Boundary Diffusion Research Articles

Effect of pressure on the sintering mechanisms of tantalum carbide ceramics

AbstractHigh pressure sintering shows great superiority in promoting the densification of the hard‐to‐densify refractory ceramics, and the underlying mechanisms are thus of great interest. In the present work, the densification process of tantalum carbide (TaC) ceramics affected by pressure was discussed through comparing the microstructural characteristics and sintering kinetics. The TaC ceramics sintered at 30–250 MPa show dense and uniform microstructure without oxide impurities and high‐density dislocations. With the increase of sintering pressure, particle rearrangement becomes prominent at initial stage, whereas grain boundary diffusion rather than lattice diffusion dominants the final stage mechanism. Grain growth is enhanced under high pressure through the plastic deformation process. Our work should facilitate to understand the mass transport mechanisms during the high pressure sintering or deformation process of carbide ceramics.

Application of TbF3 Nano-powders by sand milling process in preparing high performance grain boundary diffusion Nd-Fe-B magnets

The Electrophoretic deposition grain boundary diffusion (EPD-GBD) technology for sintered Nd-Fe-B magnets has garnered significant attention. In this study, TbF3 nano-powders were synthesized via sand milling (SM) process, and the optimal processes were subsequently explored. Notably, the SM powders exhibit two morphologies: blocky and flocculent. With prolonged SM time, the proportion of larger blocky powders decreases gradually, while the proportion of smaller flocculent powders increases. The morphology and proportion of the powders significantly impact the EPD rate and the effectiveness of GBD. Higher blocky powders proportion improved diffusion, reducing residual Tb layer and enhancing magnet coercivity. Higher flocculent powders proportion caused cracking, poor diffusion, thicker residual layer, and lower coercivity. However, a high EPD rate was achieved with a high proportion of flocculent powders. Considering both EPD rate and diffusion effect, the SM process at 1500 r/min for 30 minutes was identified as the optimal parameter. The TbF3 nano-powders prepared by these conditions exhibited superior advantage in the EPD process and the coercivity of GBD magnets compared to traditional micron-scale TbF3 powders, demonstrating the benefits of the SM process in the EPD and GBD processes of Nd-Fe-B magnets.

Molecular dynamics simulations of nanoparticle sintering in additive nanomanufacturing: insights from sintering mechanisms

The sintering behavior of nanoparticles (NPs), which determines the quality of additively nanomanufactured products, differs from conventional understanding established for microparticles. As NPs have a high surface-to-volume ratio, they are subjected to a higher influence from surface tension and a lower melting point than microparticles, resulting in variations in both crystallographic defect-mediated and surface diffusion mechanisms. Meanwhile, the interplay between these controlling mechanisms in NPs has not been well understood, primarily because sintering occurs on the nanosecond timescale, making it an exceptionally transient process. In this work, sintering of both equal and unequal sized Ag and Cu NP doublets with and without misorientation (both tilt and twist) is modeled through molecular dynamics (MD) simulations. The formation and evolution of crystallographic defects, such as vacancies, dislocations, stacking faults, twin boundaries, and grain boundaries, during sintering are investigated. The influence of these defects on plastic deformation and diffusion mechanisms, such as volume diffusion and grain boundary (GB) diffusion, is discussed to elucidate the responsible sintering mechanisms. The surface diffusion mechanism is visualized by using detailed atomic trajectories generated during the sintering process. Finally, the overall effectiveness of all diffusion sintering mechanisms is quantified. This study provides first insights into the complexity and dynamics of NP sintering mechanisms which can aid in the development of accurate predictive models.

Oxidation resistance mechanism of copper wire regulated by nano-palladium coating

The microstructure characteristics of palladium coating are the key to the oxidation resistance of palladium-coated copper wire. In this paper, the microstructure characteristics of palladium coating thickness, nanocrystalline size and amorphous band width were controlled by adjusting the process parameters of micro-area coating. The relationship between microstructure characteristics of palladium coating and oxidation resistance was studied. The results show that the wire has good oxidation resistance when the deposition temperature is 400℃ and the copper wire/mold clearances are 2.5 μm. The surface of the copper wire has a palladium coating with a thickness of 74 nm, and the palladium particles are composed of amorphous encapsulated nanocrystals. The relationship between the thickness of palladium coating and the copper wire/mould clearances is y=0.012x+44, and the increase of palladium coating thickness can inhibit the migration of copper ions. The increase in the size of the nanocrystalline and the width of the amorphous band has a certain inhibitory effect on the lattice or grain boundary diffusion of copper to the palladium coating. The multi-stage distorted nanotwins in the amorphous palladium coating can reduce the strain difference at the palladium-copper bonding interface, reduce the diffusion driving force of copper outward, and thus improve the oxidation resistance of palladium-coated copper wires.

Strategy for co-enhancement of interface adhesion and coercivity of Nd-Fe-B grain boundary diffusion magnet: TbH3 nanopowders used in screen printing

Screen printing technology (SPT) was applied to conduct grain boundary diffusion sintered Nd-Fe-B magnets using TbH3 and TbF3 nanopowders as diffusion sources. TbH3 grain boundary diffusion (HGBD) coating had better adhesion to the magnet surface than TbF3 grain boundary diffusion (FGBD) coating. When the weight gain ratio was 1.0 wt%, the HGBD magnet achieved a coercivity increment of 11.07 kOe under non-pressurized heat treatment, while effectively controlling the magnet’s residual C and O elements. However, pressure heat treatment was necessary for the FGBD magnet to improve the coercivity due to the poor adhesion between the coating and the magnet surface, resulting in more residual C and O elements inside the magnet. Moreover, the coercivity of the FGBD magnet only increased by 7.96 kOe. Compared to the FGBD magnet, Tb diffused deeper into the HGBD magnet and formed more (Nd, Tb)2Fe14B-Nd2Fe14B core–shell structures. The formation of core–shell structures greatly enhanced the nucleation field of the reverse domain, thereby increasing the coercivity. Hence, the HGBD magnet had a higher coercivity increment. In addition, the HGBD magnet possessed better coercivity temperature stability than the original and FGBD magnets. Using TbH3 nanopowders as a diffusion source for SPT can achieve higher magnetic properties and simplified processes without pressure.

Densification and properties of WC-CoCrFeNi/Co cemented carbide fabricated via spark plasma sintering

The spark plasma sintering (SPS) technique was used to employe WC-CoCrFeNi/Co cemented carbides, incorporating CoCrFeNi and Co as low-coercivity binder phases, resulting in the development of cemented carbides with exceptional mechanical properties. The densification mechanism varying with increasing sintering temperature and the effect of densification on the mechanical properties were investigated using the creep deformation model. In the temperature range of 1100 °C–1300 °C, the main factors contributing to densification behavior are particle rearrangement and grain boundary diffusion due to the viscous flow of the binder phase. The densification mechanism gradually transforms into dislocation climbing with increasing temperature. When the sintering temperature exceeded 1300 °C, the WC grain size increased significantly with increasing temperature and holding time, and the dominant mechanism for grain growth was grain boundary diffusion. The high densification resulted in a substantial increase in effective grain boundary area, thereby increasing both hardness and fracture toughness of the material. Benefiting from the fine grain strengthening effect of the high-entropy alloys (HEAs) and the excellent wettability of Co, a highly dense cemented carbide material with excellent hardness and fracture toughness is achieved at the sintering temperature of 1300 °C, characterized by a relative density of 99.3 %, a hardness of 2064.9 MPa, and a fracture toughness of 11.5 MPa m−2.

Acceleration of grain boundary diffusion during ultra-fast firing (UHS) of alumina powder compacts

The sintering rate of a ceramic at a given temperature and relative density is often higher if the heating to that sintering temperature is more rapid. This has previous been explained in terms of microstructural coarsening during slow heating, which degrades the sinterability of the body before it reaches the sintering temperature. In this work the sintering of pure alumina with conventional heating rates (CS, 100-900 °C/hour) was compared with ultra-fast firing, using the method known as ultrafast high-temperature sintering (UHS, 120-140 °C/s). Despite the wide difference in heating rate, the SEM microstructures for a given density were similar and a modified Master Sintering Curve analysis of the results showed that the small pore size reduction observed was not sufficient to account for the acceleration in sintering of UHS samples compared with CS. There remained an additional acceleration by a factor of up to ∼20 that was attributed to a higher grain boundary diffusion coefficient in UHS. It is suggested that this is the result of the boundaries forming between powder particles not having time to relax to their lowest energy, most compact structures during UHS, i.e. non-equilibrium grain boundaries. In support of this hypothesis, it is shown that powder compacts that were first partially pre-sintered with conventional heating rates and then subjected to UHS did not show the same acceleration of sintering displayed by UHS of green bodies. The implication is that the slow pre-sintering allowed relaxation of the grain boundaries to low energy structures with low diffusion coefficients.

Structural defect and activated alumina of spheric α-Al2O3 improving alumina ceramics density from the industrial coarse gibbsite

The quality of alumina ceramics generally depends on alumina powder. High-density alumina ceramics were prepared from industrial coarse gibbsite. Near-spheric α-Al2O3 with particles smaller than 500 nm was produced via the synergy of B and F from NH4BF4. The near-spheric α-Al2O3 calcined at low temperature generated alumina ceramics with high density and low porosity. Meanwhile, the wet pretreatment yielded smaller particles with structural distortion and high microstrain and aluminum hydroxide, notably increasing the relative density of alumina ceramics. 99.18% relative density, 0.017% porosity, and 27.54 GPa Vickers hardness of alumina ceramics were achieved at 1650°C for 2 h without additives and pressed sintering. In addition, the early stage of sintering of alumina green body followed particle boundary diffusion with reduced activation energy in the sintering process. The close packing of the spheric α-Al2O3, high microstrain of α-Al2O3, structural defects with considerable AlO4 units calcined at low temperature, structural distortion in the alumina, and activated alumina on the α-Al2O3 surface collectively promoted the densification of alumina ceramics, achieving remarkable hardness, high density, and tiny porosity.

Investigation for the Impact of Grain Boundary Diffusion Direction on the Magnetic Properties of Nd–Fe–B Permanent Magnets

Investigation for the Impact of Grain Boundary Diffusion Direction on the Magnetic Properties of Nd–Fe–B Permanent Magnets

Phase Field Study of Discontinuous Precipitation in a Miscibility Gap System

AbstractA phase field model is developed to study the discontinuous (cellular) precipitation (DP) reaction in a hypothetical A-B miscibility gap system. Unlike previous treatments, the model employs an interaction energy term which couples the composition and the phase field variable to enable the necessary grain boundary movement. The influence of factors such as interaction energy, interfacial mobility and grain boundary diffusivity on the transformation rate and overall microstructure evolution are discussed.

Anisotropic sintering behavior of stainless steel 316L printed by binder jetting additive manufacturing

The influence of sintering temperature and printing orientation on the anisotropic sintering shrinkage of binder jetted (BJ) stainless steel 316L (SS316L) was investigated. Near quasi-isotropic shrinkage behavior with very little anisotropy (≤4 %) in the powder bed plane was observed. The out-of-plane anisotropic shrinkage of up to 16 % was attributed to the primitive lines formed perpendicular to the build direction. This follows the power law, and the activation energy is not affected by the anisotropy. For all printing orientations, grain boundary diffusion (25–162 kJ/mol) was the dominant sintering mechanism between 1100 °C and 1300 °C. Above 1300 °C, volume diffusion (490–623 kJ/mol) takes over as the dominating sintering mechanism, leading to significantly higher shrinkage up to 15 %. The anisotropic shrinkage behavior does not affect the microstructure, as similar evolution of grain sizes, ferrite fractions, and bulk densities (up to 96 %) is observed upon sintering.

Achieving high-performance regeneration of grain boundary diffusion Nd-Fe-B sludge via an ultra-short process

The grinding sludge of grain boundary diffusion Nd-Fe-B magnet is rich in heavy rare earth and has a typical core–shell microstructure. In this work, an ultra-short process was developed to realize green, in-situ regeneration of the sludge and efficient reuse of heavy rare earth elements. The regenerated magnets showed excellent magnetic performance and good temperature stability which were fabricated after the purification of sludge, composition regulation with rare earth-rich compounds, Nd42.1FebalB0.79, NdHx, (PrNd)Hx, and sintering. Especially, the magnet prepared by using Nd42.1FebalB0.79 alloy had significant property recovery, where remanence and coercivity were restored to 12.8 kG, and 20.6 kOe, respectively, reaching 97.7 % and 162.5 % of the original magnet. The magnetization reversal behavior and the coercivity mechanism were analyzed to keep the nucleation mechanism, and the high coercivity stemmed from the repair and optimization of the grain boundary by introducing rare earth-rich alloys, the full utilization of heavy rare earth elements, and the core–shell microstructure rooted in the sludge as well.

Study on the Rheological Behaviors, Thixotropy, and Printing Characteristics of Screen Printing Slurry for Nd-Fe-B.

The rheological behavior and printing characteristics of the screen-printing slurry for Nd-Fe-B grain boundary diffusion are key factors that determine the quality of printing and magnetic performance. However, few studies have focused on the organic medium, a crucial material for slurry. In this paper, the rheology, thixotropy, and thermal decomposition behavior of the organic vehicle in Nd-Fe-B screen printing slurry were studied. The results show that the organic vehicle formed by terpineol and polyvinyl butyral (PVB) exhibits typical non-Newtonian fluid characteristics, with excellent rheology and thixotropy, ensuring that the slurry prepared from it has excellent static stability and printing consistency. Additionally, the carbon residue of the organic vehicle formed by terpineol and PVB is less than 0.1% at 900 °C, avoiding excessive carbon entering the magnet during the diffusion process. Moreover, studying the rheology and thixotropy of the organic vehicle through a rheometer can quickly screen the slurry system. This work provides valuable guidance for designing an organic vehicle for screen-printing slurry for Nd-Fe-B grain boundary diffusion in future research.

Grain boundary diffusion in additively manufactured CoCrFeMnNi high-entropy alloys: Impact of non-equilibrium state, temperature and relaxation

Grain boundary diffusion of Ni in the equiatomic CoCrFeMnNi high-entropy alloy, produced by additive manufacturing, is measured using a radiotracer technique in an extended temperature interval of 350 to 703 K. A strongly non-monotonic temperature dependence of the Ni grain boundary diffusion coefficients (with a spectacular intermittent retardation of the diffusion rates with increasing temperature) is seen and explained by relaxation of a non-equilibrium state induced by rapid solidification during fabrication. The grain boundary excess energy of the non-equilibrium state of these grain boundaries, as estimated from the diffusion data, is found to be larger than 0.3 J/m2. This corresponds to an increase of about 30% of the interface energy compared to relaxed general high-angle grain boundaries. The temperature-induced evolution of the grain boundary state is analyzed in terms of the concomitant structure evolution, segregation, phase stability and precipitation in the multi-component alloy.

(Nd,Sm,La)-Fe-B-based hot-deformed magnets with excellent comprehensive properties for variable flux permanent magnet motors

An emerging motor type, the variable flux permanent magnet (VFPM) motor, has created a demand for permanent magnets with a new set of properties. These magnets must exhibit easily tunable magnetization, demanding for moderate coercivity values, low magnetizing fields, and first-order reversal curves (FORCs) with high flatness. In this work, we report that the doping of Sm and La in Nd-Fe-B-based hot-deformed magnet, followed by the Nd-Cu diffusion process, resulted in desirable magnetic properties, such as, a moderate coercivity (µ0Hc) of 0.55 T, a low magnetizing field (µ0Hmag) of 0.94 T and a high FORC flatness of 0.96, while maintaining a high remanence (µ0Mr) of 1.3 T. The FORC flatness achieved in this work is the highest and the comprehensive properties are superior to the previously reported Nd-Fe-B magnets for VFPM motors. Microscopic investigations revealed that the high flatness achieved in this hot-deformed magnet is attributed to the formation of Fe-lean thick intergranular phases realized by Nd-Cu grain boundary diffusion. The overall combination of the magnetic properties for the diffusion processed (Nd,Sm,La)-Fe-B magnet is excellent, in comparison with a commercially available Sm2Co17-type magnet, showing the promise of the former for use in VFPM motors.

Effect of Mo addition on microstructure and mechanical properties of Cu-12.5Ni-5Sn alloy

Cu-12.5Ni-5Sn-xMo (x=0, 0.5, 1.0, 1.5 wt%) alloys were prepared using Spark Plasma Sintering (SPS) technology. The effect of Mo addition on the microstructure and mechanical properties of Cu-12.5Ni-5Sn alloy was investigated. The results indicate that adding trace amounts of Mo significantly refines the grain structure of Cu-12.5Ni-5Sn alloy, attributed to the formation of Mo-rich phases. Mo addition also inhibited the growth of discontinuous precipitation (DP) after aging treatment, reducing the DP volume fraction from approximately 25 % to 6 %. This effect is attributed to the precipitation of micron-sized Mo-rich phases at grain boundaries, which inhibit DP nucleation and growth by occupying γ-phase nucleation sites and pinning grain boundaries, thus impeding grain boundary diffusion. In addition, the interlamellar spacing of the discontinuous precipitation increases and the growth rate of lamellae decreases with increasing Mo content. Furthermore, an optimized combination of hardness (310 HB) and yield strength (586 MPa) was achieved in the Cu-12.5Ni-5Sn-1.0Mo alloy after aging treatment.

Surface Degradation of Mg2X-Based Composites at Room Temperature: Assessing Grain Boundary and Bulk Diffusion Using Atomic Force Microscopy and Scanning Electron Microscopy.

Practical application of thermoelectric generators necessitates materials that combine high heat-to-electricity conversion efficiency with long-term functional stability under operation conditions. While Mg2(Si,Sn)-based materials exhibit promising thermoelectric properties and module prototypes have been demonstrated, their stability remains a challenge, demanding thorough investigation. Utilizing atomic force microscopy (AFM) and scanning electron microscopy (SEM), we investigate the surface degradation of a composite material comprising Si-rich and Sn-rich Mg2(Si,Sn) solid solutions. The investigation reveals a pronounced dependence of stability on Sn content, with the Sn-rich phase Mg2Si0.13Sn0.87 displaying the formation of a nonprotective oxide layer. Subsequent AFM measurements provide evidence of dominating grain boundary diffusion of loosely bound Mg, compared to bulk diffusion, observed within a few days, ultimately resulting in a complete surface oxidation of the Sn-rich phase within several weeks. On the other hand, Mg2Si and Si-rich Mg2Si0.80±0.05Sn0.20±0.05 remain stable against Mg diffusion to the surface even after prolonged exposure. Comparison with previous investigations confirms that the degradation rate is found to be highly dependent on the Sn content, with markedly higher rates observed for x = 0.87 compared to x = 0.70 in Mg2Si1-xSnx. These findings contribute to a better understanding of the stability challenges associated with Mg2(Si,Sn)-based materials, essential for the development of robust thermoelectric materials for practical applications.

Daily and Half-yearly Associations between Boundary Diffusion and Parent-adolescent Relationship Quality after Divorce.

Boundary diffusion is a particular risk after divorce and has been associated with adolescents' adjustment problems. Yet, its potential impact on parent-adolescent relationship quality is less straightforward, as previous findings support both an alienation and conflict perspective. Therefore these associations (daily and half-yearly) were examined in recently divorced families, addressing both within-dyad changes and between-dyad differences. Data were collected among a sample of N = 133 (pre)adolescents (Mage = 11.76; 51.5% boys) from 76 divorced families, using a measurement burst design: Every six months, 14 consecutive days of daily diaries were collected, for 5 waves. Between dyads, adolescents who experienced more boundary diffusion than others, also reported more conflict with both their parents. Within dyads, when adolescents experienced more boundary diffusion than usual by one of their parents (actor), warmth decreased and conflict increased between this parent and the adolescent, that same and the following day. Adolescents also engaged in more conflict with the other parent that day. These findings mostly supported the conflict perspective: Post-divorce boundary diffusion appears to be a general risk factor for parent-adolescent conflict with both parents, and from day-to-day boundary diffusion was linked to a deteriorated parent-adolescent relationship quality, especially with the parent that triangulated or parentified them. There were no significant long-term associations, nor did any moderator (age, gender, living arrangement) explain heterogeneity in effects.

An STM study on the diffusion of O atoms on a CO-covered Ru(0001) surface—The role of domain boundaries

We investigate tracer diffusion at the domain boundaries in an adsorption layer, an effect that corresponds to grain boundary diffusion in 3D polycrystalline solids. Experiments were performed on adsorbed O atoms on a Ru(0001) surface in a layer of CO molecules. The CO molecules form a (3x3)R30∘ structure which displays translational domains. High-speed scanning tunneling microscopy (STM) was used to image the motion of the O atoms. The data show that single O atoms preferentially move along the domain walls which in the STM movies appear as disordered, fluctuating stripes between the ordered domains. The diffusion coefficient of the O atoms is one order of magnitude higher than the diffusion coefficient in the ordered domains. By comparison with previous experiments on completely disordered CO layers, it is concluded that the diffusion is similarly promoted by the enhanced fluctuations in the disordered domain walls.

Co-sintering of a ceramic-metal bilayer: Coupled experimental, analytical and numerical approaches

The co-sintering behavior of a ceramic-metal bilayer is carefully investigated. This bilayer is composed of a gold film deposited by screen-printing on a silica-based low-temperature cofired ceramic (LTCC) layer. First, behaviors of both materials in free sintering are finely characterized. LTCC exhibits a viscous flow sintering mechanism, whereas the densification mechanism of gold is controlled by grain boundary diffusion (with an identified apparent activation energy of 101 ± 15 kJ mol−1). Free sintering of gold is also marked by a two-step densification process due to its large bimodal particle size distribution. A methodology is proposed to estimate gold viscosity at high temperatures directly from raw experimental data in free sintering, without performing dedicated mechanical characterizations. Based on the constitutive laws identified for LTCC and gold materials, analytical and numerical models of the bilayer co-sintering process are built. During co-sintering, stresses generated by densification kinetic mismatch between gold and LTCC are progressively relaxed by camber phenomenon. Calculated evolutions of camber in co-sintering are compared to curvature monitoring by ombroscopy. Finite element simulation in 3D fits well with the experimental data, whereas the analytical solution strongly underestimates camber rates. This shows that the simplified numerical approach suggested here leads to a robust numerical model, faithfully describing thermomechanical interactions occurring in co-sintering within the ceramic-metal bilayer.