyclodextrin studies show that PCA forms 1:2 inclusion complex with α-CD and β-CD. PCA:α-CD complex form nano-sized particles (46nm) and PCA:β-CD complex form self-assembled to micro-sized tubular structures. The shape-shifting of 2D nanosheets into 1D microtubes by simple rolling mechanism were analysed by micro-Raman and TEM images. Thermodynamic parameters (ΔH, ΔG and ΔS) of inclusion process were determined from semiempirical PM3 calculations.

Micro-Sized Silicon-Carbon Composite Composed of Carbon-Coated Sub-10 Nm Si Primary Particles As High-Performance Anode Materials for Lithium-Ion Batteries

Recently, silicon has attracted considerable interest as a high-capacity anode material owing to its highest known capacity of 3580 mAh/g at room temperature, which is about 10 times larger than that of commercial graphite anodes (~370mAh/g). Despite promising advantages, practical application of silicon anodes in high performance Li-ion batteries is seriously hindered by its rapid capacity fading as a result of dramatic volume changes (>400 %) during lithium insertion and extraction processes. The huge volume changes of Si cause pulverization and loss of electrical contact between Si particles and conductive additives, as well as an unstable solid electrolyte interphase (SEI) layer, which in effect results in severe capacity fading. Constructing nanostructured silicon is a promising strategy and has been demonstrated to improve battery performance.1,2 However, there are some challenges involved in introducing these nanomaterials into electrode fabrication for practical application. One is the lower tap density of the nanoparticles, in contrast to commercially used micro-sized electrode materials, resulting in a lower volumetric energy density; the other is the difficulty with handling the chemicals, such as inhalation and explosion risks, arising from these nanoparticles in fabrication of electrodes.Thus, Control of primary particle sizes of Si below critical diameter (around 10 nm) in the micro-sized composite is more important in achieving good cycling stability as micro-sized aggregated structures of Si can expedite the capacity fading.3,4 Herein, we report a synthesis of micro-sized Si-C composites composed of carbon-coated sub-10 nm Si primary particles interconnected into secondary micro-sized particles, and its use as high-performance Li-ion anodes. This synthesis process involves facile thermal annealing of silsesquioxane with size-controlled formation of Si primary particles and subsequent carbon coating steps. The as-prepared Si-C composites anode material shows stabilized capacity of ~1600 mAh/g with excellent capacity retention (90% after 150 cycles). Moreover, the volumetric capacity can reach 1088 mAh/cm3 attributed to a high tap density (0.68 g/cm3) of micro-sized Si-C composites. Taking the facile synthesis and excellent performance of Si-C composites into account, it would be of significant interest for scalable fabrication for rechargeable lithium batteries. Furthermore, the silicon nanoparticles with controllable size are also easily used in other applications, such as light-emitting diodes, and high efficiency photovoltaic devices. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract NO. 6951378 under the Batteries for Advanced Transportation Technologies (BATT) Program

Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform

With the development of nanomaterial-based nanodevices, it became inevitable to develop cost-effective and simple nanofabrication technologies enabling the formation of nanomaterial assembly in a controllable manner. Herein, we present suspended monolithic carbon single nanowires and nanomeshes bridging two bulk carbon posts, fabricated in a designed manner using two successive UV exposure steps and a single pyrolysis step. The pyrolysis step is accompanied with a significant volume reduction, resulting in the shrinkage of micro-sized photoresist structures into nanoscale carbon structures. Even with the significant elongation of the suspended carbon nanowire induced by the volume reduction of the bulk carbon posts, the resultant tensional stress along the nanowire is not significant but grows along the wire thickness; this tensional stress gradient and the bent supports of the bridge-like carbon nanowire enhance structural robustness and alleviate the stiction problem that suspended nanostructures frequently experience. The feasibility of the suspended carbon nanostructures as a sensor platform was demonstrated by testing its electrochemical behavior, conductivity-temperature relationship, and hydrogen gas sensing capability.

Nano-/microstructure improved photocatalytic activities of semiconductors

Photocatalysis has emerged as a promising technique owing to its valuable applications in environmental purification. With the demand of building effective photocatalyst materials, semiconductor investigation experienced a developing process from simple chemical modification to complicated morphology design. In this review, the general relationship between morphology structures and photocatalytic properties is mainly discussed. Various nano-/microsized structures from zero- to three-dimensional are discussed, and the photocatalytic efficiency correspon- ding to the structures is analysed. The results showed that simple structures can be easily obtained and can facilitate chemical modification, whereas one- or three-dimensional structures can provide structure-enhanced properties such as surface area increase, multiple reflections of UV light, etc. Those principles of structure-related photocatalytic properties will afford basic ideology in designing new photocatalytic materials with more effective catalytic properties.

Effect of reducing agents on the structure of zinc oxide under microwave irradiation

Zinc oxide was synthesized from zinc sulphate using different reducing agents under microwave irradiation. The influence of sodium borohydride, hydrazine hydrate and urea on the shape and size of the products were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM showed the nano-sized spherical and rectangular shaped structures in case of sodium borohydride and hydrazine hydrate, whereas micro-sized hexagonal structures were formed in case of urea under the same irradiation power. The reducing agents played an important role in forming the various structures. Thus different shapes and size of structures were produced by only varying the reducing agent, which had wide applications in various fields.

Effects of geometric parameters for superhydrophobicity of polymer surfaces fabricated by precision tooling machines

Effects of geometric parameters are regarded as indicating factors that affect super hydrophobic surfaces. Micro-sized pillar-type structures were prepared using a precision tooling machine that was able to fabricate various structures precisely on polymer surfaces. The tooling machine has many advantages of low cost, short processing time, and precise control of geometric parameters compared to other micro-manufacturing techniques of polymers such as injection molding, replica molding, and hot embossing. Hydrophobicity of the polymer affected by surface conditions and geometric properties is considered. Furthermore, to examine the effects of low surface energy, we coated the CF2-based fluoropolymer onto various pillar structures. The some of the coated surfaces show the superhydrophobicity with a water contact angle greater than 150°. The hydrophobic and oleophobic properties are regarded as verifying the transition of the energy state and the tendency of wetting behavior depending on the geometric parameters and low surface energy property.

Additive-free solvothermal synthesis of hierarchical flower-like LiFePO4/C mesocrystal and its electrochemical performance

Three dimensional hierarchical flower-like lithium iron phosphate (LiFePO4) mesocrystals were successfully synthesized via a solvothermal approach with the utilization of a mixture of water/ethylene glycol/dimethylacetamide (H2O/EG/DMAC) as co-solvent. No other surfactant or template agent was used and beautiful micro-sized LiFePO4 mesoporous structures with a special rose-like morphology were obtained. The hierarchical LiFePO4 mesocrystals were assembled by well crystallized nano-sized LiFePO4 thin plates with a thickness around 100 nm. The characteristics and electrochemical dynamics as well as performance of the obtained hierarchical flower-like LiFePO4 mesocrystals were carefully investigated. The flower-like hierarchical LiFePO4 mesocrystals showed a high initial lithium intercalation capability of 147 mA h g−1 at a current density of 17 mA g−1 (0.1 C), which should be attributed to the high specific surface area resulting from the mesoporous superstructure and well crystallized LiFePO4 nano-plate composition units. Polyvinylpyrrolidone (PVP) was introduced during the solvothermal synthesis as an in situ carbon coating source. The obtained flower-like C-coated LiFePO4 mesocrystals exhibited even better initial lithium intercalation capability of 161 mA h g−1 at 0.1 C and showed improved lithium storage performance at high rates as well as good cyclic stability.

Out of the cleanroom, self-assembled magnetic artificial cilia

Micro-sized hair-like structures, such as cilia, are abundant in nature and have various functionalities. Many efforts have been made to mimic the fluid pumping function of cilia, but most of the fabrication processes for these "artificial cilia" are tedious and expensive, hindering their practical application. In this paper a cost-effective in situ fabrication technique for artificial cilia is demonstrated. The cilia are constructed by self-assembly of micron sized magnetic beads and encapsulated with soft polymer coatings. Actuation of the cilia induces an effective fluid flow, and the cilia lengths and distribution can be adjusted by varying the magnetic bead concentration and fabrication parameters.

The superhydrophobic properties of self-organized microstructured surfaces derived from anodically oxidized Al/Nb and Al/Ta metal layers

Al/Nb (aluminium-on-niobium) and Al/Ta (aluminium-on-tantalum) metal layers sputter-deposited onto Si wafers were anodized respectively in 0.06 and 0.1moldm−3 citric acid electrolytes under a high voltage of 480 and 400V in order to consecutively grow porous alumina layers with the longest interpore distances, this followed by growth of self-organized arrays of micro-sized goblets-like niobium oxide structures and eryngii-mushrooms-like tantalum oxide structures protruding through the alumina barrier layer. The shape and size of the metal oxide microstructures were additionally tailored by post-anodizing treatment combining partial open-circuit dissolution of the alumina pores with reanodizing the underlying metal through the as grown and expanded pores. The structured metal oxide surfaces derived after selectively dissolving away the anodic alumina layers were coated with fluoroalkyl phosphate (FAP) and tested for their non-wetting properties by measuring static contact angles for water droplets. Both the FAP-coated goblet and eryngii-mushroom surfaces showed superhydrophobic behaviour with the contact angles of 158 and 156° respectively. The effect is due to the creation of composite solid–liquid–air interfaces that allow for dramatically decreased liquid-to-solid contact area and adhesion, in accord with the Cassie–Baxter model.

Micro-sized Columnar Structures of Ni fabricated by using Negative-type Micromold made of Photocurable Resin

Micro-sized Columnar Structures of Ni fabricated by using Negative-type Micromold made of Photocurable Resin

Fabrication of Microcoils with Narrow and High Aspect Ratio Coil Line

Actuators are some of the most important elements in machines and robotics. The need for micro-fabrication of microactuators, smart sensors and microcoils is increasing. Actuators, which have a large volume and weight as part of a product, have been required to reduce their size. However, the processing required for three-dimensional micro-sized structures is very difficult. Here, the LIGA process could be used to fabricate nano- and micro-scale parts for many applications. Therefore, we have fabricated a spiral microcoil with narrow pitch and high aspect ratio coil line structures for an electromagnetic microactuator using the LIGA process. We have fabricated coil lines with a width of 10 μm and an aspect ratio of over 5 on an acrylic pipe surface using the X-ray lithography technique. Then, a void-free copper film was formed by electroforming in order to obtain coil lines. Finally, we have succeeded in obtaining a microcoil with an aspect ratio of over 2 using the isotropic etching process. We have also estimated the suction force of electromagnetic actuators using these microcoils. It is expected that these microcoils with a narrow and high aspect ratio structure could be capable of delivering high performance in spite of their miniature size.