How the Nano bead mill affect the rod-type Titania Nanoparticles
1. This article focuses on the low-energy dispersion of nanomaterials during bead milling. The effects of milling parameters (including bead size, rotation speed, and milling time) on the dispersibility of brittle rod-shaped titanium dioxide nanoparticles were studied.
2. Based on the experimental data obtained on the morphological, optical and crystalline properties of the dispersed nanoparticles, a complete dispersion of primary particles in colloidal suspension can be obtained only by using the optimal milling parameters for the bead milling process.
3. Deviation from the optimal conditions (i.e., higher rotation speed and larger bead size) leads to reagglomeration phenomena, produces smaller ellipsoidal particles, and deteriorates the crystallinity and physicochemical properties of the dispersed nanoparticles, especially the refractive index. Rate.
4. We also found that the reduction in refractive index caused by the grinding process is related to the formation of broken particles and collisions of amorphous phases on the particle surface. In addition, the current low-energy dispersion method has industrial application prospects, confirming that there are almost no obvious impurities (from the breakage of the beads) in the final product.
Ⅰ . Introduction
In recent years, the dispersion of nanomaterials has attracted great attention, because only well-dispersed and undamaged single nanoparticles show great potential in electronic, chemical, mechanical and biological applications. Well-dispersed nanoparticle suspensions are very important because they can reorganize and reassemble into larger particles or thin films with controlled structures. Various well-dispersed nanoparticles are now available, such as metals, metal oxides, metal nitrides, metal carbides and polymers.
Many methods for dispersing nanomaterials have been used, including ultrasound-assisted dispersion, jetting, ball, bead and roller milling, and homogenization. However, only partial information is available on the operating conditions, dispersion media, surface modifiers involved in current dispersion methods, and the effects of collision type and energy on the dispersion process.
Previously, we have studied the dispersion behavior of various nanomaterials such as titanium oxide, aluminum oxide, zinc oxide, boron nitride, titanium nitride, iron oxide, carbon black, and nickel metal in bead milling. Bead milling can effectively disperse nanomaterials without chemical reactions or changes in material properties. However, the effects of bead milling processing parameters on the optical properties and crystallinity of dispersed nanomaterials have not been studied in detail.
Here, we investigated the effects of bead milling dispersion parameters, including bead size, rotation speed, and milling time, on the dispersibility of nanoparticles and examined the morphological, optical, and crystalline properties of the dispersed nanoparticles.
Furthermore, to minimize the impact energy during dispersion, we used beads with a diameter of tens of microns for bead milling, whereas current milling processes use beads with a diameter of 0.1-0.3 mm . Using smaller beads helps prevent nanoparticle breakage and preserve their properties (e.g., crystallinity). Rod-type titanium dioxide (TiO 2 ) was used as a model dispersed nanomaterial. TiO 2 was chosen because it is widely available, nontoxic, and inexpensive, but most commercially available TiO 2 materials are in bulk or aggregated form. Rod-type materials were chosen to examine the ability of our current bead milling process to disperse fragile materials. To demonstrate the effectiveness of our current method in industrial applications, we also investigated the purity of the bead milling product, particularly the breakage of the beads, whereas information about purity is often ignored in current dispersion papers.
II. Dispersion Behavior of Agglomerated Nanoparticles during Bead Milling
Figure 1 shows the particle dispersion during bead milling based on the type of energy used during the dispersion process. We use two types of dispersions: high energy dispersion and low energy dispersion. In this energy classification, low energy defines the conditions of the bead milling process that provide only the breakup of agglomerated particles at the agglomeration sites.
In the case of high-energy grinding processes (the first route), the properties of the final dispersed material sometimes change. The high-energy dispersion process can completely break up the particles. The breakup location can be either the agglomerate location or the crystal body location. As a result, the dispersed slurry contains particles of various sizes. In addition, this condition is incompatible with the case of brittle materials. In addition, the grinding time is also important. Too long a grinding process can lead to re-agglomeration phenomena.
In order to disperse the material efficiently and reduce property damage, the second route can be an alternative. Since nanoparticles are usually soft agglomerated, an optimized process capable of breaking agglomerated particles only at the agglomeration sites is crucial. In fact, when a low-energy dispersion process is applied, the final slurry contains dispersed nanoparticles of a single size with properties similar to their original characteristics.
Based on the current development, in order to obtain a low-energy dispersion process in a realistic bead milling process, several operating parameters need to be considered, including milling time, temperature, beads and particle size, rotation speed, physicochemical properties and composition of the dispersion medium, and agglomerated particles. In our previous work, low-energy bead milling dispersion can be obtained when the milling process is carried out under specific conditions. This condition can be achieved when the process is carried out at a rotation speed and bead size of 10 m/s and 30 lm. Deviation from this condition will lead to a re-agglomeration phenomenon, produce smaller and broken particles, and deteriorate the crystallinity and physicochemical properties of the dispersed nanoparticles. Therefore, in this study, the effects of shorter milling time, lower rotation speed, and smaller bead size were investigated, while other operating parameters will be discussed in our future studies.
Figure 1 Schematic diagram of particle dispersion process considering the influence of dispersion energy
II. Experimental Methods
Commercial rod-type TiO2 nanoparticles (MT-01, rutile phase; Tayca Co. Ltd., Japan; surface modified with stearic acid and alumina) were used as the nanoparticle source, dispersed in toluene (Kanto Chemical Co. Ltd., Japan). Crodafos (5-oleyl phosphate and dioleyl alcohol; Croda, Japan). The raw material composition was fixed to a mass ratio of toluene/TiO2/Crodafos of 90/5/5. This suspension was called TiO2 slurry.
The TiO 2 slurry was then added to the bead milling apparatus. The schematic diagram of the bead milling apparatus is shown in Figure 2, and the detailed apparatus information was reported in our previous reports [1, 10-12]. In brief, the apparatus was a bead milling vessel with a volume of 0.15 L, equipped with a pump for supplying the nanoparticle slurry, a mixing tank, and a centrifuge for separating the beads and the TiO 2 slurry. The inner diameter and height of the bead milling vessel were 50 and 150 mm, respectively. Regarding the configuration of the bead milling vessel, we used a rotor with a diameter of 44 mm equipped with 11 rotor pins. The bead filling rate was 65% of the total volume of the vessel. Beads (zirconia beads; Nikkato Corp., Osaka, Japan) were added to the bead milling system before adding the TiO 2 slurry. In this study, the milling time, bead size, and rotation speed were varied. During the bead milling process, each suspension was sampled at a specific time to analyze the effect of milling time on the properties of the TiO 2 product, as shown in Figure 2.
To measure the particle size distribution, dynamic light scattering analysis (DLS; FPAR-1000, Otsuka Denshi. Co, Japan) was performed. The particle morphology was examined using a transmission electron microscope (TEM, JEM-3000F; JEOL Ltd., Japan). The crystallinity of the particles was determined by X-ray diffraction (XRD; RINT 2550 VHF, Rigaku Denki, Japan; operated with Cu Kα radiation in the angular domain between 3° and 120°). The optical properties of the slurries were investigated by a haze meter (NDH4000; Nippon Denshoku Co. Ltd., Japan) and a spectrophotometer (U-2810; Hitachi, Japan). To analyze the refractive index, each solution was spin-coated on a glass substrate, dried at 60 °C for 6 h, and then the refractive index was measured using a prism coupler (Model 2010; Metricon, Japan). To analyze the concentration of the slurry, and in particular to investigate the presence of zirconia beads, inductively coupled plasma (ICP; Seiko SPS-4000, Seiko Instrument Inc., Japan) was used.
In addition, regarding the calculation of crystallinity, the software provided by the XRD measuring device is used to calculate the crystallinity. In order to simplify the calculation, the percentage of crystallinity is defined as the ratio of "crystallization area" to "the sum of crystalline and non-crystalline areas". An example of the calculation method for obtaining the percentage of crystallinity is shown in FIG3. The example of the XRD spectrum of anatase shown in this figure is taken from Reference 1.
Fig.2 A schematic illustration of the bead-milling apparatus.
Fig.3 Definition of crystallinity measurement in the XRD analysis. XRD pattern of anatase shown in this figure is adopted from Ref.
III. Results
1. Particle size distribution
Figure 4 shows the DLS particle size distribution of dispersed TiO2 at different grinding times . The size distribution of the particles changes with the processing time. As the processing time increases, the maximum peak of the DLS size shifts to smaller sizes.
TiO2 dispersed with different bead sizes and rotation speeds obtained from DLS measurements is shown in Figure 5, where Figure 5a and b are the results for rotation speeds of 10 and 8 m/s, respectively. For both rotation speeds, the size of the dispersed particles decreases with the extension of the dispersion time. The size decreases faster at first (within the first 30 min) and then gradually changes during the rest of the milling process. In this study, we limited the milling time to 480 min because longer milling times are not cost-effective for industrial applications.
In addition to the rotation speed, we also found that the characteristics of size reduction depend on the size of the beads. For both rotation speeds (Figure 5a and b), the characteristics of size reduction can be divided into two groups. One is when the process uses beads of 50 lm (shown by the dotted line), and the other is when beads with a diameter of less than 30 lm are used (shown by the solid line). From these results, we can conclude that the use of larger beads promotes the formation of smaller dispersed particles. In addition, for the sample dispersed at a rotation speed of 10 m/s and a bead size of 50 lm (see Figure 5a), the particle size did not decrease until 330 min, and then gradually increased with the extension of grinding time. This may be caused by the re-agglomeration that occurred during the grinding process.
Fig.4 Particle size distribution obtained after different processing times. The beadmilling process was conducted using a bead size of 15 μm and rotation speed of 10 m/s.
Fig.5 Median size of particles obtained using different bead sizes, rotation speeds, and processing times. (a) and (b) are results for samples prepared using rotation speeds of 10 and 8 m/s, respectively.
2. Characterization of particles during bead milling
Since the milling process may lead to the formation of thermodynamically more stable materials, the crystal structure of the TiO 2 particles was analyzed (Figure 6). XRD analysis showed that the dispersed particles were TiO 2 (not shown), confirming that the bead milling process can effectively disperse the particles without changing the material phase and pattern. However, we found that the process parameters greatly affected the physicochemical properties (i.e., crystallinity) of the dispersed particles. In the case of sample crystallinity using a rotation speed and bead size of 10 m/s and 50 μm, respectively, the crystallinity of TiO 2 decreased from 62% to about 48% with the increase of dispersion time. We also found that the decrease in crystallinity depended on the rotation speed and bead size. When the bead milling process was carried out at a higher rotation speed and a larger bead size, a lower crystallinity was obtained. In addition, due to the initial conditions (t=0), the crystallinity of titanium dioxide was relatively low. The occurrence of low crystallinity may be due to the presence of an amorphous phase on the surface of the nanoparticles (because the measured crystalline material was in the form of nanoparticles). Therefore, further studies are needed to clarify the impact of the bead milling process on highly crystalline materials.
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Fig.6 Crystallinity of TiO2 dispersed with various milling times, rotation speeds, and bead sizes. Figures (a) and (b) are samples prepared using rotation speeds of 10 and 8 m/s, respectively.
Before and after the dispersion process for different rotation speeds and bead sizes are shown in Figs. 7a and bg, respectively. Before bead milling, aggregated TiO 2 nanoparticles were observed. The primary TiO 2 particles were rod-shaped with the horizontal and vertical axes being approximately 8 and 50 nm, respectively. After bead milling of the TiO 2 particles, non-agglomerated particles were obtained (Fig. 7b–g). Different morphologies were observed depending on the rotation speed and bead size. When a rotation speed (m/s)/bead size (μm) of 8/15 and 8/30 was used, the rod shape and size of the TiO 2 particles remained unchanged (Fig. 7b, c). However, under other conditions, smaller sized and elliptical TiO 2 nanoparticles were obtained (Fig. 7d–g). In addition to morphological transformations, TEM can also detect the crystallinity of the dispersed particles. Since the amorphous phase is easily destroyed by the electron beam [23], the image quality of the particle observation can be used to study the crystallinity of the particles. Clear images indicate that the particles are highly crystalline, while blurred images indicate that the sample contains an amorphous phase. Clear particle shapes were observed in Figs. 7a–c. In contrast, in Figs. 7d–g, the particle surfaces became blurred.
Fig.7 TEM images of particles (a) before and (b–g) after dispersion. (b)–(g) show samples prepared by bead milling with a rotation speed (m/s)/bead size (lm) of 8/15, 8/30, 8/50, 10/15, 10/30, and 10/50, respectively.
The effect of grinding time on sample turbidity is shown in Figure 8. Figure 8a shows the UV-visible (UV-vis) spectra of TiO2 dispersions after different treatment times. The intensity of the absorption spectrum increases with the increase in treatment time. The change in intensity corresponds to the change in sample color (as shown in Figure 8b). Longer grinding times produce more transparent solutions. Optical turbidity is observed because light can penetrate the nanoparticles. Therefore, the successful dispersion of particles can be directly observed through visible characterization. To confirm the dispersibility of the particles, UV-visible spectroscopy analysis was performed on TiO2 dispersed at different rotation speeds and bead sizes ( Figure 9). Fig. 9a and b show the analysis results of samples prepared at rotation speeds of 10 and 8m/s, respectively. The transmittance intensity increases with the increase in grinding time. However, when the process was performed using beads with a diameter of 50 μm, we observed a unique phenomenon. After a certain grinding time, the transmittance first increased and then gradually decreased. For rotation speeds of 10 and 8 m/s, the spectral intensity began to decrease after 180 minutes and 420 minutes, respectively. The transmittance of a solution depends on the size of the particles in the solution, so the results in Figures 1 and 2, Figures 8 and 9 confirm that the increase in transmittance is caused by the reduction in the size of the TiO2 particles in the solution during the bead milling process . The decrease in transmittance observed in the sample prepared using 50 μm beads in Figure 9 confirms that reagglomeration occurs after a certain period of bead milling.
Fig.8 Effect of milling time on light absorbance. Figure (a) shows the absorbance spectra of particles during the dispersion process, whereas Figure (b) is a photograph of samples after dispersion. Samples were prepared using a rotation speed of 10 m/ s and bead size of 15 μm
Fig.9 Transmittance spectra of nanoparticles during the dispersion process. Figures (a) and (b) show samples prepared using rotation speeds of 10 and 8 m/s, respectively.
Figure 10 plots the refractive index of TiO2 dispersed at various rotation speeds and bead sizes . In the sample prepared at a rotation speed of 10 m/s (Figure 10a), a significant decrease in the refractive index from 1.73 to 1.64 is observed. In contrast, the refractive index of the sample prepared at a rotation speed of 8 m/s ( Figure 10 b) shows almost no change with time.
Fig.10 Refractive indices of samples prepared with different rotation speeds and bead sizes. Figs. (a) and (b) show the results obtained for samples prepared with rotation speeds of 10 and 8 m/s, respectively.
The effects of rotation speed and bead size on the total transmittance (TT) and haze of the samples are shown in Figure 11. Similar trends in total transmittance and haze were observed with increasing milling time, except that the samples prepared with the bead size Sample outside. 50 microns. With increasing processing time, total transmittance increases, while haze decreases. These results correlate well with the above analysis, where increasing milling time decreases particle size in solution.
For the case of grinding with a bead size of 50 μm and a rotation speed of 10 m/s, an unusual trend was observed. As shown in Figure 11a, the total transmittance increased up to 200 min. Between 200 and 330 min, the transmittance The results are relatively stable. Then, after 330 minutes, a decrease in transmittance is observed. These results are very consistent with the haze results for this sample in Figure 11c.
Fig.11 Total transmittance (TT) (a, b) and haze (c, d) of samples during the dispersion process.
3. Industrial application prospects of low-energy dispersion process
Graph of the morphological and structural changes of NPs during milling, depending on the energy supplied (high or low energy). Low energy dispersion defines the conditions of bead milling, which breaks up agglomerated nanoparticles (not grinding/crushing). High energy dispersion defines the conditions that break up the nanoparticle structure and change the intrinsic properties of the nanoparticles. While high energy dispersion allows for rapid decomposition of agglomerated nanoparticles, the bulk (material itself) is damaged and broken up (crystal destruction). As a result, the dispersed solution contains particles of multiple sizes. Due to the higher energy, this condition cannot be used for fragile materials such as metals, hollow and core-shell structured particles. Milling time is also important. Too long a milling time can lead to re-agglomeration. Nanoparticles are usually soft agglomerated, so optimal conditions to break up agglomerated nanoparticles only at the agglomerate sites are critical. During low energy dispersion, the slurry contains nanoparticles of a single size with intrinsic material properties.
Fig.12 Proposal physicochemical transformation during the bead-milling dispersion process.
Reprinted with permission from Tahara et al., 2014. Copyright: (2014) Elsevier B.V.
IV . Conclusion
In this study, we investigated the effects of dispersion process parameters, including rotation speed and bead size, on the dispersion of brittle rod-type TiO 2 nanoparticles. To evaluate the performance of the dispersed TiO 2 , its morphology, optical properties, and crystallinity were analyzed. Unbroken dispersions of primary particles in colloidal suspension were obtained using the optimal dispersion conditions. However, deviations from these optimal conditions (i.e., higher rotation speed and larger bead size) resulted in reagglomeration, smaller ellipsoidal particles, and decreased crystallinity and physicochemical properties of the dispersed particles, especially the refractive index. During the milling process, the refractive index decreased due to the fragmentation of particles and the formation of an amorphous phase on the particle surface due to the large number of collisions between beads and nanoparticles. In addition, the current low-energy dispersion method, which has the potential for industrial applications, confirmed the presence of almost no significant impurities (from wear debris or bead breakage) in the final product. We believe that this study furthers the understanding of dispersion science and technology, especially related to the dispersion of fragile nanomaterials.
This article is from: Advanced Powder Technology
Reference：
Journal of Colloid and Interface Science
Powder Technology
Chemical Engineering Journal
Advanced Powder Technology
Langmuir 29 (2013)
Acta Materialia 58
Industrial and Engineering Chemistry Research 47
Journal of Society Powder Technology Japan 48
Kagaku Kogaku Ronbunshu 39
Materials Science Forum 416
Journal of Nanoparticle Research 14
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