Thermal Conductivity Of Porous Silicon Research Articles

Enhancing the temporal response of modified porous silicon-based CO gas sensor

In this paper, the role of gold nanoparticles on the temporal response of a porous silicon gas sensor has been studied. The porous layer was prepared by a laser-assisted etching process using 810 nm wavelength and 2 W infrared laser diode as an illumination photonics source. The experimental etching conditions involve 20 mA/cm2 current density and 10 min etching time. Gold nanoparticles were incorporated into porous silicon structures via the ion reduction process. The characteristics of sensors were studied comprehensively by SEM, XRD, and the electrical properties were investigated for the sensor before and after the gold nanoparticles incorporation process. The obtained results manifested that the decay time of the sensor was reduced by a factor almost greater than (50%) as compared with the bare porous silicon sensor. The results were discussed based on the role of gold nanoparticles that enhanced the thermal conductivity of porous silicon. A higher thermal conductivity of about (130 W/mK) due to the gold nanoparticles was achieved as compared with the bare porous silicon gas sensor.

Study on thermal conductivities of Si thin films and porous Si structures based on a modified lattice Boltzmann method

In this work, a modified lattice Boltzmann method (LBM) is developed to predict the thermal conductivity of silicon thin films and porous silicon structures, in which a probability parameter is introduced to determine whether a phonon collision event happens at a specific lattice point. The thickness dependent silicon thin film thermal conductivity calculated using this method shows a good agreement with prior experimental and simulation results. We also use this approach to study the porosity and pore-size dependent thermal conductivity of porous silicon structures. Furthermore, the simulated results about thermal conductivity of porous silicon are found to agree well with the previously reported data. The modified algorithm offers substantial computational improvement as compared to current LBM models and enables us to a clearer understanding about the meaning of collision step and streaming step in the LBM model.

Thermal conductivity reduction in highly doped mesoporous silicon: The effect of nano-crystal formation

The study of heat transfer properties in mesoporous silicon, fabricated from highly doped p-type and n-type silicon wafers, is presented. Measurements were performed by a laser induced transient thermal grating technique, which allowed us to determine the effective (in-plane) thermal conductivity. It is shown that the thermal conductivity undergoes a significant decrease with respect to bulk values mainly due to a reduction of the phonon mean free path of the solid matrix. This reduction can be ascribed to the formation of nano-crystalline domains, which are a consequence of the wet etching fabrication method. Additionally, the in-plane thermal conductivity was analyzed by employing a modified effective medium approach, which includes the phonon mean free path reduction due to the presence of both the nanometric pores and the nano-crystalline domains. The theoretical analysis shows good agreement with our measurements, indicating that the inclusion of phonon mean free path reduction to an effective medium approach is a well-suited method for studying the thermal conductivity of porous silicon.

Dynamical Study of Heat Transport Properties of Porous Silicon

A new technique to determine thermal conductivity of porous silicon is proposed. Transient thermoelectric voltage is measured after a pulsed laser irradiation, and knowledge of the voltage decay time constant and porosity of the structure gives the value of the thermal conductivity. For n -type Si of 70% porosity we show the value of 35 W m -1 K -1 . The method can be easily applied for any other porous or otherwise structured low-dimensional media. DOI: http://dx.doi.org/10.5755/j01.ms.21.2.5785

Amorphization and reduction of thermal conductivity in porous silicon by irradiation with swift heavy ions

In this article, we demonstrate that the thermal conductivity of nanostructured porous silicon is reduced by amorphization and also that this amorphous phase in porous silicon can be created by swift (high-energy) heavy ion irradiation. Porous silicon samples with 41%-75% porosity are irradiated with 110 MeV uranium ions at six different fluences. Structural characterisation by micro-Raman spectroscopy and SEM imaging show that swift heavy ion irradiation causes the creation of an amorphous phase in porous Si but without suppressing its porous structure. We demonstrate that the amorphization of porous silicon is caused by electronic-regime interactions, which is the first time such an effect is obtained in crystalline silicon with single-ion species. Furthermore, the impact on the thermal conductivity of porous silicon is studied by micro-Raman spectroscopy and scanning thermal microscopy. The creation of an amorphous phase in porous silicon leads to a reduction of its thermal conductivity, up to a factor of 3 compared to the non-irradiated sample. Therefore, this technique could be used to enhance the thermal insulation properties of porous Si. Finally, we show that this treatment can be combined with pre-oxidation at 300 °C, which is known to lower the thermal conductivity of porous Si, in order to obtain an even greater reduction.

Study of Thermal Conductivity of Porous Silicon Using the Micro-Raman Method

In this work, we are interesting in the measurement of thermal conductivity (on the surface and in-depth) of Porous silicon by the micro-Raman spectroscopy. This direct method (micro-Raman spectroscopy) enabled us to develop a systematic means of investigation of the morphology and the thermal conductivity of Porous silicon oxidized or no. The thermal conductivity is studied according to the parameters of anodization and fraction of silicon oxidized. Thermal transport in the porous silicon layers is limited by its porous nature and the blocking of transport in the silicon skeleton what supports its use in the thermal sensors.

Pore-size dependence of the thermal conductivity of porous silicon: A phonon hydrodynamic approach

Phonon hydrodynamics is used to analyze the influence of porosity and of pore size on reduction in thermal conductivity in porous silicon, with respect to crystalline silicon. The expressions predict that the thermal conductivity is lower for higher porosity and for smaller pore radius, as a consequence of phonon ballistic effects. The theoretical results describe experimental data better than the assumption that they only depend on porosity.

Thermal conductivity modeling of micro- and nanoporous silicon

This study aims at developing an approximate model to predict the effective thermal conductivity of porous silicon at both micro- and nano- scales. The model accounts for different transport mechanisms and reduces to the Fourier’s law and phonon radiative transport expression in the diffusive and ballistic limits respectively. The effects of the pore size, the pore shape (square, rectangular, and triangular cylinders), and the porosity on the thermal conductivity are all taken into consideration. In particular, the geometrical effect is addressed by introducing the geometry-dependent view factor and porosity function. The accuracy of the proposed model is confirmed by comparing its predictions with the Monte Carlo simulations for two pore sizes, 100 nm and 500 nm, and a porosity ranging between 0.06 and 0.31. In addition, the geometric effect on the heat transport is also confirmed, which is found more conspicuous in the ballistic limit.

Study on thermal conductivity of porous silicon thermal isolation layer based on micro-structure mathematical model

Porous silicon can be used as a thermal isolation layer because of its low thermal conductivity. Different from other models based on the mechanism that the thermal conductivity of porous silicon is attributed to complex microcosmic thermal conductivity through such as boundary scattering, the model used in this paper is based on the mechanism that the low conductivity of the porous silicon material is due to its structure factors, such as the existence and distribution of pores and porous silicon is viewed as a compound microstructure piece, which is constructed by both silicon continuous material and pore continuous material medium, connecting in parallel and series patterns. Therefore, the authors give a more understandable and simpler reason why the conductivity of such a material is at such a low level. It is pointed out that the influence of porosity on equivalent thermal conductivity can be divided into two parts: vertical and horizontal, thereby giving semi-quantitative relationships between the porosity and equivalent thermal conductivity for different pore structures and distributions. A comparison between the calculated results and experimental results shows the validity of this model, thus supporting the reason why the thermal conductivity of porous silicon material is relatively low.

Thermal Conductivity of Porous Silicon Evaluated from Phase Characteristics of Photoacoustic Spectroscopy

ABSTRACTPorous silicon (PS), well known as a visible luminescent material, has been revealed to be an attractive candidate for an ultrasonic emitter recently. Since the ultrasonic emission is based on a thermal process it is important to evaluate thermal properties of PS. We then evaluated the thermal conductivity of PS from the phase delay in a photoacoustic (PA) signal. We report the dependence of thermal conductivity on anodization time in forming PS.

Dependence of thermal conductivity of porous silicon on porosity characterized by photoacoustic technique

We have applied a photoacoustic (PA) technique to study the thermal properties of porous silicon (PS) films formed on p-type Si substrates by electrochemical anodic etching with greatly different porosities (20%–60%). From the dependence of the PA signals on the modulation frequency of excitation light measured under a transmission detection configuration, the effective thermal diffusivities for the two-layer PS on Si samples were determined. Using a two-layer model, the thermal conductivity of the PS films was evaluated and found to decrease greatly as the porosity is larger than 30%, which was two or three orders of magnitude lower than that reported for monocrystalline silicon. The results demonstrate that the thermal conductivity of PS films depends strongly on its structure.

Effects of phonon pore scattering and pore randomness on effective conductivity of porous silicon

The observed low effective thermal conductivity of porous silicon makes for its convenient fabrication and integration as a thermal insulation layer in microelectronics. The observed average pore size is controlled by the etching process and ranges between 1 and 100 nm, which on the low end is much less than the bulk phonon mean-free path. This low effective conductivity, i.e., low effective phonon mean-free path, can be explained with the inclusion of the effects of the phonon pore scattering and the pore randomness. The available two-dimensional porous silicon pore-network simulations are used along with the Boltzmann transport equation to determine the effective conductivity. It is shown that the hindering effect of the phonon pore scattering (due to reflection from the solid-pore interface) is significant for small pore size. Also, due to the dendritic structure of the pores, the hindering effect of the pore-network randomness is significant. The predictions are compared with the existing experiments and a good agreement is found.

Nanoscale nature and low thermal conductivity of porous silicon layers

Recently discovered phenomenon of low thermal conductivity of porous silicon (PS) layers is discussed in detail. A proposed theoretical model explains the considerable decrease of the thermal conductivity of nanoscale PS in comparison with meso-PS and bulk silicon. The thermal conductance dependence of Si/porous Si structures on the formation conditions of PS layers has been studied. By varying the values of anodisation current density and anodisation time it is possible to optimize the thermal conductance values of the Si/porous Si structures. In this way, an efficient thermal isolation can be obtained by forming thick PS layers.

Theoretical Thermal Conductivity of Porous Silicon: Nonlinear Behavior

AbstractThe use of porous silicon (PS) in fabricating optoelectronic devices is in progress. However, the performance of such applications still needs to be improved. In particular, in solar cells heat must be dissipated to avoid a decay in their efficiency and one of the properties of PS that must be evaluated is the effective thermal conductivity. It is well known that the thermal conductivity of silicon is temperature dependent. Thus we cannot use a standard effective medium approach to obtain its effective thermal response. In this work, we extend the averaging volume and surface methods [1] to consider nonlinear effects in the effective transport coefficients. We model PS as composed of c-Si cylindrical columns covered by different overlayers (i.e. a-Si or SiO2) immersed in another medium. In our model the effective thermal conductivity has an explicit dependence on the temperature gradient. We present a parametric analysis of the model, compare it with the c-Si behavior and evaluate the importance of the nonlinear contribution.

The Thermal Conductivity of Porous Silicon

ABSTRACTThe thermal conductivity of porous silicon is measured as prepared and after oxidation. The measurement method uses thermal wave propagation in the porous film. We investigate three types of porous silicon: Nanoporous p-type silicon, nanoporous n-type silicon and mesoporous p+-type silicon. The nanoporous material shows a thermal conductivity in the region of 1.2 W/mK to 1.8 W/mK as prepared and after oxidation. This value is close to silicon oxide. The mesoporous material shows a high thermal conductivity of 80 W/mK as prepared which drops to 2.7 W/mK after oxidation.