Phenomenological description of grain growth stagnation for nanocrystalline films and powders

At fixed hold temperatures, grain growth usually stagnates indefinitely after sufficiently long hold times. The change in the growth behavior can be very abrupt, resulting in a sudden plateau in plots of grain size versus time at fixed temperature. Standard grain growth laws do not formally predict the rapid onset of growth stagnation, merely a slow down of grain growth to imperceptible rates. Therefore, the grain size in the plateau regions for long hold times is typically not in agreement with that predicted with kinetic variables derived from the size versus time curves for short hold times where there is pronounced curvature. Standard laws lead to endpoint grain sizes with strong dependences on the hold times. The experimental observation in many cases is a nearly linear temperature dependence that is independent of the hold times after a sufficient duration. Additionally, the growth process may restart from a stagnated state with sufficient temperature increases, where again, the stagnated grain size temperature dependence is linear. For growth laws including size dependent opposing forces, endpoint grain sizes are predicted to be either independent of temperature, or exponentially temperature dependent with thermodynamic reversibility, the latter an impossibility. We derive, heuristically, a stagnation force, phenomenologically incorporating these observations: a near linear temperature dependence of endpoint grain sizes, and irreversible growth. This description reduces to standard laws commonly used for data fitting, and leads to a normal grain size distribution. Other laws are discussed and compared. Fits to size versus time data are successfully made

Phenomenological description of grain growth stagnation for nanocrystalline films and powders

At fixed hold temperatures, grain growth usually stagnates indefinitely after sufficiently long hold times. The change in the growth behavior can be very abrupt, resulting in a sudden plateau in plots of grain size versus time at fixed temperature. Standard grain growth laws do not formally predict the rapid onset of growth stagnation, merely a slow down of grain growth to imperceptible rates. Therefore, the grain size in the plateau regions for long hold times is typically not in agreement with that predicted with kinetic variables derived from the size versus time curves for short hold times where there is pronounced curvature. Standard laws lead to endpoint grain sizes with strong dependences on the hold times. The experimental observation in many cases is a nearly linear temperature dependence that is independent of the hold times after a sufficient duration. Additionally, the growth process may restart from a stagnated state with sufficient temperature increases, where again, the stagnated grain size temperature dependence is linear. For growth laws including size dependent opposing forces, endpoint grain sizes are predicted to be either independent of temperature, or exponentially temperature dependent with thermodynamic reversibility, the latter an impossibility. We derive, heuristically, a stagnation force, phenomenologically incorporating these observations: a near linear temperature dependence of endpoint grain sizes, and irreversible growth. This description reduces to standard laws commonly used for data fitting, and leads to a normal grain size distribution. Other laws are discussed and compared. Fits to size versus time data are successfully made

Field-assisted sintering of Ni nanopowders

Ni nanopowders were sintered by a field-assisted sintering technique (FAST). The influence of heating rate from 90 to 1100 ◦C/min on densification and final grain structure of sintered Ni was studied. A moderate heating rate was found to be beneficial for the densification of Ni nanopowders, whereas a high heating rate was conducive to a lower final density. Very high heating rates resulted in non-uniform densification of the samples and formation of cracks during sintering. Electric field activation and possible densification mechanisms are discussed

Field-assisted sintering of Ni nanopowders

Ni nanopowders were sintered by a field-assisted sintering technique (FAST). The influence of heating rate from 90 to 1100 ◦C/min on densification and final grain structure of sintered Ni was studied. A moderate heating rate was found to be beneficial for the densification of Ni nanopowders, whereas a high heating rate was conducive to a lower final density. Very high heating rates resulted in non-uniform densification of the samples and formation of cracks during sintering. Electric field activation and possible densification mechanisms are discussed

Field-assisted sintering of Ni nanopowders

Ni nanopowders were sintered by a field-assisted sintering technique (FAST). The influence of heating rate from 90 to 1100 â—¦C/min on densification and final grain structure of sintered Ni was studied. A moderate heating rate was found to be beneficial for the densification of Ni nanopowders, whereas a high heating rate was conducive to a lower final density. Very high heating rates resulted in non-uniform densification of the samples and formation of cracks during sintering. Electric field activation and possible densification mechanisms are discussed.UCD-ITS-RP-07-28, Civil Engineering

Athermal and thermal mechanisms of sintering at high heating rates in the presence and absence of an externally applied field.

In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50 °C/min up to 800 °C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielec. ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielec. ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate. [on SciFinder(R)