Principles of heat accumulation and heat-accumulating materials in use

Бабаев, Б. Д.

doi:10.1134/s0018151x14050010

Cited by 17 publications

(8 citation statements)

References 10 publications

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“…The degree of the crystallinity of systems is linearly related with the amount of heat that will be absorbed or released during the phase transition—the greater the degree of crystallinity, the more heat the system is able to accumulate [ 40 ]. The degree of crystallinity of the samples was estimated as the ratio of the heat of fusion of the material experimentally determined by differential scanning calorimetry (DSC) to the heat of fusion of eicosan (244.2 J/g) [ 41 ], corrected for the mass fraction of paraffin in a specific system. The introduction of additives in paraffin in the amount of 5–10 wt.…”

Section: Resultsmentioning

confidence: 99%

Modified Technogenic Asphaltenes as Enhancers of the Thermal Conductivity of Paraffin

et al. 2023

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The low thermal conductivity of paraffin and other organic phase change materials limits their use in thermal energy storage devices. The introduction of components with a high thermal conductivity such as graphene into these materials leads to an increase in their thermal conductivity. In this work, we studied the use of inexpensive carbon fillers containing a polycyclic aromatic core, due to them having a structural similarity with graphene, to increase the thermal conductivity of paraffin. As such fillers, technogenic asphaltenes isolated from ethylene tar and their modified derivatives were used. It is shown that the optimal concentration of carbon fillers in the paraffin composite, which contributes to the formation of a structural framework and resistance to sedimentation, is 5 and 30 wt. %, while intermediate concentrations are ineffective, apparently due to the formation of large aggregates, the concentration of which is insufficient to form a strong framework. It has been found that the addition of asphaltenes modified with ammonium persulfate in acetic acid significantly increases the thermal conductivity of paraffin by up to 72%.

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Section: Resultsmentioning

confidence: 99%

Modified Technogenic Asphaltenes as Enhancers of the Thermal Conductivity of Paraffin

et al. 2023

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“…If the crystallization enthalpy Δ H ideal of an ideal crystal is known, then the experimental value of crystallinity is defined as χ H = Δ H c /Δ H ideal [ 38 ]. For an ideal n-eicosane crystal, the crystallization enthalpy is known to be 284.2 kJ/kg or 80.3 kJ/mol [ 23 ]. With this value at hand and with the use of experimental data available for the crystallization enthalpy of n-eicosane at different cooling rates [ 7 ], the crystallinity χ H can easily be estimated (see Table 1 ).…”

Section: Resultsmentioning

confidence: 99%

Cooling-Rate Computer Simulations for the Description of Crystallization of Organic Phase-Change Materials

Nazarychev

Glova

Larin

et al. 2022

IJMS

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A molecular-level insight into phase transformations is in great demand for many molecular systems. It can be gained through computer simulations in which cooling is applied to a system at a constant rate. However, the impact of the cooling rate on the crystallization process is largely unknown. To this end, here we performed atomic-scale molecular dynamics simulations of organic phase-change materials (paraffins), in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold (1.2 × 1011 K/min) in the values of cooling rates exists. When cooling is slower than the threshold, the simulations qualitatively reproduce an experimentally observed abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity of paraffins upon crystallization. Beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the supercooled liquid state. Thus, a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials, which are of great promise for use in domestic heat storage devices.

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“…If the crystallization enthalpy ΔH ideal of an ideal crystal is known, then the experimental value of crystallinity is defined as χ H = ΔH c /ΔH ideal [44]. For an ideal n-eicosane crystal, the crystallization enthalpy is known to be 284.2 kJ/kg or 80.3 kJ/mol [30]. With this value at hand and with the use of experimental data available for the crystallization enthalpy of neicosane at different cooling rates [6], the crystallinity χ H can easily be estimated, see Table 1.…”

Section: Structural Properties Of Crystalline Samplesmentioning

confidence: 99%

“…The crystallization temperature T c , the crystallization enthalpy ΔH c , and the crystallinity χ H = ΔH c /ΔH ideal (where ΔH ideal = 80.3 kJ/mol is the crystallization enthalpy for an ideal n-eicosane crystal[30]). …”

mentioning

confidence: 99%

Cooling-rate computer simulations for the description of crystallization of organic phase-change materials

Nazarychev

Glova

Larin

et al. 2022

Preprint

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A molecular-level insight into the phase transformations is of great demand for the molecular systems for which the phase transition is a key process from the point of view of practical applications, organic phase-change materials being one of the most prominent examples. Such a detailed insight can be gained through cooling-rate computer simulations, i.e. simulations in which cooling is applied to a system at a constant rate. However, the ability of the simulations to give an accurate description of the phase transition process is an open question, since the cooling rates accessible in atomic-scale computer simulations are many orders of magnitude larger than those in experiments. In this paper we present the first computational study that systematically explores as to how the cooling rate affects the crystallization of organic phase-change materials such as paraffins. To this end, we performed a series of atomistic molecular dynamics simulations in which the cooling rate was varied over four orders of magnitude. Our computational results clearly show that a certain threshold in the values of cooling rates exists. When cooling is performed with the rates smaller than the threshold, computer simulations are able to qualitatively reproduce the crystallization process in paraffins. This includes an abrupt change in the temperature dependence of the density, enthalpy, and thermal conductivity upon crystallization. The crystallization enthalpy for these cooling rates was found to agree quantitatively with experiment. However, beyond this threshold, when cooling is too fast, the paraffin’s properties in simulations start to deviate considerably from experimental data: the faster the cooling, the larger part of the system is trapped in the super-cooled liquid state. As a result, one can observe a systematic decrease in the fraction of crystalline domains in a paraffin sample with cooling rate, leading eventually to a complete lack of crystallization at low temperatures. Both the crystallization temperature and the crystallinity decrease with cooling rate in line with experimental data. All the above strongly supports the conclusion that a proper choice of a cooling rate is of tremendous importance in computer simulations of organic phase-change materials such as paraffins. Cooling with the rates that are too high can lead to a completely inaccurate description of the crystallization process, as well as the thermophysical and structural properties.

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Principles of heat accumulation and heat-accumulating materials in use

Cited by 17 publications

References 10 publications

Modified Technogenic Asphaltenes as Enhancers of the Thermal Conductivity of Paraffin

Modified Technogenic Asphaltenes as Enhancers of the Thermal Conductivity of Paraffin

Cooling-Rate Computer Simulations for the Description of Crystallization of Organic Phase-Change Materials

Cooling-rate computer simulations for the description of crystallization of organic phase-change materials

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