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Gelcasting of fused silica glass using a low-toxicity monomer DMAA
Journal of Non-Crystalline Solids 379 (2013) 229–234
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Gelcasting of fused silica glass using a low-toxicity monomer DMAA☆ Wei Wan ⁎, Jian Yang, Jinzhen Zeng, Tai Qiu College of Materials Science and Engineering, Nanjing University of Technology, No. 5 Xinmofan Road, Nanjing, 210009, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 12 August 2013 Available online xxxx Keywords: Fused silica; Gelcasting; Low-toxicity; Impregnation
a b s t r a c t A low-toxicity and water-soluble monomer N,N-dimethylacrylamide (DMAA) was used as gelling agent in gelcasting of fused silica glass. In order to make high solid loading and low-viscosity slurries, acrylic acid-2acrylamido-2-methypropane sulfonic acid copolymer (AA-AMPS) was selected as dispersant. Zeta potential, pH, dispersant dosage, solid loading and milling time have been investigated to prepare the optimum gelcasting slurries. The results suggest that the best conditions were pH 4.8, dispersant dosage 0.1–0.2 wt.% and milling time 5 h. Gelation of slurries took place with the help of initiator at 65–80 °C. Flexural strength of dried fused silica green bodies with only 3.04 wt.% polymers reached up to 20.16 MPa. Nanometer silica was introduced to boost sinterability by immersing green bodies in the tetraethyl silicate solvent. Bulk density and flexural strength of sintered fused silica bodies without obvious crystallization have been increased to 1.975 g/cm3 and 81.36 MPa, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fused silica products possess prominent corrosion and thermal shock resistance (no breakage after thermal shock between 25 and 1100 °C for more than 30 times), low thermal expansion coefficient (0.54 × 10−6/°C) and dielectric constant (3.1–3.8), and good insulating property (resistivity: 1015 Ω m). These properties make fused silica glass an excellent candidate as structural and functional materials in many fields, including glass, metal, aerospace and polysilicon industry [1–3]. As we know that sintered ceramics are hard to be machined, which, to a great extent, limits their application. If green bodies can be machined, this problem will be well solved. Gelcasting is a near net shape colloidal forming process for fabricating high-strength and complex shape ceramic green bodies which can be machined without the risk of breakage [4]. First, raw material powders are dispersed in premixed solution which has been prepared by dissolving polymerizable organic monomer and crosslinker in distilled water. Second, the slurry is degassed in a vacuum deaeration mix after adding initiator and catalyst, and then casted into the mould. Finally, after soaking in a specific temperature for a period, the gelled wet green bodies are demolded and dried. Acrylicamide (AM) has been used as gel monomer in gelcasting of many ceramics including fused silica by many researchers owing to its well gelation properties [5,6], but industry has been reluctant to use gelcasting because AM is a neurotoxin. Thus, some low-toxicity systems were researched in recent years and have been used in different
☆ Foundation items: A project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions; Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), IRT1146. ⁎ Corresponding author. Tel.: +86 25 83587276; fax: +86 25 83587268. E-mail addresses: [email protected] (W. Wan), [email protected] (T. Qiu). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.08.017
ceramics, such as alumina [7–11], zirconia [12], silicon carbide [13], and aluminum nitride [14]. However, not many researches about gelcasting of fused silica using low-toxicity gel systems have been reported. 2Hydroxyethyl methacrylate (HEMA) was used as gel monomer in gelcasting of fused silica by Yu Zhang et al. [15], but the monomer concentration in their study reached 30 vol.% and the four-point flexural strength of green bodied is about 4 MPa. Dense sintering of fused silica glass is difficult, because the sintering temperature cannot be too high in order to avoid amorphous silica translating to cristobalite. In the present work, concentrated aqueous fused silica slurries were prepared by using acrylic acid-2-acrylamido-2-methypropane sulfonic acid copolymer (AA-AMPS) as dispersant. Various parameters which affect the rheological behaviors of the slurries, flexural strength and bulk density of green bodies, including zeta potential, the dosage of dispersant, pH value, solid loading, monomer concentration, gelation temperature and dosage of initiator in the low-toxicity DMAA and N,Nmethylenebisacrylamide (MBAM) gelcasting system, were investigated. In order to improve sinterability of fused silica glass, green bodies were immersed in tetraethyl silicate solvent for 2 h to fill pores in the green bodies with nanometer silica hydrolyzed by tetraethyl silicate. Solid loading and sintering temperature which also affect sinterability were researched. 2. Experimental 2.1. Raw materials Fused silica powders (Jinhua Silica Powders Co., Ltd., Donghai, China) with an average particle size of 3.98 μm (Fig. 1) were used as raw materials. The properties of fused silica powders are shown in
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Table 1. XRD analysis indicates that the phase of silica powders is almost amorphous. DMAA (Kowa Co. Ltd., Japan), MBAM (Chemical Reagent Research Institute, Tianjing, China), ammonium persulfate (APS, Lingfeng Chemical Reagent Co., Ltd., Shanghai, China), AAAMPS (Taihe Water Treatment Co., Ltd., Shandong, China) and tetraethyl silicate (Sinopharm Chemical Reagent Co., Ltd., Shanghai) were used as monomer, crosslinker, initiator, dispersant, and impregnating reagent, respectively. Latic acid (Lingfeng Chemical Reagent Co., Ltd.,) and ammonia water (Shanghai Chemical Reagent Co., Ltd.,) were used as pH conditioner. 2.2. Experimental procedure First, premix solution was prepared by dissolving DMAA (7 wt.%– 15 wt.%) and MBAM (10 wt.% of DMAA) in distilled water. Second, fused silica powders, AA-AMPS (0.05 wt.%-0.3 wt.% of fused silica powders) and milling ball were added to the premix solution. After ball milling for 5 h, the gelcasting slurry was obtained. Third, the slurry was degassed for 10 minutes in a vacuum deaeration mix after adding APS (1 wt.%–5 wt.% of DMAA) as initiator. Then, the slurry was casted into a stainless steel mould and soaked in a specific temperature for an hour. Finally, after being dried in a specific condition, green bodies were obtained, which were used for various tests, characterization and further sintering. Before sintering, green bodies were immersed in tetraethyl silicate solvent for 2 h to introduce nanometer silica to enhance sinterability.
Table 1 Properties of fused silica powders. Size (μm)
Specific gravity (g/cm3)
Composition (%)
3.98 (d50)
2.20
SiO2 99.46
Al2O3 0.454
BaO 0.022
ZrO2 0.017
CaO 0.009
MgO 0.008
Fe2O3 0.004
K2O 0.002
3 mm × 4 mm × 40 mm and a crosshead speed of 0.5 mm/min. Four samples were used to determine the average value. Archimedes method was employed to determine the bulk density and porosity of green and sintered bodies.
3. Results
X-ray fluorescence spectroscopy (XRF, ARL ADVANT'XP, Switzerland) was used to analyze the chemical composition of fused silica powders. Zeta potential was determined by the Zeta Potential Analyzer (Ver. 3.54, Brookhaven Instruments Corp., PALS). R/S Rheometer (R/S CC 25, Brookfield Corporation, USA) was used to characterize the rheological behaviors of slurries. The infrared spectroscopy was measured by Fourier transform infrared spectrometer (IR, Nexus 670, Nicolet). X-ray diffractometer (XRD, ARL, CuKα, Switzerland) and scanning electron microscope (SEM, Model JSM-5900, JEO, Tokyo, Japan) were used to observe the phase composition and microstructure of green and sintered bodies, respectively. The organic matter pyrolysis process of green body in air was performed by a thermal analyzer (NETZSCH, STA, 449C) with the heating rate of 10 °C/min. Flexural strength was examined using an universal testing machine (CMT-6203, MTS System Corporation, Shanghai, China), by the three-point flexural method with a sample dimension of
As we know stabilizing mechanisms of particles in solution mainly include electrostatic stabilization, steric stabilization, and electrical steric stabilization. According to DLVO theory, it is known that the greater the absolute value of zeta potential, the greater the electrostatic repulsion between particles and thus the more stable the slurry. Fig. 2 shows the zeta potential of fused silica slurry. It can be seen that the isoelectric point (IEP) of fused silica powders is about 2.5 and the absolute value of zeta potential increases with pH. The maximum value of zeta potential is obtained at pH about 10.5. However, as shown in Fig. 3, the higher the pH of the premix solutions in alkaline area is, the thicker the slurry is. The suitable pH of the premix solution is about 7.75. Fig. 4 shows the effect of AA-AMPS dosage and solid loading on the viscosity of slurries. It can be seen that when AA-AMPS was added viscosity of the slurry decreases sharply and the optimum dosage of AA-AMPS is 0.1 wt.%–0.2 wt.%. Also, it can be seen that the highest solid loading can reach 68 vol.% by adding 0.1 wt.% AA-AMPS as dispersant, while the viscosity of the slurry is only 0.426 Pa s at 106.12 s−1 shear rate which makes the slurry still suitable for casting. High flexural strength enables green bodies to be machined without breakage. Thus, the preparation of green bodies with high flexural strength and as few polymer as possible is undoubtedly important. In present work, parameters influencing the flexural strength of green bodies were studied, including gelation temperature, dosage of initiator, monomer concentration. The results were shown in Figs. 5 to 7. It can be seen that the highest flexural strength of 12.68 MPa and 15.35 MPa was obtained at a gelation temperature of 75 °C and APS dosage of 2 wt.%, respectively. Flexural strength of green bodies increases steadily with
Fig. 1. Particle size of fused silica powders.
Fig. 2. Zeta potential of fused silica slurry.
2.3. Characterization
W. Wan et al. / Journal of Non-Crystalline Solids 379 (2013) 229–234
Fig. 3. Effect of pH on viscosity of slurries.
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Fig. 5. Effect of gelation temperature on flexural strength of green bodies.
density and flexural strength of fused silica glass on sintering temperature is illustrated in Fig. 12. The green bodies were impregnated before sintering and the soaking time is all 4 h. It can be seen that flexural strength increases with sintering temperature before 1275 °C and the maximum value reaches 81.37 MPa. However, when sintering temperature keeps increasing, flexural strength decreases. Fig. 13 is the SEM micrographs of green and sintered bodies. Microcracks are observed in the sintered body dwelt at 1300 °C. Also, it can be seen in Fig. 14 that crystallization of fused silica is obvious when sintering temperature is 1300 °C.
monomer content, and reaches up to 20.16 MPa when monomer content is 3.04%. The functional groups can be identified through the infrared spectroscopy. Fig. 8 shows the infrared spectroscopy of the gel prepared by heating the premix solution in a certain temperature condition. There is no strong absorption peak at 3095–3300 cm−1, which proves that there is no C C and the polymerization reaction of the DMAA monomer has took place. As shown in Fig. 9, some green bodies with complex shapes were fabricated using gelcasting. The SEM micrograph (Fig. 13) shows that fused silica green bodies have a homogenous and compact structure and no cracks were observed. The pyrolysis process of organic matters of green body in air was determined via thermal analysis. From the TG-DTA (thermo-gravimetric/ differential thermal analyzer) curves shown in Fig. 10, we can see that there are mainly two exothermic peaks which occur in the temperature range of 270–440 °C and 450–550 °C. The temperature range of the first peak is the pyrolysis process of organic matters in green bodies. The temperature range of the second peak is the decomposition procedure of sllanols. Fig. 11 shows bulk density and flexural strength of fused silica glass sintered at 1260 °C. Green bodies were heated at 1 °C/min from 270 °C to 600 °C to pyrolyze the polymer. Sintering was carried out in a furnace in air and a dwell time of 4 h. It can be seen that both bulk density and flexural strength of sintered bodies with impregnation are higher than that without impregnation. The dependence of bulk
Silica is an acidic oxide, so alkalinity solution will promote its hydrolyzation which produces silicic acid. Polymerization of silica acid will form hydrosol which may have thickening function to the slurry. When pH value increases to about 10.5, the viscosity of the slurry rises sharply which suggests that massive silica powders begin to dissolve. The suitable pH of the premix is about 7.75. According to Table 2 and Fig. 2, it can be seen that the pH of the slurry using this premix is
Fig. 4. Effect of dispersant dosage and solid loading on viscosity of slurries.
Fig. 6. Effect of initiator dosage on flexural strength of green bodies.
4. Discussions 4.1. Rheological behaviors of slurries
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Fig. 9. Green bodies with complex shapes. Fig. 7. Effect of monomer concentration on flexural strength of green bodies.
about 4.8 and the zeta potential is about −50 mV which enables fused silica powders not to coagulate. AA-AMPS (molecular weight 3000–6000) has sulfonic groups whose dissociation is not affected by pH value [16]. Thus, it can produce steric stabilization and electrostatic stabilization mechanisms at the same time and show good dispersion effect. Certainly, viscosity increases with solid loading because the free water between particles can be reduced when the solid loading increases. The viscosity of slurries with high solid loading can be characterized by Quemada Model [17]: –2
ηr ¼ ð1 þ φ=φm Þ
ð1Þ
The ηr is the relative viscosity, φ is the solid loading of slurry and φm is the maximum solid loading. As can be seen from the Quemada Model, the relative viscosity increases when the solid loading increases. Fig. 10. Pyrolysis process of the green body in air.
4.2. Properties of green bodies In a certain range of temperature, increasing polymerization temperature will accelerate the polymerization reaction of monomers, but simultaneously decrease the molecular weight of polymers. The amount of primary radicals is bound up with the slurry temperature and APS amount, which influences the speed of polymerization reaction. Fast polymerization reaction speed leads to the decrease of chain length of
polymers [18], which causes incomplete three-dimensional net structure of green bodies, resulting in the decrease of flexural strength in Figs. 5 and 6. Low temperature and initiator amount bring about slow polymerization reaction which causes particles sedimentation and leads to density gradient of green bodies, resulting in low flexural strength. Increasing of monomer content in a certain range makes
Fig. 8. The infrared spectroscopy of DMAA gel.
Fig. 11. Effect of solid loading and impregnation on flexural strength and bulk density of sintered bodies.
W. Wan et al. / Journal of Non-Crystalline Solids 379 (2013) 229–234 Table 2 pH changing between premix solutions and slurries. pH of premix solutions pH of slurries
3.91 4.02
5.92 4.73
7.75 4.80
8.62 4.92
9.52 4.94
10.56 7.55
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N―H. The absorption peak at 2933.25 cm−1 shows that there is shrinking of ―CH2. The absorption peak at 1629.58 cm−1 shows that there is absorption of amide I band. The absorption peak at 1500.37 cm−1 shows that there is absorption of amide II band. There is no strong absorption peak at 3095–3300 cm−1, which proves that there is no C C. The reason is that the monomer DMAA reacts with the crosslink MBAM through the open of C C.
4.3. Properties of sintered bodies The reason why bulk density and flexural strength of sintered bodies with impregnation are higher than those without may be that the hydrolyzation of tetraethyl silicate produces nano silica which fills pores in the green bodies and promotes sintering. The hydrolyzation of tetraethyl silicate can be characterized by the following chemical equation: SiðOC2 H5 Þ4 þ 2H2 O→SiO2 þ 4C2 H5 OH
Fig. 12. Effect of sintering temperature on flexural strength and bulk density of sintered bodies.
completer net structure, arousing the increase of flexural strength in Fig. 7. Because the flexural strength of green bodies is already fully meeting the machining requirement and excessive monomer content may causes more pores in green bodies after polymers being discharged, so no other higher monomer contents were studied in this paper. The functional groups can be identified through the infrared spectroscopy. Fig. 8 shows the infrared spectroscopy of the green body. The absorption peak at 3448.16 cm−1 shows that there is shrinking of
ð2Þ
The product silica is amorphous and the powder size is on nanometer level [19,20]. In addition, impregnation can avoid high viscosity of slurries by adding nano silica in raw materials directly. Also, it can be seen that bulk density and flexural strength increase with solid loading in a certain range. High solid loading makes the packing of fused silica powders in green bodies more closely which causes the increase of bulk density and flexural strength. However, excessive solid loading brings about high viscosity slurry which will result in difficulty in degasification of the slurry and density gradient of green bodies, thus leading to the decrease of bulk density and flexural strength. The reason why flexural strength of sintered bodies dwelt at 1300 °C decreases is that high sintering temperature promotes crystallization of fused silica and produces cristobalite which has much high thermal expansion coefficient (0.27 × 10−4/ °C) than that of amorphous silica (0.54 × 10−6/ °C) and thus leads to microcracks in sintered bodies [21]. As can be seen in Figs. 13 and 14, microcracks and obvious
Fig. 13. SEM micrographs of green body (a) and sintered bodies dwelt at: (b) 1250 °C, (c) 1275 °C, and (d) 1300 °C.
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0.2 wt.%. At the optimum conditions: gelation temperature 75 °C and APS dosage 2 wt.%, flexural strength of green bodies with only 3.04 wt.% polymers reaches up to 20.16 MPa which can fully meet the machining requirement. Flexural strength and bulk density of fused silica glass with impregnation are higher than those without impregnation owing to the filling action of nano silica and its promoting effect to sintering. In a certain range of solid loading, increase of solid loading makes closer packing of powders and leads to higher bulk density and flexural strength of green and sintered bodies. Increasing sintering temperature in a certain range makes powders easy to move and leads to high bulk density and flexural strength of fused silica bodies. However, excessive sintering promotes much crystallization of fused silica and brings about microcracks which result in decrease of flexural strength. Fused silica glass sintered at 1275 °C has the maximum flexural strength of 81.36 MPa and bulk density of 1.975 g/cm3. References Fig. 14. XRD pattern of sintered bodies dwelt at different temperature.
crystallization were observed in the sintered bodies dwelt at 1300 °C. Also, phase transformation between α cristobalite and β cristobalite leads to volume variation and brings about cracks. However, bulk density of sintered bodies increases steadily with sintering temperature because high temperature makes powders easy to move together. It can be seen that silica powders have closer packing and more small powders smooth together at high sintering temperature. 5. Conclusions The low-toxicity monomer DMAA was used in gelcasting of fused silica glass. High solid loading and low-viscosity slurries were obtained at the best conditions: premix solution pH 7.75 and AA-AMPS dosage 0.1–
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Gelcasting of fused silica glass using a low-toxicity monomer DMAA☆ Wei Wan ⁎, Jian Yang, Jinzhen Zeng, Tai Qiu College of Materials Science and Engineering, Nanjing University of Technology, No. 5 Xinmofan Road, Nanjing, 210009, PR China
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 12 August 2013 Available online xxxx Keywords: Fused silica; Gelcasting; Low-toxicity; Impregnation
a b s t r a c t A low-toxicity and water-soluble monomer N,N-dimethylacrylamide (DMAA) was used as gelling agent in gelcasting of fused silica glass. In order to make high solid loading and low-viscosity slurries, acrylic acid-2acrylamido-2-methypropane sulfonic acid copolymer (AA-AMPS) was selected as dispersant. Zeta potential, pH, dispersant dosage, solid loading and milling time have been investigated to prepare the optimum gelcasting slurries. The results suggest that the best conditions were pH 4.8, dispersant dosage 0.1–0.2 wt.% and milling time 5 h. Gelation of slurries took place with the help of initiator at 65–80 °C. Flexural strength of dried fused silica green bodies with only 3.04 wt.% polymers reached up to 20.16 MPa. Nanometer silica was introduced to boost sinterability by immersing green bodies in the tetraethyl silicate solvent. Bulk density and flexural strength of sintered fused silica bodies without obvious crystallization have been increased to 1.975 g/cm3 and 81.36 MPa, respectively. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Fused silica products possess prominent corrosion and thermal shock resistance (no breakage after thermal shock between 25 and 1100 °C for more than 30 times), low thermal expansion coefficient (0.54 × 10−6/°C) and dielectric constant (3.1–3.8), and good insulating property (resistivity: 1015 Ω m). These properties make fused silica glass an excellent candidate as structural and functional materials in many fields, including glass, metal, aerospace and polysilicon industry [1–3]. As we know that sintered ceramics are hard to be machined, which, to a great extent, limits their application. If green bodies can be machined, this problem will be well solved. Gelcasting is a near net shape colloidal forming process for fabricating high-strength and complex shape ceramic green bodies which can be machined without the risk of breakage [4]. First, raw material powders are dispersed in premixed solution which has been prepared by dissolving polymerizable organic monomer and crosslinker in distilled water. Second, the slurry is degassed in a vacuum deaeration mix after adding initiator and catalyst, and then casted into the mould. Finally, after soaking in a specific temperature for a period, the gelled wet green bodies are demolded and dried. Acrylicamide (AM) has been used as gel monomer in gelcasting of many ceramics including fused silica by many researchers owing to its well gelation properties [5,6], but industry has been reluctant to use gelcasting because AM is a neurotoxin. Thus, some low-toxicity systems were researched in recent years and have been used in different
☆ Foundation items: A project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions; Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT), IRT1146. ⁎ Corresponding author. Tel.: +86 25 83587276; fax: +86 25 83587268. E-mail addresses: [email protected] (W. Wan), [email protected] (T. Qiu). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.08.017
ceramics, such as alumina [7–11], zirconia [12], silicon carbide [13], and aluminum nitride [14]. However, not many researches about gelcasting of fused silica using low-toxicity gel systems have been reported. 2Hydroxyethyl methacrylate (HEMA) was used as gel monomer in gelcasting of fused silica by Yu Zhang et al. [15], but the monomer concentration in their study reached 30 vol.% and the four-point flexural strength of green bodied is about 4 MPa. Dense sintering of fused silica glass is difficult, because the sintering temperature cannot be too high in order to avoid amorphous silica translating to cristobalite. In the present work, concentrated aqueous fused silica slurries were prepared by using acrylic acid-2-acrylamido-2-methypropane sulfonic acid copolymer (AA-AMPS) as dispersant. Various parameters which affect the rheological behaviors of the slurries, flexural strength and bulk density of green bodies, including zeta potential, the dosage of dispersant, pH value, solid loading, monomer concentration, gelation temperature and dosage of initiator in the low-toxicity DMAA and N,Nmethylenebisacrylamide (MBAM) gelcasting system, were investigated. In order to improve sinterability of fused silica glass, green bodies were immersed in tetraethyl silicate solvent for 2 h to fill pores in the green bodies with nanometer silica hydrolyzed by tetraethyl silicate. Solid loading and sintering temperature which also affect sinterability were researched. 2. Experimental 2.1. Raw materials Fused silica powders (Jinhua Silica Powders Co., Ltd., Donghai, China) with an average particle size of 3.98 μm (Fig. 1) were used as raw materials. The properties of fused silica powders are shown in
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Table 1. XRD analysis indicates that the phase of silica powders is almost amorphous. DMAA (Kowa Co. Ltd., Japan), MBAM (Chemical Reagent Research Institute, Tianjing, China), ammonium persulfate (APS, Lingfeng Chemical Reagent Co., Ltd., Shanghai, China), AAAMPS (Taihe Water Treatment Co., Ltd., Shandong, China) and tetraethyl silicate (Sinopharm Chemical Reagent Co., Ltd., Shanghai) were used as monomer, crosslinker, initiator, dispersant, and impregnating reagent, respectively. Latic acid (Lingfeng Chemical Reagent Co., Ltd.,) and ammonia water (Shanghai Chemical Reagent Co., Ltd.,) were used as pH conditioner. 2.2. Experimental procedure First, premix solution was prepared by dissolving DMAA (7 wt.%– 15 wt.%) and MBAM (10 wt.% of DMAA) in distilled water. Second, fused silica powders, AA-AMPS (0.05 wt.%-0.3 wt.% of fused silica powders) and milling ball were added to the premix solution. After ball milling for 5 h, the gelcasting slurry was obtained. Third, the slurry was degassed for 10 minutes in a vacuum deaeration mix after adding APS (1 wt.%–5 wt.% of DMAA) as initiator. Then, the slurry was casted into a stainless steel mould and soaked in a specific temperature for an hour. Finally, after being dried in a specific condition, green bodies were obtained, which were used for various tests, characterization and further sintering. Before sintering, green bodies were immersed in tetraethyl silicate solvent for 2 h to introduce nanometer silica to enhance sinterability.
Table 1 Properties of fused silica powders. Size (μm)
Specific gravity (g/cm3)
Composition (%)
3.98 (d50)
2.20
SiO2 99.46
Al2O3 0.454
BaO 0.022
ZrO2 0.017
CaO 0.009
MgO 0.008
Fe2O3 0.004
K2O 0.002
3 mm × 4 mm × 40 mm and a crosshead speed of 0.5 mm/min. Four samples were used to determine the average value. Archimedes method was employed to determine the bulk density and porosity of green and sintered bodies.
3. Results
X-ray fluorescence spectroscopy (XRF, ARL ADVANT'XP, Switzerland) was used to analyze the chemical composition of fused silica powders. Zeta potential was determined by the Zeta Potential Analyzer (Ver. 3.54, Brookhaven Instruments Corp., PALS). R/S Rheometer (R/S CC 25, Brookfield Corporation, USA) was used to characterize the rheological behaviors of slurries. The infrared spectroscopy was measured by Fourier transform infrared spectrometer (IR, Nexus 670, Nicolet). X-ray diffractometer (XRD, ARL, CuKα, Switzerland) and scanning electron microscope (SEM, Model JSM-5900, JEO, Tokyo, Japan) were used to observe the phase composition and microstructure of green and sintered bodies, respectively. The organic matter pyrolysis process of green body in air was performed by a thermal analyzer (NETZSCH, STA, 449C) with the heating rate of 10 °C/min. Flexural strength was examined using an universal testing machine (CMT-6203, MTS System Corporation, Shanghai, China), by the three-point flexural method with a sample dimension of
As we know stabilizing mechanisms of particles in solution mainly include electrostatic stabilization, steric stabilization, and electrical steric stabilization. According to DLVO theory, it is known that the greater the absolute value of zeta potential, the greater the electrostatic repulsion between particles and thus the more stable the slurry. Fig. 2 shows the zeta potential of fused silica slurry. It can be seen that the isoelectric point (IEP) of fused silica powders is about 2.5 and the absolute value of zeta potential increases with pH. The maximum value of zeta potential is obtained at pH about 10.5. However, as shown in Fig. 3, the higher the pH of the premix solutions in alkaline area is, the thicker the slurry is. The suitable pH of the premix solution is about 7.75. Fig. 4 shows the effect of AA-AMPS dosage and solid loading on the viscosity of slurries. It can be seen that when AA-AMPS was added viscosity of the slurry decreases sharply and the optimum dosage of AA-AMPS is 0.1 wt.%–0.2 wt.%. Also, it can be seen that the highest solid loading can reach 68 vol.% by adding 0.1 wt.% AA-AMPS as dispersant, while the viscosity of the slurry is only 0.426 Pa s at 106.12 s−1 shear rate which makes the slurry still suitable for casting. High flexural strength enables green bodies to be machined without breakage. Thus, the preparation of green bodies with high flexural strength and as few polymer as possible is undoubtedly important. In present work, parameters influencing the flexural strength of green bodies were studied, including gelation temperature, dosage of initiator, monomer concentration. The results were shown in Figs. 5 to 7. It can be seen that the highest flexural strength of 12.68 MPa and 15.35 MPa was obtained at a gelation temperature of 75 °C and APS dosage of 2 wt.%, respectively. Flexural strength of green bodies increases steadily with
Fig. 1. Particle size of fused silica powders.
Fig. 2. Zeta potential of fused silica slurry.
2.3. Characterization
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Fig. 3. Effect of pH on viscosity of slurries.
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Fig. 5. Effect of gelation temperature on flexural strength of green bodies.
density and flexural strength of fused silica glass on sintering temperature is illustrated in Fig. 12. The green bodies were impregnated before sintering and the soaking time is all 4 h. It can be seen that flexural strength increases with sintering temperature before 1275 °C and the maximum value reaches 81.37 MPa. However, when sintering temperature keeps increasing, flexural strength decreases. Fig. 13 is the SEM micrographs of green and sintered bodies. Microcracks are observed in the sintered body dwelt at 1300 °C. Also, it can be seen in Fig. 14 that crystallization of fused silica is obvious when sintering temperature is 1300 °C.
monomer content, and reaches up to 20.16 MPa when monomer content is 3.04%. The functional groups can be identified through the infrared spectroscopy. Fig. 8 shows the infrared spectroscopy of the gel prepared by heating the premix solution in a certain temperature condition. There is no strong absorption peak at 3095–3300 cm−1, which proves that there is no C C and the polymerization reaction of the DMAA monomer has took place. As shown in Fig. 9, some green bodies with complex shapes were fabricated using gelcasting. The SEM micrograph (Fig. 13) shows that fused silica green bodies have a homogenous and compact structure and no cracks were observed. The pyrolysis process of organic matters of green body in air was determined via thermal analysis. From the TG-DTA (thermo-gravimetric/ differential thermal analyzer) curves shown in Fig. 10, we can see that there are mainly two exothermic peaks which occur in the temperature range of 270–440 °C and 450–550 °C. The temperature range of the first peak is the pyrolysis process of organic matters in green bodies. The temperature range of the second peak is the decomposition procedure of sllanols. Fig. 11 shows bulk density and flexural strength of fused silica glass sintered at 1260 °C. Green bodies were heated at 1 °C/min from 270 °C to 600 °C to pyrolyze the polymer. Sintering was carried out in a furnace in air and a dwell time of 4 h. It can be seen that both bulk density and flexural strength of sintered bodies with impregnation are higher than that without impregnation. The dependence of bulk
Silica is an acidic oxide, so alkalinity solution will promote its hydrolyzation which produces silicic acid. Polymerization of silica acid will form hydrosol which may have thickening function to the slurry. When pH value increases to about 10.5, the viscosity of the slurry rises sharply which suggests that massive silica powders begin to dissolve. The suitable pH of the premix is about 7.75. According to Table 2 and Fig. 2, it can be seen that the pH of the slurry using this premix is
Fig. 4. Effect of dispersant dosage and solid loading on viscosity of slurries.
Fig. 6. Effect of initiator dosage on flexural strength of green bodies.
4. Discussions 4.1. Rheological behaviors of slurries
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Fig. 9. Green bodies with complex shapes. Fig. 7. Effect of monomer concentration on flexural strength of green bodies.
about 4.8 and the zeta potential is about −50 mV which enables fused silica powders not to coagulate. AA-AMPS (molecular weight 3000–6000) has sulfonic groups whose dissociation is not affected by pH value [16]. Thus, it can produce steric stabilization and electrostatic stabilization mechanisms at the same time and show good dispersion effect. Certainly, viscosity increases with solid loading because the free water between particles can be reduced when the solid loading increases. The viscosity of slurries with high solid loading can be characterized by Quemada Model [17]: –2
ηr ¼ ð1 þ φ=φm Þ
ð1Þ
The ηr is the relative viscosity, φ is the solid loading of slurry and φm is the maximum solid loading. As can be seen from the Quemada Model, the relative viscosity increases when the solid loading increases. Fig. 10. Pyrolysis process of the green body in air.
4.2. Properties of green bodies In a certain range of temperature, increasing polymerization temperature will accelerate the polymerization reaction of monomers, but simultaneously decrease the molecular weight of polymers. The amount of primary radicals is bound up with the slurry temperature and APS amount, which influences the speed of polymerization reaction. Fast polymerization reaction speed leads to the decrease of chain length of
polymers [18], which causes incomplete three-dimensional net structure of green bodies, resulting in the decrease of flexural strength in Figs. 5 and 6. Low temperature and initiator amount bring about slow polymerization reaction which causes particles sedimentation and leads to density gradient of green bodies, resulting in low flexural strength. Increasing of monomer content in a certain range makes
Fig. 8. The infrared spectroscopy of DMAA gel.
Fig. 11. Effect of solid loading and impregnation on flexural strength and bulk density of sintered bodies.
W. Wan et al. / Journal of Non-Crystalline Solids 379 (2013) 229–234 Table 2 pH changing between premix solutions and slurries. pH of premix solutions pH of slurries
3.91 4.02
5.92 4.73
7.75 4.80
8.62 4.92
9.52 4.94
10.56 7.55
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N―H. The absorption peak at 2933.25 cm−1 shows that there is shrinking of ―CH2. The absorption peak at 1629.58 cm−1 shows that there is absorption of amide I band. The absorption peak at 1500.37 cm−1 shows that there is absorption of amide II band. There is no strong absorption peak at 3095–3300 cm−1, which proves that there is no C C. The reason is that the monomer DMAA reacts with the crosslink MBAM through the open of C C.
4.3. Properties of sintered bodies The reason why bulk density and flexural strength of sintered bodies with impregnation are higher than those without may be that the hydrolyzation of tetraethyl silicate produces nano silica which fills pores in the green bodies and promotes sintering. The hydrolyzation of tetraethyl silicate can be characterized by the following chemical equation: SiðOC2 H5 Þ4 þ 2H2 O→SiO2 þ 4C2 H5 OH
Fig. 12. Effect of sintering temperature on flexural strength and bulk density of sintered bodies.
completer net structure, arousing the increase of flexural strength in Fig. 7. Because the flexural strength of green bodies is already fully meeting the machining requirement and excessive monomer content may causes more pores in green bodies after polymers being discharged, so no other higher monomer contents were studied in this paper. The functional groups can be identified through the infrared spectroscopy. Fig. 8 shows the infrared spectroscopy of the green body. The absorption peak at 3448.16 cm−1 shows that there is shrinking of
ð2Þ
The product silica is amorphous and the powder size is on nanometer level [19,20]. In addition, impregnation can avoid high viscosity of slurries by adding nano silica in raw materials directly. Also, it can be seen that bulk density and flexural strength increase with solid loading in a certain range. High solid loading makes the packing of fused silica powders in green bodies more closely which causes the increase of bulk density and flexural strength. However, excessive solid loading brings about high viscosity slurry which will result in difficulty in degasification of the slurry and density gradient of green bodies, thus leading to the decrease of bulk density and flexural strength. The reason why flexural strength of sintered bodies dwelt at 1300 °C decreases is that high sintering temperature promotes crystallization of fused silica and produces cristobalite which has much high thermal expansion coefficient (0.27 × 10−4/ °C) than that of amorphous silica (0.54 × 10−6/ °C) and thus leads to microcracks in sintered bodies [21]. As can be seen in Figs. 13 and 14, microcracks and obvious
Fig. 13. SEM micrographs of green body (a) and sintered bodies dwelt at: (b) 1250 °C, (c) 1275 °C, and (d) 1300 °C.
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0.2 wt.%. At the optimum conditions: gelation temperature 75 °C and APS dosage 2 wt.%, flexural strength of green bodies with only 3.04 wt.% polymers reaches up to 20.16 MPa which can fully meet the machining requirement. Flexural strength and bulk density of fused silica glass with impregnation are higher than those without impregnation owing to the filling action of nano silica and its promoting effect to sintering. In a certain range of solid loading, increase of solid loading makes closer packing of powders and leads to higher bulk density and flexural strength of green and sintered bodies. Increasing sintering temperature in a certain range makes powders easy to move and leads to high bulk density and flexural strength of fused silica bodies. However, excessive sintering promotes much crystallization of fused silica and brings about microcracks which result in decrease of flexural strength. Fused silica glass sintered at 1275 °C has the maximum flexural strength of 81.36 MPa and bulk density of 1.975 g/cm3. References Fig. 14. XRD pattern of sintered bodies dwelt at different temperature.
crystallization were observed in the sintered bodies dwelt at 1300 °C. Also, phase transformation between α cristobalite and β cristobalite leads to volume variation and brings about cracks. However, bulk density of sintered bodies increases steadily with sintering temperature because high temperature makes powders easy to move together. It can be seen that silica powders have closer packing and more small powders smooth together at high sintering temperature. 5. Conclusions The low-toxicity monomer DMAA was used in gelcasting of fused silica glass. High solid loading and low-viscosity slurries were obtained at the best conditions: premix solution pH 7.75 and AA-AMPS dosage 0.1–
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