Search for: All records

Charge transport mechanisms governing DC resistance degradation in ferroelectric films are influenced by defects, particularly oxygen vacancies. This paper demonstrates that oxygen vacancies migrate in lead zirconate titanate (PZT) films under a DC bias field and contribute to resistance degradation. Model PZT thin films were developed in which the concentration and distribution of oxygen vacancies were controlled via (a) changing the dopant type and concentration from 1%–4% Mn (acceptor) to 1%–4% Nb (donor) or (b) annealing undoped PZT films at varying partial pressures of PbO. The presence of associated (immobile) and dissociated (mobile) oxygen vacancies was distinguished by thermally stimulated depolarization current (TSDC) measurements. The impact of mobile oxygen vacancies on local defect chemistry and associated charge transport mechanisms was explored by electron energy loss spectroscopy (EELS). For Mn-doped PZT films, following resistance degradation, TSDC studies revealed only one depolarization peak with an activation energy of 0.6–0.8 eV; this peak was associated with ionic space charge presumably due to migration of oxygen vacancies. The magnitude of the depolarization current peak increased with increasing degradation times. A similar depolarization current peak attributed to the existence of mobile oxygen vacancies was also observed for undoped and Nb-doped PZT films; the magnitude of this peak decreased as the Nb or PbO contents in PZT films increased. An additional TSDC peak associated with polaron hopping between Ti3+ and Ti4+ was found in both Nb-doped PZT films and undoped PZT films annealed under low PbO partial pressure. Degraded Nb-doped samples exhibited a chemical shift in the TiL2,3 peak to lower energy losses and the appearance of shoulders on the t2g and eg peaks, implying a reduction of Ti cations in regions near the cathode. 

Boron nitride nanotubes (BNNTs) have attracted attention for their predicted extraordinary properties; yet, challenges in synthesis and processing have stifled progress on macroscopic materials. Recent advances have led to the production of highly pure BNNTs. Here we report that neat BNNTs dissolve in chlorosulfonic acid (CSA) and form birefringent liquid crystal domains at concentrations above 170 ppmw. These tactoidal domains merge into millimeter-sized regions upon light sonication in capillaries. Cryogenic electron microscopy directly shows nematic alignment of BNNTs in solution. BNNT liquid crystals can be processed into aligned films and extruded into neat BNNT fibers. This study of nematic liquid crystals of BNNTs demonstrates their ability to form macroscopic materials to be used in high-performance applications.