Grain Boundary Diffusion Mechanism Research Articles

Structural and phase transformations in Fe-Pd-Ag layered thin films by grain boundary diffusion

The influence of the stacking order and of an additional Ag layer on the formation of ordered phases in thin (< 50 nm) layered Fe/Pd and Fe/Ag/Pd films was investigated at 460 °C. The samples were prepared by magnetron sputtering at room temperature on Si/SiO2 substrate and were post-annealed in vacuum. Composition depth profiling and x-ray diffraction were used to characterize the processes. It has been shown, that the stacking order strongly influences the formation of ordered phases both in Fe/Pd bilayered and Fe/Ag/Pd trilayered films. In bilayered Fe/Pd films for both stacking orders the FePd3 phase appeared and it also showed L12 ordering for one stacking order. Addition of Ag layer between the Fe and Pd layers found to promote the formation of FePd phase which showed L10 ordering or A1 disordered structure depending on stacking order. Based on the analysis of the chemical depth profiles and XRD patterns the transformations were interpreted by grainboundary diffusion mechanisms including grainboundary diffusion induced grainboundary migration and solid state reaction.

A New Solution to the Grain Boundary Grooving Problem in Polycrystalline Thin Films When Evaporation and Diffusion Meet in Power Electronic Devices.

This paper constituted an extension of two previous studies concerning the mathematical development of the grain boundary grooving in polycrystalline thin films in the cases of evaporation/condensation and diffusion taken separately. The thermal grooving processes are deeply controlled by the various mass transfer mechanisms of evaporation-condensation, surface diffusion, lattice diffusion, and grain boundary diffusion. This study proposed a new original analytical solution to the mathematical problem governing the grain groove profile in the case of simultaneous effects of evaporation-condensation and diffusion in polycrystalline thin films by resolving the corresponding fourth-order partial differential equation ∂y∂t=C∂2y∂x2-B∂4y∂x4 obtained from the approximation ∂y∂x2≪1. The comparison of the new solution to that of diffusion alone proved an important effect of the coupling of evaporation and diffusion on the geometric characteristics of the groove profile. A second analytical solution based on the series development was also proposed. It was proved that changes in the boundary conditions of the grain grooving profile largely affected the different geometric characteristics of the groove profile.

A new strategy for improving the Tb utilization efficiency in sintered NdFeB magnets through pressure-assisted grain boundary diffusion

Improving the utilization efficiency of heavy rare earth is currently a research hotspot in the field of rare earth permanent magnets. In this article, the effect and mechanism of pressure-assisted grain boundary diffusion (PGBD) on improving Tb utilization efficiency was investigated. The magnetic properties, microstructure, and magnetic domain morphology of the PGBD magnets were compared and analyzed in relation to conventional grain boundary diffusion (CGBD) magnets. The results shows that the PGBD process promoted Tb diffusion into the magnets. The residual layer thickness of the PGBD magnets was thinner than that of the CGBD magnets, demonstrating that the PGBD process could allow more Tb to diffuse into the magnets. The formation of a more complete and deeper core-shell structure in the PGBD magnets significantly enhances their resistance to demagnetization fields, thereby resulting in an increased coercivity. The PGBD process exhibits a reduction in TbH3 usage by approximately 0.2 wt% compared to the CGBD process, while maintaining an equivalent level of coercivity increment. These findings highlight the efficacy of the PGBD process in enhancing Tb utilization efficiency and conserving Tb resources.

Study on Hot Flow Behaviors and Deformation/Diffusion Mechanisms of V and V–Ti Microalloyed Steels by Physical Constitutive Modeling

The hot compression tests of medium carbon V and V–Ti microalloyed steels are carried out at the temperature of 900–1100 °C and strain rate of 0.01–10 s−1. Three physical constitutive models based on creep theory are studied, the prediction accuracy is compared and analyzed by employing the correlation coefficient (R) and average absolute relative error (AARE), and the deformation/diffusion mechanisms are discussed. The results show that Ti has obvious hardening effect in V microalloyed steel and retards the onset of dynamic recrystallization. The physical constitutive model containing the theoretical value of creep exponent 5 can accurately describe the hot flow behavior of both steels, the prediction accuracy of which is comparable to that of the model containing exponent n, reflecting that the dominant deformation mechanism of both steels is dislocation climbing. Furthermore, a modified physical constitutive model combining diffusion mechanisms of lattice diffusion and grain boundary diffusion shows higher accuracy (R = 0.996, AARE = 3.81% of V steel and R = 0.994, AARE = 4.52% of V–Ti steel), indicating that the diffusion mechanism is controlled not only by lattice diffusion but also by grain boundary diffusion.

Do smaller oxide particles sinter worse? Paradoxes of the initial stages of sintering of zirconia nanoparticles

The effect of particle size on the dominant densification mechanism at the initial stage of sintering of tetragonal zirconia nanoparticles was investigated. It has been found that a decrease in the particle size of zirconia leads to a change in the diffusion mechanism. The volume diffusion mechanism identified by the value of densification parameter n = 0.5 was dominant in zirconia nanoparticles with the average particle size 32 and 26.2 nm. The grain-boundary diffusion mechanism (n = 0.3) dominated during the sintering zirconia nanoparticles with the average particle size 21.7 nm. It was established that the change in densification parameter n from 0.5 to 0.3 caused by the increase in the value of particle surface to volume ratio due to the particles size decrease from 32 to 21 nm. A further decrease in the particle size to 17–12 nm led to a decrease in the densification parameter n to 0.17–0.11, respectively. This may be due to an increase in the influence of the particle surface on the sintering process (large values of particle surface to volume ratio) and, accordingly, the action of the surface diffusion mechanism.

Densification mechanism of spark plasma sintered ZrB2 and ZrB2-SiC ceramic composites

Sintering mechanisms are investigated by determining activation energy of densification (in the temperature range of 1650 °C to 1950 °C) during spark plasma sintering of ZrB2 and SiC-reinforced ZrB2 ceramic composite. The fractured surfaces of sintered composites reveal completely dense microstructures above 1800 °C, whereas porous microstructure prevailed in ZrB2 even after sintering at 1950 °C. Activation energy of 80 ± 16 kJ/mol is obtained for ZrB2 with grain boundary diffusion as governing diffusion mechanism in the temperature range of 1650 to 1950 °C. On the other hand, dominant densification mechanism of grain boundary diffusion (activation energy of 89 ± 19 kJ/mol) estimated in the range of 1650 to 1800 °C changed to lattice diffusion (activation energy of 601 ± 10 kJ/mol) with the addition of 20 vol% SiC in ZrB2. The change of prominent densification mechanism in ZrB2 with SiC, in addition to its impact on the mechanical performance with systemic increase of sintering temperature, is unravelled here.

Mechanism of Grain-Boundary Diffusion and Grain-Boundary Segregation of 57Co in Polycrystalline Nb

Mechanism of Grain-Boundary Diffusion and Grain-Boundary Segregation of 57Co in Polycrystalline Nb

NiO–ZnO based junction interface as high-temperature contact materials

Contact materials play a crucial role in an electronic device operating at moderate and elevated temperatures where chemical and thermal stability is of great importance. Oxide materials and their interfaces are potential candidates as high-temperature contact materials due to their high chemical and thermal stabilities. In this work, polycrystalline oxides of Ni0.98Li0.02O and Zn0.98Al0.02O were used to make junction interfaces, where the solid-state synthesis method was used to obtain the individual oxide materials. After assembly of the junction interfaces, properties such as electrical, chemical, and thermal stabilities of the interfaces were investigated. The electrical properties were assessed through current-voltage (I–V) and electrochemical impedance spectroscopy (EIS) measurements, where the interface revealed a transition from electrically rectifying to slightly ohmic contact within a temperature range from 500–1000 °C. After annealing the junction interfaces at these elevated temperatures, no secondary phase was observed at the junction interface, i.e., the interfaces remain chemically stable. Moreover, the effect of isothermal annealing on the I–V characteristics curve of the junction showed an increased reverse current output over long annealing time, attributed mainly to the increased effective contact area at the junction interface and cation inter-diffusion processes. Furthermore, an investigation of the cation inter-diffusion mechanism revealed mainly lattice diffusion of Zn2+ into Ni0.98Li0.02O, while Ni2+ diffusion into Zn0.98Al0.02O exhibited both lattice and grain-boundary diffusion mechanisms.

Elucidation of Densification Experienced By LLZO Solid Electrolytes

In order to improve the energy density and minimize the safety concerns associated with the liquid electrolytes used in the present day lithium ion batteries, solid electrolytes are being actively considered for use in the next generation energy storage devices.[1] Among various ceramic based solid state electrolytes being investigated, Li7La3Zr2O12 (LLZO) with a garnet crystal structure demonstrates very high ionic conductivity and is extremely stable against lithium.[2] Hence, LLZO is being actively considered as the electrolyte for next generation lithium ion batteries. However, due to poor processing of these ceramics, they contain pores and voids.[3] These open spaces within the bulk not only decrease the effective conductivity of the electrolyte, but also provides open pathways for Li dendrites to propagate. Presence of pores within the ceramic structure can be estimated from the final relative density of the solid electrolyte. High temperature sintering induced densification of the LLZO particles are conducted to improve the relative density of the final solid electrolyte, and this process helps to minimize the void space within the structure.[3, 4] Sometime external pressures are applied to further improve the relative densities.[5] Even though several experimental studies have been conducted for constructing the LLZO solid electrolytes, very little effort has been devoted to actually understand the physical mechanisms that play major role during the densification process.In the present context, a computational methodology has been developed at the mesoscale level to understand the impact of surface and grain-boundary diffusion mechanisms on the overall grain ripening and densification experienced by the LLZO solid electrolytes during the sintering process conducted at elevated temperatures. During densification, the extent of increase in relative density depends on the activation energy associated with the diffusive mass transport processes. Here, the activation energy for LLZO will be estimated from direct comparison of relative densities obtained from the developed computational model and experimental observations.[6] Some strategies, in terms of particle sizes and size distributions, that can lead to improved relative densities within the LLZO solid electrolytes, will also be discussed in this presentation. References Zhang, Z., et al., New horizons for inorganic solid state ion conductors. Energy & Environmental Science, 2018. 11(8): p. 1945-1976. Ramakumar, S., et al., Lithium garnets: synthesis, structure, Li+ conductivity, Li+ dynamics and applications.Progress in Materials Science, 2017. 88: p. 325-411. Rangasamy, E., J. Wolfenstine, and J. Sakamoto, The role of Al and Li concentration on the formation of cubic garnet solid electrolyte of nominal composition Li7La3Zr2O12. Solid State Ionics, 2012. 206: p. 28-32. Sharafi, A., et al., Controlling and correlating the effect of grain size with the mechanical and electrochemical properties of Li7La3Zr2O12 solid-state electrolyte. Journal of Materials Chemistry A, 2017. 5(40): p. 21491-21504. Dzepina, B., D. Balint, and D. Dini, A phase field model of pressure-assisted sintering. Journal of the European Ceramic Society, 2019. 39(2-3): p. 173-182. Shin, R.-H., et al., Effect of Li3BO3 additive on densification and ion conductivity of garnet-type Li7La3Zr2O12 solid electrolytes of all-solid-state lithium-ion batteries. Journal of the Korean Ceramic Society, 2016. 53(6). Figure 1

Grain growth kinetics and grain refinement mechanism in Al2O3/WC/TiC/graphene ceramic composite

The grain growth kinetics and grain refinement mechanism of Al2O3 grains and WC grains in Al2O3/WC/TiC/graphene ceramic composite were investigated in this work. The grain growth exponent of Al2O3 was determined as 2, which conformed to the growth mechanism of grain boundary diffusion. Meanwhile, the grain growth exponent of WC was determined as 3, which conformed to the grain growth mechanism of volume diffusion or liquid-phase diffusion. The different wettability of graphene to Al2O3 and WC was responsible for their different grain growth mechanisms. By analyzing grain growth activation energy, it can be concluded that the grain growth of Al2O3 and WC went through three stages: (1) softening stage, (2) rapid growth stage and (3) stable stage. The grain refinement mechanism of Al2O3 grains was graphene pinning effect and intragranular structure, and the grain refinement of WC grains could be attributed to the strong bonding interface between WC and graphene.

Effect of Nanopowder Addition on the Sintering of Water-Atomized Iron Powder

A promising method of improving the densification of powder metallurgical steel components is to blend nanopowder with the otherwise typically used micrometre-sized powder. The higher surface-to-volume ratio of nanopowder is hypothesized to accelerate the sintering process and increase the inter-particle contact area between the powder particles. This is supposed to enhance the material transport and improve the densification. In the present investigation, water-atomized iron powder (− 45 μm) was mixed separately with pure iron and low-carbon steel nanopowder, each at a ratio of 95 to 5 pct. These powder mixes were compacted at different pressures (400, 600 and 800 MPa) and then sintered at 1350 °C in a pure hydrogen atmosphere. The sintering behavior of the powder blend compacts was compared to that of the compact with micrometre-sized powder only. Densification commenced at much lower temperatures in the presence of nanopowder. To understand this, sintering at intermittent temperatures such as 500 °C and 700 °C was conducted. The fracture surface revealed that the nanopowder was sintered at between 500 °C and 700 °C, which in turn contributed to the densification of the powder mix at the lower temperature range. Based on the sintering experiments, an attempt was made to calculate the activation energy and identify the associated sinter mechanism using two different approaches. It was shown that the first approach yielded values in agreement with the grain-boundary diffusion mechanism. As the nanopowder content increased, there was an increase in linear shrinkage during sintering.

Emission Mössbauer Spectroscopy of Grain Boundaries in Ni–34%Cu Alloy

The grain boundaries in Ni–34%Cu alloy have been analyzed by emission Mossbauer spectroscopy. The mechanism of grain-boundary Co diffusion in the alloy and the grain-boundary segregation factor of this element are determined. The grain-boundary segregation of interstitial impurities is observed at low annealing temperatures.

Synergistic effects on thermal growth of CuO nanowires

Growing cupric oxide (CuO) nanowires (NWs) through thermal oxidation of copper (Cu) in air is a facile and low-cost strategy. In a temperature range of 673–1073 K, single-crystal, bi-crystal NWs and cone-shaped nanostructure usually coexist. As far as the underlying mechanism is concerned, the stress-driven grain-boundary (GB) diffusion mechanism has been the most successful theory up to now. However, it did not include the influence of oxygen partial pressure and cannot explain the growth of long single-crystal CuO NWs of uniform diameters. Thus, the CuO NWs growth mechanism is yet to be fully understood. In this study, we present that synergistic effects of the Kirkendall effect, stress-driven GB diffusion, large surface free energy gap between Cu2O and CuO facets and an effective electric field along the CuO NWs originating from oxygen gradient are mainly involved in the thermal growth of CuO NWs.

Anisotropy of Mass Transfer During Sintering of Powder Materials with Pore–Particle Structure Orientation

A micromechanical model for the shrinkage anisotropy during sintering of metallic powders is proposed and experimentally assessed. The framework developed for modeling sintering based on the mechanism of grain boundary diffusion is extended to take into account the dislocation pipe-enhanced volume diffusion. The studied iron powder samples are pre-shaped into their green forms by uniaxial cold pressing before sintering step. The resultant green bodies are anisotropic porous structures, with inhomogeneous plastic deformation at the inter-particle contacts. These non-uniformities are considered to be the cause of the anisotropic dislocation pipe diffusion mechanisms, and thus of the undesired shape distortion during shrinkage. The proposed model describes the shrinkage rates in the compaction loading and transverse directions, as functions of both structural and geometric activities of the samples. Dislocation densities can be estimated from such equations using dilatometry and image analysis data. The reliability and applicability of the developed modeling framework are verified by comparing the calculated dislocation densities with outcomes of nanoindentation and electron backscatter diffraction-derived lattice rotations.

Densification behavior of ZrB2-MoSi2 ceramics: The formation and evolution of core-shell solid solution structures

The role of solid solution shells in the densification behavior of ZrB2-MoSi2 ceramics was analyzed for varying ZrB2 starting particle sizes and MoSi2 contents. The formation of core-shell structures in hot-pressed ceramics in the ZrB2-MoSi2 system was confirmed by SEM, TEM and X-ray diffraction analysis. Microstructure analysis established that each ZrB2 core and its (Zr,Mo)B2 solid solution shell were isostructural with no detectable crystallographic misfit. Two single phase (Zr1-XMoX)B2 solid solutions were synthesized by reactive hot pressing to confirm that ZrB2-MoB2 solid solutions obey a quasi-linear Vegard's law. Additional (Si-free) ZrB2-Mo compositions, ZrB2/Mo/ZrB2 sandwich structures, or ZrB2-MoSi2 ceramics (quenched to room temperature) were hot pressed to study a transient liquid phase and its interdependency on ZrB2 powder purity, connectivity of pathways for mass transfer, wettability, or solubility as a function of applied pressure. The solid solution shells were the result of mass transfer by competing solid state mechanisms of surface and grain boundary diffusion during sintering that may have been boosted by a fugitive Si-based transient liquid phase. Mo was incorporated in diffusion-deposited diboride material at particle-particle necks. The volume fraction of solid solution shells and their Mo contents were affected by processing temperatures. In addition, plastic deformation of MoSi2 filled some closed porosity to aid the ceramics in achieving near full density.

Study of Lanthanum Diffusion in HfO2-Based High-k Gate Stack

La2O3 has been selected as a threshold voltage (Vt) tuning material for advanced nodes because of its uniqueness of reducing effective work function towards NFET band edge by creating dipoles at SiO2/HfO2 interface [1,2]. In this paper, we study the factors which influence the diffusion of Lanthanum (La) atoms through HfO2 during anneal. These factors include: (1) Crystallization state of HfO2; (2). HfO2 thickness; (3) Drive-in anneal temperature; (4) TiN capping material during drive-in anneal. We prepare the following stack: Si/SiO2/ HfO2/ La2O3 /TiN/a-Si, and anneal the stack to diffuse the La atoms into HfO2 layer. Then a-Si, cap TiN and excessive La2O3 were sequentially removed by wet etch. Final structure Si/SiO2/La-doped HfO2 is measured by XPS/XRF to determine the amount of La atoms that were diffused in. The diffusion of La is governed by its initial dosage (La2O3 deposition thickness), its adjacent layers (bottom HfO2, and top cap TiN), and the annealing temperature. By varying these parameters, we systematically study the effects of each factor, and correlate them to final Vt shift in the devices. The Vt shift in the MOSFET device is found to be proportional to the final diffused La amount in HfO2. The Vt shift shows obvious dependence on initial La dosage, annealing temperature and HfO2 thickness. On the other hand, the crystallization of HfO2 display a surprising blocking effect for La diffusion, indicating that La diffusion doesn’t follow the common grain-boundary diffusion mechanism. We found that HfO2 starts to crystallize when post-deposition anneal temperature is greater than 880C. La diffusion through crystallized HfO2 is significantly reduced compared to the case when HfO2 is still amorphous. On the contrary, Nitrogen atoms, which were incorporated during cap TiN deposition process, shows enhanced diffusion through HfO2. Fig.1 (a) shows the La signal in gate stack prior to and after La diffusion. The final La amount is much less for crystallized HfO2 (Hk PDA temp =900C and 950C) than for amorphous HfO2 (Hk PDA temp =800C and 850C). However, N signal, in Fig.1 (b) shows increased value for crystallized HfO2. Fig. 1(c) (d) show the Vt shift from NFET and PFET devices respectively, confirming less La diffusion (less Vt shift) in crystallized HfO2. The Vt of NFET is reduced while the Vt of PFET is enhanced because of the unipolar nature of the La-Si-O dipoles. We also conducted Angle-resolved XPS (AR-XPS) and TEM/EELS analysis and they both show that in crystallized HfO2, the La distribution is away from HfO2/SiO2 interface, while in amorphous HfO2, more La reside at HfO2/SiO2 interface. The capping material is also crucial for La diffusion in HfO2. We tested two types of TiN: (1) ALD TiN using a metalorganic precusor, tetrakis (dimethylamido) titanium and Nitrogen (TDMAT-TiN), (2) ALD TiN using TiCl4 precursor and NH3 (TiC4-TiN). We observe significant Vt difference between devices using these two TiN materials. The final diffused La is much less for TiC4-TiN based cap than TDMAT-TiN based cap. This is attributed to different La diffusion rate in the capping TiN material which in turn affects the La diffusion into HfO2. In summary, we evaluate the factors that influence La diffusion in HfO2-based Hk gate stack. Both crystallization of HfO2 and the absorption of La in top TiN material have great contributions to the effectiveness of La diffusion in HfO2. Some of these factors for La diffusion are especially intertwined with the growth of SiO2 interfacial layer (IL) which affects the Tinv and Toxgl of the MOSFET devices. Cares have to be taken to engineer the stack structure and integration scheme to achieve desired Vt shift and device performance. REFERENCE: [1] H. N. Alshareef et.al. Appl. Phys. Lett. 89, 232103 (2006) [2] Koji Kita and Akira Toriumi, Appl. Phys. Lett. 94, 132902 (2009) Figure 1

On the modeling of the co2‐catalyzed sintering of calcium oxide

A comprehensive mathematical model for the CO2‐catalyzed sintering of CaO is proposed. It takes into account the mechanisms of surface diffusion and grain boundary diffusion, catalyzed by CO2 chemisorption and dissolution, respectively. In addition, the model proposed here considers the change in pore size distribution during sintering, grain growth, and the densification by lattice diffusion, which is the intrinsic sintering mechanism of the CaO. Model predictions are validated using experimental data on the sintering of two CaO samples, one of them derived from pure CaCO3 and the other from limestone. It is found that impurities in limestone‐derived CaO do not significantly affect the CO2 dissolution or chemisorption processes; however, they strongly increase the rate of sintering by lattice diffusion. It is also established that low temperatures and CO2 partial pressures promote the coarsening by surface diffusion, whereas high temperatures and CO2 partial pressures favor densification. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3286–3296, 2017

Investigation of Grain-Boundary Diffusion of Co in Mo by Radiotracer Method and Mössbauer Spectroscopy

Grain boundary diffusion of Co in Mo has been studied by radiotracer analysis and emission Mössbauer spectroscopy. The experimental results are treated based on the specified Fisher model. The mechanism of grain boundary diffusion of Co in Mo and temperature dependences of segregation factor and grain boundary diffusivity have been determined.

Special features of grain-boundary diffusion in nanostructural and submicrocrystalline materials caused by structural heterogeneity of grain boundaries

Mechanisms of grain-boundary diffusion are analyzed based on models of structurally heterogeneous grain-boundaries and multipath diffusion in a system of structural elements responsible for accelerated mass transfer. The effective (averaged) coefficients of grain-boundary diffusion are obtained in models of continuous and discrete distribution of partial activation energies. The results of originally processed diffusion data in some submicrocrystalline materials are presented. The conclusion is drawn that, at low temperatures, the mass transfer in these materials is achieved through a small portion of active diffusion paths, presumably via grain triple junctions.

Densification Kinetics of Nanocrystalline Zirconia Powder Using Microwave and Spark Plasma Sintering—A Comparative Study

Two-stage densification process of nanosized 3 mol% yttria-stabilized zirconia (3Y-SZ) polycrystalline compacts during consolidation via microwave and spark-plasma sintering have been observed. The values of activation energies obtained for microwave and spark-plasma sintering 260-275 kJ x mol(-1) are quite similar to that of conventional sintering of zirconia, suggesting that densification during initial stage is controlled by the grain-boundary diffusion mechanism. The sintering behavior during microwave sintering was significantly affected by preliminary pressing conditions, as the surface diffusion mechanism (230 kJ x mol(-1)) is active in case of cold-isostatic pressing procedure was applied.