The effect of adding different amount of spinning additives on preparation of nano Alumina fibers using a combined method of sol gel and electrospining

Fateme Mirjalili ¹ ( Department of material engineering,faculty of engineering,Islamic azad university Maybod beranch,Iran )
fyrozeh danafar ² ( Department of Chemical Engineering, University of Shahid Bahonar, Kerman, Iran )

تاريخ الإرسال : 07 الخميس , جمادى الأولى, 1441 تاريخ التأكيد : 03 الأحد , رمضان, 1441

الکلمات المفتاحية: Electrospinning, Sol-gel method, Nano alumina fiber,

ملخص المقالة :

Alumina nanofibers were produced using a combined method of sol-gel and electros pinning method. In this method, the sol was prepared by mixing of tri-isoproxide aluminum in aqueous solution onitric acid. Spinning aid such as: ethylene glycol-acid lactic and poly vinyl alcohol was added to the solution .The sol was then heated at 80 °C to obtain gel. Then, the resulting gel was converted into aluminum oxide nanofibers by electrospinning method. The nanofibers were dried at 60 °C for 24 h and finally sintered for 2 h at 1200 ° C. The results of XRD and FTIR showed that in all samples the dominant phase was the corundum phase. Finally, to decrease the sintering temperature, magnesium nitrate hexahydrate (MgN2O6 * 6H2O) and sodium ethoxylate (SiC8 H2o O4) were added to the sol containing 10% Polyvinyl alcohol. The results of XRD and FTIR indicated the presence of corundum phase when the sintering temperature of 1000°C was used. Accordingly, the energy consumption was reduced as the sintering temperature decreased about 200°C by adding the additives,. The results of SEM and TEM showed formation of non-agglomerated nano fibers with 41nm diameters at 1000°C when 10% polyvinyl alcohol and 2% sodium ethoxylate (SiC8 H20 O4) was added to the sol.

نص كامل:



 

 

 The effect of adding different amount of  spinning additives   on preparation of nano Alumina fibers using a combined method of sol gel and electrospining 

Abstract:

Alumina nano fibers were produced using a combined method of sol-gel and electros pinning method. The sol was first prepared by mixing of tri-isoproxide aluminum in aqueous solution of nitric acid. In the next step, ethylene glycol, acid lactic and poly vinyl alcohol were added to the solution .Finally, the sol was then heated at 80 °C to obtain gel.  The resulting gel was converted to nano fibers of aluminum oxide by electrospinning method. The nano fibers were dried at 60 °C for 24 h and sintered at 1200 °C for 2 h. The results of XRD and FTIR showed that, in all samples the dominant phase was the corundum phase. To decrease the sintering temperature, magnesium nitrate hexahydrate (MgN2O6 * 6H2O) and sodium ethoxylate (SiC8 H2o O4) were added to the sol containing 10% Polyvinyl alcohol. The XRD and FTIR analysis for this sample indicated the presence of corundum phase when the sintering temperature of 1000°C was used. Accordingly, the energy consumption was reduced as the sintering temperature decreased about 200°C by adding the additives. The SEM and TEM analysis also showed formation of non-agglomerated nano fibers with diameters of 41nm when 10% polyvinyl alcohol and 2% sodium ethoxylate (SiC8 H20 O4) was added to the sol.

 

Keywords: Nano alumina fiber, sol-gel method, electrospinning

 

1. Introduction:

Alumina is one of the most important materials because of, its high strength and modulus, resistance to attacks from molten metals and non-oxide materials, chemical inertness in both oxidizing and reducing atmospheres up to 1000 °C, and good electrical insulation [1-3]. Alumina (Al2O3) is a ceramic material with several metastable polymorphs (such as χ, κ,γ,δ,θ-Al2O3), and a stable phase, which is crystallized at high temperature (α-Al2O3 [4].

Alumina fibers are often used as high temperature insulating material due to their high melting point (Tm > 2040 °C) and low thermal conductivity (<0.5 W/m K) [5 ]. Convectional thermal insulating fibers are alumino silicate. An Alumina type can be used at high temperature, i.e.1700 °C, for a long period of time that provides 20% energy savings more than other types of alumino silicate fibers [6].There are two methods named melting-based method and sol–gel processing to produce ceramic fibers [7,8]. The later method (sol–gel) has several advantages, such as lower processing temperature, more homogeneity of products, uniform diameter of fibers, lower diameter and better control over final properties of fibers. The preparation of alumina fibers by spinning melt method is very difficult, due to the exceeded high melting temperature and low viscosity of the melt [9].Electrophoretic deposition (EPD) has increasingly attracted an interest as a materials processing technique for a wide range of technical applications. This technique enables the production of unique microstructures and nanostructures as well as novel and complex material combinations in a variety of macroscopic shapes, dimensions and arrangements starting from micron-sized or nanosized particles [10, 11]. Summary of relevant recent works on EPD describes the application of this technique in the processing of several traditional and advanced materials.

 

 For example, EPD has been used for the preparation of metal oxide/ceramic nano fibers such as silica, zirconia, titania, nickel oxide, barium titanate, lead zirconate titanate and other oxide materials [12-15]. 

Poly(vinyl alcohol) (PVA) is used as a biomedical polymer due to their good chemical and physical properties like water solubility, chemical resistance, high melting point and biological compatibility. In addition, it is an inexpensive, non-toxic material having high tensile strength and flexibility. Electrospinning of PVA can be performed from aqueous solutions leading to generation of homogenous nanofibers [16].Chemical cross linking of the PVA nano fibers presents a suitable way for stabilization of the fibers towards aqueous surroundings. Synthesis of electrospun PVA fibers in term of their morphological and dimensional characteristics as well as their final concentration have been well studied to find relationship among various process parameters [17-19].  

Fibers of PVA/SiO2 have higher water solubility and thermal stability comparing to pure PVA nano fibers [10, 20].  It is generally important to obtain continuous fibers of high strength. Fibers used in thermal insulation are short. Which are stable. The stable fibers are usually post-processed to obtain the necessary forms [21-23]. Obviously; it is good to couple the fiber preparation and shaping steps to get rid of or to minimize any post-processed step in centrifugal spinning method of sol–gel technology[24,25]. Chandradass et al (2006) synthesized composite fibers (Zro2/Al2O3) from an aqueous solution of aluminum powder, aluminum chloride hexahydrate and yttrium oxide by sol-gel method. They prepared nano fibers without any agglomeration in the size range of 30-35 nm [1]. Sub micrometric alumina fibers was produced by the electro-blown spinning (EBS) technique and compared with alumina fiber mats prepared using the solution blowing spinning (SBS) method. This method was successful in producing excellent flexible and continuous alumina fibers with mean diameter of about 2.75 μm. These fibers have smaller diameter compared to SBS alumina fibers, which had diameter of about 4.12 μm. Moreover, diameter distribution of the EBS fibesr was more uniform than the SBS’s [26].Venkatesh et al (2002) prepared alumina nanofibers by hydrolyzing aluminum nitrate in the presence of hexamethylenetetramine (HMTA) followed by the supercritical fluid drying. These nanofibers were recommended to be used for biomaterials and tissue engineering [27]. Tepper et al., reported the effects of processing variables on the microstructure of alumina fibes. They used organics and spin-assist materials, which changed the required heating timing and resulted in formation of high-strength fibers [17]. When organic materials are used in electro- spinning method, the surface roughness increases after bonding and thus the fibers strength decreases. Changing of heating scheduling also have an important impact on the formation of alumina fibers and a suitable scheduling leads to the formation of high-strength, high-strength fibers [28]. Researchers fabricated long fibers of alumina using the Al source in low cost and distilled water as a solvent by sol-gel method. Aluminum nitrate (AN) and malic acid (MA) were used as raw materials and, polyvinylpyrrolidone (PVP) was added as a spinning additive [29,30]. The δ-Al2O3 nano fibers were successfully synthesized, with diameter of 2 nm and length of 50 nm. The long quartz fibers have been produced by some factories, such as Mingda Co. Ltd, Jingzhou [31-32].  Continuous α-Al2O3 fibers were grown by seeding of α-Al2O3 suspension with in-situ suspension. The results showed that the calcination of 1200 °C was required for preparation of  α-Al2O3 fiber with high density [33]. 

 

 

Dense alumina fibers were produced at 1600°C by sol-gel and electrospinning at low cost.  Aluminum nitrate and polyvinyl pyrrolidone were used as a precursor ad spinning polymer, respectively. Plutonic 127 were also used as an additive to achieve a porous structure. These fibers with high mole ratio presented a mean diameter close to nanometric scale 153 ± 39 nm [4]. In other work, Alumina borate nanofibers  were fabricated by a method of electrospinning and sol-gel method combination. The results showed that if the content of PVP increased, the diameter of the alumina borate nano fibers increased. However, the crystallinity of the calcined nano fibers decreased with the increase of the PVP content [34]. Thamarai and Parag produced alumina fibers with diameter of 5–15 µm by sol-gel process through centrifugal spinning. It was concluded that viscosity between 13 and 42 Pa.s with 3000 rpm was favored for obtaining fibers containing less shots [35].

This study has driven us to verify and  prepare alumina nano fiber by sol–gel centrifugal spinning route and electrospining method. The novelties of this research are investigating

·       the effects of different percentages of organic matter and spin-aid on spin ability of sol as well as on the morphology and the microstructure properties of fibers produced.

·       the effects of adding different amounts of spinning additive (PVA) on the morphology and microstructure of the fibers are studied.

·       the effect of magnesium nitrate hexahydrate (MgN2O6 * 6H2O) and sodium ethoxylate (SiC8 H2o O4) on phase transformation of alumina nano fibers by determining the weight change percentages of additives.

·       the effect of different additives on required sintering temperature.

The target of this project is developing and evaluating of alumina nano fiber with the best properties for industrial uses.

2. Experimental procedure:

2.1 Preparation of nano alumina fiber:

Nano Alumina fibers were prepared by sol-gel and electro spinning method using Aliminum iso proxide ( C9 H21  ALO30 , Merck KGaA 8.01079.0250), NHO3 (Merck KGaA 1.00456.1000)  , Lactic acid (Merck KGaA 1.00366.1000), Hydroxylethyil cellulose (Merck KGaA 8.220.68.0100) and polyvinilalcohol (Merck KGaA 8.21038.01000) as starting materials. The alumina fibers were prepared in the processing steps as depicted in Fig. 1. The alumina sol was prepared by mixing H2O, aluminum isoproxide and nitric acid, followed by heating in water bath (80 °C) for 3 h. A proper amount of water and spinning additive (PVP) were added into the alumina gel. Then, the precursor solution was concentrated to obtain spinning sol in water bath (60 °C) for 3 h. The sol fibers were prepared by eletrospinning. Then, the sol fibers were dried at 60 °C for 24 h in an oven. The gel fibers were then sintered at 1200 °C for 2 h, with a heating rate of 2 °C/min. In the next step, different additives such as magnesium nitrate hegsahydrate  (MgN2 O6 * 6H2O, Merck KGaA 1.05853.0100) and tetra etyle silicate (Sic8H20 O4, Merck KGaA 8.00658.0250) were added. After preparation of sol, about 2 Wt% of the additives was added to the alumina gels. Then, the precursor solution was concentrated to obtain spinning sol in water bath (60 °C) for 3 h. The sol fibers were prepared by eletrospinning and dried at 60 °C for 24 h in an oven. The gel fibers were then sintered at 1000 °C for 2 h, with a heating rate of 2 °C/min. Phase identification was performed by X-ray diffraction (XRD) PW1800, of Philips Company, using nickel filtered Cu Ka radiation in the range of 2θ = 10°-60° with a scanning speed of 5° per minute. A Fourier transform infrared spectrometer (FTIR) by Perkin Elmer Spectrum 100 series was used with the universal attenuated total reflection (UATR) method. Microstructures of powders were identified by transmission electron microscope (Philips –Zeiss- Germany) and scanning electron microscopy (SEM PHENOM).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 1 Schematic view of production route for alumina fibers

 

3. Results and discussions:

3.1 XRD results for preparation of nanofiber

The X-ray diffraction patterns (XRD) of samples with 14 % PVA( A3), sample with 10 % PVA( A2) and sample without PVA( A1) after calcining at 1200 ° C for 2 h are shown in Fig. 2. The straight base line and sharp peaks of the diffractogram in Fig 2 confirmed that, the products were well crystallized. The XRD pattern in Fig 2 indicated that, the main phase in all samples are ɑ-AL2O3 phase (corundum) which is consisted with  the information contained in the standard cards (0173-10) and (10-44) and (1484-43). Consequently, in all samples, the ɑ- AL2O3 phase (corundum) was formed. Hyuk-Joon and Kug (1999) observed that completion of the most stable phase, α-alumina occurred at 1200°C. Although the thermodynamically stable α-Al2O3 phase could be obtained through a reconstructive transformations (boehmite), the morphology remains unchanged and the final products have the same shape as in the initial phases (36,37).

 

Fig 2:X-ray diffraction pattern of ɑ-AL2O3 synthesized after calcination at 1200 ° C for 2 h.

 

 

The XRD pattern of the ɑ-Al2O3 nano fiber powder containing 2% magnesium nitrate hexahydrate (MgN2O6 * 6H2O) (B1) and 2% tetraethyl ortho-silicate (SiC8 H20 O4) (B2), after calcination at 1000 ℃ for 2 h are shown in Fig 3.

 Comparing diffraction patterns of two samples, the formation of ɑ-Al2O3 phase could be accounted as the main phase in the samples of B1 and B2, which is in agreement with standard cards of (10-04-04), (1484-43(. Magnesium nitrate hexahydrate (MgN2O6 * 6H2O) (B1) and tetraethyl ortho-silicate (SiC8 H20 O4) (B2) additives have the potential synergistic property to reduce the transformation temperature. They can cause to reduce the transition alumina transformation temperature as well as the ability to modify the grain morphology. Its calcination temperature was about 1000 ◦C, which was 200 ◦C lower than general calcination temperature for preparing α-Al2O3 fibers and thus led to save the energy consumption.  Ions of Mg2+ and Si4+ in solid solution could provide solid solution with Al2O3 .Integration of limited Mg2+ and Si4+ ions in solid solution might create interfacial energy difference. The interfacial energy difference improved the interface reaction of alumina and speeded up the grain growth in some directions [37].

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 3: X-ray diffraction pattern of B1 and B2 samples after calcination at 1000 ℃ temperature for 1 h

 

3.2 FTIR results of the ɑ-Al2O3nano fiber

Fig 4 shows the FTIR spectrum of samples A1, A2 and A3 after calcining at 1200 ° C for 2 h. The FTIR shows the existence of ((AL-O)) bonds at 1447 cm-1, (ɑ-AL2O3) bands at 1493

 cm-1, (ALO4) bands at 1590 cm-1, (AL-O-AL) bands at 1640 cm-1 and (ALO6) band at 729

cm -1 in all samples [12,18].The peaks marked with (AL-OH) at wave number of 1400 cm-1 and (CO) and (C = C) at wave numbers of, 1450-1600 cm-1 respectively related to the additives. The peak marked with H2O in 3429 cm -1 belonged to the stretching vibration as well as the deformation vibration characteristics of hydroxylate group ( O-H ) of water [20,26].

 

 

 

 

 In addition, by comparing the graphs, it was observed that, when the fibers were formed the intensity of the peaks was decreased. Moreover, by comparing bands and existence of narrower widths in the range of 400-750 cm-1, A3 sample was selected as an optimal sample.

 

Fig 4: FTIR curves powder samples  of A1, A2, A3

Structural changes at B1 and B2 samples were investigated by FTIR, which could be observed in Fig 5. Peaks in the wave number of 150-500 cm-1were related to Al-O band. The (ɑ-AL2O3) bands at 560 cm-1 and ALO4 band at 645 cm-1 were excited [2,18,27]. The peaks marked with (AL-O-AL) ,  ALO6  and AL-OH were at 760 , 825 and 1381 cm-1  related to alumina groups , respectively. Peaks appeared in 1450-1627 cm-1 were related to the (C-O) and (C-C) bonds[26-28].There were the peaks at 2300cm-1 and 3448 cm-1 which were associated to (O-N) band and (O-H) bands separately. According to Fig 5, with increase of magnesium nitrate hexahydrate and tetraethyl ortho-silicate additives, the absorption band of ɑ -Al2O3 appeared at 863·37 cm−1. 

 

 

Moreover, the significant absorption bands of α-Al2O3 at 583·15 and 517·19 cm−1 to 572 and 482 cm−1, respectively. This observation showed that, the phase transformation of α-Al2O3 was promoted by the addition of additives. Also, with increasing weight percentage of additives, the intensity peak of Al-O decreased. Accordingly, the phase transformation of α-Al2O3 was promoted by the addition of additives.

 

 

Fig 5: FTIR curves powder samples B1, B2

 

3.3 Morphological properties of the ɑ-Al2O3nano fiber

Fig 6 shows the scanning electron microscopy (SEM) images of nano alumina fiber before calcined and after calcined. As it could be seen, the fiber could not growth very well   and a network of interlinked fibers form a continuous web-like structure. It might be due to insufficient amount of spinning aid (PVA), as with increasing the amount of PVA the fibers were grown and formed.

 

 

Fig 6: SEM image of sample A1 (before calcined  a), sample A1 (after calcined ,b)

 

 

Fig 7 shows SEM of A2 (10%  PVA)  sample with good appearance and fiber shape without any defect and agglomeration. They had smooth surfaces, uniform diameters and were flexible with average diameter of fibers was about 65-80 nm.

Fig 7: SEM image of sample A2 (before calcined  a), sample A2 (after calcined ,b)

 

Fig 8 shows SEM micrograph of sample A3 (14 % PVA). As it was shown, the growth of fiber was achieved and the shape of fiber was formed.

 

 

Thus, nano fiber with reasonable strength and small amount of dust formation was normally obtained. Average diameter of fibers was about 160-200nm with regular shape and some agglomeration. Regarding to Fig.6 and Fig. 7 with increasing the PVA percentages, the average diameter of fibers increased because the average diameter of gel fiber remarkably depends on the amount of  PVA. This observation is consisted with  the presented results by Xiaoling  et al., who prepared magnesia-alumina spinel/yttrium aluminum garnet composite fibers by sol-gel method. They showed the diameter of fibers depends on the amount of polyvinylpyrrolidone (PVP) and with increasing of PVP amount the diameter of fibers increased [38].

Fig 8: SEM image of sample A3 (before calcined  a), sample A3 (after calcined ,b)

 

Fig 9 shows  SEM images of sample B1(containing magnesium hexahydrate nitrate (MgN2O6 * 6H2O)) and B2 (Containing tetraethyl octosilicate (SiC8 H20 O4). As it could be seen the shape of both samples were entirely fibrous. Sample B1 had a certain amount of agglomeration with a diameter of about 90-100 nm, but sample B2 shows that, the fibers were more uniform without any agglomeration with a diameter about 40-50 nm.

 

 

 

Fig 9: SEM image of sample B1 (a: before calcined ), (b:after calcined), sample B2 (a: before calcined ), (b:after calcined)

As shown in Fig. 9, adding of different additives influenced  stabilization of transition forms of alumina and retarded the conversion to a-alumina by incorporating dopant contains and vary the number of point defects (impurity ions) in the structure. Tetraethyl octosilicate had better effect on decreasing the diameters of alumina nano fiber.  The silica in tetraethyl octosilicate affect the size asa  grain growth inhibitor instead of varying fiber alumina diameter . Ion of  Si4+ formed more liquid and enhanced the directional growth of the alumina nanostructures and hence increased the transition forms of alumina to ɑ-alumina.

 

Transmission electron microscopy (TEM) images of samples A1and B2 were used for investigating the particle size and morphology of nano alumina fibers. As Fig 10 shows, in sample A1 the fibers did not growth very well and there was a lot of agglomeration. However, sample B2 had completely fiber form without any agglomeration with average diameter about 40 nm which is consisted with SEM results.

Fig 10: TEM images of sample A1 and  sample B2

 

In summary, liquid-forming additives enhance the γ -to-α transformation of alumina. Additive effects on sintering and shrinkage were almost similar except for the added effect of reaction sintering and faster transformation kinetics due to the better contact between powders in a

compact [37]. Moreover, the average diameter of gel fiber remarkably depended on the PVA percentages and with increasing of PVA addition amount the average diameter was increased.

 

 

 

 

4: Conclusion:

1-       Nano ɑ-Al2O3 nano fiber powder with using different additives was synthesized through sol-gel and electro spinning method at 1200 ℃.

2-       Using combination of sol-gel and electro spinning techniques, different amounts of PVA (1, 10,14 %wt) were tested. The best result was obtained when 10% of PVA  was used as spinning aid.

3-       The formation of ɑ-Al2O3nano fibers was achieved with 2% additives of magnesium hexahydrate nitrate and tetraethyl octosilicate .

4-       The results of XRD showed that with increasing amount of magnesium nitrate hexahydrate (MgN2O6 * 6H2O) and tetraethyl erythrocylate (SiC8 H20 O4), the sintering temperature reduced from 1200°C to 1000°C  and ɑ-Al2O3 was formed which caused to decrease the energy consumption .

5-       By using 10% PVA as spinning aid and  2% tetraethyl octosilicate as an additive , the diameter of nano fibers  became about 41nm with regular shape and good dispersion.

 

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