Morphology Evolution and Properties of Hollow Glass Beads via Dried-Gel Particles Method.
Summary: Hollow glass beads (HGB) were prepared in a vertical furnace by adopting urea as blowing agent and liquid sodium silicate as main raw material. The effects of weight fraction of urea, firing time and average size of dried-gel particles on the formation of HGB were discussed. Experiment results showed that increasing the weight fraction of urea and the firing time could improve the morphology and the hollow structure of the products, and appropriate parameters were 4.5wt% and 2.5s, respectively. Moreover, using the dried-gel particles with small size were in favor of obtaining HGB with relatively small size, thick wall and high static pressure strength. In addition, the possible mechanism for the formation of HGB was presented.
Keywords: Hollow glass beads; Sol-gel; Morphology evolution
Recently, energy security and global warming are twin factors making hydrogen (H2) economy attractive. For H2 to replace fossil fuels, safe and efficient storage techniques with high volumetric efficiency are required. An elegant solution to the storage problem of H2 is one of the most important issues. Hollow glass beads (HGB) possessing lots of excellent properties such as light weight and isotropy have been attracted much attention to be used in the field of storage and transportation of H2 [1-2]. Until now, HGB can be obtained via various methods such as heat treatments , rotating electrical arc , plasma  and template calcinating technologies . In this work, HGB with controllable appearance and properties were fabricated in a self-designed furnace. The effects of some parameters, such as weight fraction of urea, firing time and average size of dried-gel particles on the formation of HGB were discussed in details.
The aim of this work was to obtain a simple technology for preparing HGB and provide the mechnism of the formation.
Results and discussion
Blowing Agent Analysis
The morphology evolution of products which were fabricated with different weight fraction of urea. According to Fig. 1(a), it is clear that the products were only black beads when there were not any urea in raw materials. However, as the increase of weight fraciton of urea from 1.5~4.5wt% (Fig. 1(b-d)), there were more and more light beads with black outline, indicating more and more beads possessed hollow structure. Fig. 1(d) shows that black beads (as the black arrows shown) might be solid particles which possessed reversed appearance compared to those light beads. The possible reasons for this trend were the fact that more gases were released as the weight fraction of urea increased, and they were entrapped by fussed mass during the heating. Moreover, the volume of the bead was expanded when the gases pressure exceeded its surface tension. Therefore, an appropriate weight fraction of urea for preparing HGB was 4.5wt%.
Firing Time Analysis
The morphology evolution of products which were fabricated via different firing time. It is found in Fig. 2(a) that when the firing time was only 0.5s the products were irregular particles with rough shape. However, when the firing time increased to 1.0~1.5s some products looked like a peanut, a dumbbell or a bead with a long "tail" (Fig. 2(b-c), as the black arrows shown). Furthermore, when the firing time increased to 2.5s, most products were hollow beads with relatively smooth surface (Fig. 2(e)). Fig. 2(f) shows that the products exhibited multiple colors and cross extinction, indicating the structure of the products were hollow and symmetrical. This result was confirmed by Fig. 4(g) (as the white arrows shown).
The possible reasons for this phenomenon were that all dried-gel particles acted with four forces during their dropping: buoyancy (Fb), drag force (Fd), gravity (G) and thrust (Ft) (as Fig. 3 shown). Moreover, Ft was greatly affected by air stream which was emitted from the air compressor, while other three forces were nearly unchanged. According to Fig. 3, if the firing time was less than 0.5s, corresponding to a strong Ft, the dried-gel particles would not be completely fused. This would cause beads to be solid and irregular. Moreover, with the gradual increase of fire time, particles' viscosity (e) and surface tension (s) were gradually reduced . This would cause those dumbbell-shaped feedstock to be stretched and subsequently separated (in Fig. 3(b)). Therefore, the products would look like a peanut, a dumbbell or a bead with a long "tail". However, if the firing time increased to 2.5s, corresponding to a weak Ft, hollow beads with thin wall would be fabricated.
In addition, those sphere-shaped dried-gel particles would be directly turned into HGB if they were fired for 2.5s (in Fig. 3(a)). Therefor, an appropriate firing time for preparing HGB was 2.5s.
Dried-gel Particles Analysis
According to Fig. 4 and Table-1, it is found that HGB with relatively small size, thick wall and high stacking density and static pressure strength were prepared when the dried-gel particles with small average size were used. Typically, HGB with 13.2um (average size), about 1.2um (wall thickness), 0.78g/cm3 (stacking density) and 85.7MPa (static pressure strength) were prepared when the dried-gel particles with the average size of 18.6um were used. However, HGB with those of 140.8um, about 3um, 0.18g/cm3 and 4.4MPa were prepared when the dried-gel particles with the average size of 162.9um were used. This could be explained by a revised formula .
Where "P" is the theoretical collapse strength of an individual HGB, "Ei'" is the theoretical calculation factor of Young's modulus of oxide, "xi" is the molar fraction of oxide, "l" is the strength factor which is regard as 1 if the HGB is perfect, "h" is the wall thickness of the HGB, "r" is the radius of the HGB, and "d" is the Poisson's ratio for glass.
It is clear that P, other things being equal, was directly proportional to the square of h and inversely proportional to the square of r. In other words, decreasing r and increasing h of HGB were effective ways to improve the static pressure strength of HGB.
It is found in Fig. 5(a) that there were two peaks at 1670cm-1 and 1625cm-1, which were contributed by the stretching modes of C=O of urea . However, in Fig. 5(b), no peaks corresponding to the bonds of C=O were found. Therefore, it can be infered that all urea were completely decomposed during the formation of HGB.
corresponding SXRD of those quickly cooling HGB).
Liquid sodium silicate (modulus=3.9) were purchased from Tianjin Chemicals Corp. Urea, aluminium nitrate, borax and hydrochloric acid were purchased from Beijing Chemicals Corp. All the raw materials were industrial grade.
Preparation of Dried-gel Particles
Typically, 4.0g aluminium nitrate and 40.0g borax were mixed in a 1500ml glass beaker which contained 400ml dilute hydrochloric acid. Moreover, 160.0g liquid sodium silicate, 400ml distilled water and a given amount of urea as the blowing agent (0wt%, 1.5wt%, 3wt%, 4.5wt%, based on the weight of liquid sodium silicate) were well mixed at room temperature, and then quickly added into the mentioned glass beaker. 10min later as-fabricated gel was dried at 90 for 36h. Finally, the white solids were grinded.
Preparation of HGB
As-prepared dried-gel particles were continuously added into a vertical furnace (Fig. 7) with the help of air stream, which emitted from an air compressor with adjustable varying-speed system. The furnace temperature at four separate regions (from top to bottom) were kept at 1300, 1000, 700, 400, respectively. Finally, HGB were obtained via two ways: (a): natural (the products were directly collected at the bottom of the furnace) and (b) quickly cooling (the products were guided into the water and then collected and dried).
Firing Time Test
Firing time of dried-gel particles in furnace could be approximately calculated by the following formula:
Where H = 4m is the height of the furnace in which the dried-gel particles were heated. V is the velocity of the air stream and it can be recorded from the flowmeter. In this work, V was adjusted to be 8.00m/s, 4.00m/s, 2.67m/s, 2.00m/s and 1.60m/s, respectively. Then, the firing time of the dried-gel particles was 0.5s, 1.0s, 1.5s, 2.0s and 2.5s, repectively.
Stacking Density Test
3g HGB were put into a measuring cylinder. Subsequently, the volume of HGB was recorded after vibrating the cylinder to keep the volume constant. Then the stacking density (r(cm3)) of HGB could be calculated by the following formula:
Static Pressure Strength Test
3g HGB were wrapped with semipermeable membrane. Subsequently, they were put into the cavity of the test instrument which was filled with water (as Fig. 8 shown). The pressure strength of the water in the cavity was controlled by manpower. After maintaining a given pressure for 20min, floated HGB were collected and dried (the weight was marked as m2(g)). Then, the flotation ratio (e) of HGB could be calculated by the following formula:
The static pressure strength of HGB was defined as the recorded pressure when e was equal to 90%.
The morphology of the specimens was observed by using a Hatchi S-4800 field emission scanning electron microscope (FE-SEM) and a Nikon Eclipse TE2000-U polarized optical microscope with bright field and cross-polarization field. The average particle size of dried-gel particles and HGB were measured by using a Brookhaven 90Plus ultrasonic particle size analyzer (UPSA). Fourier transform infrared spectroscopy (FT-IR) of the products were performed with a Shimadzu FT-IR-8400s spectro- meter. The structure of the products was characterized by a Rigaku D/max small-angle (SXRD) and wide-angle X-ray diffraction tester (WXRD).
In conclusion, a dried-gel particles method for preparing HGB was provided. Experiment results showed that when the urea usage and the firing time were 4.5wt% and 2.5s HGB with good appearance and structure could be fabricated. Moreover, FE-SEM, stacking density and stactic pressure strength results showed that HGB with relatively small size, thick wall and high stacking density and static pressure strength could be prepared when the dried-gel particles with small average size were used; FT-IR results confirmed that the urea were completely decomposed during the formation of HGB; XRD results illustrated that HGB was amorphous structure without mesoporous in the shell when they were prepared via quickly cooling.
The authors greatfully thank the financial support of National Natural Science Foundation of China (No. 02490220).
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1 Schoolof Materials Science and Engineering, Shijiazhuang Tiedao University, Shijiazhuang 050043, China, 2 School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China) firstname.lastname@example.org
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|Author:||Xiang-Feng Wu; Xu-Chun Li; Jian-Guo Ding; Yong-Ke Zhao; Yu-Hua Wang|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Aug 31, 2012|
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