Multifunctional sandwich materials with ROTIS structure for improved thermal and electrical properties in construction application

Abstract

There is still a lot of research space and market demand for lightweight, heat-insulating, and EMI-shielding construction materials. This paper develops and compares the thermal insulation, ohmic heating effect, and EMI shielding properties of a kind of multifunctional sandwich material with “ROTIS” and “FLAT” structures. The ROTIS-structured sample exhibits slightly higher thermal conductivity than the FLAT-structured sample, owing to its lower volume porosity and higher plated copper content per unit area. Since ROTIS technology allows for a significant increase in the thickness of thinner raw materials, this structure allows for an increase in the thermal insulation of thinner materials. Also, samples with ROTIS structures have a better ohmic heating effect than samples with FLAT structures. This is because the active layer has more plated copper content per unit area, while the insulation layer has less thermal resistance. Unsatisfactorily, the samples with the ROTIS structure show lower electromagnetic shielding effectiveness at 1-1.5 GHz, which is mainly due to their reduced volume porosity. In conclusion, this research develops sandwich materials with the ROTIS structure that exhibit excellent thermal insulation, electromagnetic shielding, and ohmic heating properties, making them suitable for use as building materials in demanding indoor temperatures and electromagnetic environments.

Keywords

Sandwich materials ROTIS structure thermal insulation property ohmic-heating effect electromagnetic interference shielding property

Introduction

Construction materials are various materials applied to construction projects. There is a wide variety of construction materials, broadly categorized as inorganic, organic, and composite materials.¹ With the development of society, many scholars have put forward the development strategy of ecological construction materials. Including the development of high-performance and long-life construction materials, greatly reducing the material consumption and service life of the construction project; the development of low energy consumption and environmentally friendly construction materials to replace the high energy consumption, environmental pollution, toxic and harmful to the human body of the construction materials; the development of construction materials with the improvement of ecological environment of the building and health care functions, such as antibacterial, deodorization, shielding of harmful radiation of the multifunctional materials, and so on.^2,3 Among them, the thermal insulation performance of buildings is an important aspect of saving energy and improving the living environment.⁴ The proportion of building energy consumption in the overall energy consumption of mankind is generally 30%–40%, the vast majority of which is the energy consumption of heating and air conditioning, so construction energy efficiency is of great significance.⁵ Furthermore, as more and more electronic products enter our lives, the electromagnetic radiation they emit poses a potential threat to our bodies. The united nations conference on the human environment has listed it as one of the major pollutants that must be controlled.⁶ Therefore, the study of construction materials with electromagnetic radiation shielding is also in line with the current direction of development and is of great significance for the protection of human beings.

At present, many experts and scholars are committed to researching the thermal insulation of construction materials and conforming to the structure of heat preservation and energy saving technology.⁷ Hamza Alahmad and other scholars concluded that the building materials commonly used for thermal insulation are extruded polystyrene foam, expanded polystyrene foam, glass wool, rock wool, polyurethane rigid foam, and phenol foam.⁸ Dong Yitong et al. have summarized some of the new insulation materials used in buildings in recent years in one of their review articles, including transparent insulation material, aerogel, closed-cell foam, vacuum insulation material, and reflective insulation material.⁹ Many other researchers have recently focused on phase change and 3D printing material, obtaining valuable research results.^10,11 At the same time, research on the construction materials for electromagnetic shielding effectiveness is relatively limited. Currently, cement-based electromagnetic shielding material and electromagnetic shielding coating are two widely used products.^12,13 For example, Aylin Akyildiz et al. focused on the addition of carbon nanomaterials to cement-based mixtures to enhance electromagnetic shielding effectiveness. The results revealed that a 1% carbon nanotube yielded maximum EM shielding at 2 GHz.¹⁴ In addition, some researchers are focusing on the sustainability of the electromagnetic shielding material and introducing the application of more environmentally friendly cellulose, biopolymers, and industrially recycled materials.¹⁵ However, there is rarely research on construction materials that combine the above two properties. This paper investigates the thermal insulation and electromagnetic shielding properties of a kind of sandwich material with the ROTIS structure. The sandwich material is composed of polyester nonwoven with an acrylic binder that has high spectral absorbance and polyester nonwoven with copper plating that has an electromagnetic shielding effect as well as an ohmic heating effect. After processing it into the ROTIS structure, which makes it a type of construction material as the barrier layer that is light, insulates heat, and shields electromagnetic interference (refer to Figure 1). Finally, by comparing the properties of the sandwich materials with and without the ROTIS structure, it can be determined how the ROTIS structure affects them.

Materials and methods

Materials

The sandwich material samples in this paper are composed of two kinds of different nonwoven materials. One is named “active layer material (AL)” and was purchased from the company BOCHEMIE a.s. It is a kind of polyester nonwoven fabric with three different contents of copper nanoparticles deposited on it, and the material’s ability to shield against electromagnetic radiation is mostly attributable to its patented method of chemically depositing metals onto surfaces.¹⁶ Additionally, the distribution of copper nanoparticles on the material’s surface results in an ohmic heating effect. It means that when a specific electric power is applied to the material, an electric current passes through it and then can generate a certain amount of heat.¹⁷ This significantly broadens the potential applications for the material. This study specifically uses it as the middle layer of sandwich materials. Another one is named “insulation layer material (IL)”, and it is a kind of polyester nonwoven fabric with an acrylic binder¹⁸ purchased from the company SINTEX a.s. It is always used for thermal insulation due to its high spectral absorbance, which aids in heat preservation. This type of fabric is typically used in multi-layer fabrics rather than on its own, and three different thicknesses of this kind of nonwoven have been selected for this article. Figure 2 and Table 1 show the basic information for AL and IL samples.

Table 1.

Basic information of AL and IL samples.

Sample code	Sample details		Fabrication method
AL10	100% polyester nonwoven with copper coating	10 g/m² polyester nonwoven with 3 g/m² copper coating	Thermal bonding
AL20		20 g/m² polyester nonwoven with 5 g/m² copper coating
AL30		30 g/m² polyester nonwoven with 7 g/m² copper coating
IL100	100% polyester nonwoven with acrylic binder		Chemical bonding
IL150
IL200

The sandwich material with the ROTIS structure is based on the ROTIS technology developed at TUL Liberec.¹⁹ It is based on the twisting of fiber ends that protrude from the web surface into what is referred to as quasi-yarns with a fineness of 250 tex. The ability to fix surface reinforcing nets to the surface of fixed structures is a benefit of this technology. Simultaneously, the machine’s pressure transforms the 2D textile material into the 3D textile material. The machine production process is referred to in Figure 3(a). The sandwich material with the FLAT structure is fed into the machine from the A subsystem, and through extrusion by machine pressure and fixation by quasi-yarns, the sandwich material with the ROTIS structure is finally out of the B subsystem of machine.²⁰ The key point in making samples is to choose the machine’s appropriate compression index, which is the ratio of the conveyor’s input velocity to its output velocity. The value of the compression index of the machine is usually taken according to the thickness of the fed sample. This is because a high compression index will increase the quasi-yarns’ tension in the sample, causing it to crumble; while a low index will result in insufficient pressure, preventing the sample from molding. Figure 3 and Table 2 display the basic information for sandwich material samples with FLAT/ROTIS structures.

Table 2.

Basic information of sandwich material samples with ROTIS and FLAT structures.

Sample configuration	Sandwich material samples with ROTIS structure				Sandwich material samples with FLAT structure
Sample configuration	Sample code	Compression index of machine	Length of the wave (mm)	Wave inclination angle (degree)	Sample code
IL100+AL10+IL100	1R				1F
IL100+AL20+IL100	2R	3.5	5	85-95	2F
IL100+AL30+IL100	3R				3F
IL150+AL10+IL150	4R				4F
IL150+AL20+IL150	5R	3	5.4	85-95	5F
IL150+AL30+IL150	6R				6F
IL200+AL10+IL200	7R				7F
IL200+AL20+IL200	8R	2.5	5.9	85-95	8F
IL200+AL30+IL200	9R				9F

Methods

Fabric physical properties and Fourier transform infrared spectroscopy

The determination of the sample’s thickness was carried out using a Schmidt thickness gauge D-200, supplied by Hans Schmidt & Co GmbH (Germany) and adhering to EN ISO 5084 standard.^22,23 The areal mass of the fabric was quantified as the mass per unit area and was computed in accordance with ISO 3801 standard.²⁴ In this research, the mass was assessed using a precision balance provided by Sartorius Lab Instruments GmbH & Co. KG (Göttingen, Germany).²⁵ Infrared spectra were acquired using a Fourier transform infrared spectrometer (FTIR) named Tensor 27 (Bruker). Powder-form samples were subjected to analysis in the attenuated total reflection (ATR) mode, employing a diamond crystal as the internal reflection element. To measure the samples, the wavelength range was from 2 to 18 µm. The acquisition parameters included a total of 50 scans at a resolution of 4⁻¹ cm.²⁶ And volume porosity was calculated via volume packing ratio according to the equation (1):

v o l u m e p o r o s i t y [%] = 1 - \frac{V_{f i b e r}}{V_{f a b r i c}} = 1 - \frac{\frac{m_{f i b e r}}{ρ_{f i b e r}}}{\frac{m_{f a b r i c}}{ρ_{f a b r i c}}} \approx 1 - \frac{ρ_{f a b r i c}}{ρ_{f i b e r}} = 1 - \frac{\frac{G S M}{t}}{ρ_{f i b e r}}

(1)

Where V_fiber and V_fabric are the volume of the fiber and the fabric; m_fiber and m_fabric are the mass of the fiber and the fabric; ρ_fiber and ρ_fabric are the density of the fiber and the fabric; GSM is the areal mass of the fabric, and t is the thickness of the fabric. All of the above tests were performed five times on different points of the fabric samples under the same conditions, and the average of these measurements was taken as the measured value.

Thermal conductivity and thermal resistance

The Alambeta device, provided by Sensora Instruments & Consulting (Czech Republic), has been selected as the measurement apparatus in this study. This investigation aims to evaluate the thermal transfer characteristics of fabric samples, specifically through the metrics of thermal conductivity and thermal resistance following ISO 5086-1 with Togmeter SDL M 259.²⁷ The operational principle of the Alambeta device involves a mathematical analysis of the heat flow dynamics across the test samples, facilitated by the temperature differential between the lower and upper measurement plates. The applied pressure during the measurement process was maintained at 200 Pa, with the contact area measuring 12 cm by 12 cm and the thickness ranging from 0.5 to 10 mm. Measurements were conducted under controlled environmental conditions, with an ambient temperature of approximately 22°C and a relative humidity of about 40%. Each fabric sample undergoes five measurements under identical conditions at disparate points, with the calculated mean of these measurements representing the final values for thermal conductivity and thermal resistance.^28,29

Ohmic-heating and emissivity

The ohmic-heating test was characterized as a methodology whereby an electric current is conducted through materials, fundamentally to induce thermal energy.¹⁷ This process was tested by an infrared camera (FLIR-E6390, FLIR Sweden) alongside a voltage regulator (S-LS-78, STAMOS Poland). The fabric’s extremities were interfaced with the electrical output of the voltage regulator, ensuring the electrode separation was sustained at 20 cm. The infrared apparatus was positioned at an elevation of 20 cm above the material’s surface to facilitate optimal thermal imaging. During the experimental procedure, an electric current was incrementally applied across the material, with voltage levels varying from 0 to 5 V. The average temperature over the middle section between the two electrodes was measured after different minutes. The test was performed five times with different fabric samples under the same conditions, and the average of these measurements was taken as the measured value. (Figure 4).

Since an infrared camera was used to measure the sample surface temperature, the emissivity of the sample surface must be measured. The emissivity of a material’s surface is its efficiency at emitting energy in the form of thermal radiation.³⁰ As shown in Figure 5(b), the whole set of equipment for testing emissivity is mainly divided into four parts. The first part is a heating plate, which can be adjusted to the temperature required for a test. In this experiment, the temperature was set to 40°, and the sample was placed on it. The second part is a temperature sensor, which is connected to the sample surface on one side and the computer on the other side. It can transfer the actual temperature of the sample surface (Tr) to the computer. The third part is an infrared imaging device (FLIR camera), and its emissivity (ε) was set to 0.95 during measurement in this experiment. The distance between the infrared detector and the test sample was half a meter. It was connected to the computer also, and the measured temperature (T) was transmitted to the computer. The fourth part is the computer, where the software calculates according to the Stefan-Boltzmann law and gets the emissivity of the sample. All testing was carried out in a confined space under the same temperature and humidity.³¹ The sample’s real emissivity (ε_r) is calculated according to Stefan-Boltzmann’s law (2):

ε_{r =} ε {(\frac{T}{T_{r}})}^{4}

(2)

Electromagnetic shielding effectiveness

The fundamental concept underlying electromagnetic shielding materials revolves around the mechanisms of wave reflection, absorption, and the multiple internal reflections of electromagnetic waves within the material, culminating in the diminution of electromagnetic wave energy.^32,33 The assessment of electromagnetic interference (EMI) shielding effectiveness (SE) was executed employing the coaxial transmission line approach in adherence to ASTM standard 4935-10.³⁴ This standard stipulates the evaluation of a shielding material’s response to plane electromagnetic waves, across a frequency spectrum ranging from 30 MHz to 3 GHz with a contact measuring area of 12 cm by 12 cm. The experimental setup incorporated a coaxial specimen holder, provided by Electro-Metrics Inc. (model EM-2107A), to facilitate the measurement process. The vector network analyzer utilized for this purpose was sourced from company Rohde & Schwarz (model ZNC3), which was connected to both the input and output terminals of the specimen holder. The SE of the material was determined by analyzing the power ratio between the output and input signals. Each fabric sample underwent five measurements under identical environmental and operational conditions, but at distinct locations on the fabric.³⁵ The mean of these measurements was subsequently computed to represent the electromagnetic shielding effectiveness of the sample.

Results and discussion

Fabric physical properties and FTIR

The basic physical properties and FTIR results of AL and IL samples are shown in Table 3, Table 4, and Figure 5. The samples of active layer material are thinner and lighter. Samples AL10, AL20, and AL30 have areal masses of 12.13 g/m², 24.81 g/m², and 36.47 g/m², respectively. And their thicknesses are 0.06 mm, 0.08 mm, and 0.11 mm. In addition to this, the important difference between these three samples is that they are plated with 3 g/m², 5 g/m², and 7 g/m² of copper on the greige fabrics of approximately 10 g/m², 20 g/m², and 30 g/m². On the contrary, the samples of insulation layer material are thicker and denser. The areal masses of samples IL100, IL150, and IL200 are 79.94 g/m², 130.56 g/m², and 228.08 g/m², respectively. And their thicknesses are 4.21 mm, 8.11 mm, and 12.93 mm. It can be seen from Figure 5 and Table 4 that the reflectance, transmittance, and absorbance of AL and IL samples are at the wavelengths of 2 µm to 18 µm. Samples of AL have a higher spectral reflectance, especially sample AL10, which is 18% at a wavelength of 10 µm. As the thickness of the sample’s greige fabric increases, so does its absorbance. In addition, samples of IL have very high spectral absorbance, all above 97%. This result also proves that IL samples are well suited for use in the sandwich material and have thermal insulation ability.

Table 3.

Basic information of AL and IL samples.

Sample code	Areal mass (g/m²)	Thickness (mm)	Volume porosity (%)
Sample code	M ± 95%CI	M ± 95%CI	M ± 95%CI
AL10	12.13 ± 0.02	0.06 ± 0.01	89.12 ± 0.05
AL20	24.81 ± 0.13	0.08 ± 0.01	82.26 ± 0.09
AL30	36.47 ± 0.41	0.11 ± 0.01	80.78 ± 0.11
IL100	79.94 ± 1.59	4.21 ± 0.02	98.64 ± 0.07
IL150	130.56 ± 3.41	8.11 ± 1.09	98.82 ± 0.04
IL200	228.08 ± 2.56	12.93 ± 0.17	98.75 ± 0.08

Table 4.

Reflectance, transmittance, and absorbance of AL and IL samples at a wavelength of 10 µm.

Sample code	Reflectance (%)	Transmittance (%)	Absorbance (%)
AL10	18	26	56
AL20	6	7	87
AL30	3	2	95
IL100	0.5	2	97.5
IL150	0.5	0.6	98.9
IL200	0.5	0.2	99.3

The basic information about sandwich material samples with FLAT and ROTIS structures is shown in Table 5. The sandwich material samples in this paper mainly consist of two layers of insulation layer material plus one layer of active layer material in the middle. The thickness of the sandwich material samples with FLAT structure increases with the increased thickness of the applied single layer samples, and they range in thickness from 8 mm to 28 mm. For the sandwich material samples with ROTIS structure, the variation in thickness of these samples is interesting. Due to the pressure of the machine, the samples take on a sinusoidal-like shape. This approach makes the thickness of thinner samples increase while the thickness of thicker samples remains constant or decreases, and they range in thickness from 17 mm to 22 mm. In addition, the areal mass of all sandwich material samples with flat structures increases with the increased areal mass of the applied single-layer samples. And the areal mass of sandwich material samples with the ROTIS structure is far higher than that of sandwich material samples with the FLAT structure under the same sample configuration. Therefore, it can be seen that the ROTIS structure makes the samples more compact and reduces the presence of air. The calculated volume porosity results show that all sandwich material samples with the ROTIS structure have a volume porosity decrease of about 2% to 3% compared to sandwich material samples with a flat structure, and this result supports the above conjecture.

Table 5.

Basic information of sandwich material samples with FLAT and ROTIS structures.

Sandwich material samples with FLAT structure				Sandwich material samples with ROTIS structure
Sample code	Areal mass (g/m²)	Thickness (mm)	Volume porosity (%)	Sample code	Areal mass (g/m²)	Thickness (mm)	Volume porosity (%)
Sample code	M ± 95%CI	M ± 95%CI	M ± 95%CI	Sample code	M ± 95%CI	M ± 95%CI	M ± 95%CI
1F	172.01 ± 1.28	8.21 ± 0.23	98.51 ± 0.02	1R	805.56 ± 0.03	17.36 ± 0.17	96.62 ± 0.03
2F	184.69 ± 0.49	10.47 ± 0.26	98.73 ± 0.04	2R	932.41 ± 0.82	18.43 ± 0.07	96.35 ± 0.07
3F	196.35 ± 0.21	10.51 ± 0.11	98.64 ± 0.07	3R	1013.64 ± 2.67	18.53 ± 0.17	96.19 ± 0.12
4F	273.25 ± 2.74	17.33 ± 0.91	98.91 ± 0.02	4R	862.22 ± 1.45	19.83 ± 0.06	96.81 ± 0.05
5F	285.93 ± 1.23	18.91 ± 0.49	98.92 ± 0.01	5R	1021.27 ± 0.77	20.76 ± 0.28	96.45 ± 0.12
6F	297.59 ± 0.88	18.81 ± 0.34	98.95 ± 0.01	6R	1147.25 ± 3.91	20.87 ± 0.28	96.17 ± 0.11
7F	468.29 ± 1.29	26.61 ± 0.29	98.73 ± 0.03	7R	1101.11 ± 0.18	20.76 ± 0.17	96.13 ± 0.07
8F	480.97 ± 0.24	27.17 ± 0.07	98.74 ± 0.04	8R	1164.82 ± 2.22	21.37 ± 0.47	96.18 ± 0.04
9F	492.63 ± 0.82	27.31 ± 0.27	98.71 ± 0.02	9R	1270.81 ± 2.04	21.83 ± 0.28	96.12 ± 0.06

Thermal conductivity and resistance

Thermal conductivity and thermal resistance are chosen to characterize the samples’ thermal insulation performance. In general, the higher the thermal conductivity, the better the ability of the thermally conductive interface material to conduct heat energy directly.³⁶ Thermal resistance can be understood as the resistance encountered by heat in the path of heat flow, and the thickness of the thermally conductive interface material is one of the most important factors affecting its thermal resistance.³⁷ Table 6 shows the thermal conductivity and thermal resistance of AL and IL samples. It can be seen that all samples have very low thermal conductivity, which means these materials have good thermal insulation properties themselves. As the plated copper contents of the AL samples increase and the thicknesses of the IL samples increase, their thermal conductivities increase slightly. Copper is an extremely thermally conductive material, so the thermal conductivity of AL samples increases as the plated copper content increases.³⁸ The increase in thicknesses leads to an increase in the acrylic binder content of the IL samples, which has a thermal conductivity of 0.192 W/m·K, thereby also contributing to the enhanced thermal conductivity of the IL samples.³⁹ Beyond that, the thermal resistance of the IL samples is much higher than that of the AL samples. When the thermal conductivity is relatively similar, the difference in thermal resistance increases as the difference in the sample’s thickness increases.

Table 6.

Thermal conductivity and thermal resistance of AL and IL samples.

Sample code	Thermal conductivity [W/m·K]	Thermal resistance [K·m²/W]
Sample code	M ± 95%CI	M ± 95%CI
AL10	0.0358 ± 0.0004	0.0018 ± 0.0002
AL20	0.0411 ± 0.0005	0.0019 ± 0.0003
AL30	0.0466 ± 0.0013	0.0022 ± 0.0001
IL100	0.0367 ± 0.0004	0.1147 ± 0.0024
IL150	0.0466 ± 0.0024	0.1733 ± 0.0157
IL200	0.0534 ± 0.0009	0.2423 ± 0.0053

Table 7 presents thermal conductivity, volume porosity and copper plated content of sandwich material samples with FLAT and ROTIS structures. A comparison of the thermal conductivity for all samples reveals that the thermal conductivity of the samples with the ROTIS structure is slightly higher than that of the samples with the FLAT structure. A reason for this phenomenon would be that when making the sample with the ROTIS structure, the machine’s pressure reduces the amount of still air inside the sample, leading to a slight increase in its thermal conductivity. In addition to this, structural changes in fabrics can also cause changes in their thermal conductivity. Scholar Bogaty et al. proposed a series-parallel theoretical model to express the relationship between the thermal conductivity of fabrics and the internal structure of the fabric and defined the effective thermal conductivity of the air and fiber assembly as equation (3):

λ = x (v_{f i b} λ_{f i b} + v_{a i r} λ_{a i r}) + y (\frac{λ_{f i b} λ_{a i r}}{v_{a i r} λ_{f i b} + v_{f i b} λ_{a i r}})

(3)

Where x and y are the effective volume fractions of fibers parallel to and perpendicular to the heat transfer direction, and x + y = 1; λ_fib and λ_air are the thermal conductivities of fiber and air; v_fib and v_air are the volume fractions of fiber and air, and v_fib+v_air = 1.⁴⁰

Table 7.

Thermal conductivity, volume porosity and copper plated content of sandwich material samples with FLAT and ROTIS structures.

Sandwich materials with FLAT structure				Sandwich materials with ROTIS structure
Sample code	Thermal conductivity [W/m·K]	Volume porosity (%)	Copper plated content (g/m²)	Sample code	Thermal conductivity [W/m·K]	Volume porosity (%)	Copper plated content (g/m²)
Sample code	M ± 95%CI	M ± 95%CI	Copper plated content (g/m²)	Sample code	M ± 95%CI	M ± 95%CI	Copper plated content (g/m²)
1F	0.0398 ± 0.0011	98.51 ± 0.02	3	1R	0.0498 ± 0.0009	96.62 ± 0.03	14.1
2F	0.0398 ± 0.0015	98.73 ± 0.04	5	2R	0.0544 ± 0.0019	96.35 ± 0.07	25.3
3F	0.0433 ± 0.0008	98.64 ± 0.07	7	3R	0.0617 ± 0.0009	96.19 ± 0.12	36.2
4F	0.0521 ± 0.0011	98.91 ± 0.02	3	4R	0.0569 ± 0.0009	96.81 ± 0.05	9.5
5F	0.0531 ± 0.0006	98.92 ± 0.01	5	5R	0.0596 ± 0.0021	96.45 ± 0.12	17.9
6F	0.0539 ± 0.0016	98.95 ± 0.01	7	6R	0.0658 ± 0.0015	96.17 ± 0.11	22.1
7F	0.0622 ± 0.0004	98.73 ± 0.03	3	7R	0.0631 ± 0.0011	96.13 ± 0.07	7.1
8F	0.0624 ± 0.0008	98.74 ± 0.04	5	8R	0.0642 ± 0.0007	96.18 ± 0.04	12.1
9F	0.0629 ± 0.0014	98.71 ± 0.02	7	9R	0.0682 ± 0.0009	96.12 ± 0.06	18.1

In this study, the thermal conductivity of the ROTIS structure is also in line with the theoretical model described above. By looking at Figure 6, it can be noticed that the samples with ROTIS structures contain fabrics both parallel and perpendicular to the direction of heat transfer. By modifying the above theoretical model, it is possible to obtain formulas (4) for the thermal conductivity that correspond to the samples with ROTIS structures as:

λ_{R O T I S} = x λ_{p a r a l l e l} + (1 - x) λ_{p e r p e n d i c u l a r}

(4)

Where x and 1-x are the effective volume fractions of fabrics parallel to and perpendicular to the heat transfer direction; λ_parallel and λ_{perpendicular} are the thermal conductivities of fabrics parallel to and perpendicular to the heat transfer direction. As can be seen from Table 8, the thermal conductivities of all samples with ROTIS structures and fabrics parallel to/perpendicular to the heat transfer direction are measured by Alambeta and displayed in the table. In addition to this, the values of x and 1-x calculated by the above formula are also shown in the table, i.e., as the effective volume fractions of fabrics parallel to and perpendicular to the heat transfer direction. Incorporating the ROTIS structure allows the samples to form two parts of the material parallel as well as perpendicular to the direction of heat transfer, thus increasing the amount of copper plated per unit area. From the Alambeta test results in Table 8, it can be found that λ_parallel is all higher than λ_{perpendicular}. Based on the relationship in equation (4), it can be calculated that λ_ROTIS is necessarily higher than λ_FLAT, because λ_FLAT is only equivalent to λ_{perpendicular}. This calculation is also in agreement with the results measured by Alambeta, it also shows that the copper plating content per unit area positively affects the thermal conductivity of their samples. In addition, since the samples’ thickness of the sandwich material with ROTIS structure is much higher than its wavelength, the material parallel to the heat transfer direction will have a greater effect on the thermal conductivity than the material perpendicular to the heat transfer direction. Figure 7 and Table 9 corroborate this inference, showing that as the copper plating content per unit area increases in ROTIS-structured sandwich materials with the same insulation layer material, the effective volume fraction of fabrics parallel to the heat transfer direction increases and that perpendicular to the heat transfer direction decreases.

Table 8.

Measured related thermal conductivity of sandwich material samples with ROTIS structure and calculated value of x, 1-x, x/1-x.

Sample code (R)	λ_ROTIS	λ_parallel	λ_{perpendicular}	x	1-x	x/1-x
1	0.0498	0.0626	0.0398	0.44	0.56	0.79
2	0.0544	0.0645	0.0398	0.59	0.41	1.44
3	0.0617	0.0681	0.0433	0.74	0.26	2.85
4	0.0569	0.0634	0.0521	0.43	0.57	0.75
5	0.0596	0.0649	0.0531	0.55	0.45	1.22
6	0.0658	0.0685	0.0539	0.81	0.19	4.26
7	0.0631	0.0641	0.0622	0.48	0.52	0.92
8	0.0642	0.0653	0.0624	0.6	0.4	1.5
9	0.0682	0.0695	0.0629	0.77	0.23	3.35

Table 9.

Linear correlation result between the copper plated content of sandwich material samples with ROTIS structure and the value of x/1-x.

Sample	Slop	Intercept	R²
Samples 1R,2R,3	0.0931	−0.6516	0.9533
Samples 4R,5R,6	0.2015	−1.584	0.8667
Samples 7R,8R,9	0.2238	−0.8591	0.9432

The variation of thermal resistance values for the two groups of samples shown in Table 10 and Figure 8 is more complex, but the results follow the Fourier’s law of heat conduction as formulas (5):

T h e r m a l r e s i s t a n c e = \frac{T h i c k n e s s}{T h e r m a l c o n d u c t i v i t y}

(5)

Table 10.

Thermal resistance, thermal conductivity, and thickness of sandwich material samples with FLAT and ROTIS structures.

Sandwich materials with FLAT structure				Sandwich materials with ROTIS structure
Sample code	Thermal resistance [K·m²/W]	Thermal conductivity [W/m·K]	Thickness (mm)	Sample code	Thermal resistance [K·m²/W]	Thermal conductivity [W/m·K]	Thickness (mm)
Sample code	M ± 95%CI	M ± 95%CI	M ± 95%CI	Sample code	M ± 95%CI	M ± 95%CI	M ± 95%CI
1F	0.2603 ± 0.0075	0.0398 ± 0.0011	8.21 ± 0.23	1R	0.3487 ± 0.0036	0.0498 ± 0.0009	17.36 ± 0.17
2F	0.2631 ± 0.0118	0.0398 ± 0.0015	10.47 ± 0.26	2R	0.3393 ± 0.0132	0.0544 ± 0.0019	18.43 ± 0.07
3F	0.2427 ± 0.0024	0.0433 ± 0.0008	10.51 ± 0.11	3R	0.3003 ± 0.0035	0.0617 ± 0.0009	18.53 ± 0.17
4F	0.3327 ± 0.0235	0.0521 ± 0.0011	17.33 ± 0.91	4R	0.3523 ± 0.0043	0.0569 ± 0.0009	19.83 ± 0.06
5F	0.3553 ± 0.0056	0.0531 ± 0.0006	18.91 ± 0.49	5R	0.3487 ± 0.0105	0.0596 ± 0.0021	20.76 ± 0.28
6F	0.3493 ± 0.0151	0.0539 ± 0.0016	18.81 ± 0.34	6R	0.3167 ± 0.0056	0.0658 ± 0.0015	20.87 ± 0.28
7F	0.4281 ± 0.0089	0.0622 ± 0.0004	26.61 ± 0.29	7R	0.3291 ± 0.0091	0.0631 ± 0.0011	20.76 ± 0.17
8F	0.4353 ± 0.0058	0.0624 ± 0.0008	27.17 ± 0.07	8R	0.3327 ± 0.0096	0.0642 ± 0.0007	21.37 ± 0.47
9F	0.4271 ± 0.0136	0.0629 ± 0.0014	27.31 ± 0.27	9R	0.3201 ± 0.0057	0.0682 ± 0.0009	21.83 ± 0.28

The thermal resistance of textiles is affected by their thermal conductivity and thickness, lower thermal conductivity and thicker thicknesses increase the thermal resistance of the material.⁴¹ The thermal resistance values of the sandwich material sample, including the IL100 and different AL samples, with the ROTIS structure are much higher than those of the FLAT structure. This is mainly because the thicknesses of the sandwich material sample with the ROTIS structure have a substantial increase, leading to significant increases in their thermal resistances. Interestingly, the thermal resistance values of the sandwich material sample, including the IL150 sample and different AL samples, with the ROTIS structure are similar to those of the FLAT structure. The reason for the lack of a significant change in their thermal resistance is twofold: on the one hand, the thickness of the samples with the ROTIS structure increases only slightly, and on the other hand, the thermal conductivity of the samples with the ROTIS structure also increases. For sandwich material samples that included IL200 and different AL samples, the thickness of the samples with the ROTIS structure was drastically reduced compared to the samples with the FLAT structure. This directly results in the thermal resistance of the sandwich samples with the ROTIS structure being lower than that of the sandwich samples with the FLAT structure. From the above experimental results, it can be concluded that ROTIS structures will greatly improve their thermal insulation performance for samples with lower thickness.

Ohmic-heating test

Ohmic heating is the process by which an electric current passes through a conductor to produce heat. Due to the distribution of copper nanoparticles on the surface of active layer materials, they exhibit the ohmic heating effect. The ohmic-heating effect of active layer materials is tested in this study, including the active layer material itself and the sandwich material samples with FLAT/ROTIS structures. As can be seen from Figure 9, the ohmic-heating test results for samples AL10, AL20, and AL30. The voltage of 1.5 V was loaded on sample AL10 and 2 V on samples AL20 and AL30, respectively. This is because when the loaded voltage is too high, generating too much heat can cause the sample to burn. The comparison shows that the three samples of the active layer can reach temperatures ranging from around 28°C to 35°C with no significant differences and also cannot reach a relatively high temperature. This is because the thicknesses of the active layer materials are very thin, and they quickly diffuse into the surrounding air when heat is generated. Therefore, this kind of material should be put in the sandwich structure and will fulfill the function of generating and retaining heat.

For the sandwich material samples with AL10, the highest values for the ohmic heating test ranged from 27°C to 34°C. However, the sandwich material samples with AL20 and AL30 showed a better ohmic heating effect. The most satisfying result was that the highest values for the ohmic heating test were around 70°C and 80°C for samples 2R and 3R. It can also be observed from the above three figures that the ohmic heating temperatures of the sandwich material samples with AL20 and AL30 are both much higher than those of the AL20 sample and AL30 sample themselves. This proves the material’s suitability for sandwich construction, where it can retain heat better. In addition, it can be observed that sandwich material samples with ROTIS structures exhibit relatively higher temperatures than those with FLAT structures. Table 11 and Figure 10 illustrate how the thermal resistance of the insulation layer and the copper content of the active layer affect the measured temperature of ohmic heating, which also rationally explains the phenomenon mentioned above. The sandwich material sample with a low thermal resistance of the insulation layer (i.e. greater heat transfer capability) and a high copper content of the active layer (i.e. generates more heat) results in a higher temperature of ohmic heating. Comparing the two structures of sandwich material samples, it can be seen that the ROTIS structure not only makes the thermal resistance of the insulation layer lower but also increases the copper content of the active layer, which inevitably leads to a higher temperature of ohmic heating. In addition, Table 12 exhibits the extent to which these two factors influence the ohmic heating of the samples by “One-way ANOVA” and “Pearson correlation coefficient”. For the sandwich material samples with the FLAT structure, the copper plated content of the active layer has a relatively significant effect on the temperature of ohmic heating. This is because the p-value of the One-way ANOVA is much less than 0.05 and its Pearson correlation coefficient is 0.96. For the sandwich material samples with ROTIS structure, the p-value of both factors is greater than 0.05, i.e., both factors affect the ohmic thermal temperature of the samples at the same time, and there is no question of which one is more significant. However, the correlation coefficient shows that the thermal resistance of the insulation layer affects the ohmic heat of the samples to a slightly greater extent than the copper plated content of the active layer. The above series of results show that the active layer material can better perform its ohmic heating effect when placed in this sandwich material with the ROTIS structure.

Table 11.

Thermal resistance of insulation layer, copper plated content of active layer, and temperature of ohmic heating for sandwich material samples with FLAT and ROTIS structure.

Sandwich materials with FLAT structure				Sandwich materials with ROTIS structure
Sample code	Thermal resistance of insulation layer [K·m²/W]	Copper plated content of active layer (g/m²)	Temperature of ohmic heating (°C)	Sample code	Thermal resistance of insulation layer [K·m²/W]	Copper plated content of active layer (g/m²)	Temperature of ohmic heating (°C)
1F	0.1147	3	27.4	1R	0.0601	14.1	34.1
2F	0.1147	5	36.4	2R	0.0601	25.3	65.7
3F	0.1147	7	60.1	3R	0.0601	36.2	78.2
4F	0.1733	3	27.3	4R	0.0919	9.5	31.9
5F	0.1733	5	35.6	5R	0.0919	17.9	55.4
6F	0.1733	7	57.8	6R	0.0919	22.1	72.1
7F	0.2423	3	26.7	7R	0.1452	7.1	29.1
8F	0.2423	5	34.4	8R	0.1452	12.1	46.7
9F	0.2423	7	54.2	9R	0.1452	18.1	70.7

Table 12.

Analysis results by “One-Way ANOVA” and “Pearson correlation coefficient”.

Sample	Factor	ANOVA p-value	Pearson correlation coefficient
Sandwich material samples with FLAT structure	Thermal resistance of insulation layer	0.81	0.092
Sandwich material samples with FLAT structure	Copper plated content of active layer	0.000036	0.96
Sandwich material samples with ROTIS structure	Thermal resistance of insulation layer	0.37	0.34
Sandwich material samples with ROTIS structure	Copper plated content of active layer	0.9	0.048

Electromagnetic interference shielding effectiveness (EM SE)

This paper compares the values from 30 MHz to 3 GHz to evaluate the electromagnetic interference shielding effectiveness of different single layer samples and sandwich material samples. This is because this frequency in the electromagnetic spectrum is most often exposed in everyday life and work environments.¹⁶ It can be observed from Figure 11(a) that the average value of electromagnetic interference shielding effectiveness for the samples AL10, AL20, and AL30 is around 48 dB, 55 dB, and 61 dB, respectively. According to the FTTS-FA-003 standard⁴² about the grading of electromagnetic shielding fabrics for professional use, all AL samples exhibit “very good” electromagnetic shielding effectiveness. Prior research indicates that the primary determinants of electromagnetic interference shielding efficiency include the conductivity and porosity of the shielding layer.⁴³ Typically, the electromagnetic shielding effectiveness of a material is directly proportional to its conductivity and inversely proportional to its porosity.^44,38 By looking at Table 13, Figure 11(b) and Table 14, it can be seen that the experimental results in this paper also conform to this objective law. There is a positive correlation between the copper plated content of the active layer materials and their electromagnetic shielding effectiveness; and a negative correlation between the porosity of the active layer materials and their electromagnetic shielding effectiveness. In addition, the insulation layer materials did not show electromagnetic shielding effectiveness in the frequency range of 30 MHz to 3 GHz. As a result, only the active layer materials provide the electromagnetic shielding effectiveness of the sandwich material samples in this paper.

Table 13.

Average value of EM SE from 30 MHz to 3 GHz, volume porosity and copper plated content for AL samples.

Sample code	Average value of EM SE from 30 MHz to 3 GHz (dB)	Volume porosity (%)	Copper plated content (g/m²)
AL10	48	89.12	3
AL20	55	82.26	5
AL30	61	80.78	7

Table 14.

Linear correlation result between the average value of EM SE and the copper plated content, the volume porosity of AL samples.

Linear correlation	Slop	Intercept	R²
EM SE – Volume porosity	0.3056	−11.617	0.9984
EM SE – Copper plated content	−0.6469	119.22	0.9035

The electromagnetic interference shielding effectiveness of sandwich material samples with different active layer materials is shown in Figure 12. It can be observed that the differences in electromagnetic interference shielding effectiveness between the sandwich material samples with FLAT structure and active layer materials are not significant. This experimental result also follows previous research showing that simply increasing the thickness of the non-EMI shielding layer material does not affect the overall EMI shielding performance of the sandwich material when there is no significant change in the conductivity as well as the porosity of the EMI shielding layer material. The sandwich material samples with the ROTIS structure showed a trough in the test frequency range from 1 to 1.5 GHz, which was not expected. This phenomenon can be explained based on the “frequency-selective surface” theory which has been studied by many scholars, that is, the geometry and size of the shielding material will produce specific responses to electromagnetic waves of specific wavelengths.⁴⁵ From the basic information in Table 5, it can be seen that the volume porosity of sandwich material samples with the ROTIS structure decreased by 2% to 3% compared to sandwich material samples with the flat structure. Thus, it can be demonstrated that the ROTIS structure minutely alters the porosity of the active layer material in the sandwich material sample. Because of this change, the sandwich material samples with the ROTIS structure show troughs in the 1–1.5 Hz band. This means that the sample is less effective at blocking electromagnetic waves in this frequency range.

Conclusions

This paper evaluated and compared the thermal insulation property, ohmic-heating effect, and electromagnetic interference shielding property of a particular type of sandwich material with “ROTIS and FLAT” structures. The machine’s pressure causes the sandwich material with the ROTIS structure to exhibit a sinusoidal-like shape. This method causes the thickness of thinner original samples to increase, while the thickness of thicker original samples either remains constant or decreases. The ROTIS structure resulted in a significant increase in the sample’s areal mass, in contrast to a decrease in its volume porosity. Experimentally, it was found that the sandwich material with the ROTIS structure had slightly higher thermal conductivity than that with the FLAT structure. On the one hand, this is because the ROTIS structure decreases the sample’s volume porosity, resulting in a slight increase in its thermal conductivity. On the other hand, the ROTIS structure makes its sample contain two parts of fabric parallel and perpendicular to the direction of heat transfer, which will substantially increase the copper plating content per unit area in the sample. Given the positive correlation between the copper content per unit area of the sample and its thermal conductivity, it is inevitable that this structural change will enhance the sample’s overall thermal conductivity. The change in thermal resistance is affected by both the change in thermal conductivity and thickness of the samples. The ROTIS structure will increase the thermal resistance of the originally thinner samples with the FLAT structure, while the thermal resistance of the initially thicker samples with the FLAT structure will remain constant or decrease. Therefore, the ROTIS structure will greatly improve their thermal insulation performance for samples with low thickness.

This paper also evaluated the ohmic heating effects of the active layer materials and all sandwich materials. The thin thickness of the active layer material quickly diffuses the generated heat into the surrounding air, limiting the test results to a range of about 28°C to 35°C. The temperature for the ohmic heating effect of the ROTIS-structured sandwich samples is relatively higher than that of the FLAT-structured sandwich samples and the active layer material itself. This is because the ROTIS structure increases the copper content per unit area of the active layer of the sample, as well as reducing the thermal resistance of the insulating layer. The most satisfactory results are the maximum ohmic heating test values for samples 3R and 4R, which are about 70°C and 80°C, respectively. The above series of results suggest that the active layer material can enhance its ohmic heating performance in the sandwich material with ROTIS structure. The EMI shielding effectiveness of active layer materials and all sandwich materials was tested from 30 MHz to 3 GHz, and all active layer samples showed “very good” EMI shielding effectiveness. The difference in EMI shielding performance between the FLAT sandwich and the active layer is negligible, while the ROTIS sandwich samples show an unexpected decrease in the frequency range of 1 to 1.5 GHz during the test. The theory of “frequency-selective surfaces” explains this phenomenon, as the ROTIS structure alters the volume porosity of the samples, thereby influencing the shielding effectiveness of electromagnetic waves in this frequency range. This means that the ROTIS-structured sandwich samples are less effective at shielding electromagnetic waves in the frequency range of 1 to 1.5 Hz.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the by the Czech Science Foundation (GACR)-project Advanced structures for thermal insulation in extreme conditions (Reg. No. 21-32510M).

Ethical statement

ORCID iDs

Dan Wang

Shi Hu

Data availability statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.*

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