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Transport critical current densities J_c and irreversibility fields B_irr of undoped and nanoparticle doped (10 wt% SiC) Fe-sheathed MgB_2 wires were measured from 2 to 40K in magnetic field B

A comparative study of pure, SiC, and C doped MgB_2 wires has revealed that the SiC doping allowed C substitution and MgB_2 formation to take place simultaneously at low temperatures. C substitution enhances H_c2, while the defects, small grain size, and nanoinclusions induced by C incorporation and low-temperature processing are responsible for the improvement in J_c. The irreversibility field (H_irr)for the SiC doped sample reached the benchmarking value of 10T at 20K, exceeding that of NbTi at 4.2K. This dual reaction model also enables us to predict desirable dopants for enhancing the performance properties of MgB_2.

Irreversibility fields Birr and critical current densities Jc of undoped and doped (10 wt.% SiC) MgB2/Cu tapes and MgB2/Fe wires were measured from 2 K up to 38 K in magnetic field B1 T, Birr(T) variation for all samples is similar, but Birr of MgB2/Fe is higher than that of corresponding MgB2/Cu tapes. However, the enhancement of Birr upon doping, Birr(dop.)/Birr(undop.)=1.4 is the same for both types of samples indicating the same pinning mechanism. Birr of doped MgB2/Fe wire reaches that of optimised NbTi wires (10 T, 4.2 K) already at 20 K and extrapolates to 21 T at 4.2 K (close to that of Nb3Sn). In spite of low core density (~1.2 g/cm3) Jc of doped MgB2/Fe wire is high and decreases little with magnetic field: Jc(4.2 K)=19 kA/cm2 in B=13 T which yields FP=JcB=2.5 GN/m3. Since Jc and Jc-B variation in our samples is limited by core porosity and defects (cracks and voids), the Jc and FP values for more dense and homogeneous SiC doped samples are probably several times larger.

A systematic study of the parameters (irreversibility field Birr, critical current density, Jc) associated with flux pinning in nanoparticle doped (Si, SiC, average size 50 nm) powder-in-tube processed MgB2/Cu composite tapes has been carried out. Birr and Jc of undoped and doped (5 and 10 wt.%) tapes were measured in the temperature range 4-38 K and in magnetic fields up to 16 T. Birr(T) of doped tapes shows a change in slope around characteristic field B_fi which increases with nanoparticle content and also depends on their type. This indicates matching effect in flux pinning, probably associated with Mg2Si nanoprecipitates (~10 nm) formed during heat treatment. The flux pinning enhancement factor Birr(doped)/Birr(undoped), was the highest around the matching field B_fi (the field at which the areal density of precipitates matches that of flux lines) but remained high ~1.5 even for Birr>>B_fi (low temperatures). This feature contrasts sharply with matching field effects in high temperature superconductors containing columnar defects (where the enhancement is confined to fields

Irreversibility fields Birr and critical current densities Jc of undoped and doped (10 wt.% SiC) MgB2/Cu tapes and MgB2/Fe wires were measured from 2 K up to 38 K in magnetic field B=1 T, Birr(T) variation for all samples is similar, but Birr of MgB2/Fe is higher than that of corresponding MgB2/Cu tapes. However, the enhancement of Birr upon doping, Birr(dop.)/Birr(undop.)~1.4 is the same for both types of samples indicating the same pinning mechanism. Birr of doped MgB2/Fe wire reaches that of optimised NbTi wires (10 T, 4.2 K) already at 20 K and extrapolates to 21 T at 4.2 K (close to that of Nb3Sn). In spite of low core density (~1.2 g/cm3) Jc of doped MgB2/Fe wire is high and decreases little with magnetic field: Jc(4.2 K)=19 kA/cm2 in B=13 T which yields FP=JcB=2.5 GN/m3. Since Jc and Jc-B variation in our samples is limited by core porosity and defects (cracks and voids), the Jc and FP values for more dense and homogeneous SiC doped samples are probably several times larger.

The irreversibility fields Birr and critical current densities Jc of undoped and Si and SiC nanoparticle doped (5, 10 and 20 wt.%) MgB2 tapes were measured in the temperature (T) range 2-38 K and in magnetic fields B>Bcr (low temperatures). The variations of Jc and the pinning force density Fp=JcB with B and T support the above findings.

MgB2/Cu wires and tapes have been prepared using the powder-in-tube and reaction in-situ techniques.

Magnetoresistivity and critical current density were measured on five Si-nanoparticle doped (c=28 K in magnetic fields B>Bphi).

Nedavno je opaženo da dodatak različitih nanočestica (npr. SiC, Si, C, Y$_{; ; ; 2}; ; ; $O$_{; ; ; 3}; ; ; $) jako pojačava zapinjanje magnetskih vrtloga u supravodiču MgB$_{; ; ; 2}; ; ; $. Mehanizam tog pojačanja za sada još nije razjašnjen. U cilju rješenja tog problema, sustavno smo mjerili magnetootpor i kritične struje dviju MgB$_{; ; ; 2}; ; ; $ traka obloženih bakrom, od koji je jedna bila dopirana s 5 tež.\% Si-nanočestica veličine 50 nm. Mjerenja izvršena u području temperatura $T\geq 28$ K i magnetskih polja $B\leq 1$ T su nam omogućila da precizno odredimo temperaturne ovisnosti ireverzibilnih polja $B_{; ; ; irr}; ; ; $ (za $B\leq B_{; ; ; irr}; ; ; $ je tok struje kroz supravodič nedisipativan) i poljne i temperaturne ovisnosti gustoća kritičnih struja $J_{; ; ; c}; ; ; $ i volumnih sila zapinjanja vorteksa $F_{; ; ; p}; ; ; =J_{; ; ; c}; ; ; B$ za oba uzorka. Monotono snižavanje $B_{; ; ; irr}; ; ; $ s porastom temperature opaženo u nedopiranom uzorku je u dopiranom zamijenjeno izrazitim koljenom na $T=35$ K što ukazuje na zapinjanje vorteksa na koreliranim jakim centrima zapinjanja. Sukladno strukturnim istraživanjima, ti centri zapinjanja su fino raspršene Mg$_{; ; ; 2}; ; ; $Si nanočestice ($\leq 10$ nm) nastale tijekom sinteze uzorka. Ponašanje kritičnih struja i volumnih sila zapinjanja također su u skladu s iznesenim zaključcima. Dok je skaliranje $F_{; ; ; P}; ; ; -B$ krivulja u nedopiranom MgB$_{; ; ; 2}; ; ; $ uzorku slično onom u Nb$_{; ; ; 3}; ; ; $Si (zapinjanje na granicama zrna), ono u dopiranom uzorku više je nalik ponašanju opaženom u visokotemperaturnim supravodičima s uvedenim stupčastim defektima (korelirani defekti).

Ranije smo pokazali da stupčasti defekti promjera $\sim 5$ nm nastali ozračivanjem termičkim neutronima Bi2223 vrpci dopiranim s $^{; ; ; 235}; ; ; $U\ (U/n procesiranjem) tvore jake individualne centre zapinjanja kvanata magnetskog toka $\Phi _{; ; ; 0}; ; ; $ koji jako povisuju ireverzibilna polja $B_{; ; ; irr}; ; ; $ (za $B\leq B_{; ; ; irr}; ; ; $ je tok struje kroz supravodič nedisipativan) i poboljšavaju poljnu ovisnost kritičnih struja $J_{; ; ; c}; ; ; (B)$ u tim materijalima na $T=77$ K te za $B\leq 1 $ T. Proširenjem tih istraživanja na široko područje temperatura $T\geq 30$ K i polja $B\leq 5$ T, pokazali smo da su pojačanje zapinjanja vrtloga (izraženo putem $B_{; ; ; irr}; ; ; (T)$ i potencijala zapinjanja $U_{; ; ; 0}; ; ; $) i s njim povezano poboljšanje $J_{; ; ; c}; ; ; (B)$ ovisnosti (izraženo poljem $B_{; ; ; \max }; ; ; $ na kojem je volumna sila zapinjanja $F_{; ; ; p}; ; ; =J_{; ; ; c}; ; ; B$ maksimalna) u tim materijalima univerzalne funkcije plošne gustoće stupčastih defekata $n_{; ; ; \phi }; ; ; $ (time i njoj pridruženog polja podudaranja $B_{; ; ; \phi }; ; ; =n_{; ; ; \phi }; ; ; \Phi _{; ; ; 0}; ; ; $). Konkretno, $B_{; ; ; \phi }; ; ; (n_{; ; ; \phi }; ; ; )$ čini univerzalnu poljnu ljestvicu pomoću koje se sva svojstva ovih sustava povezana sa zapinjanjem vrtloga mogu skalirati (u prikazu prema $B/B_{; ; ; \phi }; ; ; $ polazuju univerzalnu ovisnost). Time je iz poznatog $B_{; ; ; \phi }; ; ; $ moguće predvidjeti ponašanje danog sustava, odnosno moguće je unaprijed odabrati najpovoljniji sustav za danu uporabu. Stoga uz značajnu konceptualnu važnost (priroda zapinjanja vrtloga na nasumično usmjerenim stupčastim defektima), ova istraživanja imaju i prvorazrednu aplikativnu važnost (ukazuju na način proizvodnje supravodiča koji se može koristiti u magnetskom polju na odabranoj povišenoj temperaturi). Očekujeme da ovi rezultati dobiveni za U/n procesirane Bi2223 vrpce vrijede za bilo koji supravodič u kojemu vrtlozi zapinju na koreliranim defektima, neovisno o vrsti defekata.