B-rich Phases Research Articles

TiB1.8 single layers and epitaxial TiB2-based superlattices by magnetron sputtering using a TiB (Ti:B = 1:1) target

Sputter-deposited titanium diborides are promising candidates for protective coatings in harsh and extreme conditions. However, growing these layers from TiB2 diboride targets by DC magnetron sputtering usually leads to over-stoichiometric layers with low crystal qualities. Moreover, superlattices with TiB2 as one of the constituents have been becoming popular, owing to their superior mechanical properties compared to single layer constituents in addition to their use in other applications such as neutron optics. Here, we propose the use of a TiB (Ti:B = 1:1) sputtering target in an on-axis deposition geometry and demonstrate the growth of epitaxial sub-stoichiometric TiB1.8 thin films. Furthermore, we present the growth of CrB1.7/TiB1.8 superlattices, from TiB (Ti:B = 1:1) and stoichiometric CrB2 targets, with abrupt interfaces as promising materials system for neutron interference mirrors. The high crystal quality structure with well-defined interfaces is the common feature of superlattices which, regardless of application, should be addressed during the growth process.Utilizing TiB target, all films crystallize in the hexagonal AlB2 structure. The sub-stoichiometry of the TiB1.8 films was accompanied by the presence of planar defects embedded in the films. CrB1.7/TiB1.8 superlattices exhibited a homogeneous boron distribution within the layers with no sign of B-rich tissue phases through the layers. This study demonstrates the feasibility for TiB as sputter target material, that offers a solution for deposition of TiB2-based superlattices without the need to adjust the deposition parameters. Such adjustments would otherwise be unavoidable for tuning the TiB2 composition and could affect the growth of the other constituent materials.

A Strategy to Enhance the B-Solubility and Mechanical Properties of Ti–B–N Thin Films

The Ti–B–N system offers a wide range of possible meta(stable) phases, making it interesting for science and industry. However, the solubility for B within the face-centered cubic (fcc)-TiN lattice is rather limited and less studied, especially without forming B-rich phases. Therefore, we address how chemistries along the TiN–TiB2 or TiN–TiB tie-line influence this B-solubility. The variation between these two tie-lines is realized through non-reactive co-sputtering of a TiN, TiB2, and Ti target. We show that for variations along the TiN–TiB tie-line, even 8.9 at.% B (equivalent to 19.3 at.% non-metal fractions) can fully be incorporated into the fcc-TiNy lattice without forming other B-containing phases. The combination of detailed microstructural characterization through X-ray diffraction and transmission electron microscopy with ab initio calculations of fcc-Ti1-xNBx, fcc-TiN1-xBx, and fcc-TiN1-2xBx solid solutions indicates that B essentially substitutes N.The single-phase fcc-TiB0.17N0.69 (the highest B-containing sample along the TiN–TiB tie-line studied) exhibits the highest hardness H of 37.1±1.9 GPa combined with the highest fracture toughness KIC of 3.0±0.2 MPa·m1/2 among the samples studied. These are markedly above those of B-free TiN0.87 having H = 29.2±2.1 GPa and KIC = 2.7±<0.1 MPa·m1/2.

Bicontinuous microemulsion in binary blends of complementary diblock copolymers.

The phase behavior of binary blends of AB diblock copolymers of compositions f and 1 - f is examined using field-theoretic simulations. Highly asymmetric compositions (i.e., f ≈ 0) behave like homopolymer blends macrophase separating into coexisting A- and B-rich phases as the segregation is increased, whereas more symmetric diblocks (i.e., f ≈ 0.5) microphase separate into an ordered lamellar phase. In self-consistent field theory, these behaviors are separated by a Lifshitz critical point at f = 0.2113. However, its lower critical dimension is believed to be four, which implies that the Lifshitz point should be destroyed by fluctuations. Consistent with this, it is found to transform into a tricritical point. Furthermore, the highly swollen lamellar phase near the mean-field Lifshitz point disorders into a bicontinuous microemulsion (BμE), consisting of large interpenetrating A- and B-rich microdomains. BμE has been previously reported in ternary blends of AB diblock copolymer with its parent A- and B-type homopolymers, but in that system the homopolymers have a tendency to macrophase separate. Our alternative system for creating BμE is free of this macrophase separation.

Comparative Study on the Surface Remelting of Mo-Si-B Alloys with Laser and Electron Beam

The Mo-12Si-8.5B alloy was surface-remelted by laser and electron beam, and the microstructure of its melt pool and substrate regions were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy spectrometry (EDS) techniques. It was found that the composition of the surface phases in the Mo-12Si-8.5B alloy did not change by the high-energy beam surface remelting process, but the microstructure of the molten pool region was significantly different from that of the substrate region, and its phase distribution was more uniform. Dendrites appeared on the surface of the material under the action of both processes, and the Si- and B-rich phases were mainly gathered in the interdendritic region. In the melt pool of the laser-remelted specimens, the α-Mo phase was continuously distributed with an average dendrite length of 70 µm, while the α-Mo phase distribution in the melt pool of the electron beam remelted specimens were relatively concentrated, with a larger dendrite size and an average dendrite length of 120 µm. The dendrite size in the melt pool of the laser remelted material was smaller, and the distribution of the elements was relatively uniform. Using a laser beam as the heat source was more favorable for the next step of the additive manufacturing of the core parts of hypersonic vehicles.

A BWR control blade degradation observed in situ during a CLADS-MADE-02 test under Fukushima Dai-Ichi Unit 3 postulated conditions

ABSTRACT The paper summarizes the results of the control blade degradation test CLADS-MADE-02 performed in the Japan Atomic Energy Agency . The test focused on the beginning phase of the accident at Fukushima Dai-Ichi Nuclear Power Plant (1F) Unit 3. The investigation provided important data, especially on the temperature history, exhaust gas measurement, and in situ video of metallic debris formation and relocation to the colder elevations under the test scenario, which reproduced oxidizing conditions during the initial phase of the 1F Unit 3 reactor heat-up. Complementary post-test analysis by metallography, Raman spectroscopy, elemental scanning electron microscope/energy-dispersive X-ray spectrometry spectroscopy, and exhaust gas analysis provided additional data on the local materials interactions that gave insights on the melt progression scenarios. Based on the test results, some decommissioning-related conclusions concerning the formation of new B-rich phases containing Cr and Fe were made.

Atomic interpretation of the interface transfer coefficients for interdiffusion in AB binary phase separating system

During intermixing in phase separating A/B diffusion couple the actual atomic fractions of A atoms (cα and cβ) on the left as well as on the right hand side of the α/β interface gradually will decrease (from unity) as well as increase (from 0) until the equilibrium cα and cβ will be reached (α and β refer to the A- and B-rich phases, separated by the interface). At the same time the interface will also be shifted. Assuming that the atomic flux across the interface, JI, is proportional to the deviations from the equilibrium (Δα=cα-cα and Δβ=cβ-cβ) the JI=(1/Ω)[KIαβΔα+KIβαΔβ) relation is obtained, where Ω=ΩA=ΩB is the atomic volume. It is shown that the KIαβ and KIβα interface transfer coefficients are positive, independent of the interface velocity, equal to each other for symmetric miscibility gap, and can be given as KIβα=KIαβ=K=zvaΓIαβcαξ. Here ΓIαβ is the jump frequency from α to β phase across the interface, a is the lattice spacing, zv is the vertical coordination number.ξ=[1+exp(ZV(cα-cβ)/kT)], where Z is the coordination number, V is the well-known solid solution parameter, proportional to the heat of mixing, and kT has its usual meaning. The above expression justifies the conjecture, frequently used in the literature, that only one interface transfer coefficient is enough for the description of the mass transfer across an interface. For short diffusion times the finite value of ΓIαβ will restrict the flux, leading to finite permeability with linear kinetics of interface shift. It is also shown that for an order of magnitude estimation of the critical interface shift (giving the transition from interface to diffusion control) the xc=aΓ(cβ)/ΓIαβ, relation can be used, where Γ(cβ) is the jump frequency in the β phase at cβ. Thus xc is practically independent of the value of V and only the composition dependence of the jump frequencies is important. For composition independent jump frequencies xc≅a, (i.e. it cannot be detected), while for the case when the diffusivity changes by seven orders of magnitude from pure A to B,xc is about 150nm.

Phase separation and glass forming ability of (Fe0.72Mo0.04B0.24)100-xGdx (x = 4, 8) bulk metallic glasses

The effect of Gd addition on the Glass Forming Ability (GFA), magnetic and thermal properties in (Fe0.72Mo0.04B0.24)100-xGdx (x=4, 8) bulk metallic glasses were investigated. The examined amorphous alloys' structural, thermal and magnetic properties were investigated by using X-ray diffraction (XRD), scanning electron mi-croscopy (SEM) differential scanning calorimeter (DSC) and vibrating sample magnetometer (VSM). The results show that molten (Fe0.72Mo0.04B0.24)92Gd8 presents the fully amorphous structure and solidifies into two different Fe-rich and B-rich amorphous phases. The DSC curve shows that the (Fe0.72Mo0.04B0.24)92Gd8 has supercooled liquid region (∆Tx) of about 70K and the saturation magnetization (Js) and coercivity (Hc) for as-cast bulk metallic glasses (BMGs) was 0.53T and 223A/m respectively.

BORALSILITE AND Li,Be-BEARING “BORON MULLITE” Al8B2Si2O19, BREAKDOWN PRODUCTS OF SPODUMENE FROM THE MANJAKA PEGMATITE, SAHATANY VALLEY, MADAGASCAR

Boralsilite Al16B6Si2O37 and Li,Be-bearing “boron mullite” Al8B2Si2O19 were discovered as breakdown products of spodumene from the elbaite-subtype Manjaka granitic pegmatite, Sahatany Valley, Madagascar. The pegmatite mineral assemblage is relatively simple (Ab + Qz > Kfs > Tur + Spd > Ap + several accessory B-rich phases). This pegmatite forms an E–W-trending elongated body emplaced in an Mg-rich calc-silicate rock; the pegmatite exocontact locally contains thin reaction zones (Tur+Phl±Brl). Boralsilite assemblages are closely associated with prismatic spodumene crystals, up to 10 cm long, mostly as narrow albite > K-feldspar zones (albite > K-feldspar > boralsilite > “boron mullite”), up to ∼0.5 mm thick, or rarely as inclusions in the central parts of spodumene crystals. Long prismatic crystals and fibers of boralsilite, up to 200 μm long, and rare elongated grains of “boron mullite” Al8B2Si2O19, up to 100 μm long, are mostly enclosed in albite and rarely in K-feldspar. The EMP and LA-ICP-MS data yielded relatively variable Al/Si ratios (∼5.4–9.8; ∼3.2–4.6) and concentrations of B (∼48500–59500; ∼24000–28000 ppm), Li (688–1433; 3339–4287 ppm), and Be (288–1082; 1017–2728 ppm), in boralsilite and “boron mullite”, respectively. Textural relations of the boralsilite assemblage assuming immobile Al suggest the hypothetical reaction: 46 LiAlSi2O6 + 30 Na+ (melt/aq.) + 6 H3BO3 = Al16B6Si2O37 + 30 NaAlSi3O8 + 46 Li+ (melt/aq.) + 9 H2O + 4 O2; part of the Li may have entered rare secondary clay minerals and partly also the “boron mullite”, but most of the Li was mobilized. Fluid/melt with very high a (B2O3), a (Na), and low a (H2O), along with high a (CO2) indicated by the absence of significant amount of hydrous phases, very minor extent of exocontact reactions along the pegmatite, and secondary rhodochrosite, are responsible for the origin of boralsilite. Very late solidus or early subsolidus conditions at temperature ∼350–450 °C and pressure ∼2–3 kbar, estimated for the crystallization of the assemblage albite + boralsilite, are significantly lower than those given for fibrous boralsilite at other localities of mostly anatectic granitic pegmatites (Larsemann Hills, Antarctica; Almgjotheii, Norway; and Horni Bory, Czech Republic).

Niobium-tantalum oxide minerals in the Jezuitské Lesy granitic pegmatite, Bratislava Massif, Slovakia: Ta to Nb and Fe to Mn evolutionary trends in a narrow Be,Cs-rich and Li,B-poor dike

A complex assemblage of Nb-Ta-(Sn) oxide minerals occur in a relatively narrow (~1−2 m thick) extensively albitized, Hercynian granitic pegmatite dike intruding biotite granodiorites near Bratislava, SW Slovakia. The dike shows enrichment in beryl (locally Cs-rich) but absence of Li- and B-rich phases. Compositions and textural relationships indicate complex evolutions of Nb-Ta oxide phases with several generations presenting distinct textural and compositional features. The first generation of the Nb-Ta minerals from the quartz-microcline-muscovite zone show Ta,Fe-rich compositions with Ta# [Ta/(Ta + Nb)] = 0.52−0.70 (Ct I columbite-tantalite), 0.88−0.90 (Tap I ferrotapiolite) and 0.73−0.86 (Fw I ferrowodginite); Mn# [Mn/(Mn + Fe)] = 0.32−0.49 (Ct I), 0.06−0.10 (Tap I) and 0.33−0.41 (Fw I). The 2nd generation is represented by ferrocolumbite to ferrotantalite (Ct II) in saccharoidal albite zone, replacement zones of Ct II in Ct I, and irregular overgrowths of ferrotapiolite (Tap II) and ferrowodginite (Fw II) on Tap I grains. The minerals of the 2nd generation show decreasing of Ta# in comparison to the 1st group: 0.10−0.60 (Ct II), 0.85−0.87 (Tap II) and 0.73−0.77 (Fw II); Mn# attains 0.30−0.45 (Ct II), 0.06−0.09 (Tap II) and 0.26−0.37 (Fw II). The 3rd generation includes fissure fillings, overgrowths and replacement zones of manganocolumbite and manganotantalite (Ct III), ferrotapiolite (Tap III) and ferrowodginite (Fw III) on the older Nb-Ta phases (Ct I, Tap I, Fw I, Fw II), in the coarse-grained unit. The 3rd population displays distinct Mn# increasing (Ct III: 0.51−0.69, Tap III: 0.11−0.24, Fw III: 0.40−0.41), Ta# values reach 0.16−0.79 (Ct III), 0.88−0.92 (Tap III) and 0.80−0.81 (Fw III). The latest, 4th generation of the Nb-Ta phases represents irregular veinlets and patches of fluorcalciomicrolite, replacing Ct I, Tap I, Fw I, Ct II and Tap III. Decrease of Ta/(Ta + Nb) values in Ct II from the saccharoidal albite unit can be explained by crystallization from the albite-rich melt, which was significantly impoverished in Ta with respect to Nb, after crystallization of Ta-rich phases from the 1st generation (ferrotapiolite I, ferrowodginite I).

Three-phase coexistence with sequence partitioning in symmetric random block copolymers

We inquire into the possible coexistence of macroscopic and microstructured phases in random Q-block copolymers built of incompatible monomer types A and B with equal average concentrations. In our microscopic model, one block comprises M identical monomers. The block-type sequence distribution is Markovian and characterized by the correlation λ. Upon increasing the incompatibility χ (by decreasing temperature) in the disordered state, the known ordered phases form: for λ>λ(c), two coexisting macroscopic A- and B-rich phases, for λ<λ(c), a microstructured (lamellar) phase with wave number k(λ). In addition, we find a fourth region in the λ-χ plane where these three phases coexist, with different, non-Markovian sequence distributions (fractionation). Fractionation is revealed by our analytically derived multiphase free energy, which explicitly accounts for the exchange of individual sequences between the coexisting phases. The three-phase region is reached, either from the macroscopic phases, via a third lamellar phase that is rich in alternating sequences, or, starting from the lamellar state, via two additional homogeneous, homopolymer-enriched phases. These incipient phases emerge with zero volume fraction. The four regions of the phase diagram meet in a multicritical point (λ(c),χ(c)), at which A-B segregation vanishes. The analytical method, which for the lamellar phase assumes weak segregation, thus proves reliable particularly in the vicinity of (λ(c),χ(c)). For random triblock copolymers, Q=3, we find the character of this point and the critical exponents to change substantially with the number M of monomers per block. The results for Q=3 in the continuous-chain limit M→∞ are compared to numerical self-consistent field theory (SCFT), which is accurate at larger segregation.

Dynamic observation on microstructure of the B-rich and Nd2Fe14B phases in Nd-Fe-B alloy by means of HVEM and Mössbauer effect analysis

The B-rich phase in Nd-Fe-B magnet was studied by means of HVEM, Mossbauer spectroscopy, X-ray diffraction and magnetic measurements. It can be represented by chemical formula Nd1+eFe4B4 with ɛ, which is about 0.1. High density planar defects were observed in the B-rich phase. X-ray diffraction and electron diffraction show that Nd1+eFe4B4 compound is composed of two interpenetrating subsystems which both belong to tetragonal symmetry. The compound may be the compositional incommensurate structure in c direction. As Nd-Fe-B magnet is heated to 322 °C, the lattice of the B-rich phase is distorted and the remarkable change in microstructure can be observed. However, the microstructure of the B-rich phase at 500 °C was similar to that at room temperature. The Mossbauer investigation and magnetic measurements indicate that the B-rich phase is paramagnetic at room temperature, so it may behave as nonmagnetic inclusion to improve coercivity of the Nd-Fe-B magnet. By analysis on Mossbauer effect to Nd15Fe85−xBx (x = 0, 4, 8, 11) it follows that boron (B) content increase, Nd2Fe14B magnetism look gradually increase in alloy, As x < 4, there are no rich boron look appearance, and Nd2Fe14B descends 24%, magnet performance descends too. If x = 11, Nd2Fe14B descends 5%. It does not affect intrinsic coercivity, but enhance, Nd2Fe14B quantity and the saturation magnetization intension, properly controling boron (B) content and alloy ingredient, properly increasing Fe enhance Nd2Fe14B comprehensive effective.

Compositional evolution of tourmaline-supergroup minerals from granitic pegmatites in the Larsemann Hills, East Antarctica

In the Larsemann Hills, Prydz Bay, East Antarctica, granulite-facies metasedimentary units containing tourmaline, prismatine and grandidierite are cut by three generations (D 2 , D 3 and D 4 ) of granitic pegmatites, several of which are also unusually enriched in B-rich phases, including tourmaline (schorl – dravite – foitite solid solutions), prismatine, grandidierite, werdingite, boralsilite and dumortierite. Tourmaline can be used to gain information about the crystallization history of the pegmatites. Six microstructural varieties of tourmaline have been recognized: 1) primary tourmaline in a graphic intergrowth with quartz, 2) tourmaline overgrowths on tourmaline in the graphic intergrowth, 3) prisms of tourmaline with oscillatory zoning, 4) tourmaline that has replaced boralsilite, 5) tourmaline that has replaced prismatine, and 6) chaotically zoned tourmaline. Tourmaline compositions evolved as crystallization proceeded in the D 2 and D 3 pegmatites (considered together) and in the D 4 pegmatites, resulting in an increase in X -site vacancy, a decrease in Ti and F contents, an increase in Al at the ( Y + Z ) sites, and a decrease in the ratio Mg/(Mg + Fe). The first three changes are consistent with decreasing temperature. However, oscillatory zoning is characteristic of open systems, where there had been a mixing from different chemical sources; we consider it more likely that X -site vacancy and F content varied as a function of fluid composition rather than temperature per se. On the basis of the compositional variation and microstructural relations and comparison with the retrograde path inferred for the host rocks, we suggest that the pegmatites evolved as temperatures decreased from 700–750°C, 3–4.5 kbar when the graphic tourmaline + quartz crystallized, through 600–700°C, 3–4 kbar, the stability range considered appropriate for boralsilite, to below 600°C at~3 kbar for secondary tourmaline and dumortierite, conditions consistent with the presence of secondary andalusite.

Curvature dependence of surface free energy of liquid drops and bubbles: A simulation study

We study the excess free energy due to phase coexistence of fluids by Monte Carlo simulations using successive umbrella sampling in finite L×L×L boxes with periodic boundary conditions. Both the vapor-liquid phase coexistence of a simple Lennard-Jones fluid and the coexistence between A-rich and B-rich phases of a symmetric binary (AB) Lennard-Jones mixture are studied, varying the density ρ in the simple fluid or the relative concentration x(A) of A in the binary mixture, respectively. The character of phase coexistence changes from a spherical droplet (or bubble) of the minority phase (near the coexistence curve) to a cylindrical droplet (or bubble) and finally (in the center of the miscibility gap) to a slablike configuration of two parallel flat interfaces. Extending the analysis of Schrader et al., [Phys. Rev. E 79, 061104 (2009)], we extract the surface free energy γ(R) of both spherical and cylindrical droplets and bubbles in the vapor-liquid case and present evidence that for R→∞ the leading order (Tolman) correction for droplets has sign opposite to the case of bubbles, consistent with the Tolman length being independent on the sign of curvature. For the symmetric binary mixture, the expected nonexistence of the Tolman length is confirmed. In all cases and for a range of radii R relevant for nucleation theory, γ(R) deviates strongly from γ(∞) which can be accounted for by a term of order γ(∞)/γ(R)-1∝R(-2). Our results for the simple Lennard-Jones fluid are also compared to results from density functional theory, and we find qualitative agreement in the behavior of γ(R) as well as in the sign and magnitude of the Tolman length.

Microstructure–critical current density model for MgB2 wires and tapes

MgB 2 wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterized for their superconducting properties. Either prereacted MgB2 (ex situ) or a mixture of Mg+2B (in situ) was used as the precursor powder. In some wires the precursor powder was mixed with SiC. The critical current density (Jc) of these wires was found to differ by orders of magnitude, the highest Jc being 104 A cm−2 at 10.5 T and 4.2 K. The microstructure of these wires was investigated using quantitative electron microscopy and spectroscopy methods [B. Birajdar, N. Peranio, and O. Eibl, Supercond. Sci. Technol. 21, 073001 (2008)]: combined scanning electron microscopy, electron probe microanalysis, and transmission electron microscopy analysis with artifact-free sample preparation, elemental mapping, and advanced chemical quantification. Wires with prereacted MgB2 (ex situ) show oxygen-poor MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous oxygen-rich matrix introducing structural granularity. Wires with elemental precursors (in situ) are generally more dense but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases in comparison to the ex situ wires. SiC in the in situ wires results in the formation of Mg2Si secondary phases. In situ and mechanically alloyed samples show smaller (20–100 nm) MgB2 grains, the grain size being slightly larger than the coherence length. All samples show Mg oxide. SiC added samples annealed beyond 950 °C yield formation of Si oxide compounds, whereas Mg2Si is found for annealing temperatures of less than 650 °C. The critical current is limited due to the anisotropy but also due to structural granularity. A microstructure–critical current density model is given to explain the large, orders of magnitude, differences in the Jc of MgB2 wires and tapes. The model contains the following microstructure parameters: (1) MgB2 grain size, (2) colony size, (3) volume fraction of B-rich secondary phases, and (4) oxygen mole fraction. The logarithmic critical current densities as a function of magnetic field were parametrized and the decay field and the critical current density at zero field (Jc0) was quantitatively correlated with the parameters of the microstructure. The MgB2 grain size is negatively correlated with the decay field and the three other microstructure parameters show correlation with Jc0. Sample preparation influencing the microstructure parameters is discussed. A detailed analysis is given to correlate the microstructural data with respect to fundamental parameters of a flux-line pinning model established for anisotropic superconductors.

Stabilized in situ rectangular MgB2 wires: the effect of B purity and sheath materials

Stabilized four-filament SiC-added insitu MgB2 wires were prepared by the rectangular wire-in-tube (RWIT) technique, usingboron powders of different purity (90% and 99%), SiC powders of differentagglomerate size and sheaths of different metals. The critical current density at 11 Tand 4.2 K improved by 5–10 times by using boron powder of higher purity, SiCpowder of small agglomerate size, a chemically inert Ti barrier and mechanicalsupport provided by a Monel sheath. The decrease in critical temperature(Tc) and increase incritical current density (Jc) of wires could be explained in terms of carbon doping. Formation ofMg2Si and B-rich secondary phases, and carbon substitution was studied using EDXchemical mapping in a scanning electron microscope (SEM). The addition ofSiC results in the formation of B-rich secondary phases. Our results support themechanism of carbon doping through the reaction of SiC with Mg to formMg2Si and release free carbon. However, the extent of effective carbon doping dependson the size of the precursor SiC agglomerates, as large SiC agglomeratesare unable to react with Mg. B precursor powder of lower purity andMg2Si secondary phase formation led to incomplete phase formation ofMgB2 and consequentlyto lowered Jc.

Microstructure – critical current density model for MgB2 wires and tapes

MgB2 wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterised for their superconducting properties within the HIPERMAG project. Either pre-reacted MgB2 (ex-situ) or a mixture of Mg + 2B (in-situ) were used as the precursor powders. In some wires the precursor powders were mixed with SiC. The critical current density (Jc) of these wires was found to differ by orders of magnitude, the highest Jc's being 104Acm-2 at 10.5 T and 4.2 K. A detailed understanding of the thermodynamics in Mg-B-O and Mg-B-Si-C-O system is necessary to control the phase and microstructure formation in these systems. Therefore, thermodynamical parameters like annealing temperature and annealing time used for the synthesis of the wires were systematically varied. The microstructure of these wires was investigated using advanced electron microscopy methods [1]: combined SEM, EPMA and TEM analysis with artefact-free sample preparation, elemental mapping and chemical quantification. Ex-situ wires show oxygen-free MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous matrix introducing structural granularity. The MgB2 grains in the porous matrix are surrounded by MgSixOy layers yielding poor connectivity. In-situ wires are generally more dense, but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases in comparison to the ex-situ wires. SiC in the in-situ wires results in the formation of Mg2Si secondary phases. The size of the MgB2 grains varies between 20-1000 nm. A microstructure- critical current density model is proposed to explain the large, order of magnitude, differences in the Jc's of MgB2 wires and tapes. The model contains the following microstructure parameters: 1) colony size, 2) volume fraction of B-rich secondary phases, 3) oxygen mole fraction and 4) MgB2 grain size.

Advanced electron microscopy methods for the analysis of MgB2 superconductor

Advanced electron microscopy methods used for the analysis of superconducting MgB2 wires and tapes are described. The wires and tapes were prepared by the powder in tube method using different processing technologies and thoroughly characterised for their superconducting properties within the HIPERMAG project. Microstructure analysis on μm to nm length scales is necessary to understand the superconducting properties of MgB2. For the MgB2 phase analysis on μm scale an analytical SEM, and for the analysis on nm scale a energy-filtered STEM is used. Both the microscopes were equipped with EDX detector and field emission gun. Electron microscopy and spectroscopy of MgB2 is challenging because of the boron analysis, carbon and oxygen contamination, and the presence of large number of secondary phases. Advanced electron microscopy involves, combined SEM, EPMA and TEM analysis with artefact free sample preparation, elemental mapping and chemical quantification of point spectra. Details of the acquisition conditions and achieved accuracy are presented. Ex-situ wires show oxygen-free MgB2 colonies (a colony is a dense arrangement of several MgB2 grains) embedded in a porous and oxygen-rich matrix, introducing structural granularity. In comparison, in-situ wires are generally more dense, but show inhibited MgB2 phase formation with significantly higher fraction of B-rich secondary phases. SiC additives in the in-situ wires forms Mg2Si secondary phases. The advanced electron microscopy has been used to extract the microstructure parameters like colony size, B-rich secondary phase fraction, O mole fraction and MgB2 grain size, and establish a microstructure-critical current density model [1]. In summary, conventional secondary electron imaging in SEM and diffraction contrast imaging in the TEM are by far not sufficient and advanced electron microscopy methods are essential for the analysis of superconducting MgB2 wires and tapes.

Correlation of superconducting properties and microstructure of MgB 2 wires and tapes

Within the “Hipermag” project, MgB 2 wires and tapes were prepared by the powder in tube method using different precursor powders and processing technologies. Either pre-reacted MgB 2 (ex-situ), mixture of MgH 2 + 2B (in-situ) or mechanically alloyed (MA) MgB 2 is used. In this work superconducting properties of a long length 14-filament tape with standard ex-situ powder, MA tapes, and hydrostatically extruded wires with in-situ MgB 2 powder with SiC nanoinclusions, are correlated with their microstructure investigated using advanced electron microscopy techniques. Apart from the conventional diffraction imaging, EDX elemental maps in SEM and TEM and electron spectroscopic imaging (ESI) in TEM have been used to study the MgB 2 phase formation and granularity. The critical current densities of the wires and tapes were measured at 4.2 and 20 K for different magnetic fields. Wires and tapes prepared by different technologies show large differences by orders of magnitude in their critical current densities. MgB 2 colony formation is found as a universal phenomenon with several grains forming a dense colony. Such colonies are embedded in a matrix of reduced density introducing granularity in MgB 2 wires and tapes. Wires and tapes that do not show colony formation contain a large fraction of B-rich phases and their critical current density is limited by the MgB 2 phase formation.

Diblock Copolymer Surfactants in Immiscible Homopolymer Blends: Swollen Micelles and Interfacial Tension

We consider the effect of micellization upon the equilibrium macroscopic interfacial tension between A- and B-rich phases in a ternary mixture of immiscible A and B homopolymer with an AB diblock copolymer surfactant, for systems in which spherical micelles can form in the A-rich phase. Self-consistent field theory (SCFT) has been utilized to calculate the critical micelle concentrations (cmc's) and macroscopic interfacial tensions. The behavior of the systems of nearly balanced copolymers, which tend to form highly swollen micelles, is discussed within the context of the Helfrich theory of bending elasticity, using elastic constants obtained from SCFT simulations of weakly curved monolayers.

Combined X-ray and electron microscopy study of MgB2 powders, wires and tapes

MgB2 wires, tapes and bulk samples produced within the EU-funded HIPERMAG project have been studied by a combination of X-ray diffraction and electron microscopy. The reaction layers forming at the interface between the ceramic core and Fe or Ni sheaths can be studied with both methods. The complementary techniques enable to study both the microstructure and the formation kinetics of the interface layers. Grain sizes can be determined either by direct observation or by analysis of the shape of X-ray diffraction peaks. Electron microscopy can detect B-rich secondary phases and phases present in small fractions that are not accessible by x-ray diffraction. On the other hand, synchrotron diffraction provides a fast and non-destructive method for the study of the main phases and their development during in-situ, high-temperature investigations. The combination of the two techniques is a very valuable tool for the optimisation of MgB2-based superconducting material.