By employing a precise method for locating and directly imaging the active switching region in a resistive random access memory (RRAM) device, a nanoscale conducting channel consisting of an ...amorphous Ta(O) solid solution surrounded by nearly stoichiometric Ta2O5 is observed. Structural and chemical analysis of the channel combined with temperature‐dependent transport measurements indicate a unique resistance switching mechanism.
Joining of Cu-based dispersion-strengthened alloys to Ni-based superalloys has garnered increased attention for liquid rocket engine applications due to the high thermal conductivity of Cu-based ...alloys and high temperature tensile strength of Ni-based superalloys. However, such joints can suffer from cracking when joined via liquid state processes, leading to part failure. In this work, compositions of 15–95 wt.% GRCop42 are alloyed with Inconel 625 and characterized to better understand the root cause of cracking. Results indicate a lack of miscibility between Cu-deprived and Cu-rich liquids in compositions corresponding to 30–95 wt.% GRCop42. Two distinct morphologies are observed and explained by use of CALPHAD; Cu-deprived dendrites with Cu-rich interdendritic zones at 30–50 wt.% GRCop42 and Cu-deprived spheres surrounded by a Cu-rich matrix at 60–95 wt.% GRCop42. Phase analysis reveals brittle intermetallic phases precipitate in the 60–95 wt.% GRCop42 Cu-deprived region. Three cracking mechanisms are proposed herein that provide guidance on the avoidance of defects Ni-based superalloy to Cu-based dispersion strengthened alloy joints.
The microstructure of highly porous cellulose nanocrystal (CNC) aerogels is investigated via transmission electron microscope (TEM) tomography. The aerogels were fabricated by first supercritically ...drying a carboxylated CNC organogel and then coating via atomic layer deposition with a thin conformal layer of Al2O3 to protect the CNCs against prolonged electron beam exposure. A series of images was then acquired, reconstructed, and segmented in order to generate a three-dimensional (3D) model of the aerogel. The model agrees well with theory and macroscopic measurements, indicating that a thin conformal inorganic coating enables TEM tomography as an analysis tool for microstructure characterization of CNC aerogels. The 3D model also reveals that the aerogels consist of randomly orientated CNCs that attach to one another primarily in three ways: end to end contact, “T″ contact, and “X″ contact.
Defect mechanisms in BaTiO3‐BiMO3 ceramics Kumar, Nitish; Patterson, Eric A.; Frömling, Till ...
Journal of the American Ceramic Society,
June 2018, Letnik:
101, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Often, addition of BiMO3 to BaTiO3 (BT) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for BT. In considering one specific BiMO3 composition, that is, ...Bi(Zn1/2Ti1/2)O3 (BZT), several prospective candidates for the origin of this n‐type behavior in BT‐BZT were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in BT‐BZT ceramics (>4%) as compared to BT alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in BT‐BZT ceramics. Using experimentally determined thermodynamic parameters for BT and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in BT‐BZT ceramics as compared to BT, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in BT‐BiMO3 ceramics, to make them better suited for dielectric or other applications.
Defect mechanisms in BaTiO 3 ‐Bi M O 3 ceramics Kumar, Nitish; Patterson, Eric A.; Frömling, Till ...
Journal of the American Ceramic Society,
06/2018, Letnik:
101, Številka:
6
Journal Article
Recenzirano
Abstract
Often, addition of Bi
M
O
3
to BaTiO
3
(
BT
) leads to improvement in resistivity with a simultaneous shift to n‐type conduction from p‐type for
BT
. In considering one specific Bi
M
O
3
...composition, that is, Bi(Zn
1/2
Ti
1/2
)O
3
(
BZT
), several prospective candidates for the origin of this n‐type behavior in
BT
‐
BZT
were studied—loss of volatile cations, oxygen vacancies, bismuth present in multiple valence states and precipitation of secondary phases. Combined x‐ray and neutron diffraction, prompt gamma neutron activation analysis and electron energy loss spectroscopy suggested much higher oxygen vacancy concentration in
BT
‐
BZT
ceramics (>4%) as compared to
BT
alone. X‐ray photoelectron spectroscopy and x‐ray absorption spectroscopy did not suggest the presence of bismuth in multiple valence states. At the same time, using transmission electron microscopy, some minor secondary phases were observed, whose compositions were such that they could result in effective donor doping in
BT
‐
BZT
ceramics. Using experimentally determined thermodynamic parameters for
BT
and slopes of Kröger‐Vink plots, it has been suggested that an ionic compensation mechanism is prevalent in these ceramics instead of electronic compensation. These ionic defects have an effect of shifting the conductivity minimum in the Kröger‐Vink plots to higher oxygen partial pressure values in
BT
‐
BZT
ceramics as compared to
BT
, resulting in a significantly higher resistivity values in air atmosphere and n‐type behavior. This provides an important tool to tailor transport properties and defects in
BT
‐Bi
M
O
3
ceramics, to make them better suited for dielectric or other applications.
The purpose of the buffer layer (intrinsic wide bandgap semiconductor) in copper indium gallium selenium sulfur (CIGSS) solar cells has not been adequately explained in the literature; it is the goal ...of this dissertation to understand this purpose. New experiments completed in the course of this study have shown that materials from the top-conducting oxide layer, ZnO:Al, diffuse into the CIGSS absorber, decreasing lifetimes and degrading solar cell performance. Electronic characterization performed on ZnO:Al/CdS/CIGSS and ZnO:Al/CIGSS cells show that multi-step tunneling is a dominant loss mechanism in these structures without the buffer layer (called direct cells). The direct cells devices also had substantially less built-in voltage because of the formation of surface states or a high concentration of aluminum at the CIGSS surface. Some elementary calculations suggest that aluminum and zinc observed in the CIGSS absorber can shorten lifetimes and act as a recombination center. Based on these observations, a model of the buffer layer is discussed.
CIGSS solar cells based on CVD ZnO buffer layers Olsen, L.C.; Addis, F.W.; Liang Huang ...
Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference - 2000 (Cat. No.00CH37036),
2000
Conference Proceeding
This paper describes investigations of cells based on Siemens Solar material, CIGSS, and highly resistive ZnO (i-ZnO) buffer layers grown by MOCVD. Resistive i-ZnO buffer layers are grown on CIGSS at ...100/spl deg/C, after heating the substrate to 250/spl deg/C and using nitrogen as a carrier gas. The use of a KCN etch on the CIGSS surface prior to growth of i-ZnO buffer layers was determined to be beneficial to cell performance. XPS studies show that the etching step removes oxygen from the substrate surface that had complexed with Se to form SeO/sub 2/. Cells that do not receive a KCN etch typically have shunted I-V curves leading to relatively low open circuit voltages, fill factors and efficiencies. A tentative model is proposed for the effect of KCN. Finally, one cell (using a KCN etch) exhibited a total area efficiency of 12.7 % with an open circuit voltage of 0.577 Volts.
Role of buffer layers in CIS-based solar cells Olsen, L.C.; Eschbach, P.; Sambhu Kundu
Conference Record of the Twenty-Ninth IEEE Photovoltaic Specialists Conference, 2002,
2002
Conference Proceeding
This paper describes investigations of the effects of buffer layers on CIGSS-based solar cells. Two general types of cells have been investigated, namely, direct ZnO devices and cells with either a ...CdS or i-ZnO buffer layer. The CIGSS absorber in all cases is Siemens Solar material. The CdS buffer layers were grown by CBD at Siemens Solar, and the i-ZnO buffer layers were grown by MOCVD at WSU. Typical efficiencies for the three types of cells were; 7 to 9 % for direct ZnO/CIGSS structures; 11 to 14 % for cells with CdS or i-ZnO buffer layers. Characterization studies included current-voltage analysis to identify current loss mechanisms, and SIMS depth concentration profiles. These studies indicate that a key purpose of the buffer layer is to provide a barrier to diffusion of impurities into the absorber.
PDF
