Pore Propagation Research Articles

HYBRID METAL-SEMICONDUCTOR STRUCTURES BASED ON InP AND GaAs NANOTEMPLATES FOR ELECTRONIC AND PHOTONIC APPLICATIONS

In this scientific work are presented the results that contribute to solving an important scientific problem related to obtaining of porous templates with controlled morphology and design by replacing acidic and alkaline electrolytes, the use of which presents a danger for the environment, with neutral electrolyte (NaCl) as well as obtaining of the metal-semiconductor hybrid structures using pulsed electrodeposition that offers additional possibilities to control the localized deposition in certain portions of the porous template and allows the controlled fabrication of nanodots, nanowires, nanotubes and perforated metal nanomembranes. Mechanisms of pore propagation in InP and GaAs semiconductor substrates and electrochemical deposition of metals in the produced porous templates are identified and discussed, which allowed to control the direction of pore growth, including those parallel to the substrate surface as well as localized Au deposition.

Propagation of elastic waves in adhesive contacts: experiment and numerical model

The entry and propagation of pores inside an adhesive interface between an elastomer and a rigid sphere were studied experimentally and simulated numerically. It was shown that mutually interacting events involving attachment–detachment of different segments of the elastomer to the indenter resulted in non-trivial patterns of spatially distributed contacts between them, which were additionally influenced by air penetration of the pores.

Pore structure response at different scales in coal to cyclical liquid nitrogen treatment and its impact on permeability and micromechanical properties

The methane in the coal seams of abandoned mines is a valuable natural gas resource. However, the ultra-low permeability of coal seams restricts the extraction of coalbed methane. The liquid nitrogen fracturing technology is a novel approach suitable for enhancing the permeability of coal seams in abandoned mines. The ultra-low temperature could potentially facilitate the growth and propagation of pores and fractures in coal seams. In this study, we observed inconsistent alterations in coal properties measured by multiple instruments at different scales, whether in dry or wet coal specimens. This suggests that the mechanisms influencing the pore structure due to LN2 treatment differ across various scales in dry and wet coal specimens. For dry specimens, heterogeneous thermal deformation and freezing shrinkage exhibited opposing effects during LN2 treatment. Thermal stress-induced micro-fractures might counteract the freezing contraction of micropores in coal matrices, preventing a significant decrease in coal macropores and fractures. In wet specimens, the effects of LN2 treatment on wet coal specimens were predominantly controlled by frost heaving. However, due to low water saturation, LN2 treatment had negligible effects on coal micropores, even in the presence of local frost heaving. In field applications, water migration from smaller to larger pores could further diminish the impact of LN2 treatment on micropores.

Electrochemical etching of n-type GaN in different electrolytes

Nanosized pores were introduced into GaN epitaxial films via electrochemical (EC) etching in strong electrolytes including NaOH, HNO3, NaNO3, and NaCl. The growth behaviors of GaN nanopores show a strong correlation with the electrolyte types under the same applied bias and electrolyte concentration. Specifically, the resulting morphologies (e.g. pore growth direction and pore size) were mainly pertinent to the anions instead of cations. Pore sizes as small as a few nanometers and pore directions between the horizontal and vertical directions can be adjusted by selecting the electrolytes and etching voltages. The depletion model was employed to interpret the carrier flow pathways, and the pore propagation direction can be regarded as the sum of etch rate vectors of intended etching direction and preferential oxidation direction. Finally, the near-infrared nanoporous (NP) GaN distributed Bragg reflectors (DBRs) were manufactured by the EC etching in different electrolytes. Our study points out that the type of anions in electrolyte greatly impacts pore morphologies of EC etched GaN:Si, thereby affecting the future applications.

Degradation of physico-mechanical properties and damage mechanisms of sandstone under the combined action of freeze–thaw cycles, chemical solution, and compressive stress

Rocks in cold regions subjected to freeze–thaw cycles are often affected by the surrounding hydro-chemical solutions and external load. In this study, the evolution of physical and mechanical characteristics and the damage mechanism of sandstone under the combined action of freeze–thaw cycles, chemical solution, and compressive stress were investigated. A series of quick cyclic freeze–thaw tests in different chemical solutions and stress levels were conducted. The dry mass, wave velocity, and compressive strength decrease as the number of freeze–thaw cycles increases. The variations in dry mass, wave velocity, and rock appearance indicate that lower stress levels (<0.5σ0, σ0 is the saturated uniaxial compressive strength of the original specimen) can limit the effect of repeated freeze–thaw cycles and chemical corrosion, whereas higher stress levels (>0.5σ0) can exacerbate the combined effect on rock damage. In addition, a modified decay model is proposed that incorporates the initial properties and residual integrity of the rock to describe the evolution of rock strength and wave velocity. Both the residual strength and wave velocity of sandstone are constants that are independent of stress level and chemical solution. The degradation rate of compressive strength increases abruptly at higher stress, whereas that of wave velocity generally decreases with increasing stress level. The continuous action of the cyclic frost-heaving force, hydrolysis and chemical corrosion, and external compressive stress promote generation and propagation of pores and microcracks, causing deterioration of the mass, wave velocity, and compressive strength.

Structural evolution and performance enhancement of coal-based metal-free N,P-doped porous carbon electrocatalysts for pH-universal oxygen reduction

Structural strengths of carbon-based supports, which hold up catalytically active sites and electron transfer, need to be considered in terms of carbonaceous electrocatalysts because weak supports often give unstable or poor performance in electrochemical environments. Coal can be a reasonable option for an attractive carbon source in view of its low expense, extensive reserve and natural robustness. Conversion of coal to coal-based carbon, instead of conventionally burning it off as fuel to emit CO2, is increasingly finding its own niche in the forefront of chemical engineering. However, pore propagation and surface modification of coal are difficult to handle due to its hardness characteristics, limiting performance enhancement of coal-based catalysts. To tackle this problem, we propose a double-round activation/doping method and disclose significant influences of different reaction sequences on structure-performance correlations. The coal-based metal-free N,P-doped porous carbon materials manifest the improved performance of electrocatalytic oxygen reduction in the alkaline, acidic and neutral electrolytes, along with robust behaviours including good resistance to harmful ions and conceptual demonstration of electrochemical devices. It is also revealed that the electrocatalytic performance of coal-based catalysts is closely related both to their textural properties and to concerted effects induced by the coexistence and uniform distributions of N and P atoms doped into carbon matrices, as demonstrated by the experimental and theoretical results. Consequently, this work gives intriguing insights into structural evolution and performance enhancement of robust coal-based electrocatalysts from the perspective of reaction sequences and provides inspiration for low-carbon utilization of coal in regard to coal-to-carbon transformations.

Mixed‐mode I/II fracture properties and failure characteristics of microwave‐irradiated basalt: An experimental study

AbstractThe notched semicircular bending samples with different microwave irradiations were subjected to 3‐point bending fracture tests under mixed‐mode I/II loading to investigate the effects of microwave irradiation on the fracture characteristics of basalt. Influences of the irradiation duration on the temperature, P‐wave velocity, and mixed‐mode I/II fracture characteristics under 2 kW power were studied. The acoustic emission device was utilized to monitor the acoustic emission signals during the destruction process. The P‐wave velocity and equivalent fracture toughness are weakened by microwave irradiation, but the ductility of basalt is enhanced by microwave irradiation. The microscopic characteristics of the treated basalts were observed by scanning electron microscope. Microwave irradiation aggravated the creation and propagation of microcracks and pores. The study of the influence of microwave irradiation on mixed‐mode I/II fracture characteristics of rock provides effective theoretical support for microwave‐assisted rock fragmentation.

The α phase recrystallization mechanism and mechanical properties of a near-α titanium matrix composite

In this paper, the recrystallization mechanism of α phase, microstructure evolution and tensile fracture behavior of in-situ (TiBw + TiCp)/near-α titanium matrix composites have been investigated. The subgrain boundaries formation within αp was caused by strong recovery. The reinforcements played an important role in the recrystallization of α phase. Therefore, the continuous dynamic recrystallization, reinforcements stimulated recrystallization and static recrystallization have a common effect on the αp recrystallization. The room temperature ductility was dominated by the size of silicides, slip transmission between αp and α/β colony, and the morphology of microstructure. The large-scale silicide caused the reduction of ductility at room temperature. After annealed at 1040 °C, the ultimate tensile strength and yield strength at 650 °C of the composites was 745 MPa and 620 MPa, respectively, and the yield strength at 700 °C reached 530 MPa, which has a balance between strength and ductility. The fracture of the composites was associated with the dislocation migration hindered by the boundaries or interaction with each other and the slip of boundaries. Therefore, pores will nucleate in this region due to stress concentration, and the eventual failure were mainly caused by the propagation and connection of pores to big cracks. The above superior performances make the titanium matrix composites have a great application prospect in aerospace fields.

Research Progress of Coal Damage under Unsteady Load in China

Coalbed methane mining, suppression of coal dust, and elimination of dynamic disasters are closely related to the expansion of coal body cracks and internal damage. Understanding the expansion mechanism of pore-cracks is critical to investigate coal body damage. In this study, research from 2016 to 2021 conducted on the coal damage mechanism in China was sorted and the progress in this field was analysed to systematically investigate coal body damage. Critical topics of research in this field in recent years were identified, and load types were classified into static and dynamic loads. Dynamic loads with obvious characteristics and considerable damage-increasing effects were classified into impacting, cyclic, pulsating, and other dynamic load types. The current load-generating devices, various detection techniques and methods, research results, and the future research directions under various load types were discussed. The current coal damage research is primarily based on macrocharacteristic analysis and the stage characteristics of characterisation variables. The use of scanning electron microscopy, computerised tomography three-dimensional reconstruction technology, and acoustic emission technology can reveal the pore propagation mechanism at the micro level.

Enhancing high-temperature strength and ductility of γ-TiAl matrix composites with controllable dual alloy structure

Gamma-TiAl matrix composites can function as promising high-temperature structural materials. In this study, to improve the poor ductility and insufficient strength of composites at high temperatures, they were successfully prepared via powder metallurgy by adding the Ti–6Al–4V alloy. The experimental results indicate that the soft phases of Ti–6Al–4V were surrounded by the hard phase of Ti–48Al–2Cr–2Nb, and the special structure exhibited a unique strain hardening mechanism, which resulted in the high strength and ductility of the composites at high temperature. The 8 wt% Ti–6Al–4V composites were fractured with an elongation approximately 90.65% and tensile strength of 427.75 MPa at 800 °C and 5 × 10−5 s−1; the elongation and tensile strength were both higher than those of single unreinforced Ti–48Al–2Cr–2Nb alloy. Moreover, with an increase in the deformation strain, the deformation behavior started from the Ti–6Al–4V area, which had good ductility. The dislocation movement was hindered by the Ti–48Al–2Cr–2Nb area, and dislocation accumulation and grain deformation occurred, leading to strength increase. The stress concentration and high-density dislocations were stored in the soft phases, which delayed the initiation and propagation of pores and cracks. Moreover, besides grain sliding, evident dislocation activities occurred; therefore, the high ductility of the composites was obtained.

Statistical analysis of defects within concrete under elevated temperatures based on SEM image

Concrete is a multiphase composite material consisting of aggregates, mortar, and initial defects. This work aims to quantitatively study the distribution and evolution of initial defects under elevated temperatures. To minimize the effect of hydration process and thermal and endogenous shrinkage, concrete samples cured in water for 12 months is tested. In order to study the effect of external load on the initial distribution of defects within concrete, concrete samples without pre-loading and pre-applied with 30% of the failure load are selected as research objects. The evolution of defects within the concrete samples are characterized using scanning electron microscopy (SEM). By employing an image processing software, the two-dimensional configurations of defects are extracted from the backscattered images obtained by SEM. Each defect is identified and approximated using an optimal ellipse. Thereafter, the defects are automatically recognized and classified into microcracks and pores by utilizing the image processing software and MATLAB programming. Consequently, the distribution and propagation of microcracks and pores in concrete could be quantitatively and separately analyzed. Statistical analysis of area, perimeter, and fitting ellipse parameters (e.g., major axis, minor axis, coordinates of the center, axis orientation, aspect ratio, and solidity) can provide the distribution of initial defects within concrete. It is demonstrated that the area of microcracks consistently increases with temperature. Log-normal and normal distributions may be suitable for the distribution of aspect ratio and solidity of microcracks, respectively, for both concrete samples with and without pre-applied loading under all elevated temperatures (40, 105, 150, 200, and 250 ℃). However, the distributions of area, coordinates of the center, and axis orientation of microcracks may vary with the initial concrete loading condition and temperature. For the pre-loaded concrete sample, the distribution of parameters does not conform to any type of distributions. Moreover, for the sample without pre-loading, the distributions of area, coordinates of the center, and axis orientation of microcracks exhibit good fitting precisions with log-normal, normal, and normal distributions, respectively at lower temperatures (40–150 ℃); however, such relationships do not apply at higher temperatures (200 and 250 ℃). As reference for identifying the variations in microstructures, 150 ℃ is selected as the threshold temperature. The statistical approach employed in this study can lay the theoretical and experimental bases for determining the microstructure evolution in concrete. It can also be used for identifying the meso-defect and macro-defect and establishing their relationship with concrete properties.

Impact of water\u2013rock interaction on the pore structures of red-bed soft rock

The physical and mechanical properties of the reservoir bank slope are affected by the water–rock interaction. However, few studies considered the impact of long-term water–rock interaction on the evolution law of mesostructure. Therefore, in this study, the water–rock interaction test was conducted on a slightly weathered red-bed soft rock from the Three Gorges Reservoir area, considering the fluctuation in the reservoir water level. The corresponding pore structure parameters were measured and analyzed based on a scanning electron microscope (SEM) and digital image processing technology. The study showed that: (1) The pore size has been gradually increased, while the number of pores was increased initially and then decreased. Within 12 cycles, the maximum and average pore radius of the rock specimens was increased by 101.02% and 43.32%, respectively, and the porosity has been increased by 26.59%, whereas the number of pores decreased by 22.65%. This indicates the effect of water–rock interaction on the propagation of pores. (2) The pores were changed from oblate to slender by the water–rock interaction. The shape factor was decreased by about 15.79% within 12 cycles. In the meantime, the fractal dimension was increased from 1.20 to 1.28, and more complex structures of pores were observed. (3) The porosity evolution model for the red-bed soft rock was established based on the curve fitting technique. The results can be used as a reference to conceptualize the mesostructure damage of rocks under water–rock interaction.

Stress Corrosion Cracking Probability of Selective Laser Melted 316L Austenitic Stainless Steel under the Effect of Grinding Induced Residual Stresses

Surface quality and dimensional tolerances of the selective laser melting (SLM) process are not good enough for many industrial applications and grinding as a common finishing process introduces many surface modifications. Investigation on the effect of grinding induced surface residual stress (RS) on early stages of stress corrosion cracking (SCC) of SLM manufactured 316L austenitic stainless steel was conducted. Potentiodynamic and galvanostatic tests in a 3.5% NaCl aqueous solution, XRD, SEM and energy-dispersive X-ray spectroscopy (EDX) analysis were performed. For annealed and specimens with a low RS magnitude, the dominant observation was pit initiation from existing pores and growth in the build direction. For specimens with medium RS level, SCC initiation from pore sites and propagation along melt pool boundaries and for specimens with the highest detected RS, crack initiation from melt pool boundaries, grains, machining marks, and pore sites were observed. Cracks propagated in different directions, i.e., along melt pool boundaries, near-surface transgranular, and transgranular through columnar microstructure. Galvanostatic tests showed three distinctive regions that corresponded to crack and pit initiation and growth. The synergistic effect of high dislocation density along melt pool boundaries, stress concentration in pore sites, molybdenum segregation, and surface RS was the cause of SCC susceptibility of specimens with high RS magnitude.

Creep damage properties of sandstone under dry-wet cycles

Rock creep properties can be used to predict the long-term stability in rock engineering. In reservoir bank slopes, sandstones which are frequently used in the bank slope undergoing long-term effects of dry-wet (DW) cycles due to periodic water inundation and drainage may gradually accumulate creep deformation, resulting in rock structure’s damage or even geological hazards such as landslides. To fully investigate the effect of DW cycles on the creep damage properties of sandstone, triaxial creep tests were conducted on saturated sandstone with different DW cycles by using a triaxial rheometer apparatus. The experimental results show that both the instantaneous strain and the stabilized strain increase with the DW cycles. In addition, using the Burgers model, four kinds of functions including an exponentially decreasing function, a linearly decreasing function, a linearly increasing function and an exponentially increasing function were proposed to express the relationships between the shear modulus, viscoelastic parameters of the Burgers model and the deviatoric stress under different DW cycles. Through comparative analysis, it is found that the theoretical curves generated using proposed four kinds of functions are in good agreement with the experimental data. Furthermore, macromorphological and microstructural observations were performed on specimens after various triaxial rheological tests. For samples with small number of DW cycles, approximately X-shaped fracture surfaces were observed in shear failure zones, whereas several shear fractures including obvious axial and horizontal tensile cracks, and flaws were found for samples with relatively large DW cycles due to long-term propagation and evolution of micro-fissures and micro-pores. Furthermore, as the DW cycles increases, the variation in micro-structure of samples after creep failure was summarized into three stages, namely, a stage with good and dense structure, a stage with pore and fissure propagation, and a stage with extensive increase of pores, fissures and loose particles. It is concluded that the combination effect of permeation of water molecules through pores and fissures within sandstone, and the propagation of preexisting pores and fissures owing to the dissolution of mineral particles leads to further deterioration of the mechanical properties of sandstone as the number of DW cycles increases. This study provides a fundamental basis for evaluating the long-term stability of reservoir bank slopes under cyclic fluctuations of water level.

(Invited) Development of Nanoporous Structures and Oscillatory Behavior During Anodization of n-InP in Alkaline Electrolytes

Electrochemical etching of semiconductors under anodic conditions can lead to the growth of nanoporous structures. There has been extensive research in this area for silicon and III-V electrodes1-7. Porous structures are obtained3-7 when n-InP is anodized in aqueous KOH at concentrations of 2 mol dm-3 or greater; planar etching occurs below 1 mol dm-3. The observation of current oscillations during the anodization of semiconductors has been reported by several authors.8-11 Current oscillations on silicon in fluoride-containing electrolytes have been linked to the cyclic growth and dissolution of thin oxide layers. We have observed10,11 current oscillations on InP both in aqueous KOH and in aqueous (NH4)2S.A typical current-voltage curve of InP anodized in 5 mol dm-3 KOH is shown in Fig. 1. The anodic peak at 1.9 V (SCE) corresponds to the dissolution of InP leading to the formation of a porous InP layer that extends over 500 nm into the substrate. At higher potentials, sustained oscillations are observed.Porosity originates from pits in the surface creating domains of pores beneath a thin dense near-surface layer of ~40 nm.4 Each domain is connected to the surface via an individual channel, and eventually these domains merge to form a continuous porous layer. Pore propagation in InP in KOH is usually crystallographically oriented (CO), although it can be current-line-oriented (CLO) under certain conditions. We have proposed5 a three-step model of electrochemical nanopore formation that explains how crystallographically oriented etching can occur even though the rate-determining process (hole generation) occurs only at pore tips. The model shows that competition in kinetics between hole diffusion and electrochemical reaction determines the average diffusion distance of holes along the semiconductor surface and this, in turn, determines whether etching is crystallographic. If the kinetics of reaction is slow relative to diffusion, etching can occur at preferred crystallographic sites within a zone in the vicinity of the pore tip, leading to pore propagation in preferential directions. We believe that the model is generally applicable to electrochemical etching in III-V semiconductors.Current oscillations on n-InP in aqueous (NH4)2S occur above ~1.7 V(SCE). Similarly, in galvanostatic measurements oscillations in potential occur at higher current densities. A remarkable linear relationship is observed between the frequency of oscillation and the average current density. Thus, the oscillations exhibit a characteristic constant charge per cycle. Anodization of highly doped n-InP in aqueous KOH also shows current oscillations at higher potentials. In this case, however, the charge per cycle increases linearly with potential.The oscillatory current region in the InP/(NH4)2S system corresponds to growth of a thick porous In2S3 layer on the electrode. In contrast, the oscillatory region in the InP/KOH system corresponds to anodic dissolution of InP with the formation of a porous region near the surface. The characteristics of the InP/KOH system are very similar to reported behavior for silicon in HF, where porous silicon formation and anodic current oscillations are also observed.We will review our work on nanoporosity and oscillatory behavior in these systems and discuss the underlying mechanisms. References R.L. Smith and S.D. Collins, J. Appl. Phys 71, 8 (1992)H. Föll, J. Carstensen, S. Langa, M. Christophersen and I.M. Tiginyanu, phys. stat. sol. (a) 197, 61 (2003) N. Quill, D. N. Buckley, C. O’Dwyer and R. P. Lynch, J. Electrochem. Soc. 166, H3097 (2019)R. P. Lynch, C. O’Dwyer, N. Quill, S. Nakahara, S. B. Newcomb and D. Noel Buckley, J. Electrochem. Soc., 160, D260 (2013).R. P. Lynch, N. Quill, C. O'Dwyer, S. Nakahara and D. N. Buckley, Phys. Chem. Chem. Phys., 15, 15135 (2013).R. Lynch, C. O’Dwyer, D.N. Buckley, D. Sutton and S.B. Newcomb, ECS Trans 2, 131 (2006)C. O'Dwyer, D.N. Buckley, D. Sutton, and S.B. Newcomb, J. Electrochem. Soc. 153, G1039 (2006)S. Langa, J. Carstensen, I.M. Tiginyanu, M. Christophersen and H. Föll, Electrochem. Solid-State Lett., 4, G50 (2001)V. Parkhutik, Mater. Sci. Eng. B, 88, 269 (2002)E. Harvey, D. N. Buckley and S. N. G. Chu, Electrochem. Solid State Letters 5, G22 (2002)C. O’Dwyer, T. Melly, E. Harvey, D. N. Buckley, V. J. Cunnane, D. Sutton, and S. B. Newcomb in Proceedings of the 37th State-of-the-Art Program on Compound Semiconductors, PV 2002-14, pp. 275-285, (ISBN 1-56677-336-9, The Electrochemical Society, Pennington, NJ, 2002). Figure 1

(Invited) Development of Nanoporous Structures and Oscillatory Behavior During Anodization of n-InP in Alkaline Electrolytes

In the early stages of anodization of n-InP in KOH, isolated nanoporous domains form and eventually merge into a continuous layer. SEM and TEM cross-sections show clear evidence that these have the shape of truncated tetrahedrons and result from preferential pore progagation along the <111>A directions. We propose a three-step model of electrochemical nanopore formation that explains how crystallographically oriented etching can occur even though the rate-determining process occurs only at pore tips. The rate-determining Step 1 is the generation of holes at pore tips. If Step 2 (hole diffusion) is faster than Step 3 (electrochemical reaction) then etching can occur at preferred crystallographic sites, such as phosphorus dangling bonds, leading to pore propagation in preferential directions. Well-defined oscillations are observed under certain conditions when n-InP is anodized in either KOH or (NH4)2S. The oscillations in (NH4)2S, have a symmetric profile and the charge per cycle remains constant over a wide range of conditions: we propose a mechanism based on cyclical changes in the morphology of the InP surface beneath an In2S3 film. The exact mechanism of the oscillatory behavior in KOH is still unclear.

Etching Mechanisms in III-V Semiconductors: Electrochemical Etching of Indium Phosphide

Etching of III-V semiconductors is often observed to be anisotropic. For example both chemical and electrochemical etching of III-V semiconductors show preferential etching of {111}B planes (i.e. group-V-terminated planes).1,2 The slowest-etching plane is usually {111}A and so these facets are revealed during etching of InP, GaAs and GaP. Due to the differing etch rates of crystal planes, the formation of tetrahedral etch pits (seen as dove-tailed and v-groove voids, respectively, in (011) and (01$\\bar{1}$) cross sections) is observed on the (100) surface of III-V semiconductors. The anodic etching of many materials leads to the formation of porous structures. When semiconducting materials are anodized, localized etching can occur leading to selective removal of material such that the remaining material forms a skeletal structure that encompasses a network of pores. It is quite common for such pores to propagate along preferential directions. These directions and the pore morphology have been shown to be affected by a range of parameters including temperature, composition and concentration of electrolyte, and type, orientation and doping density of the substrate. We have investigated in detail the early stages of pore formation in InP during anodization in aqueous electrolytes (KOH, KCl etc.). Pores originate from pits in the surface and propagate along <111>A directions to form porous domains beneath a thin near-surface layer penetrated only by one surface pit per domain of pores.3-5 Eventually these domains merge to create a continuous porous layer. Although pore propagation in InP in KOH is usually crystallographically oriented (CO), it can be current-line-oriented (CLO) under certain conditions. Indeed, variation of potential, electrolyte concentration and temperature causes a gradual change in the porous layer structure and a transition from pore formation to planar etching is observed as the KOH concentration is decreased to below 2 mol dm-3. We have proposed a three-step model6,7 of electrochemical nanopore formation that explains how crystallographically oriented etching can occur even though the rate-determining process (hole generation) occurs only at pore tips. The model shows that competition in kinetics between hole diffusion and electrochemical reaction determines the average diffusion distance of holes along the semiconductor surface and this, in turn, determines whether etching is crystallographic. If the kinetics of reaction is slow relative to diffusion, etching can occur at preferred crystallographic sites within a zone in the vicinity of the pore tip, leading to pore propagation in preferential directions. We believe that the model is generally applicable to electrochemical etching in III-V semiconductors. In this paper, we will examine the effect of various parameters including temperature, electrolyte composition, potential and current density on electrochemical etching and nanopore formation in InP. We will discuss the processes leading to crystallographic etching, the factors which affect pore morphology and the transition from porous to planar etching and show that the three-step mechanism can comprehensively explain these various features. References H. C. Gatos and M. J. Lavine, J. Electrochem. Soc., 107, 427 (1960).D. N. MacFayden, J. Electrochem. Soc., 130, 1934 (1983).C. O’Dwyer, D. N. Buckley, D. Sutton, S. B. Newcomb, J. Electrochem. Soc., 153, G1039 (2006).C. O’Dwyer, D. N. Buckley, D. Sutton, M. Serantoni, S. B. Newcomb, J. Electrochem. Soc., 154, H78 (2007).R. P. Lynch, C. O’Dwyer, N. Quill, S. Nakahara, S. B. Newcomb, D. N. Buckley, J. Electrochem. Soc., 160, D260 (2013).R. P. Lynch, N. Quill, C. O’Dwyer, S. Nakahara, D. N. Buckley, Phys. Chem. Chem. Phys., 15, 15135 (2013).N. Quill, D. N. Buckley, C. O’Dwyer, R. P. Lynch, J. Electrochem. Soc. 166, H1 (2019) and references therein.

Electrochemical Deposition by Design of Metal Nanostructures

We report on the application of specially-designed masks for the purpose of electrochemical etching of InP single crystals which enables one to change in a controlled fashion the direction of propagation of pores, including those propagating in directions parallel to the top surface of substrates. The fabricated templates have been used to electrochemically deposit metallic nanostructures along predefined directions and to develop two-dimensional arrays of metallic nanotubes or nanowires embedded in semiconductor matrices.

(Invited) Electrochemical Formation of Nanoporous Indium Phosphide in KOH Electrolytes

Nanotechnology is an area of growing industrial importance and there is considerable interest in the fabrication and characterisation of nano-scale structures for various applications. Although considerable progress has been made in developing technology for such applications, the underlying fundamentals are not always well understood and many important scientific questions need to be investigated if technological progress is to be maintained. Fundamental studies of key processes such as nanopore formation therefore provide important pathways to understanding and controlling nanofabrication techniques. The anodic etching of many materials leads to the formation of porous structures. When semiconducting materials are anodized, localized etching can occur leading to selective removal of material such that the remaining material forms a skeletal structure that encompasses a network of pores. It is quite common for such pores to propagate along preferential directions. These directions and the pore morphology have been shown to be affected by a range of parameters including temperature, composition and concentration of electrolyte, and type, orientation and doping density of the substrate. Many different pore morphologies have been observed and several models have been proposed to explain them. In this paper, we review our work1-5 on nanopore formation during anodization of n-InP electrodes in aqueous KOH. When the potential of an InP electrode in a KOH solution (2 – 5 mol dm-3) is swept in the anodic direction, a current peak is obtained after which the current decreases to a low value. Cross-sectional electron microscopy of electrodes shows the formation of a porous sub-surface region of fairly uniform thickness separated from the surface by a thin non-porous region, ~40 nm in thickness. Porous InP structures can be formed both under potentiodynamic and potentiostatic conditions. Remarkably, however, porous InP layers are not formed in KOH solutions more dilute than 1.5 mol dm-3. Detailed scanning and transmission electron microscopy (SEM and TEM) shows that pores emanating from surface pits propagate along the <111>A crystallographic directions to form, in the early stages of anodization, porous domains (Fig. 1) with the shape of a tetrahedron truncated symmetrically through its center by a plane parallel to the (100) surface of the electrode. This was confirmed by comparing the predictions of a detailed model of pore propagation with SEM and TEM observations. The model considered pores originating from a pit at the (100) surface and propagating along <111>A directions at rates equal at any instant in time. It showed in detail how this leads to domains with the shape of a tetrahedron truncated by a (100) plane. Observed cross sections correspond in detail and with good precision to those predicted by the model. SEM and TEM showed that cross sections are trapezoidal and triangular, respectively, in the two cleavage planes of the wafer, and TEM showed that they are rectangular in the surface plane, as predicted. Aspect ratios and angles calculated from observed cross sections were in good agreement with predicted values. The pore patterns observed were also in good agreement with those predicted and SEM observations of the surface further confirmed details of the model. We have proposed a three-step model of electrochemical nanopore formation that explains how crystallographically oriented etching can occur even though the rate-determining process (hole generation) occurs only at pore tips. The model shows that competition in kinetics between hole diffusion and electrochemical reaction determines the average diffusion distance of holes along the semiconductor surface and this, in turn, determines whether etching is crystallographic. If the kinetics of reaction is slow relative to diffusion, etching can occur at preferred crystallographic sites within a zone in the vicinity of the pore tip, leading to pore propagation in preferential directions. The model explains the observed uniform width of pores and its variation with temperature, carrier concentration and electrolyte concentration. It also explains pore wall thickness, and deviations of pore propagation from the <111>A directions. We believe that the model is generally applicable to electrochemical pore formation in III-V semiconductors. REFERENCES R. P. Lynch, N. Quill, C. O’Dwyer, S. Nakahara and D. N. Buckley, Phys. Chem. Chem. Phys., 15, 15135-15145 (2013)R. P. Lynch, C. O’Dwyer, N. Quill, S. Nakahara, S.B. Newcomb and D. N. Buckley, J. Electrochem. Soc., 2013, 160, D260N. Quill, R. P. Lynch, C. O’Dwyer and D. N. Buckley, ECS Trans., 2013, 50 (37), 143C. O’Dwyer, D. N. Buckley, D. Sutton, M. Serantoni and S. B. Newcomb, J. Electrochem. Soc., 2007, 154, H78C. O’Dwyer, D. N. Buckley, D. Sutton and S. B. Newcomb, J. Electrochem. Soc., 2006, 153, G1039 Figure 1

Protective trivalent Cr-based electrochemical coatings for gun barrels

Conventional Cr as a protective coating on gun barrels is electro-deposited from toxic hexavalent chromium electrolyte, which can be susceptible to cracking and tends to spall off. Herein, environment friendly trivalent Cr bath was utilized to electrodeposit chromium based composite coatings on armour steel with the incorporation of 3 mol% yttria stabilised zirconia (3YSZ) and carbon nano tubes (CNT). Higher dislocation density (17 × 1015 m−2), yield strength (4.6 GPa), calculated using Taylor's model, and reduced plasticity index (0.29) resulted in an enhanced hardness (∼25 GPa) and elastic modulus (∼206 GPa) for the synergistically incorporated YSZ and CNT in Cr matrix when compared to the Cr, Cr-YSZ and Cr-CNT coatings (using micro and nano-indentation techniques). Moreover, the fretting wear testing also elicited Cr-YSZ-CNT as most adroit coating having enhanced wear resistance (wear rate ∼3.7 × 10−5 mm3/Nm) due to the efficient presence of compressive stress (−918 MPa) and fine crystallite size (11 nm). The reduction in Hertzian contact diameter, from 32 μm for Cr to 27 μm for Cr-YSZ-CNT coating, results in a decreased tribological interaction between the counter body and coating material during the wear test. The mechanism involved in Cr-YSZ-CNT coating, grafted significantly with the mechanical and tribological performance, was attributed to toughening by YSZ nanoparticles that strengthen Cr matrix and impart higher resistance to crack-propagation during fretting wear. While the lubrication and bridging phenomenon of CNTs can potentially seal-up the initiation and propagation of cracks and pores within the matrix, thereby reducing the damage due to the wear. Thus, the dexterous mechanical and tribological performance of Cr-YSZ-CNT coating over Cr, Cr-YSZ and Cr-CNT coatings, makes it suitable composite material for gun barrel protection.