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Thermal conductivity of highly porous Si in the temperature range 4.2 to 20 K.

Valalaki K, Nassiopoulou AG - Nanoscale Res Lett (2014)

Bottom Line: The reported results are the first in the literature for this temperature range.This behavior is attributed to the presence of a majority of non-propagating vibrational modes, resulting from the nanoscale fractal structure of the material.The above results complement previous results by the authors in the temperature range 20 to 350 K.

View Article: PubMed Central - HTML - PubMed

Affiliation: NCSR Demokritos/INN, Terma Patriarchou Grigoriou, Aghia Paraskevi, Athens 15310, Greece.

ABSTRACT

Unlabelled: We report on experimental results of the thermal conductivity k of highly porous Si in the temperature range 4.2 to 20 K, obtained using the direct current (dc) method combined with thermal finite element simulations. The reported results are the first in the literature for this temperature range. It was found that porous Si thermal conductivity at these temperatures shows a plateau-like temperature dependence similar to that obtained in glasses, with a constant k value as low as 0.04 W/m.K. This behavior is attributed to the presence of a majority of non-propagating vibrational modes, resulting from the nanoscale fractal structure of the material. By examining the fractal geometry of porous Si and its fractal dimensionality, which was smaller than two for the specific porous Si material used, we propose that a band of fractons (the localized vibrational excitations of a fractal lattice) is responsible for the observed plateau. The above results complement previous results by the authors in the temperature range 20 to 350 K. In this temperature range, a monotonic increase of k with temperature is observed, fitted with simplified classical models. The extremely low thermal conductivity of porous Si, especially at cryogenic temperatures, makes this material an excellent substrate for Si-integrated microcooling devices (micro-coldplate).

Pacs: 61.43.-j; 63.22.-m; 65.8.-g.

No MeSH data available.


Related in: MedlinePlus

Schematic representation of the test structure. The figure shows aschematic representation of the locally formed porous Si layer on the p-typewafer and SEM images of the porous Si surface. The SEM image in the inset ofthe principal one was obtained after a slight plasma etching of the porousSi surface in order to better reveal the porous structure. Two resistors,one on porous Si and one on bulk Si, are also depicted in the schematic ofthe test structure.
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Figure 1: Schematic representation of the test structure. The figure shows aschematic representation of the locally formed porous Si layer on the p-typewafer and SEM images of the porous Si surface. The SEM image in the inset ofthe principal one was obtained after a slight plasma etching of the porousSi surface in order to better reveal the porous structure. Two resistors,one on porous Si and one on bulk Si, are also depicted in the schematic ofthe test structure.

Mentions: The steady-state direct current (dc) method, described in detail in [18] and [21], was used to determine porous Si thermal conductivity. This method isbased on the measurement of the temperature difference across a Pt resistor lying onthe porous Si layer in response to an applied heating power. A similar resistor onbulk crystalline Si served as a temperature reference. Figure  1 shows schematically the locally formed porous Si layer with the Ptresistor on top, while the second resistor on bulk Si is also depicted. Scanningelectron microscopy (SEM) images of the specific porous Si material are alsodepicted in the same figure. The SEM image in the inset was obtained after a slightplasma etching of the porous Si surface in order to better reveal the porous Sistructure.


Thermal conductivity of highly porous Si in the temperature range 4.2 to 20 K.

Valalaki K, Nassiopoulou AG - Nanoscale Res Lett (2014)

Schematic representation of the test structure. The figure shows aschematic representation of the locally formed porous Si layer on the p-typewafer and SEM images of the porous Si surface. The SEM image in the inset ofthe principal one was obtained after a slight plasma etching of the porousSi surface in order to better reveal the porous structure. Two resistors,one on porous Si and one on bulk Si, are also depicted in the schematic ofthe test structure.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4109793&req=5

Figure 1: Schematic representation of the test structure. The figure shows aschematic representation of the locally formed porous Si layer on the p-typewafer and SEM images of the porous Si surface. The SEM image in the inset ofthe principal one was obtained after a slight plasma etching of the porousSi surface in order to better reveal the porous structure. Two resistors,one on porous Si and one on bulk Si, are also depicted in the schematic ofthe test structure.
Mentions: The steady-state direct current (dc) method, described in detail in [18] and [21], was used to determine porous Si thermal conductivity. This method isbased on the measurement of the temperature difference across a Pt resistor lying onthe porous Si layer in response to an applied heating power. A similar resistor onbulk crystalline Si served as a temperature reference. Figure  1 shows schematically the locally formed porous Si layer with the Ptresistor on top, while the second resistor on bulk Si is also depicted. Scanningelectron microscopy (SEM) images of the specific porous Si material are alsodepicted in the same figure. The SEM image in the inset was obtained after a slightplasma etching of the porous Si surface in order to better reveal the porous Sistructure.

Bottom Line: The reported results are the first in the literature for this temperature range.This behavior is attributed to the presence of a majority of non-propagating vibrational modes, resulting from the nanoscale fractal structure of the material.The above results complement previous results by the authors in the temperature range 20 to 350 K.

View Article: PubMed Central - HTML - PubMed

Affiliation: NCSR Demokritos/INN, Terma Patriarchou Grigoriou, Aghia Paraskevi, Athens 15310, Greece.

ABSTRACT

Unlabelled: We report on experimental results of the thermal conductivity k of highly porous Si in the temperature range 4.2 to 20 K, obtained using the direct current (dc) method combined with thermal finite element simulations. The reported results are the first in the literature for this temperature range. It was found that porous Si thermal conductivity at these temperatures shows a plateau-like temperature dependence similar to that obtained in glasses, with a constant k value as low as 0.04 W/m.K. This behavior is attributed to the presence of a majority of non-propagating vibrational modes, resulting from the nanoscale fractal structure of the material. By examining the fractal geometry of porous Si and its fractal dimensionality, which was smaller than two for the specific porous Si material used, we propose that a band of fractons (the localized vibrational excitations of a fractal lattice) is responsible for the observed plateau. The above results complement previous results by the authors in the temperature range 20 to 350 K. In this temperature range, a monotonic increase of k with temperature is observed, fitted with simplified classical models. The extremely low thermal conductivity of porous Si, especially at cryogenic temperatures, makes this material an excellent substrate for Si-integrated microcooling devices (micro-coldplate).

Pacs: 61.43.-j; 63.22.-m; 65.8.-g.

No MeSH data available.


Related in: MedlinePlus