Crack-free Surface Research Articles

Sol–gel based thermally stable mesoporous TiO2 nanomatrix for fiber optic pH sensing

Thermally stable acid catalyzed mesoporous titania (TiO2) nanomatrices are obtained by sol–gel method for fiber optic pH sensing. These synthesized nanoparticles are annealed at 300 °C, at two time intervals i.e., 1 h and 2 h, characterized with several analytical techniques such as FE-SEM/EDS, AFM, XRD, FTIR, TGA, and Brunauer–Emmett–Teller (BET) analysis. Microscopic analysis shows that synthesized nanoparticles have crack-free, dense and homogeneous surface with low surface roughness (4.4–5.9 nm). EDS mapping confirms the uniform distribution of Ti in all samples. XRD findings revealed the TiO2 anatase phase. BET analysis shows that the mesoporous synthesized TiO2 nanoparticles have surface areas 169 m2/g and average pore diameter 39.2 A. However, surface area is decreased to 113 and 102 m2/g and average pore diameter increased up to 62.36 and 68.18 A after 1 h and 2 h heat treatment, respectively. Furthermore, the sensing activity of phenolphthalein (phph) doped mesoporous TiO2 nanoparticles/matrix is found to be high at pH 12 without any leaching/cracking.

Electrochemical Roughening of Thin-Film Platinum for Neural Probe Arrays and Biosensing Applications

We report a method for electrochemical roughening of thin-film platinum (Pt) electrodes that increases active surface area, decreases electrode impedance, increases charge injection capacity, increases sensitivity of biosensors and improves adhesion of electrochemically deposited films. First, a well-established technique for electrochemical roughening of thick Pt electrodes (wires and foils) by oxidation-reduction pulses was modified for use on thin-film Pt. Optimal roughening of thin-film Pt electrodes with this established protocol in a sulfuric acid solution was found to occur at about four times lower frequency than that typically used for thick Pt. This modification in established procedure created a 21x surface area increase but showed nanoscale cracks from inter-grain Pt dissolution that compromised film integrity. A crack free surface with Pt nanocrystal re-deposition (20–30 nm in size) and higher enhancement in surface area (44x) was obtained when the electrolyte was switched to a non-adsorbing perchloric acid solution. These electrochemically roughened electrodes have charge injection limits comparable to titanium nitride and just below carbon nanotube-based materials. Roughened microelectrodes showed a 2.8x increase in sensitivity to hydrogen peroxide detection, indicative of improved enzymatic biosensor performance. Platinum iridium and iridium oxide coatings on these roughened surfaces showed an improvement in adhesion.

Establishing Relationship between Process Parameters and Temperature during High Speed End Milling of Soda Lime Glass

In glass machining crack free surface is required in biomedical and optical industry. Ductile mode machining allows materials removal from brittle materials in a ductile manner rather than by brittle fracture. Although end milling is a versatile process, it has not been applied frequently for machining soda lime glass. Soda lime glass is a strain rate and temperature sensitive material; especially around glass transition temperature Tg, ductility increased and strength decreased. Hence, it is envisaged that the generated temperature by high-speed end milling (HSEM) could be brought close to the glass transition temperature, which promote ductile machining. In this research, the objective is to investigate the effect of high speed machining parameters on generated temperature. The cutting parameters were optimized to generate temperature around glass transition temperature of soda lime using response surface methodology (RSM). Result showed that the most influencing process parameter is feed rate followed by spindle speed and depth of cut to generate temperature. Confirmation test showed that combination of spindle speed 30,173 rpm, feed rate 13.2 mm/min and depth of cut 37.68 µm generate 635°C, hence ductile chip removal with machined surface Ra 0.358 µm was possible to achieve.

Machinability of Soda Lime Glass in High Speed End Milling

Ductile regime machining can produce crack free surface on brittle material under certain controlled cutting conditions. Although end milling is a versatile process, it has not been frequently applied for machining soda lime glass. The brittle nature of the soda lime glass makes the machining difficult. Soda lime glass is a strain rate and temperature sensitive material. Therefore, high speed machining can be beneficiary to achieve ductile surface with low roughness. Hence, proper combination of cutting parameters needs to identify. In this paper, the effects of cutting parameters such as spindle speed, feed rate and depth cut on the surface roughness parameters Ra, Rq and Rt are investigated to assess machinability of soda lime glass during high-speed end milling. Regression models are generated and process parameters are optimized using the Central Composite Design (CCD) of Response Surface Methodology (RSM). The uncoated tungsten carbide tool was used to conduct milling operation. Experimental verification of optimal condition, spindle speed 40,000 rpm, feed rate 10 mm/min and depth of cut 34 µm confirmed that ductile surface with Ra, Rq and Rt of 0.33µm, 0.49µm and 6.59 µm respectively possible to achieve.

Preparation of TiO2 and ZnO dispersions for inkjet printing of flexible sensing devices

Research presented in this article focuses on the preparation of functional dispersions for inkjet printing of nanoparticles as sensitive layers. The stable suspensions of MOx(M = Ti, Zn) were prepared using gum arabic (GA) and Solsperse? 40000 (SO) as dispersants. A special attention was paid to the monitoring of particle size evolution during the planetary ball milling of dispersions, so that optimum ratio between milling time and particle size can be determined. After adjusting the printing parameters, prepared inks were printed on the flexible PET substrate with interdigitated electrodes (IDE). Films printed with TiO2 ink stabilized by GA exhibited highly cracked surface which resulted in low current values, whereas ZnOink stabilized by SO yielded crack-free surface and much higher current values. All investigated samples showed linear current behaviour in the range from -5 to 5 V, indicating formation of ohmic contacts between electrodes and nanoparticles, but ZnO ink produced the highest current values. Gas sensing properties, tested at room temperature at several humidity levels and for different types of alcohols, revealed that printed sensor exhibits modest sensitivity for low humidity levels and slightly higher affinity towards methanol gas. Photo sensitivity measurement showed very high photocurrent values with strong potential for optoelectronic applications.

Crack-free high surface area silica-titania nanocomposite coating as opto-chemical sensor device

This work highlights the versatility of sol-gel-derived nanocomposite silica-titania host matrix along with functionalized organic dyes i.e., bromophenol blue (C19H10Br4O5S), phenol red (C19H14O5S), cresol red (C21H17NaO5S) and phenolphthalein (C20H14O4) encapsulation with surfactant CTAB for pH sensing. The synthesized matrices before and after heat treatment (at 150 °C and 250 °C) are analyzed for chemical and physical uniformity; observed as stable, homogeneous, low surface roughness and with functionalized nano-pore surfaces. Particle size distributions in the range of 2.28 nm, 2.73 nm and 2.54 nm are established. Surface analysis shows that the nanoparticles are uniformly distributed on the surface of the mesoporous channel systems with high surface area 400 m2/g, and highly accessible pore system which has more capability to sense the broad pH range. The films show good optical qualities in the visible region with tunable refractive indices 1.57, 1.52-and 1.56. The response of the prepared sensor even shows stability towards higher pH value 12 and has linear relationship with correlation coefficient R2 ∼0.9341. The sensitivity of the proposed pH sensor is observed to be 0.614 mV/pH.

Synthesis of optically active bromophenol blue encapsulated mesoporous silica–titania nanomatrix: structural and sensing characteristics

Mesostructured silica-titania (ST) and bromophenol blue encapsulated silica–titania (BPB-ST) nanomatrices are synthesized by sol–gel method at low temperature. FESEM analysis shows uniform crack-free porous surface of ST matrix. EDX mapping analysis shows that the BPB species are uniformly distributed/immobilized throughout the ST matrix lattices. AFM images reveal 45.68 nm thick film with low average surface roughness ~1.27 nm, after encapsulation of BPB dye in ST matrix. TEM analysis exhibits uniform interior titania core and silica shell network, with average particle size 2.4 ± 0.4 nm for ST sample which reduced to 0.8 ± 0.3 nm after BPB encapsulation. Mesoporous high surface area 488 m2/g and pore volume of 0.55 cm3/g encapsulated matrix is found to be beneficial for sensing analysis. Transparent (67%) mesoporous nanoparticles with low refractive index 1.41 is obtained for encapsulated nanomatrix. The synthesized sensing material is fast responsive <1 sec and found to be linear for pH 3–12. High pKa value of 7.57 at 580 nm for ST-BPB leads to the broader sensitivity range, repetitive, easy-to use, and color indication system. The ability to change color in response to external stimulus makes ST-BPB a potential sensing material in food.

Investigation on critical crack-free cutting depth for single crystal silicon slicing with fixed abrasive wire saw based on the scratching machining experiments

The experimental research of single crystal silicon scratching machining is crucial for micro component machining, precision grinding and fixed abrasive wire saw precision slicing of single crystal silicon. The fixed abrasive wire saw slicing is the main slicing technology of large diameter single crystal silicon slicing process. During the fixed abrasive wire saw slicing process, the brittle material removal mode results in micro crack damage in wafer surface layer, which reduces the strength of sliced silicon wafer, and restricts silicon wafer thinner. The crack-free slicing surface could be obtained while the material is in ductile removal mode. In this study, the scratching machining experiments with the Nano Indenter and Berkovich diamond indenter scratching on single crystal silicon (100) plane along [010] orientation is conducted. The constant normal force 20mN, 100mN, and varied normal force from 0mN to 50mN are applied on the Berkovich indenter. The material removal mode is mainly in ductile regime while 20mN constant normal force, and the material removal mode is mainly in brittle regime and cleavage fracture emerge while 100mN constant normal force. The tangential friction force of scratching machining process changes from smooth to fluctuate when the varied normal force from 0mN to 50mN, and material removal mode from ductile regime to brittle regime. The critical normal force of ductile transition to brittle is 26mN. However, the scratching machining experimental results near the ductile regime removal mode show that the scratch exists obvious elastic recovery. The elastic recovery of scratch affects the material removal amount and machining accuracy. Based on the scratching machining experimental results, the critical crack-free cutting depth per abrasive for single crystal silicon slicing with fixed abrasive wire saw is analyzed. The results in this paper are useful for the single crystal silicon micro component machining and slicing process parameters selection of fixed abrasive wire saw.

The Effect of the Free Surface on the Singular Stress Field at the Fatigue Crack Front

Abstract Description of stress singularity in the vicinity of a free surface is presented. Its presence causes the retardation of the fatigue crack growth in that region and fatigue crack is being curved. Numerical model is used to study dependence of the stress singularity exponent on Poisson’s ratio. Estimated values are compared to those already published. Experimentally measured angles of fatigue crack on SENB specimens confirm the relation between Poisson’s ratio and the angle between crack front and free surface.

High-Performance GaN-Based LEDs on Si Substrates: The Utility of &lt;italic&gt;Ex Situ&lt;/italic&gt; Low-Temperature AlN Template With Optimal Thickness

Ex situ low-temperature (LT)-AlN templates are employed to realize GaN-based light-emitting diodes (LEDs) on Si substrates by the combination of pulsed laser deposition and metal organic chemical vapor deposition. The influence of ex situ LT-AlN template thickness on the properties and performance of as-grown LEDs has been carefully studied. The LED wafer grown on 70-nm-thick ex situ LT-AlN template exhibits a crack-free surface, good crystalline quality, and excellent multiple quantum wells (MQWs) structural property. The $500\times 500~\mu \text{m}^{\mathbf {2}}$ LED chips fabricated with lateral structure show the highest light output power of 70.2 mW at an injection current of 300 mA. Through optimizing the thickness of ex situ LT-AlN template, the light output power is greatly improved by 39%. Theoretical analysis has been taken to study the enhancement mechanism of luminous intensity in this LED. It reveals that the residual stress in as-grown wafers is significant to enhance the luminescence of LEDs. The largest residual tensile stress in the LED presents great potential to enhance the carrier injection and transportation, increase the carrier concentrations, and improve the radiative recombination intensity in MQWs, which is crucial to improve the luminous intensity of LEDs.

High-quality AlN film grown on a nanosized concave–convex surface sapphire substrate by metalorganic vapor phase epitaxy

We developed a method for fabricating high-crystal-quality AlN films by combining a randomly distributed nanosized concavo–convex sapphire substrate (NCC-SS) and a three-step growth method optimized for NCC-SS, i.e., a 3-nm-thick nucleation layer (870 °C), a 150-nm-thick high-temperature layer (1250 °C), and a 3.2-μm-thick medium-temperature layer (1110 °C). The NCC-SS is easily fabricated using a conventional metalorganic vapor phase epitaxy reactor equipped with a showerhead plate. The resultant AlN film has a crack-free and single-step surface with a root-mean-square roughness of 0.5 nm. The full-widths at half-maxima of the X-ray rocking curve were 50/250 arcsec for the (0002)/(10–12) planes, revealing that the NCC surface is critical for achieving such a high-quality film. Hexagonal-pyramid-shaped voids at the AlN/NCC-SS interface and confinement of dislocations within the 150-nm-thick high-temperature layer were confirmed. The NCC surface feature and resultant faceted voids play an important role in the growth of high-crystal-quality AlN films, likely via localized and/or disordered growth of AlN at the initial stage, contributing to the alignment of high-crystal-quality nuclei and dislocations.

Novel Strontium Doped Zinc Calcium Phosphate Conversion Coating on AZ31 Magnesium Alloy for Biomedical Applications

A simple chemical conversion coating was adopted to deposit zinc calcium phosphate (ZCP) coating and strontium doped ZCP (SZCP) coating on AZ31 magnesium alloy to induce biocompatibility and reduce the degradation rate. The surface morphology, chemical composition and functional groups of the coating were characterized by Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS), X-Ray Diffraction (XRD) studies and Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy respectively. SZCP coating formed at 20 min deposition time produced crack free surface with a high degree of crystallinity with barrier property, which ultimately retards the dissolution of magnesium in Simulated Body Fluid (SBF). The corrosion resistance of AZ31 magnesium alloy was improved by SZCP coating as evident from hydrogen evolution test (HET). Cytotoxicity evaluation with L969 cells showed that Sr doped ZCP coatings showed less toxicity on resorbable magnesium alloys.

Chemical solution deposition of ferroelectric Sr:HfO2 film from inorganic salt precursors

Strontium doped hafnium oxide (Sr:HfO2) ferroelectric thin films with strontium concentrations ranging from 0 to 20mol% were processed with inorganic hafnium source and strontium source using a chemical solution deposition technique. The co-existence of monoclinic phase and ferroelectric orthorhombic phase in the Sr doped HfO2 thin film was confirmed by x-ray diffraction and high-resolution transmission electron microscopy results. The atomic force microscope measurements were adopted and the thin films showed crack-free surface. The intrinsic ferroelectricity of the doped HfO2 thin films could be demonstrated by polarization–voltage hysteresis loops together with piezoelectric force microscope. The maximum twofold remnant polarization value of 3.02 μC/cm2 with a coercive field of 2.0 MV/cm was achieved when the Sr content was 7.5 mol%. Meanwhile, the polarization didn't show obvious degradation until 107 electric cycles, indicating the good fatigue performance of the Sr:HfO2 film. These findings indicate that it is a feasible way to prepare Sr:HfO2 ferroelectric thin films via chemical solution deposition with inorganic salt precursors.

A Novel Fabrication Technique for Electrodes of PEM Water Electrolyzers

State-of-the-art polymer electrolyte membrane water electrolyzers (PEMWEs) are assembled via sandwiching a catalyst coated membrane (CCM) between porous transport layers (PTLs). In this work we investigate electrodes deposited directly on PTLs. Together with the novel fabrication technique ‘direct membrane deposition’ (DMD, Fig 1a-c) - already successfully tested for PEM fuel cells [1,2] – this approach is supposed to lead to high performance MEAs for PEMWE. In DMD the fuel cell is assembled as two half-cells with the gas diffusion layers (GDLs) serving as the base for directly depositing the catalyst and the membrane layer. With this technique contact resistances and mass transport in PEM fuel cells were shown to be substantially improved and a reduction of the noble metal loading was enabled [3].However for the application of DMD a smooth and crack-free electrode surface is required [4]. Since state-of-the-art PTLs like Titanium fiber or powder sinter materials at the anodic side exhibit relatively large pores compared to the size of catalyst particles, the fabrication of a smooth catalyst layer on top of the PTL is the first step to establish the DMD method for PEMWEs (Fig. 1d). In this work we present our latest results on this novel topic and show current challenges and perspectives.

Ductile Regime Machining of Germanium at Varying Depth of Cut

Diamond turning is widely being used from last two decades for the fabrication of optical surfaces with nano-metric accuracy. However, obtaining ductile regime zone during machining is still challenging. Ductile regime machining helps to obtain a crack-free surface on brittle material i.e. germanium, silicon, silicon carbide. The precision manufacturing of these materials is primarily achieved by diamond turning, which normally make use of diamond tool to shear the material and achieve required surface quality with minimum subsurface deformation. This paper investigates the effect of depth of cut on germanium substrate varying from nanometer range to micrometer range (100 nm, 200 nm, 300 nm, 400 nm, 500 nm and 1000 nm, 2000 nm, 3000 nm, 4000 nm). With varying depth of cut surface morphology, surface roughness and subsurface deformation are analyzed. Interferometry technique and SEM are used for the evaluation of surface roughness and surface morphology. Best surface roughness of 3.39 nm is achieved at depth of cut of 300 nm for the machining of germanium with minimum valley and subsurface deformation.

Solution Processed p-Type Cu2ZnSnS4 Thin Films for Absorber Layer

Thin films of quaternary chalcogenide Cu2ZnSnS4 (CZTS) were prepared due to its abundance and non-toxic nature by chemical spray pyrolysis technique with 2-methoxyethanol as solvent for the precursor solution that avoids the insoluble base by-products of zinc and tin. The solution was clear without precipitates as the solvent has higher viscosity compared to DI water which resulted in the successful deposition of smoother films. The structural, morphological, optical and electrical properties of the films at different temperatures 280, 300, 350 and 400 °C were investigated. The structural formation of kesterite Cu2ZnSnS4 was confirmed with a preferential (112) phase and with an intensed 334 cm−1 Raman frequency mode.Uniform film growth and crack-free surface was obtained at a temperature of 350 °C which was confirmed from morphological studies. The optical absorption showed a continuous absorption in the UV–Visible region and the obtained band gap for the films was in the range of 1.45–1.62 eV. The electrical study using Hall measurement proved the p-type conductivity of CZTS thin films which is pertinent for an absorber layer in thin film photovoltaic cell. A p–n junction was formed with TiO2 and CZTS materials and the light and dark current response was studied.

Pesticide tailwater deeply treated by tubular porous electrode reactor (TPER): Purpose for discharging and cost saving

Pesticide tailwater often contains residual and toxic contaminants of triazole fungicides (TFs) due to their poor biodegradability which will do great harm to local aquatic systems. For this case, a novel electrochemical reactor (TPER) equipped a tubular porous RuO2-Sb2O5-SnO2 electrode was assembled and then employed to deeply treat pesticide tailwater. Characterizations of the electrode studied by SEM, EDS and XRD analysis indicated that it owns a porous structure and a compact and crack-free surface. Influence of the porous structure on electrochemical property was examined by cyclic voltammetry and normal pulse voltammetry. The results indicated that porous structure can not only enlarge electrochemical active area but also increase mass transfer efficiency by 5.7-fold in flow-through mode compared with batch mode. Furthermore, the optimal operating conditions of TPER were flow rate of 250 mL min−1 and current density of 4 mA cm−2. After 1.5 h treatment under these conditions, Tz, TC and PPC were removed by 98.9%, 99.0% and 98.4% respectively, while 81.9% of COD was also removed. Additionally, the microbial content was dropped to 0 CFU mL−1 and fecal coliform was lower than 2 MPN (100 mL)−1. All results demonstrated that the treated tailwater has met the Class 1 of National Discharge Standard of China. Especially, operating cost of TPER was only $ 0.33 per ton. The excellent performance together with the low cost indicated that TPER is a promising option for depth treatment of industrial tailwater.

Biodegradation of hydroxyapatite coated Rex-734 alloy with silver and selenium/chitosan substitutions: In vitro analysis

Abstract Hydroxyapatite (HA) based bioceramic coatings; such as single-HA, double layered with Silver-Ag, Selenium-Se and Chitosan substitutions on Rex-734 alloy were executed by using the sol-gel method. Surface morphologies and adhesion strengths of the obtained coatings were investigated and coating material weight losses and metal ion releases from the substrates were determined in SBF (Simulated Body Fluid) environment. The coatings were characterized by SEM, EDS, XRD and adhesion strengths between the substrate and bioceramic coatings were measured with adhesion tests. SEM analyses revealed that crack-free surface morphologies were obtained in all bioceramic coatings. The coatings were kept in SBF for immersion periods from one to four weeks and the weight loss (%) in coatings and changes in ion release rates were determined. HA/Ag double layered coatings applied on the Rex-734 alloy increased the adhesion strength and accordingly reduced the weight losses (%) of coating materials in SBF. Se/Chitosan double layered coatings were found to decrease the adhesion strength and increase the weight losses (%) in SBF. Additionally, all bioceramic coatings were observed to prevent the elemental ion release by physical barrier effect for the release of metal ions during the four weeks immersion period in SBF.

Artificial Multilayers of REBaCuO/YBaCuO and YBCO/CuO(Y2O3) for High-Performance Long Tapes of Coated Conductors Derived by Metal Organic Solution Deposition

Thick superconducting multilayer architectures consisting of alternate superconducting and superconducting layers DyBCO/YBCO and alternate superconducting and insulating layers YBCO/CuO(Y 2 O 3 ) are developed on buffered Hastelloy tapes using a metal organic deposition process. It is revealed that multilayer intermittent structures for superconducting layers are effective to avoid the presence of microcracks that occur with increasing thickness, which are frequently observed in monolayer films of REBa 2 Cu 3 O 7-δ (REBCO, RE = Y, Gd, or other rare earth elements). This paper shows that the thicknesses of DyBCO/YBCO multilayer films can reach 2.8 μm with a homogeneous, dense and crack-free surface morphology. The out-plane FWHM of the YBCO (005) peaks and the critical current value reach 1.1° and 365 A/cm (77 K, self-field) for such a thick film consisting of DyBCO/YBCO multilayers. It is also revealed that the introduction of a CuO layer is effective to avoid the presence of a-axis grains and voids of the YBCO films, which are frequently observed in multilayer films. Note that the introduction of CuO shows little negative influence on the texture of multilayer structure, especially for the bottom CuO. However, the in-plane texture deteriorates seriously for Y 2 O 3 /YBCO/YBCO multilayer film. The critical current density of YBCO films with middle CuO and bottom CuO are 2.0 MA/cm 2 and 2.5 MA/cm 2 (77 K, self-field), respectively. The microstructure optimization including the texture, uniformity, crack, defect, and porosity in the multilayered films is probably the reason for the enhancement of superconducting performance.

Full-band error control and crack-free surface fabrication techniques for ultra-precision fly cutting of large-aperture KDP crystals

Large-aperture potassium dihydrogen phosphate (KDP) crystals are widely used in the laser path of inertial confinement fusion (ICF) systems. The most common method of manufacturing half-meter KDP crystals is ultra-precision fly cutting. When processing KDP crystals by ultra-precision fly cutting, the dynamic characteristics of the fly cutting machine and fluctuations in the fly cutting environment are translated into surface errors at different spatial frequency bands. These machining errors should be suppressed effectively to guarantee that KDP crystals meet the full-band machining accuracy specified in the evaluation index. In this study, the anisotropic machinability of KDP crystals and the causes of typical surface errors in ultra-precision fly cutting of the material are investigated. The structures of the fly cutting machine and existing processing parameters are optimized to improve the machined surface quality. The findings are theoretically and practically important in the development of high-energy laser systems in China.