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Preparation and characterization of mesoporous silica–polyvinyl butyral hybrid coatings by electrophoretic deposition
Microporous and Mesoporous Materials 292 (2020) 109710
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Preparation and characterization of mesoporous silica–polyvinyl butyral hybrid coatings by electrophoretic deposition
T
Hideyuki Negishi∗, Yamaki Takehiro, Endo Akira Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous silica Thick film Polyvinyl butyral Electrophoretic deposition Water vapor adsorption-desorption
Thick mesoporous silica (MPS) films with sufficient mechanical strength and high porosity were prepared on aluminum plates by using electrophoretic deposition (EPD) with the addition of a small amount of polyvinylbutyral (PVB, 3.5–8.0 wt%) as binder. The porosity of the MPS films was approximately 50 vol%. The film thickness could be controlled by the deposition conditions, and a thickness of ca. 50 μm is reasonable for effectively using the whole MPS film. These MPS films exhibited good water vapor adsorption-desorption properties and sufficient stability for more than 150 adsorption-desorption cycles. When used in dehumidification applications, these MPS films had a dehumidification rate of 6.3 g-H2O/g-MPS per hour, with an adsorptiondesorption cycle as short as 2 min even when a low temperature of 60 °C was used to regenerate the adsorbent.
1. Introduction Recently, air dehumidification techniques using adsorbents have been increasingly studied [1]. Energy-efficient adsorption systems using low-temperature exhaust heat, such as desiccant cooling systems, adsorption heat pumps, and humidity control systems have attracted great attention. The reason is that due to the environmental problems [2] energy-efficient technologies are indispensable for reducing the CO2 emission globally [3]. Highly ordered mesoporous silica (MPS) has attracted growing attention as an adsorbent. With uniformly arranged mesopores (1.5–50 nm), MPS has a large specific surface area and pore volume. It also has sufficient stability and good adsorption capacity for water vapor compared with conventional adsorbents such as zeolites and silica gels [4]. Furthermore, effective regeneration of MPS is possible at temperatures as low as around 50 °C, whereas a much higher temperature (about 200 °C) is needed for the regeneration of hydrophilic zeolites [5]. As a consequence, MPS is expected to possess a relatively large effective adsorption capacity in the low relative humidity (RH) range [6]. As another example of the material, Although its maximum adsorption amount is lower than MPS, the CHA-type structure of silicoaluminophosphate zeolite (SAPO-34) is also reported to have excellent adsorption-desorption properties at low RH [7]. The systems mentioned above can be operated using low-temperature exhaust heat, if MPS is employed. For practical applications, it is important to establish a method to fix
∗
these ordered MPS powders onto a substrate [8] which is in the form of a fin-shaped heat exchanger or a wheel with a honeycomb structure, and the adsorption and desorption of water vapor are repeated under a thermal cycle. Therefore, it is useful to prepare a thick MPS film on a light-weight module substrate that has high thermal conductivity. In general, sintering or calcination in the presence of a binder is used to fix ceramic powders. However, these methods have certain difficulties when applied to MPS, because the ordered porous structure of MPS can collapse at the high temperatures employed for the sintering process [9] while the addition of binders may reduce the adsorption-desorption properties [10]. Thus, it is necessary to establish other methods to fix ordered MPS powder onto substrates without degrading their unique porous structure and adsorption properties. For example, Kimura reported that the cellulose membranes coated/filled by ordered MPS possessed a sufficient adsorption capacity for water [3]. In this study, we used the electrophoretic deposition (EPD) method to fix MPS powder to a substrate. It is a colloidal process in which a DC electric field is applied across a suspension of charged particles, attracting the particles to an oppositely charged electrode. The EPD method has some advantages over other fabrication techniques, especially with regard to reducing the fabrication costs and maintaining the structural flexibility. Therefore, this technique has been widely used for ceramic processing in a variety of technical applications [11–15]. We have previously developed an EPD-based method for fixing MPS on metal substrates, such as stainless steel and/or aluminum plates, i)
Corresponding author. E-mail address: [email protected] (H. Negishi).
https://doi.org/10.1016/j.micromeso.2019.109710 Received 29 November 2017; Received in revised form 4 September 2019; Accepted 5 September 2019 Available online 07 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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without damaging its excellent adsorption-desorption properties [8,16–19] ii) examined the influence of water content in the EPD suspension [18], and iii) studied the formation mechanism of EPD layer [20]. However, a thick MPS film prepared by EPD without any binder is not sufficiently robust. Therefore, we had tried to add a minimal amount of binder to the EPD suspension to enhance the mechanical strength of the film. Since binder addition can reduce the adsorptiondesorption properties, we had to carefully select the kind of binder and its amount, choosing tetraethyl-orthosilicate (TEOS) as it is a widely used source for the synthesis of MPS. The prepared thick MPS film became harder, and high water vapor adsorption-desorption cyclic performance was obtained [21]. However, damage of the thick MPS film was observed upon contact with the jig during the three-point bending test. Hence, although the MPS film strength against vibration was increased after using the TEOS binder, the film was still fragile against physical contact. For that reason, it is thought that the film needs to be further strengthened for use as a component in the module. In order to achieve this, a binder with a higher flexibility is required. As mentioned above, although binder addition generally reduces the adsorption properties, the polymeric binder is thought to harden the film. Polyvinyl alcohol (PVA), polyvinyl butyral (PVB), etc. are well-known polymeric binders for create the green film of ceramic forming [22]. However, because PVA dissolves in water, it is not suitable in this study because MPS is used for adsorption of water. In contrast, PVB could be dissolved in organic solvents but not in water. In EPD, a binder does not need to undergo electrophoresis themselves, as long as it is contained in the deposited film. Then, the effect of the binder emerges as the deposited film dries. For example, in the preparation of solid oxide fuel cell [15] and/or oxygen permeation membrane [23] PVB was added to acetone-based EPD suspension as a binder. In this study, we investigated the effects of adding PVB instead of TEOS as a binder in preparing robust, thick MPS film by EPD, as well as the adsorption-desorption properties of the resulting film. Accordingly, we carried out three-point bending test and examined the cyclic adsorption-desorption of water vapor using the thick MPS film, in order to confirm the increased mechanical strength and stability during thermal cycles which used reproduction (desorption) at high temperature.
Fig. 1. Electrophoretic deposition cell.19).
remove PVB by combustion. The nitrogen adsorption-desorption isotherms were measured at 77 K using a fully automatic sorption isotherm measuring equipment (Belsorp-mini, BEL Japan, Inc., Osaka, Japan). The pretreatment was carried out at 300 °C for 5 h in a nitrogen flow. The pore size distributions were determined by application of the nonlocal density functional theory (NLDFT) method [21]. The particle size distribution was measured using a laser diffraction particle size analyzer (LA-950; Horiba Ltd., Kyoto, Japan). The surface morphology of MPS films was observed with a scanning electron microscope (SEM, S4800; Hitachi High-Technologies, Tokyo, Japan), and their cross-sectional morphology was observed using a confocal laser-scanning microscope (VK9500; Keyence Corporation, Osaka, Japan). The strength of the thick MPS film was evaluated with a digital force gauge (RX50 + Model 2257; Aikoh Engineering Co., Ltd, Osaka, Japan).
2.3. Water vapor adsorption-desorption experiments 2. Experimental The cyclic water vapor adsorption-desorption test was carried out as follows, using the handmade equipment shown in Fig. 2. The MPS film was attached to the axis of the Rotator (RT-5, Taitec Co., Ltd., Nagoya, Japan). The flow paths of humid air and warm air were formed around the axis of rotation. In the adsorption area, humid air at room temperature was supplied using a humidifier (HV–W30CX-A; Sharp Corporation, Osaka, Japan). The temperature, humidity, and wind velocity were checked for each experiment. In the desorption area, dry warm air at 50 or 60 °C was supplied by a dryer (EH5402 N; Panasonic Corporation, Osaka, Japan), and its temperature and humidity were
2.1. Electrophoretic deposition The EPD procedure is the same as that described in our previous report [8]. MPS powder (TMPS-1.5; Taiyo Kagaku Co., Yokkaichi, Japan) was used as the deposition material. The mean particle size is nearly 9 μm [21]. Polyvinyl butyral (PVB) was used as a binder [15,23]. The MPS powder suspension for EPD was prepared as follows. First, a proper amount of molecular sieves (3A, 1/16) was kept in acetone for a day [18]. PVB was dissolved in the dehydrated acetone to form 0.1, 0.3, and 0.5 wt% solutions. On the other hand, SiO2 powder (MPS) was heat-treated at 120 °C for 3 h. After cooling down to room temperature, the powder was added into the PVB solution and dispersed ultrasonically for 10 min. The cell configuration used in the EPD fabrication process is shown in Fig. 1. An aluminum plate (length: 50 mm, width: 5 mm, and thickness: 0.5 mm) was used as the deposition electrode, and two stainless steel plates of the same size were used as the counter electrodes. The electrode-electrode distance was 10 mm. A DV voltage of 100 V was applied using a DC power supply (TP0360-022D; Takasago Ltd., Kawasaki, Japan) between the substrate and the counter electrode. 2.2. Characterization of MPS The amounts of PVB in the MPS film were calculated from the weight difference before and after heat treatment at 500 °C for 12 h to
Fig. 2. Rotary water vapor adsorption-desorption equipment. 2
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Fig. 3. Deposit amount and the ratio of PVB between the MPS film and the EPD suspension for different initial PVB concentrations. EPD conditions: 6 g/L of MPS in acetone, 100 V, 2 min.
monitored with a thermo-hygrometer (SK-L200TH IIα; Sato Keiryoki Mfg. Co. Ltd., Tokyo, Japan). With a given rotational speed, the MPS film passes through the humid and warm air atmospheres continuously. The amounts of water adsorption and desorption were estimated from the weight change of the MPS film.
Fig. 4. 3D images of fractured cross sections of PVB-added MPS films using EPD baths with (a) 0.1 wt% PVB (P35D434) and (b) 0.5 wt% PVB (P80D645). EPD condition: 6 g/L MPS in acetone, 100 V, 2 min.
3. Results and discussion
P80D645 film, the deposit amount is 6.45 mg/cm3, and the corresponding film porosity is 44 vol%. Clearly, a high deposit amount corresponds to a thick film, and a high porosity is achieved with a lower PVB content. Therefore, in the EPD method, films with high porosity could be obtained, and their thickness is directly related to the deposit amount. The surface morphologies of P35D434 and P80D645 observed using a SEM are shown in Fig. 5 (a) and (b), respectively. First of all, there are many open pores on both films, and the deposition densities of MPS particles are not high. This is in good agreement with the film density data described above. Further, secondary particles (average size: 9 μm, as indicated by particle size distribution measurement [21]) were not observed. Hence, it is considered that the fine particles, not the large aggregates, were deposited, because many small particles less than 9 μm were present in the EPD bath based on the particle size distribution of the MPS powder (previous report [21]), and the outmost film surface was deposited 2 min into the EPD process. Compared to the P80D645 film, agglomerated particles of 1–2 μm were observed on a rough surface in the P35D434 film. Since the EPD conditions and MPS powder concentration are the same, the difference between these two samples attributed to the effects of PVB. The film surface is flat and smooth when the content of PVB is high, which is in good agreement with the results of Fig. 4. While the exact mechanism of PVB incorporation into the MPS film is interesting, a definite answer is yet to be obtained. Nevertheless, the effect of PVB can be considered as suppressing the re-aggregation of MPS powder particles that were ultrasonically dispersed. A higher amount of PVB means more PVB molecules between the dispersed MPS particles, making the formation of secondary particles less likely. Consequently, there is a higher proportion of smaller particles in the EPD bath, and the deposited film surface became smoother. Since the formation of secondary particles also reduces the MPS powder concentration by settling, suppression of their formation also increases the deposit amount. This is consistent with the measured deposit amount in Fig. 3. To confirm the increased film strength in the presence of PVB, we carried out a three-point bending test for MPS films prepared in 6 g/L MPS suspension with various PVB concentrations. In the test, a constant force (20 N) was applied to the MPS film, and its strength was
3.1. Characterization of MPS film A suspension of MPS powder (6.0 g/L) was formed in the PVBacetone solution (0.1, 0.3, and 0.5 wt% PVB), and the MPS film was prepared by EPD at 100 V for 2 min. These conditions are the same as in our previous paper that used TEOS-added acetone [21]. The deposit amount of MPS and the content of PVB in the film are shown in Fig. 3. Here, the average deposit amounts are shown with standard deviation in the error bars. As an example, the average deposit amounts prepared using acetone containing 0.1 and 0.5 wt% of PVB are 4.34 and 6.45 mg/ cm2, respectively. When the PVB concentration in the suspension increased, the deposit amount also increased in a linear relation, as indicated in Fig. 3. However, the fitting line does not pass the origin. It is clear that PVB is contained in the deposited MPS film. Here, when 100 V of DC voltage was applied to a solution of PVB in acetone without MPS powder, the aluminum substrate did not change in weight after drying. Therefore, PVB itself does not deposit on the aluminum substrate by the electric field. The following two mechanisms are possible for PVB incorporation into the MPS film. (1) PVB adhered to MPS particles in the solution, and the particles were deposited by EPD. (2) PVB in the solution was trapped in the space between deposited MPS particles. From Fig. 3, when 0.1 wt% PVB is dissolved in the suspension, 3.5 wt% PVB was included in the film that has a deposit amount of 4.34 mg/cm2. For simplicity, the produced film sample is denoted as P35D434 ("P" + (PVB content) × 10 + "D" + (deposit amount) × 100). When using 0.5 wt% PVB suspension, the sample is similarly coded P80D645. The fractured cross sections were examined using a confocal laser scanning microscope. Fig. 4 show the 3D images of two deposited thick MPS films, with part of the substrate shown in the lower part on the left. The deposited P35D434 and P80D645 films are almost flat, with thicknesses of approximately 80 and 100 μm, respectively. The deposit amount of the former film is 4.34 mg/cm2, with 3.5 wt% PVB (density 1.07 g/cm3) and 96.5 wt% MPS particles (density: 1.15 g/cm3). The volumes of PVB and MPS in a 1 cm2 substrate are 1.42 × 10−4 and 3.64 × 10−3 cm3, respectively. Since the total film volume is 3.78 × 10−3 cm3, this translates to a film porosity of 53 vol%. For the 3
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100 μm) and contained more PVB (8.0 wt%), it can be seen that a higher PVB content in the film increased the film strength against physical contact but could cause cracks in the bent film. Nevertheless, the film did not peel off from the substrate, showing that the film adhesion was improved by the addition of PVB. From the three-point bending test results, it is considered that the PVB binder is effective for immobilizing MPS powder on the substrate. Nitrogen adsorption-desorption isotherms of the deposited MPS films containing PVB are shown in Fig. 7(a). For the P35D434 film, the amount of nitrogen adsorption-desorption decreased about 20% in comparison with that containing only MPS powder. Here, by excluding the 3.5 wt% of PVB in the film, the drop in nitrogen adsorption capacity based on the MPS weight becomes 17%. For the P80D645 film, the reduction is about 30% in comparison with that without PVB. Since the film contained 8.0 wt% PVB, the actual decrease in nitrogen adsorption capacity is 24%. The reduction of nitrogen adsorption capacity with PVB incorporation suggests that PVB covered some mesopores in the MPS. However, the difference in adsorption between P35D434 and P80D645 films was only about 7%, while the film strength improved remarkably. Interestingly, the average size of mesopores in the MPS powder is approximately 2.5 nm, whereas those in the P35D434 and P80D645 films are slightly smaller (2.3 nm), as shown in Fig. 7(b). The change of the mesopore diameter suggests that the PVB not only adhered to the surface/interface of MPS particles, but also infiltrated the larger mesopores. 3.2. Water vapor adsorption-desorption of MPS film The water vapor adsorption-desorption properties were evaluated for MPS films containing 3.5 wt% and 8.0 wt% of PVB. Samples with different deposit amounts were prepared by adjusting the deposition time. Samples coded P35D096, P35D256, and P35D295 were thus prepared containing 3.5 wt% of PVB with deposit amounts of 0.96, 2.56, and 2.95 mg/cm2, respectively. For samples containing 8.0 wt% PVB, the deposit amounts were 1.44, 2.08, 2.89, and 4.50 mg/cm2 (P80D144, P80D208, P80D289, and P80D450, respectively). First, we measured the adsorption rate of water vapor: a fully dried MPS film was placed in a 55–60% RH atmosphere at 24–28 °C, and the mass change was measured. The time-dependent mass changes in the adsorption processes are shown in Fig. 8. The mass changes scaled by MPS weight are shown in Fig. 8(a) and (c). The initial adsorption is quick and slows down slowly with time. In case of the P35D096 film, 0.21 g water vapor was adsorbed per gram of MPS within 30 s, and the adsorption became saturated at 0.24 g afterwards. In the P35D295 film, the water vapor adsorption was 0.21 g-H2O/g-MPS within 80 s, and did not change after that. Almost the same values were observed in the films with 8.0 wt% of PVB. All films required less than 60 s to reach ca. 0.21 g-H2O/g-MPS, except for P80D450 (which took about 150 s). The water vapor adsorption is slower in the film with higher MPS deposit amount. Moreover, in all films the saturated water vapor adsorption was ca. 0.24 gH2O/g-MPS. It is thought that the amount of sedimentation during EPD is almost proportional to the film thickness [24]. Based on the results of Figs. 3 and 4, the MPS film thicknesses are 18, 47, and 54 μm for the P35D096, P35D256, and P35D295 films (containing 3.5 wt% PVB); and 22, 32, 45, and 70 μm for P80D144, P80D208, P80D289, and P80D450 (containing 8.0 wt% of PVB), respectively. When the MPS film becomes thicker, the adsorption time tends to be longer. However, other than the thickest film (70 μm), the times needed for adsorption are not very different. For example, for an adsorption time of 60 s, MPS films thinner than 54 μm are effectively used; while the bottom part of the 70 μm film is not utilized. The variation of water vapor adsorbed per surface area of the MPS film is shown in Fig. 8(b) and (d). Since the amounts of MPS deposition on each film (i.e. film thickness) are different, the final amounts of adsorption are all different. However, the initial time courses overlap considerably, indicating that the initial adsorption rate
Fig. 5. Surface SEM photographs of PVB-added MPS films with (a) 0.1 wt% PVB in the EPD bath (P35D434) and (b) 0.5 wt% PVB in the EPD bath (P80D645). EPD conditions: 6 g/L MPS in acetone, 100 V, 2 min.
Fig. 6. MPS films after three-point bending test: (a) film deposited with 0.1 wt% PVB in the EPD suspension (P35D434, 80 μm thickness), (b) film with 0.5 wt% PVB in the EPD suspension (P80D645, 100 μm thickness). EPD conditions: 6 g/L MPS in acetone, 100 V, 2 min.
investigated by peeling off the MPS film. Fig. 6 shows photographs of MPS films after the test. The P35D434 film did not peel off at the bending point (Fig. 6(a)). Even though this film was nearly 80 μm thick, it merely became curved along with the substrate, showing a sufficiently high strength against the bending. However, the film collapsed in the part where the jig contacted the film, which is hardly different from the case without PVB [21]. In contrast, the thicker P80D645 film neither peeled off at the bending point nor broke in the part of physical contact with the jig, as shown in Fig. 6(b). Nevertheless, cracks occurred in the bent part of this film. Since this film was thicker (ca. 4
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Fig. 7. (a) Nitrogen adsorption-desorption isotherms at 77 K and (b) pore size distribution curve calculated using the NLDFT method employing the cylindrical pore adsorption branch model. ●: MPS powder, ■: P35D434 film, and ▲: P80D645 film. Solid symbols denote the adsorption branch, and open symbols denote the desorption branch.
Fig. 9. Time courses of mass changes during the desorption of MPS films that contained (a) 3.5 wt% of PVB (P35D295) and (b) 8.0 wt% of PVB (P80D450). (a) ■: 60 °C, 20% RH, 1–2.5 m/s; □: 50 °C, 18% RH, 1–2.5 m/s. (b) ♦: 60 °C, 20% RH, 1–2.5 m/s; ◊: 50 °C, 20% RH, 1–2.5 m/s.
60 °C, desorption of water from the P35D295 film required about 60 s. At 60 °C, the desorption from the P80D450 film also took about 60 s, while the same process took more than 120 s at 50 °C. Therefore, the P80D450 film produced with the high deposit amount takes longer to adsorb and desorb water vapor (> 60 s). On the other hand, for films with lower deposit amount than P35D295, 60 s is sufficient for adsorption and desorption. The initial desorption rates of P35D295 at 50 °C and 60 °C, and P80D450 at 60 °C are −0.033 mg-H2O/cm2⋅s, and that of P80D450 film at 50 °C is −0.022 mg-H2O/cm2⋅s. Hence, at the temperature of 60 °C, the desorption rate is a constant unaffected by the PVB content and the amount of deposited MPS. At 50 °C, the desorption rate is reduced to about 2/3. The P35D295 and P80D450 films (thicknesses: 54 and 70 μm, respectively) both desorbed completely after around 60 s at 60 °C, and the 70 μm thick film also took around 60 s. On the other hand, the 70 μm film took more than 120 s to desorb at 50 °C. If the MPS film is thinner than 54 μm, the whole film can be used effectively for complete adsorption and desorption within 60 s, under the temperature and humidity conditions of this study; while the 70 μm film requires extra time for sufficient adsorption and desorption, thereby reducing the cycle number. The ability of dehumidification is closely related to the MPS film thickness and the cycle condition. Incidentally, the amount of water vapor adsorption v (g-H2O/g-MPS) = 0 in this study is not the value at zero relative pressure of the water vapor adsorption-desorption isotherm, but the dry weight under the temperature and humidity condition of Fig. 9. When the deposit amount of MPS and content of PVB in the film are both high, the time required for desorption at 50 °C becomes longer, as
Fig. 8. Time courses of mass changes in the adsorption processes on MPS films that contained (a, b) 3.5 wt% of PVB, and (c, d) 8.0 wt% of PVB, at 24–28 °C and 55–63% RH. (a, b) △: P35D096, ◊: P35D256, and □: P35D295. (c, d) ■: P80D144, ▼: P80D208, ▲: P80D289, and ♦: P80D450.
per unit surface area is unaffected by the deposit amount or PVB content of the film (all at approximately 1.3 × 10−2 mg-H2O/cm2⋅s). In other words, the water vapor adsorption rate is a constant at the film surface. Therefore, the time to reach maximum adsorption in Fig. 8 (a) and (c) is dependent on the film thickness. Then, we measured the desorption rate of water vapor. On the P35D295 and P80D450 films with saturated water vapor adsorption, the desorption experiment was carried out under 1–2.5 m/s of wind velocity at two conditions: 50 °C, 18–20% RH and 60 °C, 20% RH. The mass changes due to desorption over time are shown in Fig. 9. The initial desorption rate is high and slows down gradually. At 50 °C and 5
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Fig. 11. Temperature histories of aluminum plate moving in 60 s cycles between 60 °C, 17% RH and 25.5 °C, 46% RH. Table 1 Conditions of the cyclic adsorption-desorption examination.
Adsorption Desorption
Temperature [°C]
Relative humidity [%]
Wind velocity [m/s]
Time [s]
20–23 60
53–58 17
2–4* 0.5–3
60 60
*Wind velocity fluctuated due to the performance of humidification device.
are listed in Table 1. The temperature of disconnection was set to 60 °C. Based on the results of Figs. 8 and 9, the times of adsorption and desorption were set to 60 s each. In other words, the cycle number is 30 rph, which is within the general rotational speed range of desiccant wheels. The water vapor adsorption-desorption properties during 150 cycles are shown in Fig. 12. The amount of water vapor adsorbed in each cycle was 0.21 g-H2O/g-MPS on the P35D239 film (44 μm thick), and 0.12 gH2O/g-MPS on the P80D450 film (70 μm thick). The adsorption amount decreased with an increase in the amount of PVB. The P35D239 film exhibited no deterioration in the amounts of adsorption-desorption during the 150 cycles, while those for the P80D450 film first reduced slightly and then became constants. Note that the desorption did not reach 100%. Because the desorption was carried out at 17% RH for only 60 s, a certain amount of water is expected to remain in the MPS films.
Fig. 10. Time-dependent mass changes during the desorption from MPS films that contained 8.0 wt% of PVB (■: P80D144, ▼: P80D208, ▲: P80D289, and ●: P80D450). (a, b): desorption at 50 °C, 21% RH; (c, d): desorption at 60 °C, 18% RH.
shown in Fig. 9(b). The desorption behaviors of the films containing 8.0 wt% PVB with different deposit amounts are shown in Fig. 10. In Fig. 10(a), the desorption at 50 °C requires longer time for the film with high MPS deposition amount. However, since the profiles in Fig. 10(b) are almost parallel to each other, the desorption rate (−0.021 mg-H2O/ cm2⋅s) is constant regardless of the deposit amount. In other words, the film containing more MPS simply took longer to desorb the water. The desorption behaviors at 60 °C in the same films are shown in Fig. 10(c) and (d). Compared with the case of 50 °C, desorption was faster at a higher temperature in all films. Again, the desorption rates at 60 °C are similar to those at 50 °C and quite constant (about −0.037 mg-H2O/ cm2⋅s) regardless of the deposit amount. Therefore, when the desorption temperature is reduced to 50 from 60 °C, the desorption becomes slower regardless of the deposit amount of MPS. Like the temperature and time of adsorption/desorption, the PVB content and deposit amount of MPS in the film are important factors that affect the water vapor adsorption-desorption characteristics. The results from Figs. 8–10 are similar to the results by Yanagihara et al. [25] claims that "the transport mechanism of water in MPS could be liquid water flow due to capillary action", and it was speculated that adsorption and desorption behavior is the same between a single MPS particle and a deposited film of MPS particles. Then, the water vapor adsorption-desorption cycles were measured. First, the temperature histories of the uncoated aluminum plate during the cyclic adsorption-desorption are shown in Fig. 11. The atmospheres were 60 °C, 17% RH and 25.5 °C, 46% RH. The temperature of the aluminum substrate clearly responded well to the temperature change of the atmosphere, due to the high thermal conductivity of aluminum. The adsorption and desorption conditions in the cyclic measurement
Fig. 12. Adsorption-desorption cycling properties of MPS films. EPD conditions: 6 g/L MPS in acetone, 100 V, 2 min ■: film with 0.1 wt% PVB (P35D239), ♦: film with 0.5 wt% PVB (P80D450). Solid symbols denote the adsorption branch, and open symbols denote the desorption branch. 6
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4. Conclusions
Table 2 Cycle condition and amount of dehumidification. PVB in EPD suspension [wt%]
0.1
PVB in MPS films [wt%] Deposition area per 1 plate [cm2] Deposit amount of MPS [mg/cm2] Desorption atmosphere
3.5 8.0 3.39 3.64 2.39 4.50 60 °C, 17% RH 60 s 20–23 °C, 53–58% RH 60 s 120 s 0.21 0.12 30 6.3 3.6 151 162
Adsorption atmosphere Time of 1 cycle Adsorption amount in 1 cycle [g/g] Cycle number [rph] Amount of dehumidification [g/g] per 1 h Amount of dehumidification per unit substrate area [g/ m2h]
In this study, we investigated an electrophoretic deposition method with the addition of a small amount of polymer binder in the bath, in order to fabricate a thick MPS film with sufficient mechanical strength and thermal stability. The addition of 3.5–8.0 wt% PVB to the EPD bath significantly improved the mechanical strength of the film (up to 100 μm in thickness), although the amount of water vapor adsorption was slightly decreased. The prepared thick MPS films also exhibited good water vapor adsorption-desorption properties and sufficient stability for more than 150 adsorption-desorption cycles. While the optimal temperature, humidity, cycle conditions, etc. remain to be determined, a film thickness of around 50 μm was reasonable. These MPS films allow a dehumidification rate of 6.3 g-H2O/g-MPS per hour in 2 min adsorption-desorption cycles, even if an exhaust heat of only 60 °C is used to regenerate the adsorbent.
0.5
The water vapor adsorption-desorption properties of each film during the cycles correspond well to the results in Figs. 8 and 9. The surface morphology of the MPS films was found to be unchanged after the cyclic tests. Therefore, the thick MPS films prepared with a small amount of PVB possessed sufficient stability for more than 150 adsorption-desorption cycles. In addition, this cycle condition was reasonable for the MPS film of 44 μm, but was not suitable for the 70 μm thick film. The dehumidification properties obtained from the adsorption-desorption cyclic tests are summarized in Table 2. The amount of dehumidification per hour was 6.3 g-H2O/g-MPS on the P35D239 film, and only 3.6 g-H2O/g-MPS on the P80D450 film, although the latter contains more MPS per unit area. When the substrate area of each MPS film is converted to 1 m2, the corresponding amounts of dehumidification are similar (151 and 162 g-H2O/m2⋅h). Tashiro et al. reported that for silica gel and zeolite, the amounts of dehumidification per hour were 1.2 and 0.75 g-H2O/g-adsorbent, respectively, when using 25 °C at 65% RH for adsorption, 70 °C at 7% RH for desorption, and a cycle period of 4 min [26]. Compared with these reported values obtained using conventional adsorbents, the dehumidification ability of these MPS film fabricated by the EPD method is 3–5 times greater. These values are also approximately equal to those of MPS films containing TEOS binder from our previous report [21] but with PVB binder which is stronger than TEOS. From the above results, it can be concluded that the thick MPS films with PVB binder prepared by the EPD method possess excellent water vapor adsorption-desorption properties, and sufficient strength and stability for more than 150 adsorption-desorption cycles. Besides, the amount of water vapor adsorption-desorption is controllable by changing the cycle conditions, under operating conditions similar to the ones used in actual applications. While the dehumidification performance can be improved by adjusting the temperature, wind speed, and humidity, we did not optimize these factors in the current study. However, once the film structure, temperature, wind speed, and humidity are specified, approximate equations of adsorption and desorption rates can be obtained (for example, from Figs. 8–10). By using these, it is possible to quantitatively relate the adsorption amount (dehumidification amount) per cycle and the time required for one cycle, allowing one to set adsorption and desorption times for more efficient dehumidification per unit time. Additionally, in order to further improve the efficiency, it is necessary to adapt the film thickness and module structure to the temperature, air flow rate, and humidity conditions. It is clear that the EPD process is a promising method for fabricating MPS films on aluminum substrates as high-performance adsorption modules. Because this method allows the MPS film to be deposited on both sides of an aluminum plate, a compact module may be realized by using a complex substrate shape, such as a honeycomb structure.
Acknowledgement This study was supported by the Industrial Technology Research Grant Program from New Energy and Industrial Technology Development Organization (NEDO) of Japan (No. 06A29202d). Mesoporous silica powder (TMPS-1.5) was supplied by Taiyo Kagaku Co. Ltd. The authors thank Mrs. Natsue Okada of National Institute of Advanced Industrial Science and Technology (AIST) and Ms. Ai Miyamoto of Tokyo University of Science (TUS) for assisting with the EPD experiments. References [1] J.C. Atuonwu, G. van Straten, H.C. van Deventer, A.J.B. van Boxtel, Ind. Eng. Chem. Res. 52 (2013) 6201–6210 https://doi.org/10.1021/ie3030449. [2] S. Nakabayashi, K. Nagano, M. Nakamura, J. Togawa, A. Kurokawa, Adsorption 17 (2011) 675–686 https://doi.org/10.1007/s10450-011-9363-1. [3] T. Kimura, Phys. Chem. Chem. Phys. 15 (2013) 15056–15601 https://doi.org/10. 1039/c3cp52207e. [4] Y. Inagi, S. Fujisaki, A. Endo, T. Yamamoto, T. Ohmori, M. Nakaiwa, F. Matsuoka, Proceedings of ECOS 1–3 (2005) 881–887 Shaping our future energy systems 2005. [5] G.H.W. van Benthem, G. Cacciola, G. Restuccia, Heat Recovery Systems & CHP 15 (1995) 531–544 https://doi.org/10.1016/0890-4332(95)90063-2. [6] A. Endo, T. Miyata, T. Akiya, M. Nakaiwa, Y. Inagi, S. Nagamine, J. Mater. Sci. 39 (2004) 1117–1119 https://doi.org/10.1023/B:JMSC.0000012958.40071.50. [7] H. Kakiuchi, S. Shimooka, M. Iwade, K. Oshima, M. Yamazaki, S. Terada, H. Watanabe, T. Takewaki, Kagaku Kogaku Ronbunshu 31 (2005) 273–277 (in Japanese), https://doi.org/10.1252/kakoronbunshu.31.273. [8] H. Negishi, A. Endo, T. Ohmori, K. Sakaki, Ind. Eng. Chem. Res. 47 (2008) 7236–7241 https://doi.org/10.1021/ie071473i. [9] L.Y. Chen, S. Jaenicke, G.K. Chuah, Microporous Mater. 12 (1997) 323–330 https:// doi.org/10.1016/S0927-6513(97)00079-5. [10] G.J. Verhoeckx, N.J.M. van Leth, Electrophoretic deposition fundamentals and applications, The Electrochemical Society Proceedings, PV vols. 2002–21 The Electrochemical Society Inc., Pennington, NJ, 2002, pp. 118–127. [11] A.R. Boccaccini, I. Zhitomirsky, Curr. Opin. Solid State Mater. Sci. 6 (2002) 251–260 https://doi.org/10.1016/S1359-0286(02)00080-3. [12] N. Koura, H. Shoji, A. Morita, Mol. Cryst. Liq. Cryst. 184 (1990) 243–247 https:// doi.org/10.1080/00268949008031769. [13] T. Uchikoshi, T.S. Suzuki, H. Okuyama, Y. Sakka, P.S. Nicholson, J. Eur. Ceram. Soc. 24 (2004) 225–229 https://doi.org/10.1016/S0955-2219(03)00242-5. [14] I. Zhitomirsky, A. Petric, J. Eur. Ceram. Soc. 20 (2000) 2055–2061 https://doi.org/ 10.1016/S0955-2219(00)00098-4. [15] H. Negishi, N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, J. Electrochem. Soc. 147 (2000) 1682–1687 https://doi.org/10.1149/1.1393418. [16] H. Negishi, A. Endo, M. Nakaiwa, H. Yanagishita, Key Eng. Mater. 314 (2006) 147–152 https://doi.org/10.4028/www.scientific.net/KEM.314.147. [17] H. Negishi, A. Miyamoto, A. Endo, Y. Inagi, K. Sakaki, T. Ohmori, Proc. 2nd Int. Cong. Ceram. (ISTEC-CNR) 6P-089 (2008) 1–6. [18] H. Negishi, A. Endo, A. Miyamoto, K. Sakaki, T. Ohmori, Key Eng. Mater. 412 (2009) 171–176 https://doi.org/10.4028/www.scientific.net/KEM.412.171. [19] H. Negishi, A. Miyamoto, A. Endo, K. Sakaki, J. Ceram. Soc. Japan 119 (2011) 168–172 https://doi.org/10.2109/jcersj2.119.168. [20] A. Miyamoto, H. Negishi, A. Endo, B. Lu, K. Sakaki, T. Ohmori, H. Yanagishita, K. Watanabe, Key Eng. Mater. 412 (2009) 131–136 https://doi.org/10.4028/www. scientific.net/KEM.412.131. [21] H. Negishi, A. Miyamoto, A. Endo, Microporous Mesoporous Mater. 180 (2013) 250–256 https://doi.org/10.1016/j.micromeso.2013.06.040. [22] H. Negishi, T. Tsuru, K. Nouzaki, T. Kitazato, K. Sakaki, H. Yanagishita, Desalin.
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pmatsci.2006.07.001. [25] H. Yanagihara, K. Yamashita, A. Endo, H. Daiguji, J. Phys. Chem. C 117 (2013) 21795–21802 https://doi.org/10.1021/jp405623p. [26] Y. Tashiro, M. Kubo, Y. Katsumi, T. Meguro, J. Mater. Sci. Soc. Jpn. 38 (2001) 166–173 (in Japanese).
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Preparation and characterization of mesoporous silica–polyvinyl butyral hybrid coatings by electrophoretic deposition
T
Hideyuki Negishi∗, Yamaki Takehiro, Endo Akira Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Mesoporous silica Thick film Polyvinyl butyral Electrophoretic deposition Water vapor adsorption-desorption
Thick mesoporous silica (MPS) films with sufficient mechanical strength and high porosity were prepared on aluminum plates by using electrophoretic deposition (EPD) with the addition of a small amount of polyvinylbutyral (PVB, 3.5–8.0 wt%) as binder. The porosity of the MPS films was approximately 50 vol%. The film thickness could be controlled by the deposition conditions, and a thickness of ca. 50 μm is reasonable for effectively using the whole MPS film. These MPS films exhibited good water vapor adsorption-desorption properties and sufficient stability for more than 150 adsorption-desorption cycles. When used in dehumidification applications, these MPS films had a dehumidification rate of 6.3 g-H2O/g-MPS per hour, with an adsorptiondesorption cycle as short as 2 min even when a low temperature of 60 °C was used to regenerate the adsorbent.
1. Introduction Recently, air dehumidification techniques using adsorbents have been increasingly studied [1]. Energy-efficient adsorption systems using low-temperature exhaust heat, such as desiccant cooling systems, adsorption heat pumps, and humidity control systems have attracted great attention. The reason is that due to the environmental problems [2] energy-efficient technologies are indispensable for reducing the CO2 emission globally [3]. Highly ordered mesoporous silica (MPS) has attracted growing attention as an adsorbent. With uniformly arranged mesopores (1.5–50 nm), MPS has a large specific surface area and pore volume. It also has sufficient stability and good adsorption capacity for water vapor compared with conventional adsorbents such as zeolites and silica gels [4]. Furthermore, effective regeneration of MPS is possible at temperatures as low as around 50 °C, whereas a much higher temperature (about 200 °C) is needed for the regeneration of hydrophilic zeolites [5]. As a consequence, MPS is expected to possess a relatively large effective adsorption capacity in the low relative humidity (RH) range [6]. As another example of the material, Although its maximum adsorption amount is lower than MPS, the CHA-type structure of silicoaluminophosphate zeolite (SAPO-34) is also reported to have excellent adsorption-desorption properties at low RH [7]. The systems mentioned above can be operated using low-temperature exhaust heat, if MPS is employed. For practical applications, it is important to establish a method to fix
∗
these ordered MPS powders onto a substrate [8] which is in the form of a fin-shaped heat exchanger or a wheel with a honeycomb structure, and the adsorption and desorption of water vapor are repeated under a thermal cycle. Therefore, it is useful to prepare a thick MPS film on a light-weight module substrate that has high thermal conductivity. In general, sintering or calcination in the presence of a binder is used to fix ceramic powders. However, these methods have certain difficulties when applied to MPS, because the ordered porous structure of MPS can collapse at the high temperatures employed for the sintering process [9] while the addition of binders may reduce the adsorption-desorption properties [10]. Thus, it is necessary to establish other methods to fix ordered MPS powder onto substrates without degrading their unique porous structure and adsorption properties. For example, Kimura reported that the cellulose membranes coated/filled by ordered MPS possessed a sufficient adsorption capacity for water [3]. In this study, we used the electrophoretic deposition (EPD) method to fix MPS powder to a substrate. It is a colloidal process in which a DC electric field is applied across a suspension of charged particles, attracting the particles to an oppositely charged electrode. The EPD method has some advantages over other fabrication techniques, especially with regard to reducing the fabrication costs and maintaining the structural flexibility. Therefore, this technique has been widely used for ceramic processing in a variety of technical applications [11–15]. We have previously developed an EPD-based method for fixing MPS on metal substrates, such as stainless steel and/or aluminum plates, i)
Corresponding author. E-mail address: [email protected] (H. Negishi).
https://doi.org/10.1016/j.micromeso.2019.109710 Received 29 November 2017; Received in revised form 4 September 2019; Accepted 5 September 2019 Available online 07 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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without damaging its excellent adsorption-desorption properties [8,16–19] ii) examined the influence of water content in the EPD suspension [18], and iii) studied the formation mechanism of EPD layer [20]. However, a thick MPS film prepared by EPD without any binder is not sufficiently robust. Therefore, we had tried to add a minimal amount of binder to the EPD suspension to enhance the mechanical strength of the film. Since binder addition can reduce the adsorptiondesorption properties, we had to carefully select the kind of binder and its amount, choosing tetraethyl-orthosilicate (TEOS) as it is a widely used source for the synthesis of MPS. The prepared thick MPS film became harder, and high water vapor adsorption-desorption cyclic performance was obtained [21]. However, damage of the thick MPS film was observed upon contact with the jig during the three-point bending test. Hence, although the MPS film strength against vibration was increased after using the TEOS binder, the film was still fragile against physical contact. For that reason, it is thought that the film needs to be further strengthened for use as a component in the module. In order to achieve this, a binder with a higher flexibility is required. As mentioned above, although binder addition generally reduces the adsorption properties, the polymeric binder is thought to harden the film. Polyvinyl alcohol (PVA), polyvinyl butyral (PVB), etc. are well-known polymeric binders for create the green film of ceramic forming [22]. However, because PVA dissolves in water, it is not suitable in this study because MPS is used for adsorption of water. In contrast, PVB could be dissolved in organic solvents but not in water. In EPD, a binder does not need to undergo electrophoresis themselves, as long as it is contained in the deposited film. Then, the effect of the binder emerges as the deposited film dries. For example, in the preparation of solid oxide fuel cell [15] and/or oxygen permeation membrane [23] PVB was added to acetone-based EPD suspension as a binder. In this study, we investigated the effects of adding PVB instead of TEOS as a binder in preparing robust, thick MPS film by EPD, as well as the adsorption-desorption properties of the resulting film. Accordingly, we carried out three-point bending test and examined the cyclic adsorption-desorption of water vapor using the thick MPS film, in order to confirm the increased mechanical strength and stability during thermal cycles which used reproduction (desorption) at high temperature.
Fig. 1. Electrophoretic deposition cell.19).
remove PVB by combustion. The nitrogen adsorption-desorption isotherms were measured at 77 K using a fully automatic sorption isotherm measuring equipment (Belsorp-mini, BEL Japan, Inc., Osaka, Japan). The pretreatment was carried out at 300 °C for 5 h in a nitrogen flow. The pore size distributions were determined by application of the nonlocal density functional theory (NLDFT) method [21]. The particle size distribution was measured using a laser diffraction particle size analyzer (LA-950; Horiba Ltd., Kyoto, Japan). The surface morphology of MPS films was observed with a scanning electron microscope (SEM, S4800; Hitachi High-Technologies, Tokyo, Japan), and their cross-sectional morphology was observed using a confocal laser-scanning microscope (VK9500; Keyence Corporation, Osaka, Japan). The strength of the thick MPS film was evaluated with a digital force gauge (RX50 + Model 2257; Aikoh Engineering Co., Ltd, Osaka, Japan).
2.3. Water vapor adsorption-desorption experiments 2. Experimental The cyclic water vapor adsorption-desorption test was carried out as follows, using the handmade equipment shown in Fig. 2. The MPS film was attached to the axis of the Rotator (RT-5, Taitec Co., Ltd., Nagoya, Japan). The flow paths of humid air and warm air were formed around the axis of rotation. In the adsorption area, humid air at room temperature was supplied using a humidifier (HV–W30CX-A; Sharp Corporation, Osaka, Japan). The temperature, humidity, and wind velocity were checked for each experiment. In the desorption area, dry warm air at 50 or 60 °C was supplied by a dryer (EH5402 N; Panasonic Corporation, Osaka, Japan), and its temperature and humidity were
2.1. Electrophoretic deposition The EPD procedure is the same as that described in our previous report [8]. MPS powder (TMPS-1.5; Taiyo Kagaku Co., Yokkaichi, Japan) was used as the deposition material. The mean particle size is nearly 9 μm [21]. Polyvinyl butyral (PVB) was used as a binder [15,23]. The MPS powder suspension for EPD was prepared as follows. First, a proper amount of molecular sieves (3A, 1/16) was kept in acetone for a day [18]. PVB was dissolved in the dehydrated acetone to form 0.1, 0.3, and 0.5 wt% solutions. On the other hand, SiO2 powder (MPS) was heat-treated at 120 °C for 3 h. After cooling down to room temperature, the powder was added into the PVB solution and dispersed ultrasonically for 10 min. The cell configuration used in the EPD fabrication process is shown in Fig. 1. An aluminum plate (length: 50 mm, width: 5 mm, and thickness: 0.5 mm) was used as the deposition electrode, and two stainless steel plates of the same size were used as the counter electrodes. The electrode-electrode distance was 10 mm. A DV voltage of 100 V was applied using a DC power supply (TP0360-022D; Takasago Ltd., Kawasaki, Japan) between the substrate and the counter electrode. 2.2. Characterization of MPS The amounts of PVB in the MPS film were calculated from the weight difference before and after heat treatment at 500 °C for 12 h to
Fig. 2. Rotary water vapor adsorption-desorption equipment. 2
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Fig. 3. Deposit amount and the ratio of PVB between the MPS film and the EPD suspension for different initial PVB concentrations. EPD conditions: 6 g/L of MPS in acetone, 100 V, 2 min.
monitored with a thermo-hygrometer (SK-L200TH IIα; Sato Keiryoki Mfg. Co. Ltd., Tokyo, Japan). With a given rotational speed, the MPS film passes through the humid and warm air atmospheres continuously. The amounts of water adsorption and desorption were estimated from the weight change of the MPS film.
Fig. 4. 3D images of fractured cross sections of PVB-added MPS films using EPD baths with (a) 0.1 wt% PVB (P35D434) and (b) 0.5 wt% PVB (P80D645). EPD condition: 6 g/L MPS in acetone, 100 V, 2 min.
3. Results and discussion
P80D645 film, the deposit amount is 6.45 mg/cm3, and the corresponding film porosity is 44 vol%. Clearly, a high deposit amount corresponds to a thick film, and a high porosity is achieved with a lower PVB content. Therefore, in the EPD method, films with high porosity could be obtained, and their thickness is directly related to the deposit amount. The surface morphologies of P35D434 and P80D645 observed using a SEM are shown in Fig. 5 (a) and (b), respectively. First of all, there are many open pores on both films, and the deposition densities of MPS particles are not high. This is in good agreement with the film density data described above. Further, secondary particles (average size: 9 μm, as indicated by particle size distribution measurement [21]) were not observed. Hence, it is considered that the fine particles, not the large aggregates, were deposited, because many small particles less than 9 μm were present in the EPD bath based on the particle size distribution of the MPS powder (previous report [21]), and the outmost film surface was deposited 2 min into the EPD process. Compared to the P80D645 film, agglomerated particles of 1–2 μm were observed on a rough surface in the P35D434 film. Since the EPD conditions and MPS powder concentration are the same, the difference between these two samples attributed to the effects of PVB. The film surface is flat and smooth when the content of PVB is high, which is in good agreement with the results of Fig. 4. While the exact mechanism of PVB incorporation into the MPS film is interesting, a definite answer is yet to be obtained. Nevertheless, the effect of PVB can be considered as suppressing the re-aggregation of MPS powder particles that were ultrasonically dispersed. A higher amount of PVB means more PVB molecules between the dispersed MPS particles, making the formation of secondary particles less likely. Consequently, there is a higher proportion of smaller particles in the EPD bath, and the deposited film surface became smoother. Since the formation of secondary particles also reduces the MPS powder concentration by settling, suppression of their formation also increases the deposit amount. This is consistent with the measured deposit amount in Fig. 3. To confirm the increased film strength in the presence of PVB, we carried out a three-point bending test for MPS films prepared in 6 g/L MPS suspension with various PVB concentrations. In the test, a constant force (20 N) was applied to the MPS film, and its strength was
3.1. Characterization of MPS film A suspension of MPS powder (6.0 g/L) was formed in the PVBacetone solution (0.1, 0.3, and 0.5 wt% PVB), and the MPS film was prepared by EPD at 100 V for 2 min. These conditions are the same as in our previous paper that used TEOS-added acetone [21]. The deposit amount of MPS and the content of PVB in the film are shown in Fig. 3. Here, the average deposit amounts are shown with standard deviation in the error bars. As an example, the average deposit amounts prepared using acetone containing 0.1 and 0.5 wt% of PVB are 4.34 and 6.45 mg/ cm2, respectively. When the PVB concentration in the suspension increased, the deposit amount also increased in a linear relation, as indicated in Fig. 3. However, the fitting line does not pass the origin. It is clear that PVB is contained in the deposited MPS film. Here, when 100 V of DC voltage was applied to a solution of PVB in acetone without MPS powder, the aluminum substrate did not change in weight after drying. Therefore, PVB itself does not deposit on the aluminum substrate by the electric field. The following two mechanisms are possible for PVB incorporation into the MPS film. (1) PVB adhered to MPS particles in the solution, and the particles were deposited by EPD. (2) PVB in the solution was trapped in the space between deposited MPS particles. From Fig. 3, when 0.1 wt% PVB is dissolved in the suspension, 3.5 wt% PVB was included in the film that has a deposit amount of 4.34 mg/cm2. For simplicity, the produced film sample is denoted as P35D434 ("P" + (PVB content) × 10 + "D" + (deposit amount) × 100). When using 0.5 wt% PVB suspension, the sample is similarly coded P80D645. The fractured cross sections were examined using a confocal laser scanning microscope. Fig. 4 show the 3D images of two deposited thick MPS films, with part of the substrate shown in the lower part on the left. The deposited P35D434 and P80D645 films are almost flat, with thicknesses of approximately 80 and 100 μm, respectively. The deposit amount of the former film is 4.34 mg/cm2, with 3.5 wt% PVB (density 1.07 g/cm3) and 96.5 wt% MPS particles (density: 1.15 g/cm3). The volumes of PVB and MPS in a 1 cm2 substrate are 1.42 × 10−4 and 3.64 × 10−3 cm3, respectively. Since the total film volume is 3.78 × 10−3 cm3, this translates to a film porosity of 53 vol%. For the 3
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100 μm) and contained more PVB (8.0 wt%), it can be seen that a higher PVB content in the film increased the film strength against physical contact but could cause cracks in the bent film. Nevertheless, the film did not peel off from the substrate, showing that the film adhesion was improved by the addition of PVB. From the three-point bending test results, it is considered that the PVB binder is effective for immobilizing MPS powder on the substrate. Nitrogen adsorption-desorption isotherms of the deposited MPS films containing PVB are shown in Fig. 7(a). For the P35D434 film, the amount of nitrogen adsorption-desorption decreased about 20% in comparison with that containing only MPS powder. Here, by excluding the 3.5 wt% of PVB in the film, the drop in nitrogen adsorption capacity based on the MPS weight becomes 17%. For the P80D645 film, the reduction is about 30% in comparison with that without PVB. Since the film contained 8.0 wt% PVB, the actual decrease in nitrogen adsorption capacity is 24%. The reduction of nitrogen adsorption capacity with PVB incorporation suggests that PVB covered some mesopores in the MPS. However, the difference in adsorption between P35D434 and P80D645 films was only about 7%, while the film strength improved remarkably. Interestingly, the average size of mesopores in the MPS powder is approximately 2.5 nm, whereas those in the P35D434 and P80D645 films are slightly smaller (2.3 nm), as shown in Fig. 7(b). The change of the mesopore diameter suggests that the PVB not only adhered to the surface/interface of MPS particles, but also infiltrated the larger mesopores. 3.2. Water vapor adsorption-desorption of MPS film The water vapor adsorption-desorption properties were evaluated for MPS films containing 3.5 wt% and 8.0 wt% of PVB. Samples with different deposit amounts were prepared by adjusting the deposition time. Samples coded P35D096, P35D256, and P35D295 were thus prepared containing 3.5 wt% of PVB with deposit amounts of 0.96, 2.56, and 2.95 mg/cm2, respectively. For samples containing 8.0 wt% PVB, the deposit amounts were 1.44, 2.08, 2.89, and 4.50 mg/cm2 (P80D144, P80D208, P80D289, and P80D450, respectively). First, we measured the adsorption rate of water vapor: a fully dried MPS film was placed in a 55–60% RH atmosphere at 24–28 °C, and the mass change was measured. The time-dependent mass changes in the adsorption processes are shown in Fig. 8. The mass changes scaled by MPS weight are shown in Fig. 8(a) and (c). The initial adsorption is quick and slows down slowly with time. In case of the P35D096 film, 0.21 g water vapor was adsorbed per gram of MPS within 30 s, and the adsorption became saturated at 0.24 g afterwards. In the P35D295 film, the water vapor adsorption was 0.21 g-H2O/g-MPS within 80 s, and did not change after that. Almost the same values were observed in the films with 8.0 wt% of PVB. All films required less than 60 s to reach ca. 0.21 g-H2O/g-MPS, except for P80D450 (which took about 150 s). The water vapor adsorption is slower in the film with higher MPS deposit amount. Moreover, in all films the saturated water vapor adsorption was ca. 0.24 gH2O/g-MPS. It is thought that the amount of sedimentation during EPD is almost proportional to the film thickness [24]. Based on the results of Figs. 3 and 4, the MPS film thicknesses are 18, 47, and 54 μm for the P35D096, P35D256, and P35D295 films (containing 3.5 wt% PVB); and 22, 32, 45, and 70 μm for P80D144, P80D208, P80D289, and P80D450 (containing 8.0 wt% of PVB), respectively. When the MPS film becomes thicker, the adsorption time tends to be longer. However, other than the thickest film (70 μm), the times needed for adsorption are not very different. For example, for an adsorption time of 60 s, MPS films thinner than 54 μm are effectively used; while the bottom part of the 70 μm film is not utilized. The variation of water vapor adsorbed per surface area of the MPS film is shown in Fig. 8(b) and (d). Since the amounts of MPS deposition on each film (i.e. film thickness) are different, the final amounts of adsorption are all different. However, the initial time courses overlap considerably, indicating that the initial adsorption rate
Fig. 5. Surface SEM photographs of PVB-added MPS films with (a) 0.1 wt% PVB in the EPD bath (P35D434) and (b) 0.5 wt% PVB in the EPD bath (P80D645). EPD conditions: 6 g/L MPS in acetone, 100 V, 2 min.
Fig. 6. MPS films after three-point bending test: (a) film deposited with 0.1 wt% PVB in the EPD suspension (P35D434, 80 μm thickness), (b) film with 0.5 wt% PVB in the EPD suspension (P80D645, 100 μm thickness). EPD conditions: 6 g/L MPS in acetone, 100 V, 2 min.
investigated by peeling off the MPS film. Fig. 6 shows photographs of MPS films after the test. The P35D434 film did not peel off at the bending point (Fig. 6(a)). Even though this film was nearly 80 μm thick, it merely became curved along with the substrate, showing a sufficiently high strength against the bending. However, the film collapsed in the part where the jig contacted the film, which is hardly different from the case without PVB [21]. In contrast, the thicker P80D645 film neither peeled off at the bending point nor broke in the part of physical contact with the jig, as shown in Fig. 6(b). Nevertheless, cracks occurred in the bent part of this film. Since this film was thicker (ca. 4
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Fig. 7. (a) Nitrogen adsorption-desorption isotherms at 77 K and (b) pore size distribution curve calculated using the NLDFT method employing the cylindrical pore adsorption branch model. ●: MPS powder, ■: P35D434 film, and ▲: P80D645 film. Solid symbols denote the adsorption branch, and open symbols denote the desorption branch.
Fig. 9. Time courses of mass changes during the desorption of MPS films that contained (a) 3.5 wt% of PVB (P35D295) and (b) 8.0 wt% of PVB (P80D450). (a) ■: 60 °C, 20% RH, 1–2.5 m/s; □: 50 °C, 18% RH, 1–2.5 m/s. (b) ♦: 60 °C, 20% RH, 1–2.5 m/s; ◊: 50 °C, 20% RH, 1–2.5 m/s.
60 °C, desorption of water from the P35D295 film required about 60 s. At 60 °C, the desorption from the P80D450 film also took about 60 s, while the same process took more than 120 s at 50 °C. Therefore, the P80D450 film produced with the high deposit amount takes longer to adsorb and desorb water vapor (> 60 s). On the other hand, for films with lower deposit amount than P35D295, 60 s is sufficient for adsorption and desorption. The initial desorption rates of P35D295 at 50 °C and 60 °C, and P80D450 at 60 °C are −0.033 mg-H2O/cm2⋅s, and that of P80D450 film at 50 °C is −0.022 mg-H2O/cm2⋅s. Hence, at the temperature of 60 °C, the desorption rate is a constant unaffected by the PVB content and the amount of deposited MPS. At 50 °C, the desorption rate is reduced to about 2/3. The P35D295 and P80D450 films (thicknesses: 54 and 70 μm, respectively) both desorbed completely after around 60 s at 60 °C, and the 70 μm thick film also took around 60 s. On the other hand, the 70 μm film took more than 120 s to desorb at 50 °C. If the MPS film is thinner than 54 μm, the whole film can be used effectively for complete adsorption and desorption within 60 s, under the temperature and humidity conditions of this study; while the 70 μm film requires extra time for sufficient adsorption and desorption, thereby reducing the cycle number. The ability of dehumidification is closely related to the MPS film thickness and the cycle condition. Incidentally, the amount of water vapor adsorption v (g-H2O/g-MPS) = 0 in this study is not the value at zero relative pressure of the water vapor adsorption-desorption isotherm, but the dry weight under the temperature and humidity condition of Fig. 9. When the deposit amount of MPS and content of PVB in the film are both high, the time required for desorption at 50 °C becomes longer, as
Fig. 8. Time courses of mass changes in the adsorption processes on MPS films that contained (a, b) 3.5 wt% of PVB, and (c, d) 8.0 wt% of PVB, at 24–28 °C and 55–63% RH. (a, b) △: P35D096, ◊: P35D256, and □: P35D295. (c, d) ■: P80D144, ▼: P80D208, ▲: P80D289, and ♦: P80D450.
per unit surface area is unaffected by the deposit amount or PVB content of the film (all at approximately 1.3 × 10−2 mg-H2O/cm2⋅s). In other words, the water vapor adsorption rate is a constant at the film surface. Therefore, the time to reach maximum adsorption in Fig. 8 (a) and (c) is dependent on the film thickness. Then, we measured the desorption rate of water vapor. On the P35D295 and P80D450 films with saturated water vapor adsorption, the desorption experiment was carried out under 1–2.5 m/s of wind velocity at two conditions: 50 °C, 18–20% RH and 60 °C, 20% RH. The mass changes due to desorption over time are shown in Fig. 9. The initial desorption rate is high and slows down gradually. At 50 °C and 5
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Fig. 11. Temperature histories of aluminum plate moving in 60 s cycles between 60 °C, 17% RH and 25.5 °C, 46% RH. Table 1 Conditions of the cyclic adsorption-desorption examination.
Adsorption Desorption
Temperature [°C]
Relative humidity [%]
Wind velocity [m/s]
Time [s]
20–23 60
53–58 17
2–4* 0.5–3
60 60
*Wind velocity fluctuated due to the performance of humidification device.
are listed in Table 1. The temperature of disconnection was set to 60 °C. Based on the results of Figs. 8 and 9, the times of adsorption and desorption were set to 60 s each. In other words, the cycle number is 30 rph, which is within the general rotational speed range of desiccant wheels. The water vapor adsorption-desorption properties during 150 cycles are shown in Fig. 12. The amount of water vapor adsorbed in each cycle was 0.21 g-H2O/g-MPS on the P35D239 film (44 μm thick), and 0.12 gH2O/g-MPS on the P80D450 film (70 μm thick). The adsorption amount decreased with an increase in the amount of PVB. The P35D239 film exhibited no deterioration in the amounts of adsorption-desorption during the 150 cycles, while those for the P80D450 film first reduced slightly and then became constants. Note that the desorption did not reach 100%. Because the desorption was carried out at 17% RH for only 60 s, a certain amount of water is expected to remain in the MPS films.
Fig. 10. Time-dependent mass changes during the desorption from MPS films that contained 8.0 wt% of PVB (■: P80D144, ▼: P80D208, ▲: P80D289, and ●: P80D450). (a, b): desorption at 50 °C, 21% RH; (c, d): desorption at 60 °C, 18% RH.
shown in Fig. 9(b). The desorption behaviors of the films containing 8.0 wt% PVB with different deposit amounts are shown in Fig. 10. In Fig. 10(a), the desorption at 50 °C requires longer time for the film with high MPS deposition amount. However, since the profiles in Fig. 10(b) are almost parallel to each other, the desorption rate (−0.021 mg-H2O/ cm2⋅s) is constant regardless of the deposit amount. In other words, the film containing more MPS simply took longer to desorb the water. The desorption behaviors at 60 °C in the same films are shown in Fig. 10(c) and (d). Compared with the case of 50 °C, desorption was faster at a higher temperature in all films. Again, the desorption rates at 60 °C are similar to those at 50 °C and quite constant (about −0.037 mg-H2O/ cm2⋅s) regardless of the deposit amount. Therefore, when the desorption temperature is reduced to 50 from 60 °C, the desorption becomes slower regardless of the deposit amount of MPS. Like the temperature and time of adsorption/desorption, the PVB content and deposit amount of MPS in the film are important factors that affect the water vapor adsorption-desorption characteristics. The results from Figs. 8–10 are similar to the results by Yanagihara et al. [25] claims that "the transport mechanism of water in MPS could be liquid water flow due to capillary action", and it was speculated that adsorption and desorption behavior is the same between a single MPS particle and a deposited film of MPS particles. Then, the water vapor adsorption-desorption cycles were measured. First, the temperature histories of the uncoated aluminum plate during the cyclic adsorption-desorption are shown in Fig. 11. The atmospheres were 60 °C, 17% RH and 25.5 °C, 46% RH. The temperature of the aluminum substrate clearly responded well to the temperature change of the atmosphere, due to the high thermal conductivity of aluminum. The adsorption and desorption conditions in the cyclic measurement
Fig. 12. Adsorption-desorption cycling properties of MPS films. EPD conditions: 6 g/L MPS in acetone, 100 V, 2 min ■: film with 0.1 wt% PVB (P35D239), ♦: film with 0.5 wt% PVB (P80D450). Solid symbols denote the adsorption branch, and open symbols denote the desorption branch. 6
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4. Conclusions
Table 2 Cycle condition and amount of dehumidification. PVB in EPD suspension [wt%]
0.1
PVB in MPS films [wt%] Deposition area per 1 plate [cm2] Deposit amount of MPS [mg/cm2] Desorption atmosphere
3.5 8.0 3.39 3.64 2.39 4.50 60 °C, 17% RH 60 s 20–23 °C, 53–58% RH 60 s 120 s 0.21 0.12 30 6.3 3.6 151 162
Adsorption atmosphere Time of 1 cycle Adsorption amount in 1 cycle [g/g] Cycle number [rph] Amount of dehumidification [g/g] per 1 h Amount of dehumidification per unit substrate area [g/ m2h]
In this study, we investigated an electrophoretic deposition method with the addition of a small amount of polymer binder in the bath, in order to fabricate a thick MPS film with sufficient mechanical strength and thermal stability. The addition of 3.5–8.0 wt% PVB to the EPD bath significantly improved the mechanical strength of the film (up to 100 μm in thickness), although the amount of water vapor adsorption was slightly decreased. The prepared thick MPS films also exhibited good water vapor adsorption-desorption properties and sufficient stability for more than 150 adsorption-desorption cycles. While the optimal temperature, humidity, cycle conditions, etc. remain to be determined, a film thickness of around 50 μm was reasonable. These MPS films allow a dehumidification rate of 6.3 g-H2O/g-MPS per hour in 2 min adsorption-desorption cycles, even if an exhaust heat of only 60 °C is used to regenerate the adsorbent.
0.5
The water vapor adsorption-desorption properties of each film during the cycles correspond well to the results in Figs. 8 and 9. The surface morphology of the MPS films was found to be unchanged after the cyclic tests. Therefore, the thick MPS films prepared with a small amount of PVB possessed sufficient stability for more than 150 adsorption-desorption cycles. In addition, this cycle condition was reasonable for the MPS film of 44 μm, but was not suitable for the 70 μm thick film. The dehumidification properties obtained from the adsorption-desorption cyclic tests are summarized in Table 2. The amount of dehumidification per hour was 6.3 g-H2O/g-MPS on the P35D239 film, and only 3.6 g-H2O/g-MPS on the P80D450 film, although the latter contains more MPS per unit area. When the substrate area of each MPS film is converted to 1 m2, the corresponding amounts of dehumidification are similar (151 and 162 g-H2O/m2⋅h). Tashiro et al. reported that for silica gel and zeolite, the amounts of dehumidification per hour were 1.2 and 0.75 g-H2O/g-adsorbent, respectively, when using 25 °C at 65% RH for adsorption, 70 °C at 7% RH for desorption, and a cycle period of 4 min [26]. Compared with these reported values obtained using conventional adsorbents, the dehumidification ability of these MPS film fabricated by the EPD method is 3–5 times greater. These values are also approximately equal to those of MPS films containing TEOS binder from our previous report [21] but with PVB binder which is stronger than TEOS. From the above results, it can be concluded that the thick MPS films with PVB binder prepared by the EPD method possess excellent water vapor adsorption-desorption properties, and sufficient strength and stability for more than 150 adsorption-desorption cycles. Besides, the amount of water vapor adsorption-desorption is controllable by changing the cycle conditions, under operating conditions similar to the ones used in actual applications. While the dehumidification performance can be improved by adjusting the temperature, wind speed, and humidity, we did not optimize these factors in the current study. However, once the film structure, temperature, wind speed, and humidity are specified, approximate equations of adsorption and desorption rates can be obtained (for example, from Figs. 8–10). By using these, it is possible to quantitatively relate the adsorption amount (dehumidification amount) per cycle and the time required for one cycle, allowing one to set adsorption and desorption times for more efficient dehumidification per unit time. Additionally, in order to further improve the efficiency, it is necessary to adapt the film thickness and module structure to the temperature, air flow rate, and humidity conditions. It is clear that the EPD process is a promising method for fabricating MPS films on aluminum substrates as high-performance adsorption modules. Because this method allows the MPS film to be deposited on both sides of an aluminum plate, a compact module may be realized by using a complex substrate shape, such as a honeycomb structure.
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