Drying of Coated Slurry in Vapor of Drying Solvent

Journal of Chemical Engineering of Japan, Vol. 43, No. 10, pp. 892 900, 2010 Short Research Communication Regular Paper Issue Drying of Coated Slurry in Vapor of Drying Solvent Yoshiyuki KOMODA 1, Ryota TAKEUCHI 1, Hironobu NISHIMURA 2, Masashi HIROMITSU 2, Takanori OBOSHI 2 and Hiroshi SUZUKI 1 1 Department of Chemical Science and Engineering, Kobe University, 1-1, Rokkodai-cho, Nada-ku, Kobe-shi, Hyogo 657-8501, Japan 2 Kansai Laboratory, Research and Development Center, Dai Nippon Printing Co., Ltd., Kami-gyoubu cho, Ukyo-ku, Kyoto-shi, Kyoto 616-8533, Japan Keywords: Pressure Controlled Drying, Saturation Vapor Pressure, Temperature Change Method, Particle Packed Structure Various thin films are produced by coating and drying of slurries. Cracks and fractures in the film, which frequently are encountered under a large drying rate condition, can be avoided by a reduction in drying rate. In this work, we propose a novel drying technique (pressure controlled drying), where the coated slurry is dried in a container filled with vapor of the drying solvent. If the inner pressure of the container is held reasonably higher than the saturation vapor pressure of the solvent, the drying rate can be successfully depressed. It was also turned out that the residual amount of the solvent in the coated slurry before and after holding the pressure has to be carefully controlled in order to work this drying technique effectively. The residual solvent is affected mainly not only by the time of holding the pressure but also by the container volume, decompressing capacity, and co-drying solvent. When the drying rate could be suppressed till the end of the constant drying rate period, the resultant film had a tightly packed cellular pattern and a smooth surface. This is probably due to the enhancement of particle arrangement with sufficient amount of the solvent and long drying time. Received on May 21, 2010; accepted on July 20, 2010 Correspondence concerning this article should be addressed to Y. Komoda (E-mail address: komoda@kobe-u.ac.jp). Introduction Thin films in the field of electronics, for example, battery electrodes, capacitor electrodes, magnetic tapes, have been manufactured by an age-old technique. That is, particles are mixed with some kinds of polymer in an aqueous or organic solvent solution, and the resultant slurry is coated on a substrate and dried. In the background with the demand for clean and efficient automobiles, the performances of fuel cells, Li-ion batteries, and super capacitors are now evolving day by day. Although the most active discipline is still the development of new materials, researchers frequently encounter difficulty in increasing efficiency due to the electrochemical limitation. In these days, the control of the internal structure of the electrodes has attracted attention because various properties often show drastic changes with the structure in spite of being the same materials. The electrode of a Polymer Electrolyte Fuel Cell (PEFC) is usually produced by coating and drying a slurry containing catalyst loaded nano-sized carbon particles and a polymer electrolyte solution. The percolations of carbon particles and networks of polymer electrolyte molecules are indispensable for electron and proton transport. The dispersed state of the particles in the slurry is reported to have a strong relationship with the uniformity of the dried product (Mustafa et al., 2004; Komoda et al., 2007). Additionally, a porous structure will also be required to carry material and product gases throughout the electrodes effectively (Reshetenko et al., 2007). Many researchers have pointed out that the internal structure of the electrode affects appreciably the cell performance (Passalacqua et al., 2001; Gode et al., 2003). However, it is difficult to control the structure in the manufacturing processes due to little knowledge on how and why the dispersed state changes. In the slurry preparation process, it was found that the mixing sequence of the materials affected the performance of a Li-ion battery (Kim et al., 1999). Also, the agglomerative characteristic of the particles in the slurry has a significant influence on the performance of PEFC (Komoda et al., 2009). The important factor in the coating process is the dispersion of agglomerates by shear flow. The void size in the dried electrode could be reduced by coating at high speed (Mustafa et al., 2004). In contrast, the drying process of the coated slurry has not been well researched because most slurries for electrodes contain a volatile solvent and usually dry quickly. Since drying is the final process to convert a well controlled slurry to the desired product, the drying process 892 Copyright 2010 The Society of Chemical Engineers, Japan

must be crucial for structure control as well as slurry preparation. In industrial drying processes, the operating conditions are sometimes changed during drying for an improvement in the structure. However, in ordinary cases these conditions are determined base on empirical knowledge. Therefore, the clear dependence of drying conditions changing during drying on the final structure will make the process design easier. In order to give clear suggestion for changing drying conditions, we have to firstly clarify the behavior with a drying condition change. Consequently, the determiner of the drying process and then the final structure of the coated slurry can emerge. Although the manufacturing process by coating and drying of a slurry is popular, the drying process has not been well studied and controlled. When drying slurries, we frequently encounter formation of fractures in the coated slurry. It is caused by the difference in the rate between drying and structure formation (Singh and Tirumkudulu, 2007). The stress development measurement by cantilever method is often applied to evaluate fracture formation (Wedin et al., 2005; Kim et al., 2009). From those results, we can learn that a rate of the drying less than or equal to that of structure formation is effective to avoid fractures and propagation. Drying rate is affected by numerous parameters, such as temperature, airflow rate and humidity. For an aqueous slurry, the drying rate is significantly influenced by the surrounding humidity (Briscoe et al., 1998). Drying under a humidity controlled condition is popular for the drying of food stuff. Similarly, even if the slurry contains a volatile organic solvent, the drying rate can be suppressed in an environment containing the vapor of the drying solvent. In this work, we investigate the drying process of the coated slurry dried in a nearly saturated vapor of the drying solvent. Here, this drying technique is named pressure controlled drying. The pressure controlled drying is composed of three stages; reducing drying pressure (first decompression), holding the pressure (constant pressure), and decompressing again to a much lower pressure (final decompression). Therefore, as well as the hold pressure, the durations of decompression and holding are controllable factors. By surveying the effects of various experimental conditions on the drying rate and process, we have clarified the factors controlling the process of the pressure controlled drying and also the effect on the final film structure. 1. Experiment 1.1 Composition and preparation of slurry There are two requirements for the slurry used. Firstly, the liquid phase of the slurry should be mainly composed of a volatile solvent to clearly elucidate the effect of a reduction in drying rate. Next, the suspended Fig. 2 Fig. 1 Apparent viscosity of sample slurry Experimental setup for drying: 1 PC, 2 Vacuum pump, 3 Pressure controller, 4 Pressure gauge, 5 Data logger, 6 Thermo couples, 7 Desiccator, 8 Coated slurry, 9 Co-drying solvent (IPA) particle must have a good affinity with the dissolved polymer to produce a homogeneous distribution of particle and polymer. Thus, we prepared a slurry consisting of silica particles (KE-P10, Nihon Shokubai Co., Ltd., d P 0.1 μm), polyethylene glycol (PEG) (WM 500,000), 2-propanol (IPA), and water. First of all, a 10 wt% aqueous PEG solution and silica-ipa slurry, the solid volume fraction of which is 0.04, were prepared using a magnetic stirrer and sonication. Afterwards, those were mixed homogeneously to obtain the sample slurry. The weight ratio of PEG to silica particles was set as 0.2. The volume fraction of silica particles in the resultant slurry was 3.5 vol%. The apparent viscosity of the slurry was measured by a stress controlled rheometer (MCR-301, Anton Paar GmBH) as shown in Figure 1. The decreasing trend in viscosity with increasing shear rate indicates that the particles form aggregates in the slurry. As stated in a previous work (Lan et al., 2006), the particles will form much larger aggregates in the coated slurry during drying. Furthermore, the particles will move as loosely packed aggregates in the coated slurry due to the flow of dispersing medium. 1.2 Drying procedure of coated slurry The slurry was coated on a glass plate by the doctor blade method in an area of 8 10 cm 2 with a coating gap be of 300 μm. The coated slurry was dried in the following steps. A slurry coated glass plate was placed into a desiccator with 1 g of IPA as shown in Figure 2. The internal pressure of the desiccator was controlled by a 893

Fig. 3 Variation of temperatures both on coated slurry and in desiccator, and pressure in desiccator in drying process at hold pressure of 70 hpa and hold time of 30 min, respectively vacuum pump and pressure controller. The desiccator was decompressed to a set pressure (hold pressure) and then the pressure was held for a set duration (hold time). Subsequently, the internal pressure of the desiccator was reduced below 20 hpa to remove the residual solvent. Actually, water cannot be completely removed in this drying procedure, but the effect of water evaporation on the structure can be neglected if the structure has fixed before the final decompression. The effective pumping speed of the vacuum pump was 2.2 10 4 m 3 /s in the experimental setup. The hold pressure and hold time were changed from 30 to 100 hpa and from 5 to 60 min, respectively. The drying process was monitored by the internal pressure and the temperatures of the desiccator and the coated slurry. Both temperatures were measured using thermocouples having a diameter of only 150 μm. Since the heat capacity of the thermocouples themselves is very small and the thermocouples were immersed in the thin coated slurry, the temperature variation during drying could be accurately measured. The surface of the film after drying was observed by a digital microscope (KH-1300, Hirox Co., Ltd). 2. Drying Behavior of Pressure Controlled Drying Various techniques have been proposed to measure the drying rate in a drying process. Gravimetry is the most popular, but difficult to apply to this system because the weight change of the coated slurry is very small and most of the balances cannot be used in the decompressed container filled with organic solvent vapor. Another prediction procedure of the drying rate using temperature data was proposed (Nishimura et al., 2007). The basis of their procedure is very clear. The temperature keeps a constant value during a constant drying rate period, and in falling drying rate period the sample temperature increases gradually. In the procedure to calculate the drying rate, various physical properties including enthalpy of evaporation and specific heat capacity have to be known in advance, but it is very difficult to measure the pressure dependence of those properties to apply to pressure controlled drying. Nevertheless, the temperature of the coated slurry will give us reliable information concerning the drying rate. The temperature difference between the slurry and surrounding is a measure of the drying rate because the temperature in the desiccator changes during drying. Typical changes of the temperatures and pressure are shown in Figure 3. In pressure controlled drying, the drying process is divided into three stages, i.e., initial decompression, constant pressure, and final decompression. In the initial decompression stage, the desiccator temperature decreased suddenly just after the beginning of decompression because of adiabatic expansion, and subsequently increased gradually due to an influx of heat from the outside of the desiccator. At the same time, not only the decrease in the desiccator temperature, but also the latent heat of solvent evaporation cooled down the coated slurry. In the constant pressure stage, the temperature difference frequently shows a constant value, although both temperatures are approaching the room temperature. The constant temperature difference indicates that the slurry temperature was determined not only by the latent heat of evaporation, but also by the heat influx from the surrounding, and corresponds to a constant drying rate period. If the desiccator pressure was kept at the hold pressure longer, the slurry temperature turned to increase more rapidly and then came near that of the desiccator. As a consequence, the temperature difference decreased gradually to zero, which means a falling drying rate period. Whether the falling rate period can be observed or not in the constant pressure stage depends on both the hold time and hold pressure. In the final decompression stage, when the temperature difference was still almost zero, the residual solvent in the wet coated slurry evaporates in this stage at a very low drying rate. On the other hand, if the falling rate period has not appeared or the temperature difference has not reduced to zero in the former stage, the temperature of the coated slurry meets that of surrounding in this stage. 894 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

The drying process of the coated slurry is dominated by evaporation from the surface, the mass transport in the wet film, and the heat flow from the outside. In the early stage of drying, the solvent has good accessibility to the drying interface due to the relatively large volume fraction of solvent. Thus, the drying rate is dominated by mass transport at the surface. By balancing the latent heat of evaporation and heat influx from the surroundings, the slurry temperature is determined. In a case of a large heat influx or low evaporation rate, the slurry temperature increases gradually, but shows a constant temperature difference from the surroundings. On the contrary, when the solvent evaporates quickly, the slurry is cooled down more significantly and will not show an increasing trend in temperature. As drying progresses, the suspended particles are packed and the drying solvent needs to pass through the voids in a particlepacked structure for evaporation. Therefore, as the particles are packed further and the void space becomes smaller, a mass transportation will be more suppressed. It leads to a gradual decrease in drying rate and corresponds to a falling drying rate period. It has been reported that crack formation and stress development occurs in the falling rate period (Lan and Xiao, 2006). Therefore, the beginning of the fall in drying rate indicates the formation of a particle-packed layer in which the small void space in the layer is filled with residual solvent. Thus, most of the solvent should be removed at the end of the constant pressure stage so as not to change the particle array anymore in the final decompression. If not, the suppressed drying rate in the constant pressure stage will be in vain because the final particle array will be mostly dominated by the quick evaporation at the beginning of the final decompression. One of the advantages of pressure controlled drying is that the drying rates both in constant and falling drying rate periods can be easily controlled by the hold pressure. If the drying rate in the constant rate period can be reduced, a well packed structure of the suspended particles will be formed in a wet and then dried film. In the determination of the operating conditions, we have to pay attention to the temperature difference, which reflects the drying rate. In the following sections, the effects of various drying conditions on the drying behavior, especially on the temperature difference, are shown and discussed. Fig. 4 Effect of co-drying solvent on the drying process 3. Effect of Controllable Conditions on Drying Behavior 3.1 Effect of co-drying solvent In the initial decompression stage, a part of the solvent in the coated slurry evaporates because of the decrease in the surrounding pressure. Especially under the condition of low hold pressure, a large amount of solvent is removed and the particle arrangement will be fixed before the constant pressure stage. In such a situation, the pressure control becomes meaningless because the particles cannot change their arrangement anymore. Consequently, a co-drying solvent was introduced in the desiccator with the coated slurry before the decompression, as shown in Figure 2, so as to reduce evaporation of the solvent from the coated slurry. The co-drying solvent is a solvent that mainly evaporates during drying and has the highest saturation vapor pressure among liquid components included. In the system investigated, where the solvent is composed of water and IPA, we chose IPA having higher vapor pressure. In the initial decompression stage, the co-drying IPA will evaporate superiorly to IPA in the coated slurry. As a result, the particle arrangement will proceed under a sufficient amount of the solvent in the constant drying rate period. For the purpose of confirming the effect of the codrying solvent, comparative drying experiments with and without co-drying IPA were conducted, and the results are shown in Figure 4. Since the saturated vapor pressure of IPA ranges from 24 (11 C) to 36 (17 C) hpa during drying, the hold pressure was set to 40 hpa, which is slightly higher than the saturation vapor pressure. The desiccator was decompressed again 10 min after zero temperature difference. Good agreement of the desiccator temperatures with and without co-drying IPA implies that the heat flow into the dessicator is constant. On the contrary, the slurry temperature showed quite different behavior. In the case of no co-drying IPA, the slurry temperature was cooled more in the initial decompression stage and became constant in the following constant pressure stage. It suggested that evaporation of VOL. 43 NO. 10 2010 895

co-drying IPA enables a decrease in the evaporation of solvent from the slurry. The constant temperature in the case of no co-drying IPA means that the heat leaving the slurry by solvent evaporation was large and comparable with that entering the slurry. We can conclude that the amount of evaporation with the first decompression and the drying rate in the constant pressure stage can be suppressed by using a codrying solvent. However, too large an amount of co-drying solvent requires a very long hold time and elongates the total drying time. In this case, the co-drying IPA remained at the beginning of the constant pressure stage and almost disappeared at the end. Therefore, we could confirm that the amount of IPA used in this work was appropriate. 3.2 Effects of hold pressure and hold time The inner pressure of the desiccator in the constant pressure stage (hold pressure) has a significant influence on the drying rate, and then the time required for the initial decompression. Further, the residual amount of the solvent in the coated slurry after holding the constant pressure can be controlled by the hold time. As referred to in the previous section, the fraction of solvent in the coated slurry is a fundamental factor, determining if the pressure controlled drying can work effectively or not. Therefore, the effects of hold pressure and hold time should be discussed in the same section. Drying behaviors at various hold pressures with a constant hold time of approximately 15 min are shown in Figure 5. Although the temperature variations at the hold pressures of 50 and 70 hpa were similar to that in Figure 3, quite different behaviors could be observed at the lowest or highest hold pressure condition. When the pressure of the desiccator was kept at 30 hpa, the temperature trend was similar to that without the co-drying solvent (Figure 4(a)). Since the pressure of the desiccator could not decrease monotonously because of the low pumping capacity, the constant pressure stage started virtually 3 min before holding the pressure. Additionally, because the hold pressure was almost the same with the saturation vapor pressure of IPA (23 hpa at 10 C), the drying rate became large and the slurry showed constant temperature in the constant pressure stage due to small heat influx. Therefore, fundamentally the hold pressure should be sufficiently higher than the saturation vapor pressure of the solvent so as to suppress the drying rate. In contrast, at the hold pressure of 100 hpa (Figure 5(d)), the slurry temperature dropped at the beginning of the final decompression and approached the ambient temperature in a short time. This indicates that the amount of the residual solvent in the slurry after the constant pressure stage was comparatively large and evaporated quickly by decompression. Paying attention to the temperature differences among experimental conditions, it was also found that the drying rate in the constant pressure stage could be suppressed by increasing the hold pressure. Fig. 5 Effect of hold pressure on drying behavior From the discussion above, it can be perceived that the appropriate hold time would be clearly dependent on the hold pressure. That is, if the hold time is much longer than 15 min at the hold pressure of 100 hpa, the temperature drop might be avoidable and an ordinal temperature trend would be obtained as shown in Figure 3. In Figure 6, the validity of the scenario was confirmed by varying the hold time at the hold pressure of 70 hpa. At the shortest hold time of 10 min (Figure 6(a)), the 896 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

Fig. 7 Change in drying behavior by reduction of pumping speed in final decompression by 75% Fig. 6 Effect of hold time on the drying behavior temperature drop was also observed at the beginning of the final decompression similarly to the case in Figure 5(d). Also, in the case of the hold time of 15 min (Figure 6(b)), careful examination of the slurry temperature revealed that the slurry temperature dropped slightly at the beginning of the final decompression although the temperature difference started decreasing before the end of the constant pressure. This result showed that residual solvent in the slurry could be evaporated quickly by the final decompression, even if the drying rate has begun to fall and the void in the particle-packed layer is filled with the solvent. At the condition of sufficient hold time (Figure 6(c)), no slurry temperature drop was observed in the second decompression. If the hold pressure is determined to be a reasonably higher than the saturation vapor pressure of the drying solvent, we can realize the reduced drying rate in the formation process of the particle packed structure and the minimum drying time. 3.3 Effect of pumping speed In the pressure controlled drying, it is important to suppress the drying rate in a constant drying rate period so as to promote rearrangement of the particles before the consolidation of particle layer. The conclusion in the last section was that the drying conditions in the constant pressure stage should be decided while paying attention to the residual solvent at the beginning and end of the constant drying rate period. This is because the drying rate increases again by the second decompression when the coated slurry contains an excessive amount of solvent. Consequently, it was supposed that the reduction in pumping speed only in the final decompression can reduce the drying rate in that stage, even if the constant pressure stage is too short to remove solvent sufficiently. At the hold pressure of 70 hpa, Figures 6(b) and (c) indicate that the drying rate begins to fall at the time of 12 13 min. Thus, at the hold time of 10 min, when the amount of residual solvent at the end of constant pressure stage was large enough for particles rearrangement, the effect of a pumping-speed reduction was investigated as shown in Figure 7. After the constant pressure stage, the desiccator was decompressed again at the pumping speeds of (a) 2.2 10 4 and (b) 4.7 10 5 m 3 /s, respectively. Thus, the drying conditions in Figure 7(a) and Figure 6(a) are identical. As a result, the temperature difference at the reduced pumping speed became roughly constant again in the final decompression stage, but was VOL. 43 NO. 10 2010 897

larger than that in the constant pressure stage. This result suggested that the drying rate successfully depressed by reducing the pumping speed in the final decompression, even if the coated slurry contains an excessive amount of solvent. However, it required a long time for the temperature difference to decrease to zero in the final decompression. Thus, in order to avoid significant evaporation in the final decompression and elongation of drying time, as reduction of the pumping speed is not a good choice, the amount of solvent before the final decompression has to be carefully controlled. 3.4 Effect of drying container volume The amount of evaporated solvent in the initial decompression determines the effectiveness of the pressure controlled drying and the total drying time. As can be expected from the discussions so far, the initial decompression stage must be affected by the co-drying solvent, hold time, pumping speed, and also desiccator volume. At the early stage of drying, the co-drying solvent primarily evaporates compared with the solvent in the slurry because the solvent evaporation from slurry is slightly restricted by suspended particles and dissolved polymer. Thus, a small drying container results in a short first decompression and a corresponding small amount of solvent evaporation. We reduced here the desiccator volume by half by inserting an aluminum block having half the volume of the desiccator. The hold pressure was determined as 40 hpa, where the solvent evaporates significantly in the initial decompression, so that the effect of the desiccator volume reduction could be clearly observed. The desiccator pressure was held till the temperature difference attained zero, and consequently the hold times were not same. The drying behaviors were compared in Figure 8. At the normal desiccator volume condition, which is identical with Figure 4(b), both temperatures of the slurry and desiccator increased gradually, roughly showing the constant temperature difference for the 10 min at the beginning of the constant pressure stage. In contrast, at the reduced desiccator volume, an interesting change in the slurry temperature was observed. The slurry temperature decreased sharply because of quick decompression, and subsequently began to increase before starting the constant pressure stage, and finally became constant. Although it is true that a constant slurry temperature was observed in the case without co-drying IPA (Figure 4(a)) or at the lowest hold pressure (Figure 5(a)), the mechanism and the resultant constant temperature were completely different. Under this condition, in spite of there being half the amount of air compared with the normal setup, the heat flow into the desiccator does not change. As a result, more heat from the outside of the desiccator could flow into the slurry and the slurry temperature initially increased. Afterwards, when the latent heat of evaporation was comparable with the heat flow from the outside, the slurry showed constant temperature. Since the amount of solvent evaporated with decompression Fig. 8 Change in drying behavior by reducing drying container volume by 50% could be reduced and the constant drying rate period became long when reducing the container volume, the rearrangement of particles at constant pressure could be promoted. However, it was supposed that the drying rate in the constant rate period could not be depressed because the slurry temperature range was almost the same. Therefore, it was concluded that the reduction in drying container volume can increase residual solvent before holding pressure. However, since large amount of residual solvent requires long hold time, thereby elongating the total drying time, the residual solvent should be reduced by decreasing the amount of co-drying solvent. 3.5 Surface appearance change by drying conditions The structure change with drying conditions by applying a pressure controlled drying technique to coated slurries is also of great interest. From the discussion so far, the amounts of residual solvent at the end of the initial decompression and constant pressure stages are significantly related with particle rearrangement. On the surfaces of all dried films of coated slurries, two dimensional polygonal domains separated by boundaries (cellular pattern) are observed. The cellular pattern is caused by convective flow in the wet coated slurry (Yiantsios and Higgins, 2006). It was also found that the cellular patterns were classified into 4 groups as shown in Figure 9 by drying conditions. In natural drying (Figure 9(a)), where the coated slurry was dried at room temperature and atmospheric pressure, most clear cellular patterns and relatively large aggregates of the primary particles could be observed. This indicates that the slurry used tends to induce remarkable convective flow because of the high drying rate of IPA. In general, particles are frequently accumulated in the drying interface and form a surface skin layer if the solvent evaporates quickly. Distinct boundaries of cells imply that a surface skin layer would not be formed and loosely packed aggregates moved as a whole in the coated slurry due to convective flow. The surfaces of the coated slurries dried at the hold pressure of 100 hpa, which is much higher than saturation vapor pressure, and various hold times are shown in 898 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN

and resulted in a rough surface. Such a surface structure was always obtained at the hold time much longer than the duration of the constant drying rate period. If the constant pressure was kept until just the end of the falling drying rate period at this hold pressure, a well packed and smooth cellular pattern could be obtained. These results suggest that convective flow is mainly formed in the initial decompression, and the convective cells are packed tightly in the constant drying rate period under a low drying rate and sufficient amount of residual solvent, and the packed structure was not changed in the falling drying rate period. Fig. 9 Surface appearances of dried films of coated slurries at various drying conditions Figures 9 (b), (c) and (d). The appearances changed drastically with drying conditions. Nevertheless, the cellular patterns were fundamentally formed and no surface skin layer was observed, even if the coated slurry was dried quickly in the first decompression stage. As well as in the natural drying, convective flow significantly affects the final structure of the dried film and could keep the slurry roughly homogeneous during drying. At the hold time of 16 min, when a large amount of solvent was evaporated at the beginning of the final decompression as shown in Figure 5(d), a loosely packed cellular pattern having uncertain boundaries was observed. A similar pattern could be observed at the lowest hold pressure of 30 hpa, where most of the solvent evaporated in the first decompression. When the desiccator was decompressed again after the temperature difference changes from constant to falling (20 min of hold time), a dried film with a flat surface and tightly packed cellular pattern with well defined boundaries (Figure 9(c)) could be obtained. Under the conditions of reduced pumping speed in the final decompression with a short hold time and reduced drying container volume with long hold time, we could obtain films having a similar surface appearance. At the hold time of 60 min, the coated slurry was kept in the desiccator filled with IPA vapor for 30 min because the temperature difference became zero at 30 min. It was true that cells were packed tightly in the film having a rough surface (Figure 9(d)). This implies that a well packed cellular structure was formed in the constant pressure stage under the condition of a sufficient amount of residual solvent and sufficient hold time. However, the quick evaporation of residual solvent disrupts the surface structure of the particle-packed layer Conclusions The drying process of the coated slurry in the closed container filled with the vapor of the drying solvent showed quite interesting behavior with temperature variation. The temperature difference was used to evaluate the drying rate and often became constant during the constant drying rate period because sufficient heat flows into the coated slurry compared with the evaporation of heat. The drying rate can be depressed under a surrounding pressure higher than the saturation vapor pressure of the drying solvent, and the residual solvent in the slurry can be controlled by the hold pressure, hold time, and various experimental conditions. In the case of sufficient hold time with a sufficient amount of residual solvent, the rearrangement of suspended particles was enhanced and the resultant film became well packed having a smooth surface. Although drying is one of the key processes to control the properties of the final film products from slurries, it was found that the drying rate, drying time and solvent content during the constant drying rate period are the most important factors for the structure control. In future research, taking into account these experimental results, a model to explain the drying behavior should be developed for process design. Acknowledgments This work was partially supported by the Grant-in-Aid for Young Scientists (B) (18760567). Literature Cited Briscoe, B. J., G. L. Biundo and N. Özkan; Drying Kinetics of Water- Based Ceramic Suspensions for Tape Casting, Ceram. Int., 24, 347 357 (1998) Gode, P., F. Jaouen, G. Lindbergh, A. Lundblad and G. Sundholm; Influence of the Composition on the Structure and Electrochemical Characteristics of the PEFC Cathode, Electrochim. Acta, 48, 4175 4187 (2003) Kim, K. M., W. S. Jeon, I. J. Chung and S. H. Chang; Effect of Mixing Sequences on the Electrode Characteristics of Lithium-Ion Rechargeable Batteries, J. Power Sources, 83, 108 113 (1999) Kim, S., J. H. Sung, K. H. Ahn and S. J. Lee; Drying of the Silica/PVA Suspension: Effect of Suspension Microstructure, Langmuir, 25, 6155 6161 (2009) Komoda, Y., Y. Ikeda, H. Suzuki, H. Usui, T. Ioro and T. Kobayashi; Effect of the Composition and Coating Condition on the Struc- VOL. 43 NO. 10 2010 899

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