High Interfacial Energy Research Articles

Effect of crystallographic orientation and grain boundary character on fatigue cracking behaviors of coaxial copper bicrystals

Four coaxial copper bicrystals were employed to study the slip morphologies and fatigue cracking behaviors during cyclic deformation. Three of them had high-angle grain boundaries (GBs) with nearly the same misorientation and one bicrystal had a twin boundary (TB). Different slip bands (SBs) operated near the GBs and TB, generating different dislocation arrangements, which are mainly determined by the crystallographic orientations of the component grains. The GBs suffered impingement or shear damage caused by slip difference from both sides. It is suggested that there is an energy increase in the interfaces between matrix and persistent slip bands (PSBs), GBs and TBs per cycle during cyclic deformation due to the accumulation of lattice defects, which would make the interface unstable. After a certain number of cycles, fatigue cracks initiated firstly at GBs for some bicrystals while fatigue cracking occurred preferentially at PSBs for the others. It is confirmed that the energy growth rate is an increasing function of the shear stress, strain amplitude and strain incompatibility, which results from slip differences on both sides of the interfaces. Interfaces with different energies and strain incompatibilities have different fatigue cracking resistance. It is found that GBs with defective and complex structure, and hence high interfacial energy accompanied by high modulus of the residual GB dislocation (GBD), are preferential sites for fatigue cracking, while the fatigue cracking appeared predominantly at PSBs when the modulus of the residual GBD is lower than that of a perfect dislocation with simple GB structure and low interfacial energy. The present model for the energy can predict well which kind of interface would form cracks preferentially during cyclic deformation in one coaxial bicrystal and which GB would need more cycles to initiate fatigue cracking between coaxial bicrystals with different GB characters.

The Use of High Pressure Direct Squeeze Casting for Semi-Solid Processing of Aluminum Base Nano Composites

The production of nano-reinforced aluminium alloys in volume and quality suitable for subsequent shape casting has been problematic. Large specific surface area and high interfacial energy of the particles combined with high surface tension of the aluminium melt makes it difficult to add appreciable numbers of particles to the melt, even when later de-agglomerated by techniques such as ultrasonic cavitation. The objective of this work was to develop a technique to incorporate particles using pressure applied while the alloy was in a semi-solid state. The composites produced could be used as a master alloy to inoculate large batches of metal for subsequent casting using any suitable technique. The results show excellent distribution of 50 nm alumina particles in 2014 material. The procedure appears to have broad applicability to a full range of aluminium alloys and particle reinforcements.

Characteristics of A356/SiCp and A390/SiCp composites

Aluminum alloy matrix composites reinforced with SiC particles (AMC) are potential materials applicable to weight reduction of fric- tion materials. We developed two AMC: A356/SiC and A390/SiC. 30 µm and 60 µm SiC particles with volume fraction of 20% were dispersed in the matrix of A356 and A390 alloys. When the temperature of the liquid alloy was higher than that of the melting point, SIC particles floated up to the surface because of the low viscosity, high surface tension and interfacial energy. To mix the ceramic particles, the aluminum alloy was in the mushy state near the solidus temperature, 610℃ for A356 and 645℃ for A390. Metallurgical, mechani- cal and tribological characteristics of the manufactured composites were evaluated.

Three-Dimensional Porous LiFePO4: Design, Architectures and High Performance for Lithium Ion Batteries

The olivine-structured lithium ion phosphate (LiFePO4) is one of the most competitive candidates of cathode materials for the sustainable lithium ion battery (LIB) systems. However, the major drawback of olivine-structured LiFePO4 is the poor intrinsic electronic and lithium ion conductivities arising from the lack of mixed valency and the onedimensional lithium ion diffusion, which influence its high electrochemical performance, especially high rate capability. Nano-structured LiFePO4 materials offer a potential solution to enhance surface-to-volume ratio and reduce transport length for mobile charges, but they have high interfacial energy, aggregate easily and need more agglutinant in electrode, which seriously impact the electrochemical performance and practical applications of LiFePO4. Furthermore they continue to experience limitations as energy and power requirements escalate with the evolution of technology. Recently, three-dimensional (3D) porous LiFePO4 architectures have been widely designed and studied. This has led to increased interest in the development of cathode materials and processing capabilities necessary to enable high-performance, nextgeneration LIB system that can deliver large amounts of energy at high rates. In this review, we focus on 3D porous LiFePO4 architectures for high power LIBs, summarize and discuss its structure, synthesis, electrochemical behaviors, mechanism, and the problems encountered in its application. The major goal is to highlight the recent progress of 3D porous LiFePO4 architectures with high rate capability, high energy density and application.

Abstract 2373: An integrated approach for isolating CTCs to enable the interrogation of cellular heterogeneity using IFAST

Abstract Background: The isolation of circulating tumor cells (CTCs) from whole blood is enabling for the study of malignant cells from patients with metastatic disease through a minimally invasive procedure. The analytical methods following the isolation of CTCs can be used to gain information about the disease, but require a highly purified population of cells that are minimally disturbed from the isolation procedures. While enumeration of CTCs during therapy has been shown to predict overall survival in men with metastatic prostate cancer, current methodology is limited in its ability to obtain information about these cells beyond count. Recent findings also suggest that CTC populations are heterogeneous (e.g. EMT, stem cell markers), and enumeration of CTCs based solely on EpCAM surface expression does not permit evaluation of CTC subpopulations. Therefore, methods that can isolate these cells based on their heterogeneity and further evaluate surface markers in an integrated platform will be beneficial in progressing the clinical relevance of CTC collection. Method: We present a method to sequentially isolate cells from a single input sample by cellular surface markers. Using immiscible filtration assisted by surface tension (IFAST) technology, we have demonstrated the ability to purify cells from a mixed cell population while maintaining the viability of these cells. The principle of the technology is that a high interfacial energy between the cell sample and oil creates a barrier that is normally impassable due to surface tension. Magnetic beads are added that have antibodies specific to a cellular surface marker and bind to the cells. A magnetic force is then applied to move the cells to and across the interface between the fluid and the oil, leaving anything unbound to the beads behind, thus isolating the cells of interest. CTC isolation is challenging due to potential cell loss in pipette transfer and wash steps. Using the IFAST device, we are able to add whole buffycoat into the input of the device and sequentially isolate, stain, and image CTCs without any pipette transfer or wash steps. Results: We have demonstrated the ability to have a single source solution in which two unique cell populations are present representing a basal and luminal cell type (DU145s, LNCaPs) and demonstrate the ability to individually isolate each of the two populations using a cellular surface antibody unique to one of the cell types (DU145 - CD44, LNCaPs - EpCAM) from a buffycoat containing high background PBMCs with greater than 80% of the resulting cells being the cell of interest in each situation, and demonstrate immunostaining of the isolated cell population (vimentin, HLA-A2). Conclusions: IFAST technology can be used to isolate multiple fractions from a single buffycoat solution using sequential bead additions and pull-throughs, demonstrating an advantage of this technology in the assessment of CTC heterogeneity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2373. doi:1538-7445.AM2012-2373

Precipitation behavior of grain boundary M23C6 and its effect on tensile properties of Ni–Cr–W based superalloy

Precipitation behavior of grain boundary (GB) M23C6 and its effect on tensile properties at elevated temperature were investigated systematically in a Ni–Cr–W based superalloy. The results show that the M23C6 precipitation behavior is influenced obviously by grain boundary character (GBC) and interfacial energy. The Σ≤9 GBs and low angle GBs have low interfacial energy, and no M23C6 carbide precipitates at these GBs. Plenty of M23C6 carbide particles precipitate at the large angle GBs with high interfacial energy. The coherent orientation relationship between M23C6 and the matrix plays an important role on the precipitation morphology of M23C6. M23C6 carbides with four typical morphologies distribute at the large angle GBs, including lamellar carbide which grows into the matrix near one side or both sides of the GBs, rod-like carbide and small lamellar carbide both of which grow along GBs. Moreover, the decrease of both tensile and yield strength of the aged alloy is mainly caused by the lamellar M23C6 carbide breaking. The tensile properties vary irregularly with increasing aging time.

Interfacial strength and fracture energy of individual carbon nanofibers in epoxy matrix as a function of surface conditions

The interfacial shear strength (IFSS) and fracture energy of individual carbon nanofibers embedded in epoxy were obtained for different surface conditions and treatments by novel, MEMS-based, nanoscale fiber pull-out experiments. As-grown vapor grown carbon nanofibers (VGCNFs) with turbostratic surface and 5nm peak-to-valley surface roughness exhibited high IFSS and interfacial fracture energy, averaging 106±29MPa and 1.9±0.9J/m2, respectively. Subsequent high temperature heat treatment and graphitization resulted in drastically reduced IFSS of 66±10MPa and interfacial fracture energy of 0.65±0.14J/m2. The smaller IFSS values and the reduced standard deviation were due to significant reduction of the fiber surface roughness to 1–2nm, as well as a decrease in surface defect density during conversion of turbostratic and amorphous carbon to highly ordered graphitic carbon. For both grades of VGCNFs failure was adhesive with clear nanofiber surfaces after debonding. Oxidative functionalization of high temperature heat-treated VGCNFs resulted in much higher IFSS of 189±15MPa and interfacial fracture energy of 3.3±1.0J/m2. The debond surfaces of functionalized nanofibers had signs of matrix residue and/or shearing of the outer graphitic layer of the VGCNFs, namely the failure mode was a combination of cohesive matrix and/or cohesive fiber failure which contributed to the high IFSS. For all three grades of VGCNFs the IFSS was independent of fiber length and diameter. The findings of this experimental study emphasized the critical role of nanofiber surface morphology and chemistry in determining the shear strength and fracture energy of nanofiber interfaces, and shed light to prior composite-level strength and fracture toughness measurements.

Visualizing Lipid Reactivity of HIV-1 Antigen and Neutralizing Antibodies using Atomic Force Microscopy

Evidence suggests that lipid membrane interactions with rare, broadly neutralizing antibodies (NAbs), 2F5 and 4E10, play a critical role in HIV-1 neutralization. The objective of this research is to understand how lipid membrane properties, such as lipid domain organization contribute to 2F5/4E10 membrane interactions and antigen localization at the membrane interface, with the ultimate vision of guiding immunogen designs. Recent immunization studies have shown that induction of antibodies that avidly bind the gp41-MPER antigen is not sufficient for neutralization. Rather, it is required that antigen designs induce polyreactive antibodies that recognize MPER antigens as well as the viral lipid membrane. However, the mechanistic details of how membrane properties influence NAb-lipid and NAb-antigen interactions remain unknown. Furthermore, it is well established that the native viral membrane is heterogeneous, representing a mosaic of lipid rafts and protein clustering. However, the size, physical properties, and dynamics of these regions are poorly characterized and their potential roles in HIV-1 neutralization are also unknown. Methods: To understand how membrane properties contribute to 2F5/4E10 membrane interactions, we have engineered biomimetic supported lipid bilayers (SLBs) and are able to use atomic force microscopy to visualize membrane domains, antigen clustering, and antibody-membrane interactions.Our results showed that lipid domains were easily observed for simple binary membrane constructs and for complex, biomimetic HIV-1 model membranes. Localized binding of HIV-1 antigens (MPER656) and NAbs were observed to interact preferentially with the most fluid membrane domain. This supports the theory that NAbs may interact with regions of low lateral lipid forces that allow antibody insertion into the bilayer. NAbs were also observed to cluster at the edge of certain domain interfaces suggesting NAbs affinity for high interfacial energy regions of the lipid membrane.

Characterizations of the interfacial energy in glass/resin laminates

A study is made of the characterization of the interfacial energy in glass/resin laminates by tensile and flexural tests. Glass/resin laminates, such as slide glass/resin/slide glass, LCD glass/resin/slide glass, and LCD glass/resin/silicon wafer layer structures, are constructed. The effects of the lubricant, mold/substrate materials and resin thickness on the interfacial energy are investigated. The application of lubricant in the laminates critically reduces the interfacial energy during the demolding process. The interfacial energy is determined not only by the mold/resin interface but also by the resin/substrate interface. Although the interfacial energy is influenced by increased surface of micro-sized patterns, the patterns are conserved after the mold is demolded by the introduction of lubricant. A higher interfacial energy increases the adhesive force between the mold and the resin when the resin is thinner, indicating the need for lubricant. In contrast, thinner resin is profitable for designing strong laminate applications.

Bonding of immiscible Mg and Fe via a nanoscale Fe 2Al 5 transition layer

Bonding of immiscible systems is difficult due to the sharp interface and high interfacial energy. A nanoscale transition layer with lattice matching to both sides could be a solution. To examine this hypothesis, Mg and Fe were successfully bonded by a precoated nanoscale Fe2Al5 layer, which in crystallographic terms matched well with both lattices. The interplanar mismatches of both Fe2Al5/Fe and Fe2Al5/Mg interfaces were less than 5% and both interfaces were found to be semicoherent.

Purification of cell subpopulations via immiscible filtration assisted by surface tension (IFAST)

The selective isolation of a sub-population of cells from a larger, mixed population is a critical preparatory process to many biomedical assays. Here, we present a new cell isolation platform with a unique set of advantages over existing devices. Our technology, termed Immiscible Filtration Assisted by Surface Tension, exploits physical phenomena associated with the microscale to establish fluidic barriers composed of immiscible liquids. By attaching magnetically-responsive particles to a target cell population via immunocapture, we can selectively transport this population across the immiscible barrier and into a separate aqueous solution. The high interfacial energy associated with the immiscible phase / aqueous phase boundaries prevents unwanted cells or other contaminants from inadvertently crossing the immiscible phase. We have demonstrated, using fluorescent particles, stromal cells, and whole blood as "background", that we can successfully isolate ~70% of a target breast cancer cell population with an average purity of >80%. Increased purity was obtained by coupling two immiscible barriers in series, a modification that only slightly increases operational complexity. Furthermore, several samples can be processed in parallel batches in a near-instantaneous manner without the requirement of any washing, which can cause dilution (negative selection) or significant uncontrolled loss (positive selection) of target cells. Finally, cells were observed to remain viable and proliferative following traverse through the immiscible phase, indicating that this process is suitable for a variety of downstream assays, including those requiring intact living cells.

Unstable graphite films on grain boundaries in crustal rocks

The origin of high electrical conductive anomalies in the lower crust is a long-standing and controversial problem. Although it has been proposed that saturated saline water or partial melt increases electrical conductivity, graphite film has also been recognized as a potential cause of high conductivity since the discovery of fine graphite films on the grain boundaries of high-grade metamorphic rocks. To investigate the stability of graphite film on grain boundary of silicate minerals under lower crustal conditions, electrical conductivity of graphite film on synthetic grain boundaries of quartz bicrystals was measured by means of impedance spectroscopy at 1 GPa and up to 1200 K in a multianvil apparatus. At first heating, the electrical conductivity of the thin graphite film with thickness less than 100 nm was initially very high but decreased with time during annealing. Under high temperature conditions (> 1000 K), the conductivity of a thin carbon film rapidly decreases and approaches the quartz conductivity. This indicates that graphite film on a grain boundary between two quartz crystals is not stable at high temperatures. Optical microscopic observation of the run products suggested a disconnected feature of graphite on a quartz grain boundary. Disconnection of graphite film can be caused by higher interfacial energy between graphite and silicate minerals than that of the grain boundary energy. Therefore, a thin graphite film is not a likely candidate to account for high conductivity anomalies in the middle and lower continental crust.

Discussion of the paper “Dissolution theory applied to the induction period in alite hydration” by P. Juilland et al., Cem. Concr. Res. 40 (2010) 831–844

The new mechanism proposed by Juilland et al. [1] to explain the induction period in alite hydration is discussed with reference to the free energy of dissolution under typical hydration conditions calculated from published data. It is concluded that the proposed mechanism is theoretically possible but requires an unusually high interfacial energy for step formation, plus an unusually large increase in dissolution rate over small changes in the degree of solution undersaturation, compared to typical published values for silicate minerals. It is suggested that comparisons with the hydration rates of CaO and MgO might shed further light on the mechanism.

Effects of impurity segregation to grain boundary on intergranular cracking in 2.25Cr-1W steel

Intergranular cracking behaviors in water quenched 2.25 Cr-1.5W heat-resistant steels are understood in the light of the formation of grain boundary carbides and the impurities segregation to grain boundaries or carbide interface at the grain boundaries. Before stress rupture tests, all materials were water-quenched after holding at 1050°C for 1h. The microstructure showed a typical lath martensite. Heating rate was 1200°C/h to a testing temperature and stress rupture test was performed without any soaking. At a fixed temperature, time to failure increased as the applied stress decreased. Irrespective of the bulk contents of P, the fracture mode was intergranular except for a condition corresponding to a longer time to failure. From AES analyses, the crater interface where grain boundary carbides had existed showed a segregation concentration of P much higher than that at the carbide-free grain boundaries, resulting from the much higher interface energy. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. These results suggest that the intergranular cracking follows the sequence from the crater interface to the carbide-free grain boundaries.

Phase‐field modeling of the effect of interfacial energy on pyrolytic carbon morphology in chemical vapor deposition

AbstractA conserved phase‐field model is proposed to investigate the effect of interfacial energy on the morphological evolution of the pyrolytic carbon deposit in chemical vapor deposition. The equilibrium geometry of carbon deposit islands is analytically predicted, of which the contact angle was controlled through the boundary conditions of the phase‐field parameter at the substrate surface according to the Young‐Laplace equation. Simulations of deposit growth are carried out for single and multi island nucleation. It is clarified that the island morphology depends on the magnitude of the interface energy. It is also observed that high interface energy results in large island size fluctuation. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Synthesis and enrichment of a macromolecular surface modifier PP- b-PVP for polypropylene

Polypropylene-block-poly(vinylpyrrolidone) copolymer was synthesized through the esterification of dicarboxyl-terminated polypropylene with monohydroxyl-terminated poly(vinylpyrrolidone) and used as a macromolecular surface modifier to improve the surface hydrophilicity of PP. The results of ATR-FTIR and contact angles measurements indicated that PP -b-PVP could diffuse preferably onto the surface by the inducement of high energy interface and lower the water contact angle of polypropylene. Lower loading and lower molecular weight of PP and PVP segment would result in higher surface selective enrichment.

Centrifuge assisted percolation of Fe–S melts in partially molten peridotite: Time constraints for planetary core formation

The mechanism which segregates molten Fe–S into metallic cores of planetary bodies is still not fully understood. Due to the high interfacial energy and wetting angle between Fe–S melts and silicate mantle minerals, the continuous percolative flow of such melts cannot be efficient for the core segregation in planetary bodies. A series of percolation experiments has been realized on a partially molten fertile garnet peridotite, employing a centrifuging piston cylinder. A high temperature garnet peridotite with Mg# ~0.90 composed of 60 vol.% olivine, 15 vol.% orthopyroxene, 6 vol.% clinopyroxene and 19 vol.% garnet has been used as the silicate matrix. Peridotite powders with the 100–200 or 20–30 µm grain size were mixed with 5–30 vol.% Fe–S of eutectic composition Fe 70S 30. The aggregates were centrifuged at 500–700 g at temperatures below and above the melting point of the peridotite. The centrifuge experiments revealed a negligible percolation of Fe–S melts through the unmolten peridotite matrix. Only at T > 1260 °C, i.e. above the solidus of the peridotite, and starting with 5 vol.% of Fe 70S 30 the vertical melt gradient achieved 1–2 vol.%/mm. In samples with 15 vol.% Fe 70S 30 the vertical separation achieved 2–2.5 vol.%/mm after 10 h of centrifuging at 500 g. An increase in the degree of partial silicate melting in the peridotite leads to an increase of the Fe–S separation rate from the peridotite matrix. Fe–S contents >10 vol.% cause an increase of the Fe–S melt droplet size and of the effective percolation velocity of Fe–S melt. A threshold dividing fast (>10 cm per year) and slow percolations (<1 mm per year) of Fe –S melt is found around 14–15 vol.% of Fe 70S 30. The experimentally determined permeabilities of Fe–S melt in the unmolten peridotite with 7–10 vol.% of Fe 70S 30 melt are 10 − 18 –10 − 19 m 2, which is 2–3 orders of magnitude lower than the values calculated previously from static experiments. The presence of the silicate melt increases the segregation velocity of Fe–S melt in a partially molten peridotite by more than one order of magnitude with respect to the unmolten peridotite matrix. This could provide an effective segregation of Fe–S melt in a planetary mantle down to 2.5 vol.% of residual Fe–S melt. The extremely slow percolation of Fe–S melt in the absence of the partial silicate melting precludes a scenario of metallic core formation via percolation before temperatures allow a substantial partial melting of mantle silicates in planetary bodies.

Study of W Metallic Coating Layer on Diamond Surface by Hydrothermal Method

As the diamond surface has higher interface energy, the adhesion of surface and matrix is bad, and it makes the diamond shatter easily. For solving this, the microstructures and properties of the metallic coating on the diamond surface which contains W (tungsten) by hydrothermal method are studied. The different temperatures such as 110 ~140°C and 5h insulating were adopted to be coated in high pressure reactor respectively. The metallic coating on the diamond surface was studied by SEM，XRD and so on. The results show that the continuous and compact coating of Ni and W on the diamond surface is formed by the hydrothermal coating technology at 130°C. The heat treatment temperatures were maintained at 850°C for 1h during the heat treatment process. After the corresponding heat treatment, the better layer coatings, of which thickness is about 10μm, are formed for protecting diamonds and reducing the trend of oxidization. The carbide WC (tungsten carbide) at the interface between the coating and diamond surface is formed. The chemical and metallurgical bonding between diamond and coating is obtained. Because of the coating layers formed and the defects on diamond surface made up, the compress strength of diamond after hydrothermal and heat treatment is enhanced. The compressive strength after hydrothermal and heat treatment is higher than that of un-coating particles(those are 29.45% and 83.64% respectively).

Biomimetically Triggered Inorganic Crystal Transformation by Biomolecules: A New Understanding of Biomineralization

Phase transformation is an important strategy in biomineralization. However, the role of biomolecules in the mineral transition is poorly understood despite the fact that the biomineralization society greatly highlights the organic controls in the formation of the inorganic phase. Here, we report an induced biomimetic phase transformation from brushite (a widely used calcium phosphate precursor in biological cement) to hydroxyapatite (main inorganic composition of skeletal mineral) by citrate (a rich organic component in bone tissue). The transformation in the absence of the organic additive cannot be spontaneously initiated in an aqueous solution with a pH of 8.45 (no phase transition is detected in 4 days), which is explained by a high interfacial energy barrier between brushite-solution and hydroxyapatite-solution interfaces. Citrate can oppositely regulate these two interfaces, which decreases and increases the stabilities of brushite and hydroxyapatite surfaces in the solution, respectively. Thus, the interfacial energy barrier can be greatly reduced in the presence of citrate and the reaction is triggered; e.g., at 1 mM citrate, the total transformation from brushite to hydroxyapatite can be completed within 3 days. The relationship between the transition kinetics and citrate concentration is also studied. The work reveals how the organic components direct solid-solid phase transformation, which can be understood by an energetic control of the interfacial barrier. It is emphasized that the terms of interfacial energy must be taken into account in the studies of phase transformation. We suggest that this biomimetic approach may provide an in-depth understanding of biomineralization.

First-principles simulations of copper diffusion in tantalum and tantalum nitride

To explore potential applications of tantalum and tantalum nitrides (TaN) as diffusion barrier materials in integrated circuits with Cu interconnects, we carry out detailed first-principles simulations of Cu/Ta and Cu/TaN systems. Various interfacial structures between Cu- and Ta-based compounds are examined by considering different surface orientations, in-plane arrangements, surface terminations, and chemical compositions. The coexistence of strong Cu-N ionic bonding and Ta-Cu covalent/metallic bonding dictates the stable interfacial structures. Using nudged elastic band method, we calculate the diffusion energy barriers of Cu to the pre-existing vacancies across Cu/Ta, Cu/TaN interfaces, and in bulk TaN compounds. As a comparison, Cu diffusion in Si is also studied. It was found that Cu can easily diffuse into Si either spontaneously or with small energy barriers. On the other hand, although the Cu/Ta interfacial diffusion barrier is low, the high vacancy formation energy in Ta renders Cu diffusion difficult. We find that fcc TaN is an excellent candidate for diffusion barrier material owing to its extremely high interfacial diffusion energy barrier. The bulk diffusion barrier of Cu in fcc TaN is also very high.