Crystallization Mechanism Research Articles

EFFECTS OF DIFFERENT LOADING DIRECTIONS ON THE ORGANIZATION AND PROPERTIES OF MAGNESIUM SINGLE CRYSTALS BASED ON MOLECULAR DYNAMICS

Magnesium and its alloys are widely used as high-quality metal materials in various fields. In this study, the tensile properties of magnesium single crystals under different loading directions were investigated based on the molecular dynamics theory. By analyzing the changes in stress, potential energy, crystal structure, and dislocation lines, the following conclusions can be drawn: There are differences in strain values, dislocation line arrangement laws, and crystal structures of magnesium single crystals when dislocation lines are generated under different loading directions. However, the types of dislocation lines are generally the same, with 1/3­ dislocations and dislocations with an unknown structure being dominant. Furthermore, the results indicate that the number of 1/3­ dislocations is larger than that of dislocations with an unknown structure. These findings are of great significance for a deeper understanding of the deformation mechanism of magnesium single crystals.

Investigation on Crystallization of CaO‐Al2O3‐B2O3‐BaO Slag Using Differential Scanning Calorimeter and In Situ High‐Temperature Raman Spectroscopy

CaO‐Al2O3‐B2O3‐based slag is among the most promising “nonreactive” mold fluxes for continuous casting of high‐aluminum steel. However, CaO‐Al2O3‐B2O3‐based slag system exhibits a stronger crystallization ability compared to traditional mold fluxes. Herein, calcium oxide in CaO‐Al2O3‐10%B2O3 slag is partially replaced by the same mass of barium oxide (5 and 10 mass%) to adjust the crystallization ability. The crystallization of glassy slags is then investigated using a differential scanning calorimeter and their crystallization kinetics are analyzed using the Matusita–Sakka model. The structural evolution of glassy slags from room temperature to 1200 °C is also investigated using in situ Raman spectroscopy. The kinetic analysis shows that the crystallization process of CaO‐Al2O3‐B2O3‐BaO glassy slags follows the surface crystallization mechanism. The predominant crystalline products are calcium monoaluminate (CaAl2O4) and calcium borate (Ca3(BO3)2). The partial replacement of calcium oxide by barium oxide inhibits the growth of CaAl2O4 in the slag by increasing the activation energy of crystal growth. The high‐temperature Raman spectroscopy study shows that the band between 700 and 900 cm−1 gets weaker relative to the band between 450 and 650 cm−1 at high temperatures. This indicates the strengthening of bending vibration of Al‐O‐Al, which is consistent with the precipitation of CaAl2O4 with full oxygen bridging.

Crystallization of CoWB ternary boride in nickel-based metallic glass

The crystallization kinetics of CoWB ternary boride were investigated through non-isothermal differential scanning calorimetry experiments, using a Ni-based metallic glass precursor as the starting material. The effects of isothermal heat treatment temperature on the phase evolutions were also systematically studied. The results showed that the melt crystallization and glass crystallization behaviors of the alloy differ from each other. The continuous heating transformation diagram plotted from non-isothermal analyses is consistent with the results of isothermal heat treatments. A broad ternary boride precipitation zone was demonstrated, involving the gradual precipitation of CoWB crystals. Avrami exponent values for the early crystallization stage showed that the crystallization mechanism of CoWB is governed by the interface-controlled three-dimensional growth mechanism, and nucleation rate is high. As the crystallization progresses, a diffusion-controlled growth occurs and the nucleation rate decreases. This study offers insights into the heat treatment of CoWB-reinforced nickel matrix nanocomposites, benefiting future research and industrial production processes.

Thermodynamic and kinetic investigation on crystallization of photosensitive glass-ceramics via molecular dynamics

Photosensitive glass-ceramics serves as a crucial support for advancement of integrated circuits in post Moore era. Nevertheless, current research lacks an in-depth discussion of crystallization and nucleation mechanisms induced by nucleating agents. Here, we select new AgSbO3 nucleating agents and conduct a detailed thermodynamic and kinetic investigation of its promoting effects in Li2SiO3 using first-principles molecular dynamics. Statistical results from pair distribution functions indicate that in the presence of AgSbO3 nucleating agents in Li2SiO3, the structure tends to exhibit a more crystalline distribution. The diffusion coefficient of Li ions in AgSbO3–Li2SiO3 is 0.47 × 10–6 cm2/s, which is one-tenth of the value of Li ions in amorphous Li2SiO3. Additionally, a molecular dynamics approach was employed to screen and analyze motion trajectories of 16 lithium ions. All of these data suggest that AgSbO3 nucleating agents can accelerate crystallization of Li2SiO3. This work provides an atomic-scale discussion of nucleating agents' effects in photosensitive glass-ceramics.

Dynamic simulation-driven analysis of cadmium, nickel, cobalt, and iron adsorption mechanisms in zeolite LTA synthesized from Bentonite

A practical and cost-effective method was successfully developed for synthesizing high-performance zeolite LTA from bentonite clay by fine-tuning activation steps and crystallization parameters. The optimal synthesis conditions and crystallization mechanism were investigated. The synthesized zeolites were characterized using XRD, FTIR, and SEM-EDS techniques. The results highlight the significant influence of factors such as crystallization temperature, duration, and the effect of sodium hydroxide concentration on the formation of zeolites. Optimal conditions set at a crystallization temperature of 97°C, duration of 24 hours, and NaOH concentration of 4M yielded pure zeolite LTA, boasting high crystallinity levels. Achieving a peak crystallinity of 82%. The obtained zeolite LTA showed an exceptional Cd (II) ion exchange capacity. A mechanism involving adsorption of Cd2⁺, Ni2⁺, Co2⁺, and Fe2⁺ ions in zeolite LTA at the α and β-cages has been proposed using dynamic simulation. This mechanism supports all experimental results, in particular for LTA- Cd2⁺, Cd2⁺ ions are predominantly distributed in both α and β-cages, with a denser distribution in the α-cages, indicating a strong preference for these sites due to their geometric and electronic environment. The resulted zeolite LTA demonstrated ability for successful Cd (II) removal, affirming its utility as an efficient material in environmental remediation industries.

Investigating single-chain structure during the crystallization process by atomic force microscopy

Crystalline polymers have gained significant traction in various fields owing to their exceptional thermal and mechanical properties. However, the precise mechanism underlying polymer crystallization remains a subject of ongoing debate. In this study, we employed atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS), also called nanofishing to investigate the mechanical properties, chain-level structure, and interactions of individual polymer chains, thereby shedding light on the crystallization mechanism. Through control of the crystallization temperature and time of polyethylene oxide (PEO) in solution, in conjunction with differential scanning calorimetry (DSC), we present compelling evidence that polymer crystallization begins with the folding of individual chains into cluster-like forms and the subsequent aggregation of these clusters to form larger crystalline structures. The clusters adopt chain-folding structures with lengths ranging from 7.9 to 9.1 nm at distinct crystallization temperatures. Remarkably, we observed the formation of these clusters in the initial stages, even preceding the 5-min mark. Over time, these clusters progressively aggregate and grow, ultimately leading to lamella structures. This suggests that the primary chain-folding process occurs in the early stage of crystallization, presenting a significant advancement in our understanding of the behavior of polymers at the chain level during the crystallization process. These insights hold immense value in the design and development of advanced crystal-based polymer materials.

Kinetic study of a-Si crystallization induced by an intermetallic compound

The problem under study is concerned with amorphous silicon (a-Si) crystallization which is induced by an intermetallic compound, with this compound chosen to be Al2Cu, formed by a solid-state reaction between nanolayers of aluminum and copper in a Cu/Al/a-Si multilayer system. In the case of crystallization initiated by the intermetallic compound Al2Cu, the crystallization temperature of amorphous silicon was found to be ≈ 300°С (upon heating at a rate of 5–10 °C/min), which was significantly higher than in the case of pure Al (≈170°С), but lower as compared to using pure Cu (≈485°С). The higher crystallization temperatures in comparison with aluminium-induced crystallization are assumed to be caused by a decrease in the number of free electrons due to the presence of copper and formation of the Al2Cu phase. By kinetic modeling, it was revealed that the mechanism of a-Si crystallization induced by the intermetallic compound Al2Cu was similar to the mechanism of Al-induced crystallization of a-Si in multilayer (Al/a-Si)n films, i.e the process of a-Si crystallization occurred in two subsequent stages: Cn-X→Fn. Kinetic parameters of silicon crystallization for each reaction step were obtained. The enthalpy of a-Si crystallization initiated by Al2Cu was estimated to be ΔH = 12.3 ± 0.5 kJ/mol.

Fully hot Air-Processed All-Inorganic CsPbI2Br perovskite solar cells for outdoor and indoor applications

The all-inorganic α-CsPbI2Br perovskite has emerged as a promising material for photovoltaic applications due to its superior optoelectronic properties and thermal stability. However, achieving a defect-free, crystalline CsPbI2Br perovskite film remains challenging due to the rapid crystal growth induced via high-temperature hot plate processing, which causes surface defects including pinholes and voids. This study introduces a novel hot plate-free methodology in which the perovskite film is fully processed using a dynamic hot air treatment under ambient conditions. Crystallographic and microscopic analyses reveal that hot air-assisted method produces highly crystalline, uniform, and compact perovskite film. The controlled crystal growth mechanism facilitated by the hot air process involves the formation of an intermediate phase (CsI-DMSO:PbX2), followed by gradual solvent evaporation, resulting in improved film quality. The resulting perovskite solar cells (PSCs) exhibited a champion power conversion efficiency (PCE) of 13.74 % and maintained around 51.60 % of its initial PCE after thermal aging at 85 ℃ under ambient conditions over 1440 h. Furthermore, under indoor white LED illumination (3200 K, 1000 lx), the optimized PSC achieved an impressive PCE of 22.50 %. These findings demonstrate that the dynamic hot air process is a viable and scalable technique for producing high-performance, stable PSCs under ambient conditions.

Effective attainment of a substantial proportion of the PbTiO3 crystalline phase through a non-isothermal crystallization process

The crystallization kinetics studies were performed on a lead–titanium borate glass with a composition 50PbO–25TiO2–25B2O3 (mol %) to well understand the crystallization process by a non-isothermal method in an attempt to obtain a substantial proportion of the lead titanate (PbTiO3) crystalline phase. Based on the DTA result, the glassy material was heat-treated to induce crystallization and then produced a glass-ceramic. The broad hump observed in the XRD patterns of a glassy sample affirmed its amorphous nature. However, XRD studies conducted on a glass-ceramic revealed the evolution of the crystalline phase of PbTiO3 and a minor phase of TiO2. The phase relationship of PbTiO3 resulting from the reaction between PbO and TiO2 during a non-isothermal crystallization process has been examined. The activation energy for the crystallization of PbTiO3 is evaluated to be 135 kJ/mol (at PbO/TiO2 ratio = 2/1 mol %). When the Pb/Ti ratio exceeds 1, the process progresses nearly to completion by the first crystallization peak, and the PbTiO3 phase formation is easier. The fraction of PbTiO3 phase in the present glass is 98.3 % at PbO/TiO2 ratio = 2/1 mol %. In addition, FTIR spectra provided the presence of Ti4+ ions in the glass-ceramic. According to the values of crystal growth and Avrami index, surface crystallization and one-dimensional growth mechanisms are suitable mechanisms for contributing to the crystallization of this glassy sample. The obtained findings provided details on the crystallization behaviour of this glassy sample and excellent insight into obtaining the PbTiO3 crystalline phase in a large proportion, which is of great importance in future technological applications.

Effective CuSO4 Concentration on Phase Formation and Optical Characteristics of Electrodeposited Ni-Cu Alloy

Ni-Cu alloys play a critical role in various industrial applications due to their exceptional characteristics. Despite extensive study of these alloys for their mechanical and corrosion characteristics, their optical properties have received limited attention. In this research, the electrodeposition of Ni-Cu alloy coatings was conducted using copper sulfate concentrations ranging from 1.25 to 10 g/L, leading to successful film formation with various phases. XRD examination associated with the Rietveld calculation exhibited modified phase fraction, crystallite size, micro-strain, and lattice constant. Scanning Electron Microscopy (SEM) analysis revealed a different thickness and a dendritic microstructure. As an increase in CuSO4, there was observed an increase in the values of the absorption edge (showing 1.61 - 1.90 eV), texture coefficient (0.46 - 0.82), and film thickness (0.63 - 1.74 µm). These various morphologies enhanced the material’s properties. In more added CuSO4 samples, the coatings significantly reveal a modification in the crystal, film growth, and optical properties. The advancements in visible light absorption suggest an enhanced coating for optical technology applications. HIGHLIGHTS The texture coefficient (TC) values of the (111) plane of Cu81Ni0.19 phase were found to increase with higher CuSO4 concentrations. Specifically, the TC values for 1.25Cu-Ni, 2.5Cu-Ni, 5.0Cu-Ni, 7.5Cu-Ni, and 10Cu-Ni were 0.46, 0.48, 0.77, 0.77, and 0.82, respectively. These values, all below 1, indicate low crystallinity but show a trend of increasing crystallinity with higher CuSO4 concentrations. The film thickness also increased with higher CuSO4 The thicknesses were 0.63 µm for 1.25Cu-Ni, 0.65 µm for 2.5Cu-Ni, 1.36 µm for 5.0Cu-Ni, 1.54 µm for 7.5Cu-Ni, and 1.74 µm for 10Cu-Ni. The grain morphology of the coatings evolved from small, elongated platelets in lower CuSO4 concentrations to larger, more dendritic structures at higher concentrations. This change in morphology is attributed to the increased CuSO4 concentration, which affects the crystal growth mechanisms during electroplating. GRAPHICAL ABSTRACT

Unconventional Near‐equilibrium Nucleation of Graphene on Si‐terminated SiC(0001) Surface

The transfer‐free character of graphene growth on Silicon Carbide (SiC) makes it compatible with state‐of‐the‐art Si semi‐conductor technologies for directly fabricating high‐end electronics. Although significant progress has been achieved in epitaxial growth of graphene on SiC recently, the underlying nucleation mechanism remains elusive. Here, we present a theoretical study to elucidate graphene near‐equilibrium nucleation on Si‐terminated hexagonal‐SiC(0001) surface. It is found that the ultra‐large lattice mismatch between SiC(0001) surface and graphene and the highly localized electron distribution on SiC(0001) surface lead to a distinctive nucleation process: (i) Most of the magic carbon clusters on SiC(0001) show only C1 symmetry and are mainly composed of pentagonal rings; (ii) Two possible nucleation pathways are revealed, i.e, longitudinal and circular modes; (iii) Carbon clusters are more stable on flat terraces than near atomic step edges. Based on above findings, a graphene nucleation diagram on SiC(0001) is established and experimentally observed contradictories for graphene growth on SiC(0001) are answered. Our in‐depth understanding on graphene nucleation on SiC(0001) extends nucleation mechanisms of 2D crystals and will benefit high‐quality graphene growth on SiC(0001).

Unconventional Near-Equilibrium Nucleation of Graphene on Si-Terminated SiC(0001) Surface.

The transfer-free character of graphene growth on Silicon Carbide (SiC) makes it compatible with state-of-the-art Si semiconductor technologies for directly fabricating high-end electronics. Although significant progress has been achieved in epitaxial growth of graphene on SiC recently, the underlying nucleation mechanism remains elusive. Here, we present a theoretical study to elucidate graphene near-equilibrium nucleation on Si-terminated hexagonal-SiC(0001) surface. It is found that the ultra-large lattice mismatch between SiC(0001) surface and graphene and the highly localized electron distribution on SiC(0001) surface lead to a distinctive nucleation process: (i) Most of the magic carbon clusters on SiC(0001) show only C1 symmetry and are mainly composed of pentagonal rings; (ii) Two possible nucleation pathways are revealed, i.e., longitudinal and circular modes; (iii) Carbon clusters are more stable on flat terraces than near atomic step edges. Based on above findings, a graphene nucleation diagram on SiC(0001) is established and experimentally observed contradictories for graphene growth on SiC(0001) are answered. Our in-depth understanding on graphene nucleation on SiC(0001) extends nucleation mechanisms of 2D crystals and will benefit high-quality graphene growth on SiC(0001).

Study on the Vibration-Damping Mechanism of a New Phononic Crystal Suspension Equipped on Underwater Gliders

The vibration caused by the movement of internal actuating components within an acoustic underwater glider can interfere with onboard sensors. However, as a new vibration-damping material, phononic crystals can effectively reduce this impact. Using simulation and an underwater test, this work studied the vibration-damping mechanism of the phononic crystal suspension (PCS) designed by Tianjin University, China. The bandgaps and the modes of PCS were calculated first, which offered basic data for the following simulation. Then, the relationship between the modes and attenuation zones (AZs) were broadly considered to reveal the variation law of the AZs with the change in modes, both in the air and under water. Finally, an underwater test was carried out to verify the good vibration-damping effect of the PCS. The results show that the cutoff frequency of the AZs could be predicted by finding the relevant modes. The PCS showed a good vibration-damping effect from 170 Hz to 5000 Hz in the underwater test, with a maximum decrease of 6 dB at 2000 Hz. Finally, the damping of the PCS could suppress the overlap of modes that resulted from Bragg scattering. This work will also provide theoretical guidance for further study on the optimization of phononic crystal mechanisms for vibration damping.

Three Strikingly Different Crystal Habits of Tadalafil: Design, Characterization, Pharmaceutical Performance, and Computational Studies.

The present study aims at improving the physicochemical properties of a widely used drug Tadalafil through crystal habit modification, without changing the polymorphic form. Three distinct types of crystal habits, namely, needle, plate, and block, were obtained under controlled crystallization protocols with optimized solvent compositions. Complete characterization of these three crystal habits was carried out using powder X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, and Fourier transform infrared spectroscopy. Morphological features were studied by optical and scanning electron microscopy. Evaluation of the pharmaceutical performance of different crystal habits reveals significant improvement in compressibility and flow properties for the block-shaped crystals in comparison to the needle- and plate-shaped crystals. Also, a more linear tablet compression behavior was noted for the plate and block morphologies of the API compared to their needle counterpart. In vitro dissolution studies showed distinct release profiles for the same API form with different crystal habits, i.e., needle > plate > block. Insights into crystal growth mechanism and the role of solvents in affording the observed crystal habits are presented based on molecular dynamics simulations of intermolecular interactions with crystal facets, in conjunction with the experimental crystal face indexing of the single crystals of different habits. These observations were further supported by interaction topology analysis and the electrostatic features on different crystal facets.

Managing Solvent Complexes to Amplify Ripening Process by Covalent Interaction Driving Force Under External Field for Perovskite Photovoltaic

AbstractUp to now, post‐annealing is most commonly used to post treat the perovskite film to accelerate the ripening process. Nonetheless, the top‐down crystallization mechanism impedes the efficient desolvation of solvent complexes. Thus, residual solvent complexes tend to accumulate at the bottom of the film during the ripening process and deteriorate the device. Here, a new strategy with unique concept is promoted to amplify ripening process of perovskite film, in which a nematic thermotropic liquid crystal (LC) molecular is introduced to facilitate the conversion of solvent complexes by utilizing the liquid crystalline behavior under external field. Upon the concurrent application of thermal and force fields, the covalent interaction between LC and solvent complexes generates a driving force, which promotes upward migration of solvent complexes, thereby facilitating their engagement in the ripening process. In addition, the driving force under external fields assists the flattening of grain boundary grooves. Therefore, film quality is improved efficiently with amplified ripening process and adequately handled buried interface. Based on the positive effects, the devices achieve a champion efficiency of 25.24%, and sustained ≈75% of its initial efficiency level even after undergoing a damp heat test (85 °C/85% RH) for 1400 h.

Transmembrane Ion Channels: From Natural to Artificial Systems.

Natural channel proteins allow the selective permeation of ions, water or other nutritious entities across bilayer membranes, facilitating various essential physiological functions in living systems. Inspired by nature, chemists endeavor to simulate the structural features and transport behaviors of channel proteins through biomimetic strategies. In this review, we start from introducing the inherent traits of channel proteins such as their crystal structures, functions and mechanisms. Subsequently, different kind of synthetic ion channels including their design principles, dynamic regulations and therapeutic applications were carefully reviewed. Finally, the potential challenges and opportunities in this research field were also carefully discussed. It is anticipated that this review could provide some inspiring ideas and future directions towards the construction of novel bionic ion channels with higher-level structures, properties, functions and practical applications.

Transmembrane Ion Channels: From Natural to Artificial Systems

Natural channel proteins allow the selective permeation of ions, water or other nutritious entities across bilayer membranes, facilitating various essential physiological functions in living systems. Inspired by nature, chemists endeavor to simulate the structural features and transport behaviors of channel proteins through biomimetic strategies. In this review, we start from introducing the inherent traits of channel proteins such as their crystal structures, functions and mechanisms. Subsequently, different kind of synthetic ion channels including their design principles, dynamic regulations and therapeutic applications were carefully reviewed. Finally, the potential challenges and opportunities in this research field were also carefully discussed. It is anticipated that this review could provide some inspiring ideas and future directions towards the construction of novel bionic ion channels with higher‐level structures, properties, functions and practical applications.

Ultra-micro Liquid Crystal in Cosmetic Emulsion with Excellent Moisture Retention and Delivery Selectivity.

Compared with ordinary emulsified cosmetics, liquid crystal emulsified cosmetics have obvious advantages. In this paper, an ultra-micro liquid crystal (ULC) emulsion with particle size less than 4 μm was successfully synthesized using a simple oil-in-water emulsification method, and large liquid crystal (LLC) and non-liquid crystal (NLC) emulsions were also prepared as controls. Three kinds of emulsions were examined by polarizing microscopy, X-ray diffraction (XRD), small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM). Rheological tests, in vivo experiments, reconstructed Raman maps tracking selected markers and thermogravimetric (TG) results showed that the ULC emulsion expressed excellent viscoelasticity, long-term moisturizing property with barrier function and better selective delivery of active substances compared with LLC and NLC emulsions. Molecular dynamics simulation on the ULC system and deuterium order parameter plots respectively indicated that GYY (glyceryl stearate citrate) showed a tendency to form a stable structure by reducing the total interaction energy and enhancing system lamellar ordering. This work not only demonstrates the great potential of ultra-micro liquid crystals in for commercial applications in cosmetics but also offers a deeper understanding of the intrinsic properties and growth mechanisms of ultra-micro liquid crystals at the microscopic level.

Stabilization and crystallization mechanism of amorphous calcium carbonate

Amorphous phases hold great promise in diverse applications and are widely used by organisms as precursors to produce biominerals with complex morphologies and excellent properties. However, the stabilization and crystallization mechanisms of amorphous phases are not fully understood, especially in the presence of additives. Here, using amorphous calcium carbonate (ACC) as the model system, we systematically investigate the crystallization pathways of amorphous phases in the presence of poly(Aspartic acid) (pAsp) with various chain lengths. Results show that pure ACC transforms into a mixture of calcite and vaterite via the typical dissolution–recrystallization mechanism and 3 % of Asp monomer exhibits negligible effect. However, pAsp with a chain length of only 10 strongly inhibits the aggregation-induced formation of vaterite spheres while slightly delaying the growth of calcite via classical ion-by-ion attachment, thus kinetically favoring the formation of calcite. Moreover, the inhibition effect of calcite growth from solution ions becomes more prominent with the increase of pAsp chain length or concentration, which significantly improves the stability of the amorphous phase and leads to crystallization of spherical or elongated calcite via the nonclassical particle attachment mechanism after pseudomorphic transformation of ACC into vaterite nanoparticles. These results allow us to reach a more comprehensive understanding of the stabilization and crystallization mechanism of ACC in the presence of additives and provide guidelines for controlling the polymorph selection and morphology of crystals during the crystallization of amorphous precursors.

Investigation of etch rate distribution by micro-hemisphere for the anisotropic wet etching of Ga-face gallium nitride crystal

The Ga-face gallium nitride (GaN) crystal has recently attracted significant research interest in the micro/nano device due to its exceptional properties. Therefore, exploring low-cost and high-efficiency anisotropic wet etching processes for GaN material is essential. This research investigates the measurement of the overall anisotropic etching rates by introducing a novel micrometer-level hemispherical structure. For the first time, we report the full anisotropic etching rate distribution for Ga-face GaN crystal planes based on the etched results of the micro hemisphere and wagon-wheel structure specimen. The etching experiments were carried out in 85 %wt H₃PO₄ etchant at 130°C and 150°C. The etching result of Ga-face GaN crystal orientations displays a hexagonal symmetry pattern. In addition, the overall etch rates measurement enables the identification of the crystal planes with maximum and minimum etch rates at different temperatures, and the experiment also shows the etch rate of primary crystallographic orientations, such as +c-plane, m-plane, and a-plane. Finally, the atomic structures of GaN crystallographic planes are analyzed to explain the anisotropy in the etching process on different crystal zones by the step-flow mechanism. The resulting etch rate database allows the numerical simulation of the etch front and helps understand the etching mechanism of the GaN crystal.