More resilient polyester membranes for high-performance reverse osmosis desalination.

Thin-film composite reverse osmosis membranes have remained the gold standard technology for desalination and water purification for nearly half a century. Polyamide films offer excellent water permeability and salt rejection but also suffer from poor chlorine resistance, high fouling propensity, and low boron rejection. We addressed these issues by molecularly designing a polyester thin-film composite reverse osmosis membrane using co-solvent-assisted interfacial polymerization to react 3,5-dihydroxy-4-methylbenzoic acid with trimesoyl chloride. This polyester membrane exhibits substantial water permeability, high rejection for sodium chloride and boron, and complete resistance toward chlorine. The ultrasmooth, low-energy surface of the membrane also prevents fouling and mineral scaling compared with polyamide membranes. These membranes could increasingly challenge polyamide membranes by further optimizing water-salt selectivity, offering a path to considerably reducing pretreatment steps in desalination.

Revealing the room temperature superplasticity in bulk recrystallized molybdenum.

Body-centered cubic refractory metallic materials exhibit excellent high-temperature strength, but often suffer from brittle intergranular fracture due to the recrystallization-induced enrichment of trace elements at grain boundaries (GBs). Here, we report a fully-recrystallized pure molybdenum (Mo) material with room temperature (RT) superplasticity, fabricated by a facile method of powder metallurgy, Y-type hot rolling and annealing. By engineering the ultralow concentration of O at GBs, the inherent GB brittleness of Mo can be largely eliminated, which, in conjunction with high fractions of soft texture and low angle GBs, enables a significant development of ordered dislocation networks and the effective dislocation transmission across low angle GBs. Synergy of these factors greatly suppress the brittle intergranular fracture of Mo, contributing to an enhanced deformability of 108.7% at RT. These findings should have general implication for fabricating a broad class of refractory metals and alloys toward harsh applications.

Synergy of multiple precipitate/matrix interface structures for a heat resistant high-strength Al alloy.

High strength aluminum alloys are widely used but their strength is reduced as nano-precipitates coarsen rapidly in medium and high temperatures, which greatly limits their application. Single solute segregation layers at precipitate/matrix interfaces are not satisfactory in stabilizing precipitates. Here we obtain multiple interface structures in an Al-Cu-Mg-Ag-Si-Sc alloy including Sc segregation layers, C and L phases as well as a newly discovered χ-AgMg phase, which partially cover the θ' precipitates. By atomic resolution characterizations and ab initio calculations, such interface structures have been confirmed to synergistically retard coarsening of precipitates. Therefore, the designed alloy shows the good combination of heat resistance and strength among all series of Al alloys, with 97% yield strength retained after thermal exposure, which is as high as 400 MPa. This concept of covering precipitates with multiple interface phases and segregation layers provides an effective strategy for designing other heat resistant materials.

Tailoring planar slip to achieve pure metal-like ductility in body-centred-cubic multi-principal element alloys.

Uniform tensile ductility (UTD) is crucial for the forming/machining capabilities of structural materials. Normally, planar-slip induced narrow deformation bands localize the plastic strains and hence hamper UTD, particularly in body-centred-cubic (bcc) multi-principal element high-entropy alloys (HEAs), which generally exhibit early necking (UTD < 5%). Here we demonstrate a strategy to tailor the planar-slip bands in a Ti-Zr-V-Nb-Al bcc HEA, achieving a 25% UTD together with nearly 50% elongation-to-failure (approaching a ductile elemental metal), while offering gigapascal yield strength. The HEA composition is designed not only to enhance the B2-like local chemical order (LCO), seeding sites to disperse planar slip, but also to generate excess lattice distortion upon deformation-induced LCO destruction, which promotes elastic strains and dislocation debris to cause dynamic hardening. This encourages second-generation planar-slip bands to branch out from first-generation bands, effectively spreading the plastic flow to permeate the sample volume. Moreover, the profuse bands frequently intersect to sustain adequate work-hardening rate (WHR) to large strains. Our strategy showcases the tuning of plastic flow dynamics that turns an otherwise-undesirable deformation mode to our advantage, enabling an unusual synergy of yield strength and UTD for bcc HEAs.

Nanoscale Chemical Heterogeneity Ensures Unprecedently Low Resistance Drift in Cache-Type Phase-Change Memory Materials.

Phase-change random access memory is a promising technique to realize universal memory and neuromorphic computing, where the demand for robust multibit programming drives exploration for high-accuracy resistance control in memory cells. Here in ScxSb2Te3 phase-change material films, we demonstrate thickness-independent conductance evolution, presenting an unprecedently low resistance-drift coefficient in the range of ∼10-4-10-3, ∼3-2 orders of magnitude lower compared to conventional Ge2Sb2Te5. By atom probe tomography and ab initio simulations, we unveiled that nanoscale chemical inhomogeneity and constrained Peierls distortion together suppress structural relaxation, rendering an almost invariant electronic band structure and thereby the ultralow resistance drift of ScxSb2Te3 films upon aging. Associated with subnanosecond crystallization speed, ScxSb2Te3 serves as the most appropriate candidate for developing high-accuracy cache-type computing chips.

A nanodispersion-in-nanograins strategy for ultra-strong, ductile and stable metal nanocomposites.

Nanograined metals have the merit of high strength, but usually suffer from low work hardening capacity and poor thermal stability, causing premature failure and limiting their practical utilities. Here we report a "nanodispersion-in-nanograins" strategy to simultaneously strengthen and stabilize nanocrystalline metals such as copper and nickel. Our strategy relies on a uniform dispersion of extremely fine sized carbon nanoparticles (2.6 ± 1.2 nm) inside nanograins. The intragranular dispersion of nanoparticles not only elevates the strength of already-strong nanograins by 35%, but also activates multiple hardening mechanisms via dislocation-nanoparticle interactions, leading to improved work hardening and large tensile ductility. In addition, these finely dispersed nanoparticles result in substantially enhanced thermal stability and electrical conductivity in metal nanocomposites. Our results demonstrate the concurrent improvement of several mutually exclusive properties in metals including strength-ductility, strength-thermal stability, and strength-electrical conductivity, and thus represent a promising route to engineering high-performance nanostructured materials.

Designing against phase and property heterogeneities in additively manufactured titanium alloys.

Additive manufacturing (AM) creates digitally designed parts by successive addition of material. However, owing to intrinsic thermal cycling, metallic parts produced by AM almost inevitably suffer from spatially dependent heterogeneities in phases and mechanical properties, which may cause unpredictable service failures. Here, we demonstrate a synergistic alloy design approach to overcome this issue in titanium alloys manufactured by laser powder bed fusion. The key to our approach is in-situ alloying of Ti-6Al-4V (in weight per cent) with combined additions of pure titanium powders and iron oxide (Fe2O3) nanoparticles. This not only enables in-situ elimination of phase heterogeneity through diluting V concentration whilst introducing small amounts of Fe, but also compensates for the strength loss via oxygen solute strengthening. Our alloys achieve spatially uniform microstructures and mechanical properties which are superior to those of Ti-6Al-4V. This study may help to guide the design of other alloys, which not only overcomes the challenge inherent to the AM processes, but also takes advantage of the alloy design opportunities offered by AM.

Mechanically Reinforced Artificial Enamel by Mg2+-Induced Amorphous Intergranular Phases.

Amorphous intergranular phases in mature natural tooth enamel are found to provide better adhesion and could dramatically affect their mechanical performance as a structure reinforcing phase. This study successfully synthesized an amorphous intergranular phase enhanced fluorapatite array controlled by Mg2+ (FAP-M) at room temperature. Furthermore, atom probe tomography (APT) observation presents that Mg2+ is enriched at grain boundaries during the assembly of enamel-like fluorapatite arrays, leading to the formation of intergranular phases of Mg-rich amorphous calcium phosphate (Mg-ACP). APT results also demonstrated that the segregation of Mg2+ caused the chemical gradient in nanocrystalline attachment and realignment under the drive of inherent surface stress. These results indicate that the amorphous intergranular phases served like glue to connect each nanorod to reinforce the enamel-like arrays. Therefore, the as-received FAP-M artificial enamel exhibits excellent mechanical properties, with hardness and Young's modulus of 2.90 ± 0.13 GPa and 67.9 ± 3.4 GPa, which were ∼8.3 and 2.2 times higher than those of FAP arrays without controlled by Mg2+, respectively.

Uniting tensile ductility with ultrahigh strength via composition undulation.

Metals with nanocrystalline grains have ultrahigh strengths approaching two gigapascals. However, such extreme grain-boundary strengthening results in the loss of almost all tensile ductility, even when the metal has a face-centred-cubic structure-the most ductile of all crystal structures1-3. Here we demonstrate that nanocrystalline nickel-cobalt solid solutions, although still a face-centred-cubic single phase, show tensile strengths of about 2.3 gigapascals with a respectable ductility of about 16 per cent elongation to failure. This unusual combination of tensile strength and ductility is achieved by compositional undulation in a highly concentrated solid solution. The undulation renders the stacking fault energy and the lattice strains spatially varying over length scales in the range of one to ten nanometres, such that the motion of dislocations is thus significantly affected. The motion of dislocations becomes sluggish, promoting their interaction, interlocking and accumulation, despite the severely limited space inside the nanocrystalline grains. As a result, the flow stress is increased, and the dislocation storage is promoted at the same time, which increases the strain hardening and hence the ductility. Meanwhile, the segment detrapping along the dislocation line entails a small activation volume and hence an increased strain-rate sensitivity, which also stabilizes the tensile flow. As such, an undulating landscape resisting dislocation propagation provides a strengthening mechanism that preserves tensile ductility at high flow stresses.

Effects of atom probe analysis parameters on composition measurement of precipitates in an Al-Mg-Si-Cu alloy.

Defining analysis parameter space for reliable composition measurement of materials is of significance for atom probe tomography applications. This research carefully explores the influence of specimen temperature and ultraviolet laser energy on measured compositions of precipitates and the matrix in an Al-Mg-Si-Cu alloy with atom probe tomography using voltage pulsing and laser pulsing. Low specimen temperature and high laser energy are beneficial to reduce background noise and improve mass resolution. Under both voltage pulsing and laser pulsing, the detected compositions and Mg/Si ratios of precipitates are highly sensitive to specimen temperature in the range of 20 - 80 K. In contrast, at a fixed temperature of 20 K, laser energy variation from 40 to 80 pJ provides consistent measurements in composition and Mg/Si ratio of precipitates. Related field-induced preferential evaporation and surface migration of certain elements have been discussed.

A medium-range structure motif linking amorphous and crystalline states.

Amorphous materials have no long-range order, but there are ordered structures at short range (2-5 Å), medium range (5-20 Å) and even longer length scales1-5. While regular6,7 and semiregular polyhedra8-10 are often found as short-range ordering in amorphous materials, the nature of medium-range order has remained elusive11-14. Consequently, it is difficult to determine whether there exists any structural link at medium range or longer length scales between the amorphous material and its crystalline counterparts. Moreover, an amorphous material often crystallizes into a phase of different composition15, with very different underlying structural building blocks, further compounding the issue. Here, we capture an intermediate crystalline cubic phase in a Pd-Ni-P amorphous alloy and reveal the structure of the medium-range order, a six-membered tricapped trigonal prism cluster (6M-TTP) with a length scale of 12.5 Å. We find that the 6M-TTP can pack periodically to several tens of nanometres to form the cube phase. Our experimental observations provide evidence of a structural link between the amorphous and crystalline phases in a Pd-Ni-P alloy at the medium-range length scale and suggest that it is the connectivity of the 6M-TTP clusters that distinguishes the crystalline and amorphous phases. These findings will shed light on the structure of amorphous materials at extended length scales beyond that of short-range order.

Rapid collagen-directed mineralization of calcium fluoride nanocrystals with periodically patterned nanostructures.

Collagen fibrils present periodic structures, which provide space for intrafibrillar growth of oriented hydroxyapatite nanocrystals in bone and contribute to the good mechanical properties of bone. However, there are not many reports focused on bioprocess-inspired synthesis of non-native inorganic materials inside collagen fibrils and detailed forming processes of crystals inside collagen fibrils remain poorly understood. Herein, the rapid intrafibrillar mineralization of calcium fluoride nanocrystals with a periodically patterned nanostructure is demonstrated. The negatively charged calcium fluoride precursor phase infiltrates collagen fibrils through the gap zones creating an intricate periodic mineralization pattern. Later, the nanocrystals initially filling the gap zones only expand gradually into the remaining space within the collagen fibrils. Mineralized tendons with organized calcium fluoride nanocrystals acquire mechanical properties (indentation elastic modulus ∼25.1 GPa and hardness ∼1.5 GPa) comparable or even superior to those of native human dentin and lamellar bone. Understanding the mineral growth processes in collagen may facilitate the development of tissue engineering and repairing.

Self-inhibition effect of metal incorporation in nanoscaled semiconductors.

There has been a persistent effort to understand and control the incorporation of metal impurities in semiconductors at nanoscale, as it is important for semiconductor processing from growth, doping to making contact. Previously, the injection of metal atoms into nanoscaled semiconductor, with concentrations orders of magnitude higher than the equilibrium solid solubility, has been reported, which is often deemed to be detrimental. Here our theoretical exploration reveals that this colossal injection is because gold or aluminum atoms tend to substitute Si atoms and thus are not mobile in the lattice of Si. In contrast, the interstitial atoms in the Si lattice such as manganese (Mn) are expected to quickly diffuse out conveniently. Experimentally, we confirm the self-inhibition effect of Mn incorporation in nanoscaled silicon, as no metal atoms can be found in the body of silicon (below 1017 atoms per cm-3) by careful three-dimensional atomic mappings using highly focused ultraviolet-laser-assisted atom-probe tomography. As a result of self-inhibition effect of metal incorporation, the corresponding field-effect devices demonstrate superior transport properties. This finding of self-inhibition effect provides a missing piece for understanding the metal incorporation in semiconductor at nanoscale, which is critical not only for growing nanoscale building blocks, but also for designing and processing metal-semiconductor structures and fine-tuning their properties at nanoscale.

Ultrastrong low-carbon nanosteel produced by heterostructure and interstitial mediated warm rolling.

Ultrastrong materials can notably help with improving the energy efficiency of transportation vehicles by reducing their weight. Grain refinement by severe plastic deformation is, so far, the most effective approach to produce bulk strong nanostructured metals, but its scaling up for industrial production has been a challenge. Here, we report an ultrastrong (2.15 GPa) low-carbon nanosteel processed by heterostructure and interstitial mediated warm rolling. The nanosteel consists of thin (~17.8 nm) lamellae, which was enabled by two unreported mechanisms: (i) improving deformation compatibility of dual-phase heterostructure by adjusting warm rolling temperature and (ii) segregating carbon atoms to lamellar boundaries to stabilize the nanolamellae. Defying our intuition, warm rolling produced finer lamellae than cold rolling, which demonstrates the potential and importance of tuning deformation compatibility of interstitial containing heterostructure for nanocrystallization. This previously unreported approach is applicable to most low-carbon, low-alloy steels for producing ultrahigh strength materials in industrial scale.

Revealing Solute Clusters in Coalescence by Atom Probe Tomography Analysis.

Experimentally revealing dynamic evolution and growth behavior of small solute clusters in alloys remains a technical challenge. To date, the coalescence of the solute clusters has seldom been experimentally addressed. To address the challenge, we used atom probe tomography (APT) to access boundary information of solute clusters and identify those in close contact. By systematically investigating the population and size evolution of the clusters in close contact with aging time, we unveiled important information regarding the clusters in coalescence with the exsitu experimental technique. In this work, the maximum separation method was employed to identify clusters in APT datasets of naturally aged Al­Zn­Mg alloy. Coalescence was found to significantly contribute to the growth of small clusters and remained predominant for the formation and growth of large Guinier­Preston II ${\rm \lpar G}{\rm P}_{{\eta }^{\prime}}\rpar$ zones after 3 months aging.

Nanoscale pathways for human tooth decay - Central planar defect, organic-rich precipitate and high-angle grain boundary.

Understanding the pathways and mechanisms of human tooth decay is central to the development of both prophylaxes and treatments, but only limited information is presently available about the initiation of caries at the nanoscale. By combining atom probe tomography and high-resolution electron microscopy, we have found three distinct initial sites for human dental enamel dissolution: a) along the central dark line (CDL) within carbonated apatite nanocrystals, b) at organic-rich precipitates and c) along high-angle grain boundaries. 3D maps of the atoms within hydroxyapatite nanocrystallites in sound and naturally-decayed human dental enamel reveal a higher concentration of Mg and Na in the CDL. The CDL is therefore thought to provide a pathway for the exchange of ions during demineralization and remineralization. Mg and Na enrichment of the CDL also suggests that it is associated with the ribbon-like organic-rich precursor in amelogenesis. Organic-rich precipitates and high-angle grain boundaries were also shown to be more vulnerable to corrosion while low-angle grain boundaries remained intact. This is attributed to the lower crystallinity in these regions.

Atom Probe Tomography Characterization of Dopant Distributions in Si FinFET: Challenges and Solutions.

Atom probe tomography (APT) has emerged as an important tool in characterizing three-dimensional semiconductor devices. However, the complex structure and hybrid nature of a semiconductor device can pose serious challenges to the accurate measurement of dopants. In particular, local magnification and trajectory aberration observed when analyzing hybrid materials with different evaporation fields can cause severe distortions in reconstructed geometry and uncertainty in local chemistry measurement. To address these challenges, this study systematically investigates the effect of APT sampling directions on the measurement of n-type dopants P and As in an Si fin field-effect transistor (FinFET). We demonstrate that the APT samples made with their Z-axis perpendicular to the center axis of the fin are effective to minimize the negative effects that result from evaporation field differences between the Si fin and SiO2 on reconstruction and achieve improved measurement of dopant distributions. In addition, new insights have been gained regarding the distribution of ion-implanted P and As in the Si FinFET.

Atom Probe Tomography Analysis of TiCx Powders Synthesized by SHS in Al/Fe/Cu-Ti-C Systems.

The stoichiometry of titanium carbide (TiCx) particles is important in determining particle properties. Spherical TiCx powders with particle sizes of 1-5 µm were produced by self-propagating high-temperature synthesis (SHS) in 30 wt.% Al-, 30 wt.% Cu-, and 30 wt.% Fe-Ti-C systems, respectively. To measure the compositions of the carbide powders, atom probe tomography (APT) tip specimens were carefully prepared by using a focus ion-beam milling method. APT analysis revealed that the TiCx particles formed in Al-, Cu-, and Fe-Ti-C systems are highly substoichiometric. The results are consistent with observations of the TiCx particles with a high content of oxygen and a certain amount of secondary metallic elements (Al, Cu, and Fe).