Α2 Phase Research Articles

Emergence of Hexagonally Close-Packed Spheres in Linear Block Copolymer Melts.

The hexagonally close-packed (HCP) sphere phase is predicted to be stable across a narrow region of linear block copolymer phase space, but the small free energy difference separating it from face-centered cubic spheres usually results in phase coexistence. Here, we report the discovery of pure HCP spheres in linear block copolymer melts with A = poly(2,2,2-trifluoroethyl acrylate) ("F") and B = poly(2-dodecyl acrylate) ("2D") or poly(4-dodecyl acrylate) ("4D"). In 4DF diblocks and F4DF triblocks, the HCP phase emerges across a substantial range of A-block volume fractions (circa fA = 0.25-0.30), and in F4DF, it forms reversibly when subjected to various processing conditions which suggests an equilibrium state. The time scale associated with forming pure HCP upon quenching from a disordered liquid is intermediate to the ordering kinetics of the Frank-Kasper σ and A15 phases. However, unlike σ and A15, HCP nucleates directly from a supercooled liquid or soft solid without proceeding through an intermediate quasicrystal. Self-consistent field theory calculations indicate the stability of HCP is intimately tied to small amounts of molar mass dispersity (Đ); for example, an HCP-forming F4DF sample with fA = 0.27 has an experimentally measured Đ = 1.04. These insights challenge the conventional wisdom that pure HCP is difficult to access in linear block copolymer melts without the use of blending or other complex processing techniques.

Addition of Ge to Cr–Ta–Si laves phase-based alloys

The Cr–Cr2Ta system is of interest as a potential candidate for structural applications at temperatures that exceed those that can be tolerated by nickel-based superalloys. However, the oxidation resistance of alloys based on this system is currently insufficient. Previous work on the Cr–Cr3Si system suggests that Ge additions may also improve the oxidation resistance of alloys based on the Cr–Cr2Ta system. To ascertain whether such additions may be appropriate, the effect of Ge additions on the microstructure and mechanical properties of a Cr–10Ta–7Si alloy was investigated. The Laves phase in this system was unexpectedly found to decompose into a lamellar structure comprising a Cr solid solution and a cubic phase with an Fd-3m structure. The addition of Ge promotes this decomposition, as well as the formation of the A15 Cr3(Si,Ge) phase, at the expense of the Cr solid solution. The formation of the A15 phase increases the hardness of the alloy but has a detrimental effect on its ductility.

Research on Hot Deformation Behavior of Mo‐Containing Ti2AlNb‐Based Alloy

Herein, the hot compression deformation behavior and microstructure instability of the different Mo‐containing Ti2AlNb‐based alloys are investigated in the temperature range of 900−1150 °C and strain rate range of 0.001−1 s−1. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to characterize the microstructure evolution during the deformation process. The results reveal that Mo element addition can refine the lamellar α2/O phase of Ti2AlNb‐based alloy. In addition, it has a certain influence on the phase‐transition temperature of the alloy, which leads to the narrowing of the O + B2 + α2 phase region. During the hot compression process, the spheroidization of the lamellar structure in Ti2AlNb−0.5Mo alloy is mainly through the transformation of O→B2 + α2, whereas the microstructure of Ti2AlNb−2Mo alloy is mainly converted from the O phase to the B2 phase. In addition, it is found that the high content of Mo element causes the segregation of Al element and leads to poor plasticity and stability of the Ti2AlNb−2Mo alloy. As such, the hot deformation performance of the Ti2AlNb−0.5Mo alloy is significantly better, which provides theoretical guidance for the composition design of the Ti2AlNb‐based alloy.

Microstructure and mechanical properties of as-cast and directionally solidified large size Ti–45Al–8Nb alloys by electromagnetic confinement

Microstructure, fracture toughness and high-temperature tensile property of as-cast and directionally solidified Ti–45Al–8Nb alloys were investigated. The microstructure of as-cast alloy consists of equiaxed grains, in which there are a large number of reticular B2 phases and massive γ phases. Compared with as-cast alloy, the transverse grain boundaries of the directionally solidified alloy were eliminated, and the lamellar orientation was controlled. The volume fraction of α2 phase increased from 11.6% in as-cast alloy to 19.3% in directionally solidified alloy and the volume fraction of γ phase changed little. From as-cast condition to directional solidification, the morphology of B2 phase changed from reticular shape to elongated shape, and the volume fraction decreased from 10.2% to 1.4%. The lamellar spacing of the alloy decreased from 1500 nm to 1020 nm and it becomes more uniform. The room temperature fracture toughness increases from 21.1 MPa m1/2 in as-cast alloy to 29.5 MPa m1/2 in directionally solidified alloy. The high-temperature tensile strength and the elongation at 850 °C of the alloy increased from 376.6 ± 16 MPa and 4.18 ± 0.9% in as-cast alloy to 592.5 ± 29.5 MPa and 11.6 ± 1.5% in directionally solidified alloy. Correspondingly, the fracture mode changed from intergranular fracture in as-cast alloy to translamellar fracture in directionally solidified alloy. In addition, the toughening mechanism and strengthening mechanism were discussed.

Evolution of the magnetic hyperfine field profiles in an ion-irradiated Fe60Al40 film measured by nuclear resonant reflectivity.

Nuclear resonant reflectivity (NRR) from an Fe60Al40 film was measured using synchrotron radiation at several grazing angles near the critical angle of total external reflection. Using laterally resolved measurements after irradiation with 20 keV Ne+ ions of gradually varying fluence of 0-3.0 × 1014 ions cm-2, the progressive creation of the ferromagnetic A2 phase with increasing ion fluence was confirmed. The observed depth selectivity of the method has been explained by application of the standing wave approach. From the time spectra of the nuclear resonant scattering in several reflection directions the depth profiles for different hyperfine fields were extracted. The results show that the highest magnetic hyperfine fields (∼18-23 T) are initially created in the central part of the film and partially at the bottom interface with the SiO2 substrate. The evolution of the ferromagnetic onset, commencing at a fixed depth within the film and propagating towards the interfaces, has been directly observed. At higher fluence (3.0 × 1014 ions cm-2) the depth distribution of the ferromagnetic fractions became more homogeneous across the film depth, in accordance with previous results.

Microstructure tailoring of Al-containing compositionally complex alloys by controlling the sequence of precipitation and ordering

Refractory metal-based, Al-containing compositionally complex alloys (RCCA) are promising candidates for high-temperature structural applications. To shed light on the complex phase transitions, thermodynamic calculations were performed to select two representative alloys with different sequences in phase transitions. Samples of these compositions were synthesized by arc melting of pure elements followed by a homogenization treatment to experimentally verify the room temperature microstructure and assess the phase transitions. Differential scanning calorimetry (DSC), scanning (SEM) and transmission electron microscopy (TEM) studies after the homogenization and quenching revealed multiple distinct sequences of transitions: (i) 82(TaMoTi)-8Cr-10Al (in at.%) exhibits a solid-state phase separation concurrent with ordering of the precipitates. This results in a disordered matrix with ordered precipitates. Thermal analysis indicates that while cooling from the high-temperature A2 phase, the phase separation and ordering are spread out over a large temperature range (approx. 750 – 1250°C), with a peak at 1055°C. (ii) In the 77(TaMoTi)-8Cr-15Al alloy, a continuous phase transition at 1155°C leads to a single-phase B2 matrix with planar faults. At slightly lower temperatures (approx. 1096°C), phase separation occurs resulting in a B2 matrix with segregation at planar faults and A2 precipitates. In both investigated compositions, the A2 phase is enriched in Ta and Mo. Conversely, the B2 phase is enriched in Al and Ti, while Cr is uniformly distributed in the phases.

Enhancing ion transport in charged block copolymers by stabilizing low symmetry morphology: Electrostatic control of interfaces

Recently, the interest in charged polymers has been rapidly growing due to their uses in energy storage and transfer devices. Yet, polymer electrolyte-based devices are not on the immediate horizon because of the low ionic conductivity. In the present study, we developed a methodology to enhance the ionic conductivity of charged block copolymers comprising ionic liquids through the electrostatic control of the interfacial layers. Unprecedented reentrant phase transitions between lamellar and A15 structures were seen, which cannot be explained by well-established thermodynamic factors. X-ray scattering experiments and molecular dynamics simulations revealed the formation of fascinating, thin ionic shell layers composed of ionic complexes. The ionic liquid cations of these complexes predominantly presented near the micellar interfaces if they had strong binding affinity with the charged polymer chains. Therefore, the interfacial properties and concentration fluctuations of the A15 structures were crucially dependent on the type of tethered acid groups in the polymers. Overall, the stabilization energies of the A15 structures were greater when enriched, attractive electrostatic interactions were present at the micellar interfaces. Contrary to the conventional wisdom that block copolymer interfaces act as "dead zone" to significantly deteriorate ion transport, this study establishes a prospective avenue for advanced polymer electrolyte having tailor-made interfaces.

Investigation on transformation-related recrystallization behavior of Ti2AlNb-based alloy

To achieve grain refinement, the recrystallization behavior of B2 matrix phase in a Ti2AlNb-based alloy was investigated by performing a series of thermal compression tests at temperatures in the range of 950~1025°C with strain rates from 0.001s−1 to1s−1. Instead of water quenching immediately after deformation, samples were further held at deformation temperature for a certain time, in order to make the heat-affecting time at various strain rates consistent. Electron back scattered diffraction (EBSD) technique was employed for microstructure analysis, in combination with grain orientation spread (GOS) method to distinguish recrystallized grains from non-recrystallized grains. The results reveal that: (i) dynamic recrystallization (DRX) is not easy to occur in this alloy, and increasing strain rate leads to a decrease of DRX fraction; (ii) when evaluated on the same time scale, the sample at strain rate of ε˙=0.1s−1 has the highest recrystallized fraction among all applied strain rates, which provides sufficient nucleation rate of DRX as well as notable grain growth during post-dynamic recrystallization (PDRX); and (iii) the recrystallization evolution with temperature changes is closely linked to the phase transformation in Ti2AlNb. The presence of O and α2 phases affects recrystallization process via pinning effect and particle stimulated nucleation (PSN) mechanism. This work therefore provides a new understanding of recrystallization behavior during and post deformation of Ti2AlNb-based alloy, which is meaningful to refine grains by combining the contributions of DRX and PDRX.

Cracking mechanism and a novel strategy to eliminate cracks in TiAl alloy additively manufactured by selective laser melting

Additive manufacturing of TiAl alloys by selective laser melting (SLM) remains a challenge due to serious cracking during the print process. In this work, the cracking mechanism of the SLM-fabricated Ti-48Al-2Cr-2Nb alloy is studied. Experimental results show that the cracks mainly occurred in the Nb-poor regions and near the interfaces between α2 and B2 phases. A novel strategy to eliminate cracks is thus proposed: eliminating the Nb-poor regions and reducing the interface between different phases. It is found that by increasing Nb content, a nearly α2 single-phase structure without heterogeneous interface formed along with the elimination of Nb-poor regions in the SLM-fabricated Ti–48Al–2Cr–8Nb alloy. The as-SLMed alloy is crack-free and of high strength, verifying the effectiveness of the proposed strategy.

Effects of standing and walking training using a laser pointer based on stimulus-driven attention for behavioural outcome in spatial neglect: A single-case study

ABSTRACT The therapy for unilateral spatial neglect (USN) is unclear. This case report investigated the effect of standing and walking training using a laser pointer based on stimulus-driven attention for USN. The patient was a right-handed 79-year-old man with cardiogenic cerebral embolism in the right middle and posterior cerebral arteries. Initially, we evaluated the absence of hemiparalysis in the lower limb and sensory disorder; almost all daily activities were performed independently. Intervention effects were verified using the BABA method. The course of the four phases (B1, A1, B2, A2) was conducted for 5 days. In the B1 and B2 phases, standing and walking training using a laser pointer was performed additionally to conventional physical therapy. Outcomes were measured using the Behavioural Inattention Test conventional subtest (BIT-c), Catherine Bergego Scale (CBS), and modified Posner task (MPT). The BIT-c remained unchanged in each phase. CBS scores improved after B1 and B2. In the MPT, the reaction time in the left space reduced after B1 and B2 compared with those in the A1 and A2 control phases. In this case, training may have contributed to the improvement in the response to the neglected space and behavioural assessment of USN.

Random copolymerization of macromonomers as a versatile strategy to synthesize mixed‐graft block copolymers

AbstractThe random copolymerization of norbornene‐functionalized macromonomers was explored as a method of synthesizing mixed‐graft block copolymers (mGBCPs). The copolymerization kinetics of a model system of polystyrene (PS) and poly(lactic acid) (PLA) macromonomers was first analyzed, revealing a gradient composition of side chains along the mGBCP backbone. The phase separation behavior of mGBCPs with PS and PLA side chains of various backbone lengths and side chain molar ratios was investigated, and increasing the backbone length was found to stabilize the phase‐separated nanostructures. The graft architecture was also demonstrated to improve the processability of the mGBCP, compared to a linear counterpart. Investigations of mGBCPs comprised of polydimethylsiloxane and poly(ethylene oxide) side chains exemplified the diverse self‐assembled morphologies, including a Frank‐Kasper A15 phase, that can be obtained with mGBCPs synthesized by random copolymerization of macromonomers. Lastly, a ternary mGBCP was synthesized by the copolymerization of three macromonomers.

Structure and Properties of Ti-Al-Ta and Ti-Al-Cr Cladding Layers Fabricated on Titanium

Being one of the most high-demand structural materials, titanium has several disadvantages, including low resistance to high-temperature oxidation and wear. The properties of titanium and its alloys can be improved by applying protective intermetallic coatings. In this study, 2 mm thick Ti-Al-Ta and Ti-Al-Cr layers were obtained on titanium workpieces by a non-vacuum electron-beam cladding. The microstructure and phase compositions of the samples were different for various alloying elements. The Cr-containing layer consisted of α2, γ, and B2 phases, while the Ta-containing layer additionally consisted of ω′ phase (P3¯m1). At the same atomic concentrations of aluminum and an alloying element in both layers, the volume fraction of the B2/ω phase in the Ti-41Al-7Ta alloy was significantly lower than in the Ti-41Al-7Cr alloy, and the amount of γ phase was higher. The Ti-41Al-7Cr layer had the highest wear resistance (2.1 times higher than that of titanium). The maximum oxidation resistance (8 times higher compared to titanium) was observed for the Ti-41Al-7Ta layer.

Microstructure and Superconducting Properties of Rapid Heating, Quenching, and Transformation (RHQT) Powder‐in‐Tube Nb3Al Wires Doped with Sn Element

Properties of rapid heating, quenching, and transformation (RHQT)‐treated powder‐in‐tube (PIT) Nb3Al1−xSnx wires are studied herein. After the RHQ process, the body‐centered cubic (bcc) phase is formed in the Sn‐doped Nb3Al wires. The transformed superconducting A15 phase shows a uniform microstructure and homogenous component distribution feature. With the increase of the Sn doping level, the critical transition temperature (Tc), critical current density (Jc), and irreversibility field (Birr) of the superconducting wires increase first and then reduce, achieving the highest Tc value of 17.6 K and the smallest ΔTc of 0.6 K in the 4% Sn doped Nb3Al wire. The 4% Sn addition helps to improve the layer Jc of the sample to 2.1 × 105 A cm−2 at 8 K and 5 T, which is about 2.5 times that of the pure Nb3Al sample. Furthermore, the Nb3Al1−xSnx (x = 0.04) wire gives the highest Birr value of 17.22, 13.23, and 9.11 T at 8, 10, and 12 K, respectively. The addition of Sn helps to achieve stoichiometric Nb3Al A15 phase and formation of precipitations with the size of 10 nm, which might act as the flux‐pinning center.

Vacuum brazing TiAl-based intermetallics using novel Ti–Fe–Mn eutectic brazing alloy

Ti–22Fe–23Mn(at.%) eutectic brazing alloy ribbon fabricated by vacuum arc remelting and rapid solidification as filler metal used to vacuum braze Ti–45Al–2Mn–2Nb–1B(at.%) alloy. The effects of brazing temperature and time on the interfacial microstructure and joining properties were investigated in detail. The microstructure of the Ti–20Fe–21Mn brazing alloy presented excellent diffusion ability, and the brazed joints mainly consisted of α2/γ lamellar phase, single α2 phase, B2 phase, and boride. The brazed joint's tensile strength first increased and then decreased with the brazing temperature in the range of 1170 °C–1250 °C, and the holding time varied from 5 min to 60 min. The highest tensile strength at room temperature was 540 MPa when the joint brazed at 1230 °C for 45 min. The high-temperature tensile strength first increased and then decreased with the testing temperature. The high-temperature tensile strength of Ti–22Fe–23Mn(at.%) alloy was superior to Ti–Zr–Cu–Ni (at.%) alloy. The high-temperature tensile strength was lower than 45XD alloy below 850 °C. However, the tensile strength was closer to 45XD alloy when the testing temperature varied from 900 °C to 1000 °C. All the cracks nucleated and propagated in the poor boride zone, and the fracture surfaces presented typical brittle cleavage fracture characteristics.

Interaction mechanism, magnetic properties and microstructure of Ce-Fe-B/Alnico spark plasma sintered magnets

By combining rare earth-based Ce-Fe-B and rare earth free Alnico powders, composite Ce-Fe-B/Alnico spark plasma sintered (SPS) magnets have been fabricated and reported for the first time. It is observed that the two phases exit in the alloys interacting magnetostatically. The Alnico phase constitutes α1 and α2 phases as a consequence of spinodal decomposition process, while the Ce-Fe-B phase contained Ce2Fe14B and CeFe2 phases. By increasing the content of Alnico phase, the remanence magnetization and the maximum energy density of the composite alloys increased owing to the higher saturation magnetization of Alnico phase. Magnetic properties, such as remanence magnetization Js = 0.93 T, intrinsic coercivity Hcj = 202 kA/m and maximum energy density (BH)max = 19.8 kJ/m3 were obtained in spark plasma sintered composite magnets containing 15 wt% Alnico. The recoil loops demonstrated the presence of anisotropy variations in the alloys specifically at high applied magnetic field. Analysis of the Curie temperature and elemental analysis validated that the constituent phases of SPS magnets exist in the alloys. This work may shed light on the development of composite magnets with distinct phases and enhanced magnetic properties.

Tribo-oxidation of Ti-Al-Fe and Ti-Al-Mn cladding layers obtained by non-vacuum electron beam treatment

The research was devoted to studying unlubricated tribological behaviors of γ-TiAl-based coatings in the temperature range 25–400 °C. These γ-TiAl-based intermetallic coatings alloyed with Fe or Mn were formed on CP-Ti workpieces using a non-vacuum electron beam deposition. The microstructure, phase and elemental analyses proved the formation of γ-TiAl and appearance of the ternary phases (Laves and G-phases). It was shown that friction coefficients obtained for three materials were barely different but their fluctuations were strongly dependent on surface oxidation and the phase composition. It was found that the Ti-45Al-8Mn (at.%) coatings consisting mainly of γ and α2 phases had lower wear resistance in comparison with the Ti-42Al-7Fe (at.%) and Ti-57Al-13Fe (at.%) coatings. Whereas the Ti-57Al-13Fe (at.%) coating contained γ and G phases possessed better wear resistance as compared to that of Ti-42Al-7Fe (at.%) coating with the structure represented by the γ matrix and a eutectic (α2 + Laves phases). The microprobe analysis, scanning electron and optical microscopy, and XPS measurements revealed the formation of highly oxidized mechanically mixed layers after the sliding tests. The Ti-45Al-8Mn (at.%) coating subsurface consisted of Al2O3, Al, TiO2, and Ti sub-oxides, whereas for both Ti-Al-Fe coatings the formation of an initial oxide film consisted of a titanium dioxide (rutile structure), alumina, and a low amount of iron oxide (hematite structure) was revealed. Thermal softening of the counter body provoked the iron oxide growth in the wear traces of the coatings alloyed with Fe. Tribooxidation behaviors of Ti-Al-Fe coatings may be interpreted as an example of adaptation the as-deposited structure and phases to high-temperature sliding condition and therefore these coatings can be recommended for protection of the titanium alloy components.

Crowding-Induced Unconventional Phase Behaviors in Dendritic Rodlike Molecules via Side-Chain Engineering.

Dendritic molecules with a fanlike or conelike conformation are common molecular building blocks to construct supramolecular columnar or spherical phases. Although it is well-accepted that the preferred molecular conformation of dendritic molecules dictates their packing schemes, manipulation of this crucial parameter usually requires significant changes in molecular structures and tedious synthetic efforts. Herein, we report a simple yet highly efficient strategy to tune the molecular conformation of dendritic rodlike molecules by adjusting the length of alkyl side chains tethered to the rods. Strikingly, tiny chemical structure differences can largely change the "crowding" near the branching point to induce the "fanlike to conelike" conformational transitions and thus result in the formation of diverse supramolecular structures, including the columnar phase, double gyroid phase, and the unconventional Frank-Kasper σ and A15 phases. Our study provides a practical platform for further investigation of unconventional structure formation and phase transitions in soft matter.

Thermal Stability and Lattice Strain Evolution of High‐Nb‐Containing TiAl Alloy under Low‐Cycle‐Fatigue Loading

The micromechanical behavior and the effect of temperature on the micromechanical mechanism of high‐Nb‐containing TiAl alloy during low‐cycle fatigue still remain uncertain. Herein, in situ and ex situ synchrotron‐based high‐energy X‐ray (HEXRD) experiment results reveal that the γ and ωo phases suffer compressive lattice strains but the lattice strain in the α2 phase evolves from tensile to compressive during low‐cycle fatigue at 900 °C. In addition, the three phases suffer compressive lattice strains during cooling to room temperature, which could result in larger compressive lattice strains in γ and ωo phases and the change of the lattice strain state in the α2 phase. The peak‐broadening results show γ recrystallization is dominant in the interrupted low‐cycle‐fatigue samples, whereas inhomogeneous deformation occurs in the failed low‐cycle‐fatigue samples. The performed synchrotron diffraction experiments offer a deeper insight into the phase transformations and micromechanism of TiAl alloy during low‐cycle fatigue.

Phase relations in the CuI-SbSI-SbI3 composition range of the Cu–Sb–S–I quaternary system

The phase equilibria in the Cu-Sb-S-I quaternary system were studied by differential thermal analysis and X-ray phase analysis methods in the CuI-SbSI-SbI3 concentration intervals. The boundary quasi-binary section CuI-SbSI, 2 internal polythermal sections of the phase diagram, as well as, the projection of the liquidus surface were constructed. Primary crystallisation areas of phases, types, and coordinates of non- and monovariant equilibria were determined. Limited areas of solid solutions based on the SbSI (b-phase) and high-temperature modifications of the CuI (α1- and α2- phases) were revealed in the system. The formation of the α1 and α2 phases is accompanied by a decrease in the temperatures of the polymorphic transitions of CuI and the establishment of metatectic (3750C) and eutectoid (2800C) reactions. It was also shown, that the system is characterised by the presence of a wide immiscibility region that covers a significant part of theliquidus surface of the CuI and SbSI based phases

A15 Nb3Si: a ‘high’ T c superconductor synthesized at a pressure of one megabar and metastable at ambient conditions

A15 Nb3Si is, until now, the only ‘high’ temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb3Si and its production from tetragonal Nb3Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb3Si material to trace T c as a function of pressure. T c is suppressed to ∼5.2 K at 88 GPa. Then, using these T c (P) data for A15 Nb3Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb3Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb3Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb3Si, which produced 50%–70% A15 material, T c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb3Si structural transformation. Our theoretical calculations show that A15 Nb3Si has an enthalpy vs the tetragonal structure that is 70 meV atom−1 smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom−1. The fact that ‘annealing’ the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom−1.