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Superconducting magnesium diboride has been prepared with intra-granular TiB precipitates that have appropriate size and spacing for the pinning of superconducting vortices. The TiB particles have a size ranging from 1 nm to 20 nm and a spacing about 5 times as large. By co-depositing Ti and B on a commercial carbon coated silicon carbide substrate with chemical vapor deposition methods, it was possible to obtain a starting fiber with Ti well dispersed in the B. Reacting these fibers in a Mg vapor then transformed the fiber into a superconductor capable of carrying 10,000 A/cm^2 at 25K and 1 T. The Ti-B bonds are strong enough that there is relatively little migration of the Ti as the MgB2 grains form around the TiB precipitates. TiB2 precipitates, if they are present, can not be seen in the selected area diffraction pictures that show the TiB precipitates., Comment: 6 pages, 4 figures. Applied Physics Letters (ssubmitted)

A series of doping experiments are reported for MgB2 conductors that have been synthesized using doped boron fibers prepared by chemical vapor deposition(CVD) methods. Undoped MgB2 samples prepared from CVD prepared fibers consistently give critical current densities, Jc, in the range of 500,000 A/cm^2 in low field at 5K. These values fall by a factor of about 100 as the magnetic field increases to 3T. For heavily Ti-doped boron fibers where the B/Ti ratio is comparable to 1, there is a substantial suppression of both Tc, superconducting volume fraction, and Jc values. If, however, a sample with a few percent Ti in B is deposited on a carbon coated SiC substrate and reacted at 1100 degrees C for 15 min, then Tc is suppressed only a couple of degrees Kelvin and critical current densities are found to be approximately 2-5 x 10^6 A/cm^2 for superconducting layers ranging from 4-10 micrometers thick. These materials show Jc values over 10,000 A/cm^2 at 25K and 1.3 T., Comment: 10 pages, 6 figures

Processing methods are described for the development of magnesium diboride wire using the chemical vapor deposition (CVD) to produce long lengths of suitably doped starting boron fiber. It is found that titanium can be co-deposited with the boron to make long lengths of doped fiber that contain both TiB and TiB2. When this fiber is reacted in Mg vapor to transform boron into MgB2, the resulting conductor has a superconducting critical current density of about 5 times 10^6 A/cm^2 at 5K and self field. The critical current density at 25K and 1 Tesla is 10,000 A/cm^2. Using optical methods to define grain boundaries and energy dispersive X-rays to determine Ti and Mg concentration, these samples show a fine dispersion of $Ti$ through out the grains and no conspicuous precipitation of TiB2 on the MgB2 grain boundaries. This is to be contrasted with the precipitation of TiB2 on MgB2 grain boundaries observed for samples prepared by solid state reaction of Ti, Mg, and B powders. Introducing Ti impurities into the B during the CVD deposition of the B gives a distribution of TiB2 in the resulting MgB2 different from solid state reaction of powders., Comment: 14 pages and 10 figures. submitted to Physica C

Sintered powders and wires of superconducting $MgB_2$ have been fabricated under a variety of conditions in order to determine details of the diffusion of the $Mg$ into $B$ and to study the types of defects that arise during growth. For samples prepared by exposure of boron to $Mg$ vapor at $950^{\circ}C$, the conversion of particles of less than $100\mu m$ size particles to $MgB_2$ is complete in about $2 h$. The lattice parameters of the $MgB_2$ phase determined from X-ray are independent of the starting stoichiometry and the time of reaction. Wire segments of $MgB_2$ with very little porosity have been produced by reacting $141 \mu m$ diameter boron fibers in an atmosphere of excess $Mg$ vapor at $950^{\circ}C$. Defects in the reacted fibers are predominantly the voids left as the boron is converted to $MgB_2$.