**4. Smart nanocomposite coatings: Self-heating**

Surface heating is a challenge for several applications, and it is usually carried out by different approaches such as external heating source (portable equipment) or heating elements positioned on the surface (i.e., electrical resistances) which, in fact, modifies the surface quality of the parts where they are positioned. Heat is required in organic coatings for different purposes:


These applications will be further explored after an explanation of basic aspects of self-heating coatings based on the introduction of conductive nanoparticles inside polymer matrices, such as CNTs and GNPs. Nevertheless, these types of coatings are not limited to these applications, and they can find a potential field to be implemented in any product that need to be heated such as heating seats for commercial vehicles [45], floor heating, heating textiles, etc. where temperatures required are usually below 100°C [46].

### **4.1 Fundamentals of self-heating by Joule effect**

The addition of carbon nanoparticles inside a polymer matrix above the percolation threshold, which has been previously explained, allows getting an electrically conductive material. The electrical current that flows through the material will generate heat according to Joule's law (Eq. (1)), which is commonly known as Joule effect in materials:

$$\mathbf{Q} = \dot{\mathbf{r}}^2 \mathbf{R} \mathbf{t} \tag{3}$$

#### *Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

superficial defects by analyzing the changes of its surface resistivity. In this context, their effectiveness has been widely demonstrated in polymer coatings for the detection of superficial cracks [43], where the electrical resistance measurement between adjacent channels can easily detect an artificial damage (**Figure 5**), as well as in sensing skins for spatial pressure mapping, where the strain induced by the applied pressure is monitored [44]. Here, the main issue is correlated to the positioning of the electrodes and the data processing, which usually involves the use of complex mathematical tools. However, the results for SHM applications are very promising and give a new functionality to nanoreinforced polymer coatings.

Surface heating is a challenge for several applications, and it is usually carried out by different approaches such as external heating source (portable equipment) or heating elements positioned on the surface (i.e., electrical resistances) which, in fact, modifies the surface quality of the parts where they are positioned. Heat is

• Deicing systems: ice accretion to surfaces when subjected to cold and humid environments is something very common that requires the use of deicing alternatives, and, among them, heat of the surface to create a liquid film can be

• Self-curing coatings: the use of coatings with curing temperatures above 23°C is often limited because of the need of external heating sources that makes the production more difficult. Nevertheless, the glass transition temperatures or hardness are usually higher for higher cross-linked coating, thus making more

• Heat activated self-healing mechanisms: as previously mentioned, the main source for self-healing activation is UV radiation and temperature.

Nevertheless, the requirement of a heating source limits the application of these promising coatings to structures with easy access to be heated.

These applications will be further explored after an explanation of basic aspects

The addition of carbon nanoparticles inside a polymer matrix above the percolation threshold, which has been previously explained, allows getting an electrically conductive material. The electrical current that flows through the material will generate heat according to Joule's law (Eq. (1)), which is commonly known as Joule

> *Q* ¼ *i* 2

*Rt* (3)

of self-heating coatings based on the introduction of conductive nanoparticles inside polymer matrices, such as CNTs and GNPs. Nevertheless, these types of coatings are not limited to these applications, and they can find a potential field to be implemented in any product that need to be heated such as heating seats for commercial vehicles [45], floor heating, heating textiles, etc. where temperatures

interesting the use of higher curing/post-curing temperatures.

**4. Smart nanocomposite coatings: Self-heating**

*21st Century Surface Science - a Handbook*

required in organic coatings for different purposes:

of great interest.

required are usually below 100°C [46].

effect in materials:

**216**

**4.1 Fundamentals of self-heating by Joule effect**

where *Q* is the heat generated, *i* is the current flow, *R* is the electrical resistance, and t is the time the current is applied.

The first thing that can be analyzed from Eq. (1) is that higher current intensity would lead to higher heat generated and, consequently, higher contents of carbon nanoparticles will be desired for this purpose in order to increase the temperature reached or to reduce the voltage required. Although all common carbon nanostructures can be used for this purpose (carbon black, carbon nanotubes, graphene nanoplatelets, or even graphite flakes), the importance of reaching high intensity values usually gives the best results for CNT-filled materials [47]. In fact, very high CNT amounts can be found in the literature in order to increase the electrical conductivity and, consequently, the current flowing at lower voltages applied. This is the case of the study based on ABS as matrix where CNT was added up to 15 wt.% in order to allow reaching temperatures over 200°C when voltages of only 12 V were applied [48] or the research carried out by Chu et al. where similar results in terms of temperature and voltage at contents of 7.5 wt.% of CNT in PDMS were found [49]. The interest in the use of low voltages is based on the use of batteries commonly installed in cars and trucks, among others.

Apart from the heat generated, there is an important fact regarding these percolated electrically conductive networks, which is the homogeneous distribution of heat through the coating. Two important effects must be taken into account for this aspect: (1) thermal conductivity of polymers which is particularly low, thus making heat transfer through the coating more difficult, and (2) homogeneous presence of the carbon nanoparticles through the polymer matrix, which is not always easily reached.

Both CNTs and GNPs show extremely good thermal conductivity individually. Nevertheless, in spite of their similar intrinsic thermal conductivity, the morphology of GNPs makes them more interesting for this purpose, even when compared to SWCNT [50]. Even at the same content of both types of nanoparticles, Zakaria et al. found that the thermal conductivity was higher for GNP nanocomposites than the MWCNT ones. In fact, although for electrical properties, higher contents of GNPs are usually required to meet similar properties to the ones found for MWCNT, in that case, at only 3 wt.% of nanoreinforcement, GNP nanocomposites showed an increase of 126.4% in thermal conductivity while 3 wt.% of MWCNT only increased this property by 60.2% [51]. In fact, experimental values of thermal conductivity are usually lower than those predicted theoretically, and it has been attributed mainly to waviness, dispersion, alignment, interfacial resistance, and contact resistance [52].

Proper exfoliation of GNPs causes an important increment on thermal conductivity related to an increase of the aspect ratio. Chu et al. [53] proposed a model to calculate the thermal conductivity of nanocomposites based on randomly oriented nanoparticles which takes into account geometrical aspects of the nanoparticles (aspect ratio) as well as differences in the intrinsic thermal conductivity of the nanofiller in each direction. In the case of GNPs, these aspects will be strongly related to their exfoliation and dispersion in the polymer matrix. On the other hand, the waviness of the nanoreinforcement may reduce the effective aspect ratio of the nanofillers which lead to propose few layers GNPs as an optimal solution instead of individual monolayers that tend to roll up easier during dispersion stage.

These self-heating coatings do not require extremely high thermal conductivities, but they should be high enough to ensure good heat transfer through the whole surface for the purposes mentioned above.

The formation of aggregates is very common in this type of materials, and this may cause that at very low carbon nanoparticle loadings, some resin areas are free of nanoreinforcement, which leads to nonuniform heating of the samples. In that

seems to be one of the most promising ones. To avoid the use of additional membranes or layers, multifunctional coatings with heating capability by thermoresistive methods at the same time they protect the underlying

The use of carbon nanostructures has been studied for this purpose trying to create an active method based on Joule effect heating at the same time that hydrophobicity is increased by the addition of these nanoreinforcements. With this purpose, several research efforts have been already done reaching very promising results in terms of temperature, homogeneity, heat rate, and power consumption. By the use of GNPs, Redondo et al. achieved 35°C of temperature increments at 800 V with a heat rate of 13.6°C/min and a power consumption of less than 3 W. This temperature increment should be enough to produce ice melting on the coating and, consequently, separation of the ice accreted to the surface, even at severe ambient temperatures below 20°C [54]. When using CNTs, the electrical conductivity of the materials is usually higher than GNPs, thus increasing the value of the intensity at a constant voltage which is useful for heat generation according to Eq. (1). CNT/PVA films with very high CNT concentration have been produced in order to get very low electrical resistance and, consequently, higher electroresistive heating. The same procedure, using high MWCNT loadings (10 wt.% in poly-1,3,4-oxadiazole), allowed to reach temperatures above 100°C by the application of only 40 V in these nanoreinforced films. Prolongo et al. compared the temperature reached by the use of CNTs and GNPs as nanoreinforcements, and with contents of 0.5 wt.% of CNTs, the temperature reached almost 100°C with only 90 V applied, while 300 V were required to go over 65°C when 8% GNP were added [47]. Nevertheless, the authors claimed that temperature was more homogeneous when GNPs were used as nanoreinforcement. Finally, the authors are currently exploring the possibility of using the electrical network of CNTs to sense the temperature and, consequently, activate the voltage application when required because of the weather conditions measured by the coating itself. Coatings based on CNT/PDMS were manufactured and showed an effect-denominated negative temperature coefficient (NTC). This effect on electrical response was nonlinear with temperature changes, being the sensitivity more than six times higher in the range 5 to 5°C than at room temperature, which makes them potential candidates for temperature measurement for smart coatings being able to detect temperature and activate the voltage required accordingly [60].

Based on the same basics of previous applications, self-heating nanoreinforced polymers have been developed in order to allow curing by electrical voltage application. This issue has been already explored to take advantage of the uniform heat as far as the heat is generated from the material itself and, also, of the absence of power loses associated to heat transmission from the heating element to the material itself, as it happens when ovens are used [61]. Mas et al. proved the efficiency of this curing method by the addition of MWCNT to an epoxy matrix, and they found uniform thermal properties in the resin cured by Joule heating. In fact, they were able to control the real curing temperature by the coupling of thermocouples to the voltage source with a PID controller. The input of the thermocouples was used by the PID to adjust the power supply in order to keep the curing temperature constant during the process. One of the main advantages they found was the high heating

This fact was also proved by Jang et al. in the research conducted to prove that curing by Joule heating of thin films of PDMS reinforced with high loadings of

structural material are very interesting.

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

**4.3 Self-curing coatings**

**219**

rate as the heat emerges from the material itself.

**Figure 6.** *Joule effect heating of epoxy nanocomposites containing (a) 8 wt.%, (b) 10 wt.%, and (c) 12 wt.% of GNPs [54].*

cases, the thermal conductivity of the sample is even more important, as heat will not be homogeneously generated, thus making more important its thermal conduction. Prolongo et al. showed this effect when comparing MWCNT loaded with GNP ones, and they found that differences between maximum and minimum temperatures were much higher in those specimens based on MWCNT [47]. When adding GNPs, also this effect was found as lower contents of GNPs lead to areas with lower nanoreinforcement concentration, thus leading to higher temperature differences between different areas in the same sample, while the samples containing higher GNP contents (12 wt.%) showed more uniform heating (**Figure 6**).

#### **4.2 Self-heating as deicing system**

Icing on structure surface can seriously affect the function of the system, and, even, it may cause its damage and consequently the need for replacement which leads to economic, environmental, and security issues. Wind turbines or aircraft surfaces are examples in which ice accretion has a detrimental effect on operation conditions by modifying the aerodynamic profile, structural weight, etc. [55, 56]. Most strategies currently used are based on two different approaches that affect the coatings used:


#### *Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

seems to be one of the most promising ones. To avoid the use of additional membranes or layers, multifunctional coatings with heating capability by thermoresistive methods at the same time they protect the underlying structural material are very interesting.

The use of carbon nanostructures has been studied for this purpose trying to create an active method based on Joule effect heating at the same time that hydrophobicity is increased by the addition of these nanoreinforcements. With this purpose, several research efforts have been already done reaching very promising results in terms of temperature, homogeneity, heat rate, and power consumption. By the use of GNPs, Redondo et al. achieved 35°C of temperature increments at 800 V with a heat rate of 13.6°C/min and a power consumption of less than 3 W. This temperature increment should be enough to produce ice melting on the coating and, consequently, separation of the ice accreted to the surface, even at severe ambient temperatures below 20°C [54]. When using CNTs, the electrical conductivity of the materials is usually higher than GNPs, thus increasing the value of the intensity at a constant voltage which is useful for heat generation according to Eq. (1). CNT/PVA films with very high CNT concentration have been produced in order to get very low electrical resistance and, consequently, higher electroresistive heating. The same procedure, using high MWCNT loadings (10 wt.% in poly-1,3,4-oxadiazole), allowed to reach temperatures above 100°C by the application of only 40 V in these nanoreinforced films. Prolongo et al. compared the temperature reached by the use of CNTs and GNPs as nanoreinforcements, and with contents of 0.5 wt.% of CNTs, the temperature reached almost 100°C with only 90 V applied, while 300 V were required to go over 65°C when 8% GNP were added [47]. Nevertheless, the authors claimed that temperature was more homogeneous when GNPs were used as nanoreinforcement.

Finally, the authors are currently exploring the possibility of using the electrical network of CNTs to sense the temperature and, consequently, activate the voltage application when required because of the weather conditions measured by the coating itself. Coatings based on CNT/PDMS were manufactured and showed an effect-denominated negative temperature coefficient (NTC). This effect on electrical response was nonlinear with temperature changes, being the sensitivity more than six times higher in the range 5 to 5°C than at room temperature, which makes them potential candidates for temperature measurement for smart coatings being able to detect temperature and activate the voltage required accordingly [60].

#### **4.3 Self-curing coatings**

Based on the same basics of previous applications, self-heating nanoreinforced polymers have been developed in order to allow curing by electrical voltage application. This issue has been already explored to take advantage of the uniform heat as far as the heat is generated from the material itself and, also, of the absence of power loses associated to heat transmission from the heating element to the material itself, as it happens when ovens are used [61]. Mas et al. proved the efficiency of this curing method by the addition of MWCNT to an epoxy matrix, and they found uniform thermal properties in the resin cured by Joule heating. In fact, they were able to control the real curing temperature by the coupling of thermocouples to the voltage source with a PID controller. The input of the thermocouples was used by the PID to adjust the power supply in order to keep the curing temperature constant during the process. One of the main advantages they found was the high heating rate as the heat emerges from the material itself.

This fact was also proved by Jang et al. in the research conducted to prove that curing by Joule heating of thin films of PDMS reinforced with high loadings of

cases, the thermal conductivity of the sample is even more important, as heat will not be homogeneously generated, thus making more important its thermal conduction. Prolongo et al. showed this effect when comparing MWCNT loaded with GNP ones, and they found that differences between maximum and minimum temperatures were much higher in those specimens based on MWCNT [47]. When adding GNPs, also this effect was found as lower contents of GNPs lead to areas with lower nanoreinforcement concentration, thus leading to higher temperature differences between different areas in the same sample, while the samples containing higher

*Joule effect heating of epoxy nanocomposites containing (a) 8 wt.%, (b) 10 wt.%, and (c) 12 wt.% of GNPs [54].*

Icing on structure surface can seriously affect the function of the system, and, even, it may cause its damage and consequently the need for replacement which leads to economic, environmental, and security issues. Wind turbines or aircraft surfaces are examples in which ice accretion has a detrimental effect on operation conditions by modifying the aerodynamic profile, structural weight, etc. [55, 56]. Most strategies currently used are based on two different approaches that affect the coatings used:

• Passive methods which do not require external energy source. The main advantage of these methods is their lack of energy consumption to operate; nevertheless, their effectiveness is usually lower, so they are commonly used in combination with active methods to reduce the power needed [57]. Hydrophobic coatings are one of the passive methods most widely accepted which can be achieved by different approaches such as the addition of nanoparticles or tuning of surface textures in multiple length scales based, among others, in biomimetic

techniques [58, 59]. This aspect has been discussed in Section 2.2.

• Active methods which require external energy source. These methods are usually more effective to avoid icing problem and can be used in combination with passive methods. Among these active methods, heating systems are the most reliable ones in spite of the power consumption although mechanical ice breakage by means of inflatable rubber boots can be found in small airplanes. When heating the surface, several approaches can be used (infrared heating and warm air conduction, among others), but electrical resistance heating

GNP contents (12 wt.%) showed more uniform heating (**Figure 6**).

**4.2 Self-heating as deicing system**

*21st Century Surface Science - a Handbook*

**Figure 6.**

**218**

CNT (7 wt.%) could lead to even better performance than the equivalent ovencured samples. They measured the mechanical properties of both materials, and they found that stiffness of samples cured by Joule effect heating was slightly higher, which was associated to faster and more uniform heating of the whole material volume. So they conclude that this curing technique could allow obtaining materials at shorter curing times, more homogenous, and with higher cross-linked structures [62].

allowed heating the blends up to 100°C which would be enough to activate selfhealing mechanisms in blends containing PCL as healing phase [67]. All these recent studies reveal that the use of self-heating can help to develop self-healing polymerbased materials with the main advantages to avoid the use of external heat sources, use electrical voltage that could be remotely activated, allow heat to emerge from the material itself avoiding heating other materials parts, and reduce losses due to

Self-cleaning is a surface property consisting in keeping the surface clean under severe environmental conditions [68]. It is inspired from lotus effect with a specific micro- and nano-hierarchical surface morphology and low surface energy, providing superhydrophobicity. A surface can be considered as superhydrophobic when the water contact angle is higher than 150° (WCA > 150°) and the low sliding angle is lower than 10° (SA < 10°). Another opposite approach for getting self-cleaning surfaces is the incorporation of photocatalytic fillers affording hydrophilic surfaces (WCA < 5–10°) able to keep free of organic contaminants and moisture. The selfcleaning surfaces can be developed with nanodoped polymer coatings and with neat

One of the most studied self-cleaning materials is based on titanium oxide (TiO2) and zinc oxide (ZnO) due to their superhydrophobicity and photocatalytic

manufactured by sol-gel process. Modifying these nanofillers with graphene or its derivatives can enhance their visible light response. Hybrid TiO2/graphene

nanofillers can exhibit strong electronic overlap and high interfacial binding energy; thus, photoexcited carriers can transfer from TiO2 to graphene, and its band gap is

Another possibility to develop superhydrophobic self-cleaning coatings is the use of hydrophobic polymer matrix, such as polysiloxanes or fluoro-polymers,

decomposition ability of organic pollutants. These coatings are usually

reduced, improving the visible light photoresponse [69, 70]. Nevertheless, graphene enhances the photocatalytic efficiency of ZnO due to that graphene accepts the electron from ZnO nanoparticles, preventing the recombination of photo-generated electron hole in the semiconductor. These nanocoatings show superhydrophobicity when they are irradiated with visible light. An interesting alternative approach is the impregnation of cotton fibers to manufacture industrial

self-cleaning textiles [71]. The treated fabrics exhibit an increase anti-

*Scheme of the main approaches of self-cleaning coatings with carbon nanoparticles.*

**5. Smart nanocomposite coatings: Self-cleaning**

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

graphitic coating, such as it is shown in **Figure 7**.

bacteriological behavior and high biocompatibility.

heat transfer.

**Figure 7.**

**221**

Other carbon nanostructures different from CNTs have been also studied to cure thermoset materials out of the oven, such as the previously study from Mas et al. established [61]. Xia et al. proposed the use of GNP to cure epoxy matrices by adding contents over 8.5 wt.% as they found this value to be the percolation threshold for the GNP morphology and dispersion technique used. They found curing degrees similar to those found for oven-cured samples but with much faster heating rates and more homogenous curing, similar to the studies previously mentioned. Nevertheless, they found preferential orientation of the GNPs which was associated to the presence of an electric field during curing stage as it was not observed in oven-cured samples. This fact is very important as they found improved electrical and mechanical performance in this direction, but this anisotropic behavior must be taken into account when designing elements with these materials [63].

So, the application of curing by Joule effect heating has been proved as possible in electrically conductive networks based on carbon nanoparticles inside polymer matrices with particular interest due to faster curing cycles, more uniform curing degree, and easy application in large structures/surfaces.

#### **4.4 Self-healing coatings**

Any thermally activated mechanism could be beneficiated from the previously explained self-heating by thermoresistive heating. Thermo-reversible Diels-Alder reactions are one of these examples, in which the Diels-Alders and retro-Diels-Alder reactions are favored at different temperatures allowing the restoration of the covalent bonds, thus repairing the cracks generated. This method was used by Willocq et al. to produce MWCNT nanoreinforced polymer matrices with selfhealing capabilities by Joule effect heating at low voltages of 25 V which were enough to reach the retro-Diels-Alder reaction temperature in the vicinity of the macroscopic damage (crack) due to local higher heating around the crack. This local temperature increment around the crack was strongly dependent on the position of the electrodes with respect to the direction of the crack [64].

Huang et al. used GNPs dispersed in a thermoplastic polyurethane (TPU) matrix in order to create a percolated network and reach 98% self-healing efficiency by the application of 220 V to the material. The use of GNPs in this case allowed improving alternative self-healing approaches such as heating by IR radiation absorption which is also enhanced by the presence of GNPs in the matrix, thus allowing different alternatives to improve the thermal self-healing process of the TPU [65].

Also, this Joule effect-based heating can be used to activate self-healing in thermosetting/thermoplastic blends which self-healing capabilities. This mechanism is activated by a temperature increase above the melting temperature of the thermoplastic material used and high enough to allow its proper flow through the cracks created. Several materials have been studied for this purpose, and, among them, polycaprolactone (PCL) has been widely studied due to its low melting temperature which can allow self-healing mechanism to take place at temperatures around 100°C to allow its proper flow [66]. Zhang et al. proved the efficiency of the addition of CNT to EVA/PCL composites with shape memory purposes, which

CNT (7 wt.%) could lead to even better performance than the equivalent ovencured samples. They measured the mechanical properties of both materials, and they found that stiffness of samples cured by Joule effect heating was slightly higher, which was associated to faster and more uniform heating of the whole material volume. So they conclude that this curing technique could allow obtaining materials at shorter curing times, more homogenous, and with higher

Other carbon nanostructures different from CNTs have been also studied to cure thermoset materials out of the oven, such as the previously study from Mas et al. established [61]. Xia et al. proposed the use of GNP to cure epoxy matrices by adding contents over 8.5 wt.% as they found this value to be the percolation threshold for the GNP morphology and dispersion technique used. They found curing degrees similar to those found for oven-cured samples but with much faster heating rates and more homogenous curing, similar to the studies previously mentioned. Nevertheless, they found preferential orientation of the GNPs which was associated to the presence of an electric field during curing stage as it was not observed in oven-cured samples. This fact is very important as they found improved electrical and mechanical performance in this direction, but this anisotropic behavior must be taken into account when designing elements with these

So, the application of curing by Joule effect heating has been proved as possible in electrically conductive networks based on carbon nanoparticles inside polymer matrices with particular interest due to faster curing cycles, more uniform curing

Any thermally activated mechanism could be beneficiated from the previously explained self-heating by thermoresistive heating. Thermo-reversible Diels-Alder reactions are one of these examples, in which the Diels-Alders and retro-Diels-Alder reactions are favored at different temperatures allowing the restoration of the covalent bonds, thus repairing the cracks generated. This method was used by Willocq et al. to produce MWCNT nanoreinforced polymer matrices with selfhealing capabilities by Joule effect heating at low voltages of 25 V which were enough to reach the retro-Diels-Alder reaction temperature in the vicinity of the macroscopic damage (crack) due to local higher heating around the crack. This local temperature increment around the crack was strongly dependent on the position of

Huang et al. used GNPs dispersed in a thermoplastic polyurethane (TPU) matrix in order to create a percolated network and reach 98% self-healing efficiency by the application of 220 V to the material. The use of GNPs in this case allowed improving alternative self-healing approaches such as heating by IR radiation absorption which is also enhanced by the presence of GNPs in the matrix, thus allowing different alternatives to improve the thermal self-healing process of the TPU [65].

Also, this Joule effect-based heating can be used to activate self-healing in thermosetting/thermoplastic blends which self-healing capabilities. This mechanism is activated by a temperature increase above the melting temperature of the thermoplastic material used and high enough to allow its proper flow through the cracks created. Several materials have been studied for this purpose, and, among them, polycaprolactone (PCL) has been widely studied due to its low melting temperature which can allow self-healing mechanism to take place at temperatures around 100°C to allow its proper flow [66]. Zhang et al. proved the efficiency of the addition of CNT to EVA/PCL composites with shape memory purposes, which

degree, and easy application in large structures/surfaces.

the electrodes with respect to the direction of the crack [64].

cross-linked structures [62].

*21st Century Surface Science - a Handbook*

materials [63].

**220**

**4.4 Self-healing coatings**

allowed heating the blends up to 100°C which would be enough to activate selfhealing mechanisms in blends containing PCL as healing phase [67]. All these recent studies reveal that the use of self-heating can help to develop self-healing polymerbased materials with the main advantages to avoid the use of external heat sources, use electrical voltage that could be remotely activated, allow heat to emerge from the material itself avoiding heating other materials parts, and reduce losses due to heat transfer.
