Grain Growth Evolution Research Articles

Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation

In polycrystalline compound semiconductor thin films, structural defects such as grain boundaries as well as lateral stress can form during film growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ or ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$, temporary Cu excess during film growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-efficiency chalcopyrite thin film solar cells. However, the mechanisms responsible for the beneficial effect of Cu excess are yet not fully clarified. Here, we investigate the evolution of lateral stress, grain growth, and Cu-Se segregation during Cu-Se deposition onto Cu-poor ${\mathrm{CuInSe}}_{2}$. Real-time x-ray diffraction and fluorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diffusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the film can be already reached without the necessity of an overall Cu-rich film composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber films.

New understanding of the role of coincidence site lattice boundaries in abnormal grain growth of aluminium alloy

The sequential microstructure evolution of abnormal grain growth (AGG) in the aluminium alloy (AA5052) was investigated to analyse the migration behaviour of coincidence site lattice (CSL) boundaries, which are known to play an important role in inducing AGG. The sequential evolution showed that CSL boundaries tend to disappear more slowly than general boundaries at the growth front of abnormally growing grains. Especially, the migration rate of Σ9 boundaries was noticeably low, which is contrary to the previous suggestions.

Comparative Study on Cu-CVD Nucleation Using β-diketonato and Amidinato Precursors for Sub-10-nm-Thick Continuous Film Growth

We demonstrate the growth of sub-10-nm-thick continuous Cu films using chemical vapor deposition (CVD) for next-generation Cu interconnects for ultra-large-scale integration (ULSI). The thickness of such films is equivalent to that of Cu during coalescence, and optimized operating conditions and substrate materials are required to form high-density nucleates. Ru was used as an underlayer, and the time evolution of nucleation and grain growth were studied with systematically varied conditions using two Cu precursors: conventional β-diketonato and newly developed amidinato precursor compounds. The revealed geometry of the initial nano-scale Cu grains prior to coalescence suggests the required nucleate density for 7-nm-thick continuous film growth, and which was 2.4 × 1011 /cm2. The maximum nucleate density was achieved with the lowest deposition temperature and highest precursor concentration for both precursors; i.e., 6.9 × 1011 /cm2 for β-diketonato at 100°C, and 4.6 × 1011 /cm2 for amidinato at 150°C. A 10-nm-thick continuous Cu film was formed using amidinato under the optimized conditions. Furthermore, the framework used in this study to enable a high nucleate density suggests that it is possible to form thinner (4 nm∼) Cu films using amidinato. Because of the inherent good step coverage of CVD, this process is a promising candidate for next-generation ULSI Cu interconnects.

Growth and characterization of RF-sputtered ZnS thin film deposited at various substrate temperatures for photovoltaic application

RF-sputtered ZnS thin film was grown under various substrate temperatures with the aim of investigating its effects on the structural, surface morphology and optical properties. Investigated substrate temperature in this study was in the range of 25°C–300°C and the structural and optical properties were investigated in order to elucidate the changes induced by the varying thermal energy during the growth process. Structural determination by XRD method indicates all sputtered films have cubic structure with (111) as the preferential orientation. However, higher substrate temperature up to 200°C increases the film's crystallinity and grain size evident by the increase in peak intensity. Slight peak shift indicates ZnS lattice undergoes strain relaxation process mediated through the increase in the lattice constant from 5.32Å to 5.40Å. SEM image of surface morphology clearly shows the evolution of grain growth in which sputtered film at 200°C has the largest grains with distinct grain boundaries. Calculation from the obtained transmission spectra indicates optical band gap is in the range of 3.6–3.9eV. Theoretical analysis in terms of lattice parameter between ZnS with several upcoming photovoltaic absorber layers shows that lattice matched ZnS buffer layer can be grown by varying the substrate temperature.

Microstructure and microwave properties of inkjet printed barium strontium titanate thick-films for tunable microwave devices

Abstract Barium strontium titanate (BST) is a promising material for passive tunable microwave devices such as phase shifters, tunable filters or tunable matching networks. This publication covers the preparation of BST thick-films for microwave applications through inkjet printing. A Ba 0.6 Sr 0.4 TiO 3 ink was prepared, printed on alumina substrates and sintered at temperatures between 1100 °C and 1200 °C. The microstructure of the thick-films reveals the evolution of grain growth and porosity with increasing sintering temperature. Furthermore, a reaction with the substrate was observed for T ≥ 1175 °C. A maximum tunability of 36% was observed at temperatures right below the onset of the substrate reactions. This process conditions were used for the preparation of a loaded line phase shifter, which successfully shows the capability of the inkjet printing process for future microwave device fabrication.

Microstructural evolution and mechanism of grain growth in magnesia ceramics prepared by high pressure and temperature with ultra-high heating rate

The fast densification method of combustion reaction plus quick pressing was adopted to prepare nanocrystalline ceramics. The densification process of magnesia compact with a particle size of 100 nm was investigated, under the applied pressure of up to 170 MPa, and the temperature range of 1740–2080 K with ultra-high heating rate (above 1700 K/min). High-purity magnesia ceramics with a relative density of 98.8% and an average grain size of 120 nm was obtained at 1740 K, and the grain growth during the densification process was effectively restrained. The characteristic morphology of evaporation-condensation was observed in the compact prepared at 2080 K, which revealed the actual process of mass transfer by gas diffusion. Moreover, the investigation on the microstructure evolution and mechanism of grain growth was carried out, on the basis of as-preserved nanocrystalline ceramics. The result indicated that the grain growth of the nanocrystalline MgO was controlled by the mechanism of evaporation-condensation rather than surface diffusion. Furthermore, the pressure had an influence of restraining the grain growth based on solid diffusion and strengthening the effect of gas diffusion with the increasing temperature. Under the particular conditions, there existed an appropriate temperature for the densification of nanocrystalline magnesia, while the excessive temperature would exaggerate grain growth and impede densification.

Preparation of Integrated Passive Microwave Devices Through Inkjet Printing

Barium strontium titanate (BST) is a promising material for passive tunable microwave devices such as phase shifters or tunable matching networks. This publication covers the preparation of BST thick-films for microwave applications through inkjet printing. Two barium strontium titanate (BST) inks were prepared, printed on alumina substrates and sintered at different temperatures. The first ink was prepared with pure BST and sintered between 1100°C and 1200°C. The second ink was prepared with a BST–ZnO–B2O3 composition and was suitable to reduce the sintering temperature down to 800°C. The microstructure of the thick-films reveals the evolution of grain growth with increasing sintering temperature in the thick-films. Furthermore, a reaction with the substrate was observed for both inks at high sintering temperatures. The microwave characterization of the thick-films shows that for the permittivity and the tunability of the films, the effect of grain growth and reaction with the substrate compete against each other. Hence, the optimal microwave properties were achieved at a transition temperature, where first additional phases could already be observed. Even though, the properties are poorer for lower sintering temperatures, the investigations show that the preparation of silver- or gold-based metal–insulator–metal (MIM) structures through inkjet printing is possible with this composition. This allows various new design concepts for partly or fully inkjet printed passive microwave devices. Furthermore, it gives the opportunity for a future integration of passive tunable microwave devices in a low temperature co-fired ceramic (LTCC) fabrication process.

Effect of the Energy of Triple-Lines on 3D Grain Growth

The effect of the triple line energy on grain growth was studied by means of computer simulations with a network model. The results showed that the driving force stemming from the triple lines can influence significantly the evolution of grain growth.

Ex Situ Observation of Microstructure Evolution During Abnormal Grain Growth in Aluminum Alloy

The sequential microstructural evolution of abnormal grain growth in the aluminum alloy was traced by ex situ observation using electron-backscattered diffraction during secondary recrystallization. Near the growth front of abnormally growing grains, many three- or four-sided grains with a negative curvature, which had the same orientation as abnormally growing grains, were penetrating the surface, indicating that abnormal grain growth occurs dominantly by repeated events of triple-junction wetting. Also, abnormally growing grains were shown exclusively to have sub-boundaries.

Monte Carlo simulation of influence of reheat temperature on austenite grain growth in C–Mn steel

The evolution and topology of a two-dimensional austenite grain growth of C–Mn steel are simulated by the Monte Carlo (MC) model during the reheating process. Simulated microstructural maps have been generated in a square lattice (400×400) at various MC steps and temperatures. Normalised grain size distribution has also been computed at different reheating temperatures, assuming a uniform temperature distribution in the slab. Two important model parameters, namely, the grain growth exponent and the model constant, have been estimated to predict the average grain size. The activation energy for grain growth has been calculated using the Arrhenius relationship. The grain growth behaviour as a function of both MC and physical time has been computed, and the effect of temperature on the rate of growth has been analysed. The predicted temporal evolutions of grain growth and model parameters have been validated with the published literature and found to be in good agreement.

On abnormal grain growth in nanocrystalline materials induced by small particles

Abstract A duality in the behaviour of small particles, i. e. their ability to migrate along with some grain boundaries and, simultaneously, inability to move along with the others, has been shown to affect the character of the grain growth process. It is found by numerical simulations that a decrease in the particle mobility favours the evolution of abnormal grain growth and that the particle volume fraction, at low particle mobility, also affects its development. It has been supposed that these results explain the evolution of abnormal grain growth in compacted nanocrystalline materials where small voids could play the part of particles.

Tracking the Evolution of Abnormal Grain Growth in Grain-Oriented Electrical Steels by Coercivity Measurements

The measurements of coercivity were utilised to investigate the main evolution stages of abnormal grain growth in grain-oriented electrical steels. The investigated grain-oriented steels were taken from industrial line after final cold rolling. The steels were subjected to short time annealing at elevated temperatures with high heating rates. The coercivity measurements carried out within the present work were confronted with microstructure and texture analysis in order to show the direct dependence of the measured coercivities on metallurgical characteristics of the investigated steel. It is shown that the coercivity measurements technique could capture main microstructural changes taking place in recrystallization.

Simulation of the Dendritic Growth of Aluminum Casting during Solidification Based on CA Method

The dynamic evolution of grain growth in the process of aluminum casting and the impact of different casting conditions on the grain growth were simulated by using the Cellular Automation(CA) method in this paper. The simplified binary alloy was used to simulate the growth of the grain in the undercooled melt, Finite Difference Method (FDM) combined with relative motion was used, and dynamic evolution of microstructures in the process of aluminum twin-roll casting was achieved. Visual Fortran programming language was adopted to calculate and realize the image post-processing. based on the growth of the grain in the undercooled melt, the impact of the undercooling triggered by cooling and the casting speed on aluminum strip in the casting process was simulated. The results indicate that, in the condition of a certain nucleation rate, as the cooling intensity increases, bigger the grain is, which provides a basis for optimizing the twin-roll casting process parameters.

Grain Growth Simulation Using Cellular Automata

It is well known that controlling the microstructure of most industrial materials is the key to control its mechanical and physical properties. In particular grain growth is very important phenomenon in material science. However, it is very difficult to examine the dynamic solidification microstructure evolution at high temperatures or during deformation processes. Computer simulations have been used as an effective solution for that difficulty. Cellular Automaton (CA) is one of the techniques that have been used to simulate the evolution of grain growth. In this study 2D grain growth simulations of Al-1%Mg alloy was simulated using CA model based on different transition principles. The first is based on low energy principle. In this method the changes of boundary energy value is compared for each cell and then choosing the one that can minimize the energy system to the largest extent. The second method is based on changes of thermal energy that is computed for each grain boundary. The transition only occurs at the highest energy value. The third method is based on starting with a number of random distributed nucleuses within the simulation area with different orientations. At each CA steps these nucleuses will grow into a grain without effecting in the other grains. The morphology and grain kinetics are studied and discussed for each case.

Kinetics of Recrystallization and Grain Growth of Cold Rolled TWIP Steel

Hot rolled, laboratory-cast, TWIP steel samples (5.4 mm thick) of 22% Mn - 0.6% C (in mass-%) were cold rolled to different reductions (from 40 % to 70 %) and subsequently isothermally annealed for various times at temperatures ranging from 450º C to 1100º C. The evolution of recrystallization and grain growth was followed by control of the softening kinetics complemented by metallographic, OIM and microtexture observations. A map of the recovery, recrystallization and grain growth in the temperature-time space was obtained. In all instances, the grain size at the end of recrystallization was very fine, D ≤ 2 µm and larger grain sizes were the result of grain growth. A range of grain sizes 2 µm ≤ D ≤ 50 µm was covered by the grain growth experiments. A phenomenological grain growth equation that is useful for the annealing control of this steel was derived from the measurements.

Influences of Flow Intensity, Cooling Rate and Nucleation Density at Ingot Surface on Deflective Growth of Dendrites for Al-based Alloy

The dendrite tip growth kinetics in the flow field and the decentred quadrilateral growth algorithm for describing the evolution of grain growth are combined in Cellular Automaton model to predict the deflective growth of dendrites inclined toward upstream direction. The influences of flow intensity, cooling rate (or solidification rate), nucleation density at ingot surface on the deflective growth of dendrites are discussed. The increase of flow intensity dominantly forces the dendrites to grow in upstream direction; on the contrary, the increases of nucleation density at ingot surface and cooling rate suppress slightly this deflective growth. The relations predicted among deflection angle, flow intensity and solidification rate for AI-Si alloy and Al-Cu alloy show the same tendency as that in Okano et al.'s empirical expression deduced from experiments on steel. The deflection angle predicted for Al-Cu alloy fits well with previous experimental results.

Morphology and grain structure evolution during epitaxial growth of Ag films on native-oxide-covered Si surface

Epitaxial nanocrystalline Ag films were grown on initially native-oxide-covered Si(001) substrates using radio-frequency magnetron sputtering. Mechanisms of grain growth and morphology evolution were investigated. An epitaxially oriented Ag layer (∼5 nm thick) formed on the oxide-desorbed Si surface during the initial growth phase. After a period of growth instability, characterized as kinetic roughening, grain growth stagnation, and increase of step-edge density, a layer of nanocrystalline Ag grains with a uniform size distribution appeared on the quasi-two-dimensional layer. This hierarchical process of film formation is attributed to the dynamic interplay between incoming energetic Ag particles and native oxide. The cyclic interaction (desorption and migration) of the oxide with the growing Ag film is found to play a crucial role in the characteristic evolution of grain growth and morphology change involving an interval of grain growth stagnation.

Phase Field Model Simulation of Grain Growth in Three Dimensions under Isotropic and Anisotropic Grain Boundary Energy Conditions

Phase-field model (PFM) in multiple orientation fields was used to simulate the grain growth in three-dimensions (3-D) for isotropic and anisotropic grain boundary energy. In the simulation, the polycrystalline microstructure was described by a set of non-conserved order parameters and each order parameter describes each orientation of grains. For isotropic grain boundary energy, the simulation showed the microstructure evolution of normal grain growth. For anisotropic grain boundary energy, however, the simulation showed that certain grains which share a high fraction of low energy grain boundaries with other grains have a high probability to grow by wetting along triple junctions and can grow abnormally with a growth advantage of solid-state wetting. The PFM simulation shows the realistic microstructural evolution of island and peninsular grains during abnormal grain growth by solid-state wetting.

An investigation of descaling spray on microstructural evolution in hot rolling

High pressure water spray is used to remove oxide scales in hot rolling of steel plate. This would reduce the temperature temporarily on the workpiece surface. To investigate the effect of temperature variation due to the descaling spray on the microstructural evolution, a thermal-mechanical coupled FE model has been established. The heat loss due to the spray is experimentally measured and modelled using the established FE model. The heat transfer coefficient is determined using an inverse engineering method. A set of unified viscoplastic constitutive equations is implemented into the commercial FE solver MARC through the user-defined subroutine CRPLAW. The effect of descaling water spray on the surface temperature variation is numerically studied. Furthermore, the effect of surface temperature variation on the evolution of dislocation density, recrystallisation and grain growth is investigated for various hot rolling conditions.

Cellular automata simulation of grain growth in three dimensions based on the lowest-energy principle

The microstructure and morphology evolution of grain growth were studied by 3D simulation using the cellular automata (CA) model based on the lowest-energy principle. In the present CA model, the transition of cells during the grain growth has a typical physical meaning due to the application of the lowest-energy principle. The results show that the kinetics of grain growth follows Burke equation with the growth exponent as 2. The average number of grain faces is 13.6 and the highest frequency of grain faces is 10 faces. The grain size distribution follows Weibull function. The relationship between the number of faces of a grain and the average number of faces of its adjacent grains follows the Aboav–Weaire law. There is a correlation between the topologies of the simulated 2D and 3D grain growth. The average number of sides per face for all grains is 5.65 and the average number of sides per face is about equal to 6 when the grain aces is larger than 35.