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Effect mechanism of spinel (MgAl2O4) reinforced corundum ceramics on microstructure and properties
Journal of Alloys and Compounds 793 (2019) 146e154
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Effect mechanism of spinel (MgAl2O4) reinforced corundum ceramics on microstructure and properties Qiang Ren, Yuhan Ren*, Xiulan Wu, Wenni Bai, Jinle Zheng, Ou Hai** School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi'an, 710021, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 January 2019 Received in revised form 4 April 2019 Accepted 15 April 2019 Available online 18 April 2019
High strength spinel reinforced corundum ceramics were successfully prepared by using a bauxite mixture with additions of varying mixtures of magnesium ore - clay. Controlled corundum grains were obtained tightly arranged crystal structure by controlling the rate of Al2O3 dissolution-precipitation process through magnesium ore addition in ceramic system. At the same time, the size of the pores was controlled by adjusting the content of the liquid phase in the system. Finally, high strength ceramics were obtained by modifying corundum grain boundary through controlling the content of the magnesium ore. The effects of magnesium ore on microstructure and properties of corundum ceramics were studied. The phase compositions and microstructures were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Fracture mechanism and sintering behavior of corundum ceramics were preliminary analyzed. The flexural strength, density and water absorption of ceramics as functions of magnesium ore contents were systematically investigated. When additions of magnesium ore were 7 wt%, corundum ceramic performances of flexural strength and density were reached a maximum of 291.64 MPa and 3.28 g/cm3. High strength corundum ceramics from bauxite are important to broaden the application fields of bauxite-based ceramics. Alternatively, the formula of the highest strength ceramic (7 wt% magnesium ore) was selected to be the final formula to manufacture proppants, the performances were 4.88% of breakage ratio (under the closed pressure of 86 MPa) and 1.86 g/cm3 of bulk density. The approach opens new opportunities for corundum ceramic as a proppant material since the ceramic system displays high strength. © 2019 Elsevier B.V. All rights reserved.
Keywords: Corundum ceramics Spinel Flexure strength Microstructure Proppants Bauxite
1. Introduction Corundum ceramic is one of the widest application range materials due to its many excellent properties, such as high mechanical strength, high melting point, high chemical stability, and good thermal shock resistance [1e4]. It can be widely used in abrasive, refractory, electric porcelain and optics materials owing to those outstanding performance [5e9]. Proppant is one of the important areas of corundum ceramic for hydraulic fracturing in oil and gas production. However, large of oil and gas was buried below 4 km, most of these oilfields are low-permeability oil and gas reservoirs with high well temperature and high closing pressure, such deep well oil and gas fields require high strength and low breakage
** Corresponding author. * Corresponding author. E-mail addresses: [email protected] (Y. Ren), [email protected] (O. Hai). https://doi.org/10.1016/j.jallcom.2019.04.151 0925-8388/© 2019 Elsevier B.V. All rights reserved.
proppants [10e12]. It is generally to increase the strength by increasing the alumina content in proppants, and some proppants were prepared by using pure alumina, which undoubtedly increases the cost. Consequently, it is necessary to improve the mechanical strength and microstructure design of corundum ceramics. In general, corundum ceramics can be achieved from bauxite minerals or pure alumina sintered more than 1500 C [13]. However, there are few studies for research on high strength corundum ceramic sintered below 1400 C. Adding suitable additives can effectively reduce the firing temperature and improve properties when preparing ceramics. These additives are usually divided into three types according to their main roles in sintering. The first type is to form a solid solution. Additive ions (Fe3þ, Mn4þ, Ti4þ, etc.) enter the corundum to distortion of host lattices to promote crystal growth. Xiangchen Kong et al. [14] investigated the pyrolusite additive on the microstructure and mechanical strength of corundumemullite ceramics supports by bauxite. Ceramic support exhibits relative density reaches approximately 91% and
Q. Ren et al. / Journal of Alloys and Compounds 793 (2019) 146e154
compressive strength reaches 20 MPa. Tingting Wu et al. [15] have reported the acid resistance of ceramic proppants produced by commercial Al2O3 powder and TiO2 have the acid solubility as low as 0.13 wt% heated at 1500 C. The second type is to form a liquid phase, such as Na2O, CaO, MgO, etc, promoting the formation of a liquid phase in the sintering process to densify the ceramics. Haiqiang Ma et al. [16] researched a low-cost bauxite-based ceramic proppant by feldspar addition, the result shown that the sintering temperature reduced by 100 C when 4 wt% feldspar was added compared with the proppants without adding feldspar. Zuolei Liu et al. [17] fabricated low-temperature sintering of bauxite-based fracturing proppants containing CaO and MnO2 additives with apparent breakage ratio of less than 7% at 69 MPa. The third type is to form a new phase to enhance ceramics. The first and second classes additives were widely used in the corundum ceramics. But the third type additive was rarely reported in corundum ceramics especially applying in ceramic proppants. Therefore, this study aims to further improve the performance of corundum ceramic, which make possible to prepare of highdensity, high-strength ceramic proppants for used in deep well fracturing. In our work, with the additive of magnesium ore were prepared spinel reinforce corundum ceramics by bauxite. The effects of magnesium ore on microstructures and properties of corundum ceramic samples were systematically investigated. Meanwhile, fracture mechanism and sintering behavior of spinel enhanced corundum ceramics were further discussed. Based on the optimization crystal structure and pore structure of corundum ceramic, ceramic proppants were synthesized and their properties were also studied. 2. Material and methods 2.1. Raw materials The raw materials were obtained from Dianchi Dehui Petroleum Proppant Co., Ltd., Sanmenxia City, Henan Province, China. Chemical compositions of them were displayed in Table 1. 2.2. Materials design Al2O3 and MgO were provided by bauxite and magnesium ore, respectively. According to the Al2O3eMgO binary phase diagram [18] shown in Fig. 1, spinel (MgAl2O4) can be introduced into the corundum ceramics by adding an appropriate MgO. Series formulas with different contents of MgO were designed as showed in Table 2. 2.3. Experimental procedure About 300 g of the designed sample, 420 mL of water and 900 g of ballstone were mixed and the mixtures were homogenized by wet milling for 15 min under 480 r/min to attain proper particles which have a relatively narrow size distribution (1e8 mm). The average particle size was 5.40 mm. The slurry was oven dried at 120 C. After drying, the dried mixture was crushed and passed through a sieve with an aperture size of 120 meshes. (a): Corundum ceramics were prepared by dry pressing. The mixture was
147
Fig. 1. Phase diagram of MgO and Al2O3.
uniaxially pressed at 16 MPa using water as a binder to prepare 56 mm 10 mm 5 mm rectangular samples. The samples were dried at 80 C for 8 h to ensure them don't crack on the rapid evaporation of water during the sintering process. (b): Ceramic proppants are prepared by sugar-film coating machine. Parts of the fine powders were put into machine to grow into the cores of the proppants for about 2 h. Then the nucleuses were coated densely and grew into spherical green bodies with certain size after rolling 3 h. At last, the ceramics and ceramic proppants were sintered in the box type electrical resistance furnace. Heating rates were 12 C/ min from room temperature to 1250 C and 2 C/min up to sintering temperature. After cooling to room temperature, the samples were tested.
2.4. Characteristic Water absorption (Wa) was measured through immersing the dry fired specimen in the water for 12 h and then calculated by the following Eq. (1).
Wa ¼
ðm1 m0 Þ 100% m0
(1)
where m0 is the weight of the dry fired specimen, m1 is the weight of dry fired specimen which have been immersed in water for 12 h, respectively. The flexure strength was measured by three-point bend test and tested by Engineering Material Strength Tester (Ltd.SGW, Xiangtan Xiangyi Instrument and Meter Co, PR China). The loading rate was 200 N/s. The flexure strength (sf) was calculated by the following Eq. (2).
sf ¼
3FL 2wh2
(2)
where F is the fracture load, L is the distance between the two outer points, w is the width of the fired specimen and h is the height of
Table 1 Chemical composition of the mineral raw materials.
Bauxite Clay Magnesium ore
SiO2
Al2O3
Fe2O3
TiO2
K2O
Na2O
CaO
MgO
IL
8.39 42.33 19.58
70.74 37.4 9.42
1.28 2.53 2.54
4.82 2.95 0.25
0.57 0.6 0.12
0.01 0.23 e
0.23 0.42 2.22
0.23 0.38 47.06
14.12 12.73 19.03
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the fired specimen, respectively. The bulk density (rbulk) was measured by Archimedes' method and calculated with the following formula Eq. (3).
rbulk ¼
mp vcyl
(3)
where mp was the net weight of proppant filled with proppant in the cylinder (g) and vcyl was the volume of the cylinder (cm3). The breakage ratio was measured by the following the SY/ T5108-2014 standards. After crushing, the breakage ratio of the proppants was calculated by the following Eq. (4).
f ¼
mc 100% m0
(4)
where f is the breakage ratio of proppant specimens (%). Mc and M0 are the weight of the particle produced during the experiment (g) and the start weight of proppant specimens (g), respectively. The density was measured by Archimedes' method and electronic densimeter (Ohaus, America). The phase compositions of the specimen were identified by X-ray diffraction (XRD D/Max-2200 PC, Rigaku, Japan) using Cu ka radiation (l ¼ 0.154 nm). The microstructure of the specimen was analyzed by scanning electronic microscope (SEM FEI Verios 460, FEI, America). The sintering process identified by synchronous thermal analyzer (STA409PC, Netzsch, German). 3. Results and discussion 3.1. Phase composition of corundum ceramics Fig. 2 shows the XRD pattern of all samples. Main crystal phase of all samples was confirmed with PDF#46-1212 (Al2O3). It can be demonstrated that the main crystalline phase of all samples was corundum. With the magnesium ore contents increased, the diffraction peaks decreased gradually, which indicated magnesium ore acts as a flux to increase the amount of liquid phase during the heat treatment and promote the sintering process at low temperature. Meanwhile, over the addition of 5 wt% magnesium ore, new diffraction peaks were gradually revealed, new crystal phase was confirmed with PDF#21-1152 (MgAl2O4). When the contents increased to 8 wt%, the spinel contents reached a maximum. 3.2. Microstructure of corundum ceramics The strength of the ceramic generally decreases with the increase of the pores, because the pores not only reduces the loading area, but also causes stress concentration near the pore, which reduces the loading capacity of the ceramic. Therefore, the pore structure plays an important role in the strength of corundum ceramics [19,20]. Here, we modified the Al2O3 system by adding magnesium ore to control pore size. Fig. 3 shows the original fracture surface pore structure of different samples fired at 1350 C for 30 min (without hydrofluoric acid corrosion). The diameter of
Fig. 2. XRD pattern of different samples fired at 1350 C for 30 min.
the most pores was between 2 and 20 mm. The sample only 3 wt% magnesium ore was added, the pores in the sample was larger in diameter, which even greater than 30 mm (Fig. 3(a)). With the magnesium ore contents increased, the diameter of the pores greatly decreased (Fig. 3(b) and (c)). The degree of densification reached a maximum at an additional amount of 7 wt%. Lastly, when the addition of magnesium ore reached 8 wt%, the diameter of pores was slightly increased (Fig. 3(d)). The change of pore structure was closely associated with the liquid phase. Fig. 3 clearly shows that the contents of vitreous increased with the increased of magnesium ore. It can be explained by the addition of MgO to promote formation of the low viscosity liquid phase [21]. The size of pores gradually decrease was caused by filling of the amorphous glass phase as the magnesium ore was increased. Fig. 3(d) clearly shows that MgO acts as a flux to significantly increase the amorphous in the ceramic. During the sintering process, large amount the liquid phase was produced in a short time, and some large-sized pores are quickly wrapped in liquid phase, resulting in an increase in the size of the pores. Defects are mostly present at the grain boundaries, enhance the grain boundaries of corundum crystals were also important to increase the ceramic strength. The sample 1 (3 wt% magnesium ore) was only granular crystals (Fig. 4(a)). Moreover, with magnesium ore was increased, a number of short columnar crystals can be observed between granular crystals (Fig. 4(b) and (c)). The energydispersive spectrometry (EDS) (Fig. 4(d)) of granular crystals and short columnar crystals confirms that they consist of (39.46 mol% Al and 60.54 mol% O) and (10.02 mol% Mg, 23.18 mol% Al and 66.80 mol% O), respectively. They are a nice approximation of the molecular composition of corundum and spinel, respectively. It is known that corundum formation in reaction sintering of Al2O3 is controlled by dissolution-precipitation process [22e24]. We thus conclude that the morphology of corundum will be
Table 2 Designed composition (wt%). Sample
Bauxite
Clay
Magnesium ore
SiO2
Al2O3
Fe2O3þ TiO2
K2O þ Na2O
CaO
MgO
1 2 3 4 5 6
96 95 94 93 92 91
1 1 1 1 1 1
3 4 5 6 7 8
10.53 10.66 10.80 10.93 11.07 11.21
79.62 78.95 78.29 77.62 76.95 76.28
6.96 6.93 6.89 6.86 6.82 6.79
0.66 0.66 0.65 0.65 0.64 0.64
0.34 0.36 0.39 0.41 0.43 0.46
1.90 2.45 2.99 3.54 4.09 4.63
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Fig. 3. Pore structures of different samples original fracture surface fired at 1350 C for 30 min: (a) sample 1, (b) sample 3, (c) sample 5, (d) sample 6.
Fig. 4. Crystal grains identify of different samples fired at 1350 C for 30 min: (a) sample 1, (b) sample 3, (c) sample 5, (d) EDS.
controlled by adjusting dissolution amount of Al2O3. The addition of magnesium ore was promoting formation of the low viscosity liquid phase. Al2O3 moved faster in the low viscosity liquid phase to promote the dissolution-precipitation process. Some small corundum grains were gradually dissolved, and then precipitate on the surface of large grains, which was conducive to the further growth of large grains. Therefore, the sample having higher contents of low viscosity liquid phase has a larger crystal size. However, as can be seen from Fig. 5(a) and (b), corundum grain size of sample
5 with a higher amount glass phase was smaller than the sample 2 with a lower amount glass phase. From the grain size distribution (Fig. 5(c) and (d)), which had been calculated from image analysis of SEM images of sintered samples using Nano Measurer 1.2 software, average grain size of the sample 2 and 5 were calculated to be 5.7 mm and 4.26 mm, respectively. It can be explained that the shortcolumnar spinel pinned at the grain boundary of the corundum to inhibit the precipitation process. The spinel reduces the rate of movement of the grain boundary, which prevents the abnormal
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Fig. 5. Crystal structures and size of different samples fired at 1350 C for 30 min: (a) (c) sample 2, (b) (d) sample 5.
growth of the corundum and makes the grain size small and distribution narrower [25,26]. But in the sample 2, there was no spinel to inhibit the precipitation process, which means that some of the corundum particles grew rapidly in the low viscosity liquid phase, accompanied by dissolution of some of the corundum grains, resulting in a wide grain size distribution, larger grain crystals are likely to generate 5e10 mm pores when aligned (Fig. 5(a)). Based on the above discussion, it can be demonstrated that MgO from magnesium ore acted as flux to increase the liquid phase, which flowed to fill up the pores and accelerate the densification process. When the addition of magnesium ore above 5 wt%, spinel crystals appeared in the samples, the spinel was pinned at the corundum grain boundaries to reduce the rate of corundum grains growth. At the best addition of 7 wt% magnesium ore (sample 5), the size distribution of corundum grains was narrower, and the higher corundum grains packing was obtained. 3.3. Crushing behavior of proppants As a fundamental step toward the understanding of the compression behavior of proppants in hydraulic fracturing, study on the crushing behavior of proppants is essential. Fig. 6 shown the fracture surface of corundum ceramic. The fracture surface is smooth, and on characteristic surface features (cleavage faces) point to the origin of the fracture. On cleavage faces, complete corundum grains wrapped by vitreous phase can be clearly observed. Therefore, corundum crystalline ceramics fail by cleavage along widely spaced, closely packed planes. The ceramic proppants will open tight shale or sandstone formation to form a high flow guiding channel, proppant grains are subjected to compressive stress from the rock wall [27]. Corundum ceramic proppant failure occurs by formation of pores at the
Fig. 6. The fracture surface of corundum ceramic.
intersection of grain boundaries and precipitation of additional pores along grain boundaries by diffusion processes [28,29]. Fig. 7 showed the possible crushing mechanism of ceramic proppant. When ceramic proppant without columnar spinel, corundum crystal size is large, microcracks are generated at the defect and rapidly expand along the grain boundary under the compressive stress, the crack propagation path is short. After introduced the columnar spinel into the ceramic proppant, corundum crystals were refined, crack propagation required through a longer path. In addition, the crack along the grain boundaries encountered the
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Fig. 7. Possible crushing mechanism of ceramic proppants.
columnar spinel which are pinned between corundum grains, part of the crack propagation is prevented, and the other part requires more force to break the columnar spinel. Therefore, spinel can effectively improve the mechanical strength of corundum ceramic proppant. 3.4. Sintering behavior of corundum ceramics It is necessary to study on spinel synthesis temperature due to its play a pivotal role in corundum ceramics. Mixed raw materials have losses in weight and different thermal changes at different temperatures in the process of sintering, these thermal behaviors could be analyzed using TG-DSC. Therefore, we generally use these special points to determine the chemical reactions that occur during sintering. The synthesis temperature of spinel can be inferred by comparing the special points on the TG-DSC curve of the sample 2 (without spinel) and the sample 5 (with spinel) in Fig. 8. In the similar peaks of samples 2 and 5, the first endothermic peak is close to 76 C indicated that the thermal event is raw materials lost adsorbed water corresponding TG curve small decline; the second endothermic peak approximate at 515 C indicated that the thermal event is diaspore lost structural water corresponding TG curve drops sharply [30]; the last exothermic peak about 1215 C is due to corundum crystal transformation corresponding TG curve slow changes. Exothermic peak at 1057.2 C in Fig. 8(b) possibly can be associated with forming of spinel. XRD patterns with different sintering temperature are shown in Fig. 9 which can be proved
151
Fig. 9. XRD patterns of sample 5 with different temperature.
thermal behavior at 1057.2 C of sample 5. The appearance of spinel initially occurred with 1100 C. As the temperature increases, the relative intensity of the characteristic peaks of spinel gradually increases, which indicated a higher crystallinity in higher temperature. The possible reactions occurring in the mixture of bauxitebased ceramic can be described by Eqs. (5)e(7).
2AlOðOHÞ/g Al2 O3 þ H2 O
(5)
MgO þ Al2 O3 /MgAl2 O4
(6)
g Al2 O3 /a Al2 O3
(7)
3.5. Property of corundum ceramics Fig. 10 shown the variation in water absorption at different sintering temperature. With the increase of magnesium ore addition, the sintered initial temperature (water absorption less than 0.5% for the first time) of samples showed a decreasing tendency. At 1350 C, compared water absorption of sample 1 (3 wt% magnesium ore) with sample 3 (5 wt% magnesium ore), it was clear that the result of sample 3 reduced sharply. This is mainly due to the MgO present in magnesium ore to form lower viscosity liquid phase which flowed easily to fill up the pores. When the content of
Fig. 8. TG-DSC curve of the samples: (a) Sample 2, (b) Sample 5.
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Fig. 10. Variation in water absorption with sintering temperature.
magnesium ore more than 5 wt%, the water absorption was not significantly reduced. It can be explained that excess MgO was used to synthesize spinel crystals. Moreover, with the increase of temperature, the water absorption of all samples showed a decreasing tendency before the initial sintering temperature. After the initial sintering temperature, the water absorption was close to 0. The decreased water absorption of samples was due to liquid phase formation which filled the pores and accelerated the densification process. There is no doubt that the increase of sintering temperature can be effectively densification of ceramics, but the fuel is wasted by increasing the sintering temperature. Therefore, it is preferred to heat in the initial sintering temperature for a suitable period of time to improve the densification process. As can be seen from Fig. 10, the initial sintering temperature of most samples was 1350 C, so the sintering system of all the samples was kept at 1350 C for 30 min. Fig. 11 illustrates variation in properties with samples fired at 1350 C for 30 min. The water absorption of all samples was less than 0.5% (Fig. 11(a)), which indicated significant densification was obtained. The density and the flexural strength of the samples were increased first and then decreased with increasing magnesium ore contents (Fig. 11(b) and (c)). However, from samples 1 to 5, the increase rate of density was gradually slow and the increase rate of flexural strength was gradually sharp. This may be result from that the MgO only acted as flux in corundum ceramics when magnesium ore less than 5 wt%, increased liquid was filled pores to rapidly increases the density while increasing the strength. When the magnesium ore was greater than 5 wt%, part of the MgO was used to form the spinel, so the density growth was slowed, but the spinel greatly increased the flexural strength. There are two main reasons: (a) The spinel refined corundum crystal; (b) The spinel prevented crack expanding. Moreover, when the contents of magnesium ore were reached 8 wt%, the main reason for the decreased in density and flexural strength was the formation of a large amount of brittle vitreous and the pore size becomes larger (Fig. 3(d)). At the best additions of 7 wt% magnesium ore (sample 5), the performances of flexural strength and density were reached a maximum of 291.64 MPa and 3.28 g/cm3, respectively. Table 3 is a comparison of the properties of similar ceramics reported and the corundum ceramics (Al2O3 contents 76.95 wt%) we have prepared. Spinel-reinforced corundum ceramics show outstanding performance compared with other bauxite-based ceramics. Meanwhile, the flexural strength obviously higher than the standard of 90 and 95 alumina ceramics (Chinese Electronic
Fig. 11. Variation in properties with samples fired at 1350 C for 30 min: (a) water absorption (b) density (c) flexural strength.
Components Structural Ceramic Materials standard GB/T 55932015), it also has higher or closer to some reported of aluminum ceramics. High flexural strength of spinel enhanced corundum ceramics has broad application prospects for expanding the application field of bauxite-based ceramic materials. At the same time, it is greater significance to reduce the production cost of alumina ceramics. The sample 5 was selected to be the final formula to prepare ceramic proppants. Fig. 12 shown variation in bulk density and breakage ratio of sample 5 with sintered temperatures. Bulk density increased and breakage ratio (under 86 MPa closed pressure) decreased at sintering temperatures of 1320 Ce1350 C. It was due to liquid phase formation which filled the pores to increase densification. For optimum sintering temperature of 1350 C, the performances were 4.88% of breakage ratio and 1.86 g/cm3 of bulk density. According to the Chinese Industry Standards SY/T5108-2014, The proppant breaking rate of the ceramic proppants prepared by using raw bauxite as raw material at 1350 C under the closed pressure of 86 MPa was 4.88% < 9%. It achieved in 12.5 K grade standard ceramic proppant. 4. Conclusions In this study, high strength spinel reinforced corundum ceramics were successfully prepared by reaction sintering using ‘magnesium ore - clay’ catalytic system in bauxite at 1350 C. By optimizing the additive system, this method has the potential to provide a facile and controllable strategy for the preparation of high strength corundum ceramic dense structures. The short columnar spinel mainly enhanced the strength of the corundum ceramics by
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Table 3 Comparison of reported similar samples with our sample.
Bauxite-based ceramic [23] Bauxite-based ceramic [13] Aluminum ceramic [31] Aluminum ceramic [32] GB/T 5593-2015 90/95 Our sample
Main raw materials
Reaction temperature
Phase composition
Flexural strength (MPa)
bauxite, fly ash bauxite high purity Al2O3 powders analytical grade Al2O3 powders e bauxite
1350 C 1600 C 1500 C 1450 C e 1350 C
corundum, mullite corundum, mullite corundum corundum e corundum, spinel
199.89 168 269 220e342 230/280 291.64
[5]
[6]
[7]
[8]
[9]
[10] Fig. 12. Variation in bulk density and breakage ratio of sample 5 with sintered temperatures.
hindering crack propagation. When additions of magnesium ore were 7 wt%, corundum ceramic performances of flexural strength and density were reached a maximum of 291.64 MPa and 3.28 g/ cm3. The performances of proppants were 4.88% of breakage ratio (under the closed pressure of 86 MPa) and 1.86 g/cm3 of bulk density. Hence, high strength spinel reinforced corundum ceramics from this work are a promising candidate for the production of deep well fracturing proppants. We anticipate that this method to synthesize high strength corundum ceramics from bauxite as a general approach has significance for widening the bauxite-based ceramics field.
[11]
[12]
[13]
[14]
[15]
[16]
Acknowledgements This work was financially supported by the Innovation of Science and Technology Plan Projects of Shaanxi Province, China (Grant No. 2013KTDZ03-02-01, 2017TSCXL-GY-07-02), Scientific Research Program Funded by Shaanxi Provincial Education Department (Program No. 18JK0115), Research Starting Foundation from Shaanxi University of Science and Technology (Grant no.2016BJ-41), and the Graduate Innovation Fund of Shaanxi University of Science and Technology (Grant no. SUST-A04).
[17]
[18]
[19]
[20]
References [1] I. Dinaharan, Influence of ceramic particulate type on microstructure and tensile strength of aluminum matrix composites produced using friction stir processing, J. Asian Ceram. Soc. 4 (2016) 209e218, https://doi.org/10.1016/ j.jascer.2016.04.002. [2] H.H. Lee, S.H. Kim, B. Joshi, S.H. Cho, S.W. Lee, High strength and toughness Al2O3 composite materials for the improvement of ceramic channel in SRL hot dipping system, Mater. Sci. Forum 658 (2010) 416e419, https://doi.org/ 10.4028/www.scientific.net/MSF.658.416. [3] H.N. Yoshimura, N.E. Narita, A.L. Molisani, H. Goldenstein, High temperature flexural strength and fracture toughness of AlN with Y2O3 ceramic, J. Mater. Sci. 44 (2009) 5773e5780, https://doi.org/10.1007/s10853-009-3809-9. [4] T. Wu, B. Wu, Corrosion resistance of ceramic proppant in BaO-CaO-P2O5-
[21]
[22]
[23]
[24]
Al2O3 system, Corros. Sci. 63 (2012) 399e403, https://doi.org/10.1016/ j.corsci.2012.06.025. Q. Lü, X. Dong, Z. Zhu, Y. Dong, Environment-oriented low-cost porous mullite ceramic membrane supports fabricated from coal gangue and bauxite, J. Hazard Mater. 273 (2014) 136e145, https://doi.org/10.1016/ j.jhazmat.2014.03.026. J. Chen, H. Zhao, J. Yu, H. Zhang, Z. Li, J. Zhang, Synthesis and characterization of reaction-bonded calcium alumino-titanate-bauxite-SiC composite refractories in a reducing atmosphere, Ceram. Int. 44 (2018) 15338e15345, https://doi.org/10.1016/j.ceramint.2018.05.183. Y. Meng, G. Gong, Z. Wu, Z. Yin, Y. Xie, S. Liu, Fabrication and microstructure investigation of ultra-high-strength porcelain insulator, J. Eur. Ceram. Soc. 32 (2012) 3043e3049, https://doi.org/10.1016/j.jeurceramsoc.2012.04.015. Z.D. Taszic, Improving the microstructural and physical properties of alumina electrical porcelain with Cr2O3, MnO2 and ZnO additives, J. Mater. Sci. 28 (1993) 5693e5701. G. Xiong, B. Wu, T. Wu, Effects of Pr6O11 addition on the acid resistance of ceramic proppant, Materials (Basel) 10 (2017) 81e85, https://doi.org/10.3390/ ma10040427. J. Szymanska, P. Wisniewski, P. Wawulska-Marek, M. Malek, J. Mizera, Selecting key parameters of the green pellets and lightweight ceramic proppants for enhanced shale gas exploitation, Procedia Struct. Integr. 1 (2016) 297e304, https://doi.org/10.1016/j.prostr.2016.02.040. S. Yang, X. Wu, L. Han, J. Wang, Y. Feng, Migration of variable density proppant particles in hydraulic fracture in coal-bed methane reservoir, J. Nat. Gas Sci. Eng. 36 (2016) 662e668, https://doi.org/10.1016/j.jngse.2016.11.009. K.M. Bartko, C.I. Arocha, T.S. Mukherjee, L. Sierra, J.M. Terracina, P. Lord, First application of high density fracturing fluid to stimulate a high pressure and high temperature tight gas producer sandstone formation of Saudi Arabia, SPE Hydraul. Fract. Technol. Conf. (2009), https://doi.org/10.2118/118904-MS. A.V. Maldhure, H.S. Tripathi, A. Ghosh, Mechanical properties of mullitecorundum composites prepared from bauxite, Int. J. Appl. Ceram. Technol. 12 (2015) 860e866, https://doi.org/10.1111/ijac.12291. X. Kong, Y. Tian, Y. Chai, P. Zhao, K. Wang, Z. Li, Effects of pyrolusite additive on the microstructure and mechanical strength of corundum-mullite ceramics, Ceram. Int. 41 (2015) 4294e4300, https://doi.org/10.1016/ j.ceramint.2014.11.116. T. Wu, J. Zhou, B. Wu, Effect of TiO2 content on the acid resistance of a ceramic proppant, Corros. Sci. 98 (2015) 716e724, https://doi.org/10.1016/ j.corsci.2015.06.012. H. Ma, Y. Tian, Y. Zhou, G. Li, K. Wang, P. Bai, Effective reduction of sintering temperature and breakage ratio for a low-cost ceramic proppant by feldspar addition, Int. J. Appl. Ceram. Technol. 15 (2018) 191e196, https://doi.org/ 10.1111/ijac.12774. Z. Liu, J. Zhao, Y. Li, Z. Zeng, J. Mao, Y. Peng, Y. He, Lowerature sintering of bauxite-based fracturing proppants containing CaO and MnO2 additives, Mater. Lett. 171 (2016) 300e303, https://doi.org/10.1016/ j.matlet.2016.02.090. S. Ma, W.Q. Quek, Q.F. Li, Y.F. Zhang, J.Y.H. Fuh, L. Lu, Sintering of translucent alumina, J. Mater. Process. Technol. 209 (2009) 4711e4715, https://doi.org/ 10.1016/j.jmatprotec.2008.10.056. I. Vida-Simiti, N. Jumate, V. Moldovan, G. Thalmaier, N. Sechel, Characterization of gradual porous ceramic structures obtained by powder sedimentation, J. Mater. Sci. Technol. 28 (2012) 362e366, https://doi.org/10.1016/S10050302(12)60068-1. , P. Svan K. Maca, V. Pouchlý, K. Bodisova c arek, D. Galusek, Densification of fine-grained alumina ceramics doped by magnesia, yttria and zirconia evaluated by two different sintering models, J. Eur. Ceram. Soc. 34 (2014) 4363e4372, https://doi.org/10.1016/j.jeurceramsoc.2014.06.030. Y. Dong, S. Hampshire, J. er Zhou, Z. Ji, J. Wang, G. Meng, Sintering and characterization of flyash-based mullite with MgO addition, J. Eur. Ceram. Soc. 31 (2011) 687e695, https://doi.org/10.1016/j.jeurceramsoc.2010.12.012. X. Wu, Z. Huo, Q. Ren, H. Li, F. Lin, T. Wei, Preparation and characterization of ceramic proppants with low density and high strength using fly ash, J. Alloys Compd. 702 (2017) 442e448, https://doi.org/10.1016/j.jallcom.2017.01.262. X. Chen, T. Li, Q. Ren, X. Wu, H. Li, A. Dang, T. Zhao, Y. Shang, Y. Zhang, Mullite whisker network reinforced ceramic with high strength and lightweight, J. Alloys Compd. 700 (2017) 37e42, https://doi.org/10.1016/ j.jallcom.2017.01.075. X. Chen, T. Li, Q. Ren, X. Wu, A. Dang, H. Li, T. Zhao, Fabrication and morphology control of high strength lightweight mullite whisker network,
154
[25]
[26]
[27]
[28]
Q. Ren et al. / Journal of Alloys and Compounds 793 (2019) 146e154 J. Alloys Compd. 729 (2017) 285e292, https://doi.org/10.1016/ j.jallcom.2017.09.150. S. Lahiri, S. Sinhamahapatra, H.S. Tripathi, K. Dana, Rationalizing the role of magnesia and titania on sintering of a-alumina, Ceram. Int. 42 (2016) 15405e15413, https://doi.org/10.1016/j.ceramint.2016.06.189. I. Ahmad, M. Islam, M.A. Dar, F. Xu, S.I. Shah, Y. Zhu, Magnesia tuned multiwalled carbon nanotubes-reinforced alumina nanocomposites, Mater. Char. 99 (2015) 210e219, https://doi.org/10.1016/j.matchar.2014.12.002. R.L. Michalowski, S.S. Nadukuru, Static fatigue, time effects, and delayed increase in penetration resistance after dynamic compaction of sands, J. Geotech. Geoenviron. Eng. 138 (2012) 564e574, https://doi.org/10.1061/ (ASCE)GT.1943-5606.0000611. R.H. Brzesowsky, S.J.T. Hangx, N. Brantut, C.J. Spiers, Compaction creep of sands due to time-dependent grain failure: effects of chemical environment,
[29]
[30]
[31]
[32]
applied stress, and grain size, J. Geophys. Res. Solid Earth 119 (2014) 7521e7541, https://doi.org/10.1002/2014JB011277. S. Man, R. Chik-Kwong Wong, Compression and crushing behavior of ceramic proppants and sand under high stresses, J. Pet. Sci. Eng. 158 (2017) 268e283, https://doi.org/10.1016/j.petrol.2017.08.052. Q. Ren, H. Li, X. Wu, Z. Huo, O. Hai, F. Lin, Effect of the calcining temperatures of low-grade bauxite on the mechanical property of mullite ceramics, Int. J. Appl. Ceram. Technol. 15 (2018) 554e562, https://doi.org/10.1111/ijac.12815. C.J. Xiao, H.T. Zhang, L.Y. Zhu, Z.X. Li, Effect of BaTiO3 addition on mechanical properties of BaTiO3/Al2O3 composite, Appl. Mech. Mater. 422 (2013) 57e61, https://doi.org/10.4028/www.scientific.net/AMM.422.57. H. Li, X. Xi, J. Ma, K. Hua, A. Shui, Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength, Ceram. Int. 43 (2017) 5108e5114, https://doi.org/10.1016/j.ceramint.2017.01.024.
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Effect mechanism of spinel (MgAl2O4) reinforced corundum ceramics on microstructure and properties Qiang Ren, Yuhan Ren*, Xiulan Wu, Wenni Bai, Jinle Zheng, Ou Hai** School of Materials Science and Engineering, Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, Shaanxi University of Science and Technology, Xi'an, 710021, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 January 2019 Received in revised form 4 April 2019 Accepted 15 April 2019 Available online 18 April 2019
High strength spinel reinforced corundum ceramics were successfully prepared by using a bauxite mixture with additions of varying mixtures of magnesium ore - clay. Controlled corundum grains were obtained tightly arranged crystal structure by controlling the rate of Al2O3 dissolution-precipitation process through magnesium ore addition in ceramic system. At the same time, the size of the pores was controlled by adjusting the content of the liquid phase in the system. Finally, high strength ceramics were obtained by modifying corundum grain boundary through controlling the content of the magnesium ore. The effects of magnesium ore on microstructure and properties of corundum ceramics were studied. The phase compositions and microstructures were investigated by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Fracture mechanism and sintering behavior of corundum ceramics were preliminary analyzed. The flexural strength, density and water absorption of ceramics as functions of magnesium ore contents were systematically investigated. When additions of magnesium ore were 7 wt%, corundum ceramic performances of flexural strength and density were reached a maximum of 291.64 MPa and 3.28 g/cm3. High strength corundum ceramics from bauxite are important to broaden the application fields of bauxite-based ceramics. Alternatively, the formula of the highest strength ceramic (7 wt% magnesium ore) was selected to be the final formula to manufacture proppants, the performances were 4.88% of breakage ratio (under the closed pressure of 86 MPa) and 1.86 g/cm3 of bulk density. The approach opens new opportunities for corundum ceramic as a proppant material since the ceramic system displays high strength. © 2019 Elsevier B.V. All rights reserved.
Keywords: Corundum ceramics Spinel Flexure strength Microstructure Proppants Bauxite
1. Introduction Corundum ceramic is one of the widest application range materials due to its many excellent properties, such as high mechanical strength, high melting point, high chemical stability, and good thermal shock resistance [1e4]. It can be widely used in abrasive, refractory, electric porcelain and optics materials owing to those outstanding performance [5e9]. Proppant is one of the important areas of corundum ceramic for hydraulic fracturing in oil and gas production. However, large of oil and gas was buried below 4 km, most of these oilfields are low-permeability oil and gas reservoirs with high well temperature and high closing pressure, such deep well oil and gas fields require high strength and low breakage
** Corresponding author. * Corresponding author. E-mail addresses: [email protected] (Y. Ren), [email protected] (O. Hai). https://doi.org/10.1016/j.jallcom.2019.04.151 0925-8388/© 2019 Elsevier B.V. All rights reserved.
proppants [10e12]. It is generally to increase the strength by increasing the alumina content in proppants, and some proppants were prepared by using pure alumina, which undoubtedly increases the cost. Consequently, it is necessary to improve the mechanical strength and microstructure design of corundum ceramics. In general, corundum ceramics can be achieved from bauxite minerals or pure alumina sintered more than 1500 C [13]. However, there are few studies for research on high strength corundum ceramic sintered below 1400 C. Adding suitable additives can effectively reduce the firing temperature and improve properties when preparing ceramics. These additives are usually divided into three types according to their main roles in sintering. The first type is to form a solid solution. Additive ions (Fe3þ, Mn4þ, Ti4þ, etc.) enter the corundum to distortion of host lattices to promote crystal growth. Xiangchen Kong et al. [14] investigated the pyrolusite additive on the microstructure and mechanical strength of corundumemullite ceramics supports by bauxite. Ceramic support exhibits relative density reaches approximately 91% and
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compressive strength reaches 20 MPa. Tingting Wu et al. [15] have reported the acid resistance of ceramic proppants produced by commercial Al2O3 powder and TiO2 have the acid solubility as low as 0.13 wt% heated at 1500 C. The second type is to form a liquid phase, such as Na2O, CaO, MgO, etc, promoting the formation of a liquid phase in the sintering process to densify the ceramics. Haiqiang Ma et al. [16] researched a low-cost bauxite-based ceramic proppant by feldspar addition, the result shown that the sintering temperature reduced by 100 C when 4 wt% feldspar was added compared with the proppants without adding feldspar. Zuolei Liu et al. [17] fabricated low-temperature sintering of bauxite-based fracturing proppants containing CaO and MnO2 additives with apparent breakage ratio of less than 7% at 69 MPa. The third type is to form a new phase to enhance ceramics. The first and second classes additives were widely used in the corundum ceramics. But the third type additive was rarely reported in corundum ceramics especially applying in ceramic proppants. Therefore, this study aims to further improve the performance of corundum ceramic, which make possible to prepare of highdensity, high-strength ceramic proppants for used in deep well fracturing. In our work, with the additive of magnesium ore were prepared spinel reinforce corundum ceramics by bauxite. The effects of magnesium ore on microstructures and properties of corundum ceramic samples were systematically investigated. Meanwhile, fracture mechanism and sintering behavior of spinel enhanced corundum ceramics were further discussed. Based on the optimization crystal structure and pore structure of corundum ceramic, ceramic proppants were synthesized and their properties were also studied. 2. Material and methods 2.1. Raw materials The raw materials were obtained from Dianchi Dehui Petroleum Proppant Co., Ltd., Sanmenxia City, Henan Province, China. Chemical compositions of them were displayed in Table 1. 2.2. Materials design Al2O3 and MgO were provided by bauxite and magnesium ore, respectively. According to the Al2O3eMgO binary phase diagram [18] shown in Fig. 1, spinel (MgAl2O4) can be introduced into the corundum ceramics by adding an appropriate MgO. Series formulas with different contents of MgO were designed as showed in Table 2. 2.3. Experimental procedure About 300 g of the designed sample, 420 mL of water and 900 g of ballstone were mixed and the mixtures were homogenized by wet milling for 15 min under 480 r/min to attain proper particles which have a relatively narrow size distribution (1e8 mm). The average particle size was 5.40 mm. The slurry was oven dried at 120 C. After drying, the dried mixture was crushed and passed through a sieve with an aperture size of 120 meshes. (a): Corundum ceramics were prepared by dry pressing. The mixture was
147
Fig. 1. Phase diagram of MgO and Al2O3.
uniaxially pressed at 16 MPa using water as a binder to prepare 56 mm 10 mm 5 mm rectangular samples. The samples were dried at 80 C for 8 h to ensure them don't crack on the rapid evaporation of water during the sintering process. (b): Ceramic proppants are prepared by sugar-film coating machine. Parts of the fine powders were put into machine to grow into the cores of the proppants for about 2 h. Then the nucleuses were coated densely and grew into spherical green bodies with certain size after rolling 3 h. At last, the ceramics and ceramic proppants were sintered in the box type electrical resistance furnace. Heating rates were 12 C/ min from room temperature to 1250 C and 2 C/min up to sintering temperature. After cooling to room temperature, the samples were tested.
2.4. Characteristic Water absorption (Wa) was measured through immersing the dry fired specimen in the water for 12 h and then calculated by the following Eq. (1).
Wa ¼
ðm1 m0 Þ 100% m0
(1)
where m0 is the weight of the dry fired specimen, m1 is the weight of dry fired specimen which have been immersed in water for 12 h, respectively. The flexure strength was measured by three-point bend test and tested by Engineering Material Strength Tester (Ltd.SGW, Xiangtan Xiangyi Instrument and Meter Co, PR China). The loading rate was 200 N/s. The flexure strength (sf) was calculated by the following Eq. (2).
sf ¼
3FL 2wh2
(2)
where F is the fracture load, L is the distance between the two outer points, w is the width of the fired specimen and h is the height of
Table 1 Chemical composition of the mineral raw materials.
Bauxite Clay Magnesium ore
SiO2
Al2O3
Fe2O3
TiO2
K2O
Na2O
CaO
MgO
IL
8.39 42.33 19.58
70.74 37.4 9.42
1.28 2.53 2.54
4.82 2.95 0.25
0.57 0.6 0.12
0.01 0.23 e
0.23 0.42 2.22
0.23 0.38 47.06
14.12 12.73 19.03
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the fired specimen, respectively. The bulk density (rbulk) was measured by Archimedes' method and calculated with the following formula Eq. (3).
rbulk ¼
mp vcyl
(3)
where mp was the net weight of proppant filled with proppant in the cylinder (g) and vcyl was the volume of the cylinder (cm3). The breakage ratio was measured by the following the SY/ T5108-2014 standards. After crushing, the breakage ratio of the proppants was calculated by the following Eq. (4).
f ¼
mc 100% m0
(4)
where f is the breakage ratio of proppant specimens (%). Mc and M0 are the weight of the particle produced during the experiment (g) and the start weight of proppant specimens (g), respectively. The density was measured by Archimedes' method and electronic densimeter (Ohaus, America). The phase compositions of the specimen were identified by X-ray diffraction (XRD D/Max-2200 PC, Rigaku, Japan) using Cu ka radiation (l ¼ 0.154 nm). The microstructure of the specimen was analyzed by scanning electronic microscope (SEM FEI Verios 460, FEI, America). The sintering process identified by synchronous thermal analyzer (STA409PC, Netzsch, German). 3. Results and discussion 3.1. Phase composition of corundum ceramics Fig. 2 shows the XRD pattern of all samples. Main crystal phase of all samples was confirmed with PDF#46-1212 (Al2O3). It can be demonstrated that the main crystalline phase of all samples was corundum. With the magnesium ore contents increased, the diffraction peaks decreased gradually, which indicated magnesium ore acts as a flux to increase the amount of liquid phase during the heat treatment and promote the sintering process at low temperature. Meanwhile, over the addition of 5 wt% magnesium ore, new diffraction peaks were gradually revealed, new crystal phase was confirmed with PDF#21-1152 (MgAl2O4). When the contents increased to 8 wt%, the spinel contents reached a maximum. 3.2. Microstructure of corundum ceramics The strength of the ceramic generally decreases with the increase of the pores, because the pores not only reduces the loading area, but also causes stress concentration near the pore, which reduces the loading capacity of the ceramic. Therefore, the pore structure plays an important role in the strength of corundum ceramics [19,20]. Here, we modified the Al2O3 system by adding magnesium ore to control pore size. Fig. 3 shows the original fracture surface pore structure of different samples fired at 1350 C for 30 min (without hydrofluoric acid corrosion). The diameter of
Fig. 2. XRD pattern of different samples fired at 1350 C for 30 min.
the most pores was between 2 and 20 mm. The sample only 3 wt% magnesium ore was added, the pores in the sample was larger in diameter, which even greater than 30 mm (Fig. 3(a)). With the magnesium ore contents increased, the diameter of the pores greatly decreased (Fig. 3(b) and (c)). The degree of densification reached a maximum at an additional amount of 7 wt%. Lastly, when the addition of magnesium ore reached 8 wt%, the diameter of pores was slightly increased (Fig. 3(d)). The change of pore structure was closely associated with the liquid phase. Fig. 3 clearly shows that the contents of vitreous increased with the increased of magnesium ore. It can be explained by the addition of MgO to promote formation of the low viscosity liquid phase [21]. The size of pores gradually decrease was caused by filling of the amorphous glass phase as the magnesium ore was increased. Fig. 3(d) clearly shows that MgO acts as a flux to significantly increase the amorphous in the ceramic. During the sintering process, large amount the liquid phase was produced in a short time, and some large-sized pores are quickly wrapped in liquid phase, resulting in an increase in the size of the pores. Defects are mostly present at the grain boundaries, enhance the grain boundaries of corundum crystals were also important to increase the ceramic strength. The sample 1 (3 wt% magnesium ore) was only granular crystals (Fig. 4(a)). Moreover, with magnesium ore was increased, a number of short columnar crystals can be observed between granular crystals (Fig. 4(b) and (c)). The energydispersive spectrometry (EDS) (Fig. 4(d)) of granular crystals and short columnar crystals confirms that they consist of (39.46 mol% Al and 60.54 mol% O) and (10.02 mol% Mg, 23.18 mol% Al and 66.80 mol% O), respectively. They are a nice approximation of the molecular composition of corundum and spinel, respectively. It is known that corundum formation in reaction sintering of Al2O3 is controlled by dissolution-precipitation process [22e24]. We thus conclude that the morphology of corundum will be
Table 2 Designed composition (wt%). Sample
Bauxite
Clay
Magnesium ore
SiO2
Al2O3
Fe2O3þ TiO2
K2O þ Na2O
CaO
MgO
1 2 3 4 5 6
96 95 94 93 92 91
1 1 1 1 1 1
3 4 5 6 7 8
10.53 10.66 10.80 10.93 11.07 11.21
79.62 78.95 78.29 77.62 76.95 76.28
6.96 6.93 6.89 6.86 6.82 6.79
0.66 0.66 0.65 0.65 0.64 0.64
0.34 0.36 0.39 0.41 0.43 0.46
1.90 2.45 2.99 3.54 4.09 4.63
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Fig. 3. Pore structures of different samples original fracture surface fired at 1350 C for 30 min: (a) sample 1, (b) sample 3, (c) sample 5, (d) sample 6.
Fig. 4. Crystal grains identify of different samples fired at 1350 C for 30 min: (a) sample 1, (b) sample 3, (c) sample 5, (d) EDS.
controlled by adjusting dissolution amount of Al2O3. The addition of magnesium ore was promoting formation of the low viscosity liquid phase. Al2O3 moved faster in the low viscosity liquid phase to promote the dissolution-precipitation process. Some small corundum grains were gradually dissolved, and then precipitate on the surface of large grains, which was conducive to the further growth of large grains. Therefore, the sample having higher contents of low viscosity liquid phase has a larger crystal size. However, as can be seen from Fig. 5(a) and (b), corundum grain size of sample
5 with a higher amount glass phase was smaller than the sample 2 with a lower amount glass phase. From the grain size distribution (Fig. 5(c) and (d)), which had been calculated from image analysis of SEM images of sintered samples using Nano Measurer 1.2 software, average grain size of the sample 2 and 5 were calculated to be 5.7 mm and 4.26 mm, respectively. It can be explained that the shortcolumnar spinel pinned at the grain boundary of the corundum to inhibit the precipitation process. The spinel reduces the rate of movement of the grain boundary, which prevents the abnormal
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Fig. 5. Crystal structures and size of different samples fired at 1350 C for 30 min: (a) (c) sample 2, (b) (d) sample 5.
growth of the corundum and makes the grain size small and distribution narrower [25,26]. But in the sample 2, there was no spinel to inhibit the precipitation process, which means that some of the corundum particles grew rapidly in the low viscosity liquid phase, accompanied by dissolution of some of the corundum grains, resulting in a wide grain size distribution, larger grain crystals are likely to generate 5e10 mm pores when aligned (Fig. 5(a)). Based on the above discussion, it can be demonstrated that MgO from magnesium ore acted as flux to increase the liquid phase, which flowed to fill up the pores and accelerate the densification process. When the addition of magnesium ore above 5 wt%, spinel crystals appeared in the samples, the spinel was pinned at the corundum grain boundaries to reduce the rate of corundum grains growth. At the best addition of 7 wt% magnesium ore (sample 5), the size distribution of corundum grains was narrower, and the higher corundum grains packing was obtained. 3.3. Crushing behavior of proppants As a fundamental step toward the understanding of the compression behavior of proppants in hydraulic fracturing, study on the crushing behavior of proppants is essential. Fig. 6 shown the fracture surface of corundum ceramic. The fracture surface is smooth, and on characteristic surface features (cleavage faces) point to the origin of the fracture. On cleavage faces, complete corundum grains wrapped by vitreous phase can be clearly observed. Therefore, corundum crystalline ceramics fail by cleavage along widely spaced, closely packed planes. The ceramic proppants will open tight shale or sandstone formation to form a high flow guiding channel, proppant grains are subjected to compressive stress from the rock wall [27]. Corundum ceramic proppant failure occurs by formation of pores at the
Fig. 6. The fracture surface of corundum ceramic.
intersection of grain boundaries and precipitation of additional pores along grain boundaries by diffusion processes [28,29]. Fig. 7 showed the possible crushing mechanism of ceramic proppant. When ceramic proppant without columnar spinel, corundum crystal size is large, microcracks are generated at the defect and rapidly expand along the grain boundary under the compressive stress, the crack propagation path is short. After introduced the columnar spinel into the ceramic proppant, corundum crystals were refined, crack propagation required through a longer path. In addition, the crack along the grain boundaries encountered the
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Fig. 7. Possible crushing mechanism of ceramic proppants.
columnar spinel which are pinned between corundum grains, part of the crack propagation is prevented, and the other part requires more force to break the columnar spinel. Therefore, spinel can effectively improve the mechanical strength of corundum ceramic proppant. 3.4. Sintering behavior of corundum ceramics It is necessary to study on spinel synthesis temperature due to its play a pivotal role in corundum ceramics. Mixed raw materials have losses in weight and different thermal changes at different temperatures in the process of sintering, these thermal behaviors could be analyzed using TG-DSC. Therefore, we generally use these special points to determine the chemical reactions that occur during sintering. The synthesis temperature of spinel can be inferred by comparing the special points on the TG-DSC curve of the sample 2 (without spinel) and the sample 5 (with spinel) in Fig. 8. In the similar peaks of samples 2 and 5, the first endothermic peak is close to 76 C indicated that the thermal event is raw materials lost adsorbed water corresponding TG curve small decline; the second endothermic peak approximate at 515 C indicated that the thermal event is diaspore lost structural water corresponding TG curve drops sharply [30]; the last exothermic peak about 1215 C is due to corundum crystal transformation corresponding TG curve slow changes. Exothermic peak at 1057.2 C in Fig. 8(b) possibly can be associated with forming of spinel. XRD patterns with different sintering temperature are shown in Fig. 9 which can be proved
151
Fig. 9. XRD patterns of sample 5 with different temperature.
thermal behavior at 1057.2 C of sample 5. The appearance of spinel initially occurred with 1100 C. As the temperature increases, the relative intensity of the characteristic peaks of spinel gradually increases, which indicated a higher crystallinity in higher temperature. The possible reactions occurring in the mixture of bauxitebased ceramic can be described by Eqs. (5)e(7).
2AlOðOHÞ/g Al2 O3 þ H2 O
(5)
MgO þ Al2 O3 /MgAl2 O4
(6)
g Al2 O3 /a Al2 O3
(7)
3.5. Property of corundum ceramics Fig. 10 shown the variation in water absorption at different sintering temperature. With the increase of magnesium ore addition, the sintered initial temperature (water absorption less than 0.5% for the first time) of samples showed a decreasing tendency. At 1350 C, compared water absorption of sample 1 (3 wt% magnesium ore) with sample 3 (5 wt% magnesium ore), it was clear that the result of sample 3 reduced sharply. This is mainly due to the MgO present in magnesium ore to form lower viscosity liquid phase which flowed easily to fill up the pores. When the content of
Fig. 8. TG-DSC curve of the samples: (a) Sample 2, (b) Sample 5.
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Fig. 10. Variation in water absorption with sintering temperature.
magnesium ore more than 5 wt%, the water absorption was not significantly reduced. It can be explained that excess MgO was used to synthesize spinel crystals. Moreover, with the increase of temperature, the water absorption of all samples showed a decreasing tendency before the initial sintering temperature. After the initial sintering temperature, the water absorption was close to 0. The decreased water absorption of samples was due to liquid phase formation which filled the pores and accelerated the densification process. There is no doubt that the increase of sintering temperature can be effectively densification of ceramics, but the fuel is wasted by increasing the sintering temperature. Therefore, it is preferred to heat in the initial sintering temperature for a suitable period of time to improve the densification process. As can be seen from Fig. 10, the initial sintering temperature of most samples was 1350 C, so the sintering system of all the samples was kept at 1350 C for 30 min. Fig. 11 illustrates variation in properties with samples fired at 1350 C for 30 min. The water absorption of all samples was less than 0.5% (Fig. 11(a)), which indicated significant densification was obtained. The density and the flexural strength of the samples were increased first and then decreased with increasing magnesium ore contents (Fig. 11(b) and (c)). However, from samples 1 to 5, the increase rate of density was gradually slow and the increase rate of flexural strength was gradually sharp. This may be result from that the MgO only acted as flux in corundum ceramics when magnesium ore less than 5 wt%, increased liquid was filled pores to rapidly increases the density while increasing the strength. When the magnesium ore was greater than 5 wt%, part of the MgO was used to form the spinel, so the density growth was slowed, but the spinel greatly increased the flexural strength. There are two main reasons: (a) The spinel refined corundum crystal; (b) The spinel prevented crack expanding. Moreover, when the contents of magnesium ore were reached 8 wt%, the main reason for the decreased in density and flexural strength was the formation of a large amount of brittle vitreous and the pore size becomes larger (Fig. 3(d)). At the best additions of 7 wt% magnesium ore (sample 5), the performances of flexural strength and density were reached a maximum of 291.64 MPa and 3.28 g/cm3, respectively. Table 3 is a comparison of the properties of similar ceramics reported and the corundum ceramics (Al2O3 contents 76.95 wt%) we have prepared. Spinel-reinforced corundum ceramics show outstanding performance compared with other bauxite-based ceramics. Meanwhile, the flexural strength obviously higher than the standard of 90 and 95 alumina ceramics (Chinese Electronic
Fig. 11. Variation in properties with samples fired at 1350 C for 30 min: (a) water absorption (b) density (c) flexural strength.
Components Structural Ceramic Materials standard GB/T 55932015), it also has higher or closer to some reported of aluminum ceramics. High flexural strength of spinel enhanced corundum ceramics has broad application prospects for expanding the application field of bauxite-based ceramic materials. At the same time, it is greater significance to reduce the production cost of alumina ceramics. The sample 5 was selected to be the final formula to prepare ceramic proppants. Fig. 12 shown variation in bulk density and breakage ratio of sample 5 with sintered temperatures. Bulk density increased and breakage ratio (under 86 MPa closed pressure) decreased at sintering temperatures of 1320 Ce1350 C. It was due to liquid phase formation which filled the pores to increase densification. For optimum sintering temperature of 1350 C, the performances were 4.88% of breakage ratio and 1.86 g/cm3 of bulk density. According to the Chinese Industry Standards SY/T5108-2014, The proppant breaking rate of the ceramic proppants prepared by using raw bauxite as raw material at 1350 C under the closed pressure of 86 MPa was 4.88% < 9%. It achieved in 12.5 K grade standard ceramic proppant. 4. Conclusions In this study, high strength spinel reinforced corundum ceramics were successfully prepared by reaction sintering using ‘magnesium ore - clay’ catalytic system in bauxite at 1350 C. By optimizing the additive system, this method has the potential to provide a facile and controllable strategy for the preparation of high strength corundum ceramic dense structures. The short columnar spinel mainly enhanced the strength of the corundum ceramics by
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Table 3 Comparison of reported similar samples with our sample.
Bauxite-based ceramic [23] Bauxite-based ceramic [13] Aluminum ceramic [31] Aluminum ceramic [32] GB/T 5593-2015 90/95 Our sample
Main raw materials
Reaction temperature
Phase composition
Flexural strength (MPa)
bauxite, fly ash bauxite high purity Al2O3 powders analytical grade Al2O3 powders e bauxite
1350 C 1600 C 1500 C 1450 C e 1350 C
corundum, mullite corundum, mullite corundum corundum e corundum, spinel
199.89 168 269 220e342 230/280 291.64
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[10] Fig. 12. Variation in bulk density and breakage ratio of sample 5 with sintered temperatures.
hindering crack propagation. When additions of magnesium ore were 7 wt%, corundum ceramic performances of flexural strength and density were reached a maximum of 291.64 MPa and 3.28 g/ cm3. The performances of proppants were 4.88% of breakage ratio (under the closed pressure of 86 MPa) and 1.86 g/cm3 of bulk density. Hence, high strength spinel reinforced corundum ceramics from this work are a promising candidate for the production of deep well fracturing proppants. We anticipate that this method to synthesize high strength corundum ceramics from bauxite as a general approach has significance for widening the bauxite-based ceramics field.
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[13]
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Acknowledgements This work was financially supported by the Innovation of Science and Technology Plan Projects of Shaanxi Province, China (Grant No. 2013KTDZ03-02-01, 2017TSCXL-GY-07-02), Scientific Research Program Funded by Shaanxi Provincial Education Department (Program No. 18JK0115), Research Starting Foundation from Shaanxi University of Science and Technology (Grant no.2016BJ-41), and the Graduate Innovation Fund of Shaanxi University of Science and Technology (Grant no. SUST-A04).
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References [1] I. Dinaharan, Influence of ceramic particulate type on microstructure and tensile strength of aluminum matrix composites produced using friction stir processing, J. Asian Ceram. Soc. 4 (2016) 209e218, https://doi.org/10.1016/ j.jascer.2016.04.002. [2] H.H. Lee, S.H. Kim, B. Joshi, S.H. Cho, S.W. Lee, High strength and toughness Al2O3 composite materials for the improvement of ceramic channel in SRL hot dipping system, Mater. Sci. Forum 658 (2010) 416e419, https://doi.org/ 10.4028/www.scientific.net/MSF.658.416. [3] H.N. Yoshimura, N.E. Narita, A.L. Molisani, H. Goldenstein, High temperature flexural strength and fracture toughness of AlN with Y2O3 ceramic, J. Mater. Sci. 44 (2009) 5773e5780, https://doi.org/10.1007/s10853-009-3809-9. [4] T. Wu, B. Wu, Corrosion resistance of ceramic proppant in BaO-CaO-P2O5-
[21]
[22]
[23]
[24]
Al2O3 system, Corros. Sci. 63 (2012) 399e403, https://doi.org/10.1016/ j.corsci.2012.06.025. Q. Lü, X. Dong, Z. Zhu, Y. Dong, Environment-oriented low-cost porous mullite ceramic membrane supports fabricated from coal gangue and bauxite, J. Hazard Mater. 273 (2014) 136e145, https://doi.org/10.1016/ j.jhazmat.2014.03.026. J. Chen, H. Zhao, J. Yu, H. Zhang, Z. Li, J. Zhang, Synthesis and characterization of reaction-bonded calcium alumino-titanate-bauxite-SiC composite refractories in a reducing atmosphere, Ceram. Int. 44 (2018) 15338e15345, https://doi.org/10.1016/j.ceramint.2018.05.183. Y. Meng, G. Gong, Z. Wu, Z. Yin, Y. Xie, S. Liu, Fabrication and microstructure investigation of ultra-high-strength porcelain insulator, J. Eur. Ceram. Soc. 32 (2012) 3043e3049, https://doi.org/10.1016/j.jeurceramsoc.2012.04.015. Z.D. Taszic, Improving the microstructural and physical properties of alumina electrical porcelain with Cr2O3, MnO2 and ZnO additives, J. Mater. Sci. 28 (1993) 5693e5701. G. Xiong, B. Wu, T. Wu, Effects of Pr6O11 addition on the acid resistance of ceramic proppant, Materials (Basel) 10 (2017) 81e85, https://doi.org/10.3390/ ma10040427. J. Szymanska, P. Wisniewski, P. Wawulska-Marek, M. Malek, J. Mizera, Selecting key parameters of the green pellets and lightweight ceramic proppants for enhanced shale gas exploitation, Procedia Struct. Integr. 1 (2016) 297e304, https://doi.org/10.1016/j.prostr.2016.02.040. S. Yang, X. Wu, L. Han, J. Wang, Y. Feng, Migration of variable density proppant particles in hydraulic fracture in coal-bed methane reservoir, J. Nat. Gas Sci. Eng. 36 (2016) 662e668, https://doi.org/10.1016/j.jngse.2016.11.009. K.M. Bartko, C.I. Arocha, T.S. Mukherjee, L. Sierra, J.M. Terracina, P. Lord, First application of high density fracturing fluid to stimulate a high pressure and high temperature tight gas producer sandstone formation of Saudi Arabia, SPE Hydraul. Fract. Technol. Conf. (2009), https://doi.org/10.2118/118904-MS. A.V. Maldhure, H.S. Tripathi, A. Ghosh, Mechanical properties of mullitecorundum composites prepared from bauxite, Int. J. Appl. Ceram. Technol. 12 (2015) 860e866, https://doi.org/10.1111/ijac.12291. X. Kong, Y. Tian, Y. Chai, P. Zhao, K. Wang, Z. Li, Effects of pyrolusite additive on the microstructure and mechanical strength of corundum-mullite ceramics, Ceram. Int. 41 (2015) 4294e4300, https://doi.org/10.1016/ j.ceramint.2014.11.116. T. Wu, J. Zhou, B. Wu, Effect of TiO2 content on the acid resistance of a ceramic proppant, Corros. Sci. 98 (2015) 716e724, https://doi.org/10.1016/ j.corsci.2015.06.012. H. Ma, Y. Tian, Y. Zhou, G. Li, K. Wang, P. Bai, Effective reduction of sintering temperature and breakage ratio for a low-cost ceramic proppant by feldspar addition, Int. J. Appl. Ceram. Technol. 15 (2018) 191e196, https://doi.org/ 10.1111/ijac.12774. Z. Liu, J. Zhao, Y. Li, Z. Zeng, J. Mao, Y. Peng, Y. He, Lowerature sintering of bauxite-based fracturing proppants containing CaO and MnO2 additives, Mater. Lett. 171 (2016) 300e303, https://doi.org/10.1016/ j.matlet.2016.02.090. S. Ma, W.Q. Quek, Q.F. Li, Y.F. Zhang, J.Y.H. Fuh, L. Lu, Sintering of translucent alumina, J. Mater. Process. Technol. 209 (2009) 4711e4715, https://doi.org/ 10.1016/j.jmatprotec.2008.10.056. I. Vida-Simiti, N. Jumate, V. Moldovan, G. Thalmaier, N. Sechel, Characterization of gradual porous ceramic structures obtained by powder sedimentation, J. Mater. Sci. Technol. 28 (2012) 362e366, https://doi.org/10.1016/S10050302(12)60068-1. , P. Svan K. Maca, V. Pouchlý, K. Bodisova c arek, D. Galusek, Densification of fine-grained alumina ceramics doped by magnesia, yttria and zirconia evaluated by two different sintering models, J. Eur. Ceram. Soc. 34 (2014) 4363e4372, https://doi.org/10.1016/j.jeurceramsoc.2014.06.030. Y. Dong, S. Hampshire, J. er Zhou, Z. Ji, J. Wang, G. Meng, Sintering and characterization of flyash-based mullite with MgO addition, J. Eur. Ceram. Soc. 31 (2011) 687e695, https://doi.org/10.1016/j.jeurceramsoc.2010.12.012. X. Wu, Z. Huo, Q. Ren, H. Li, F. Lin, T. Wei, Preparation and characterization of ceramic proppants with low density and high strength using fly ash, J. Alloys Compd. 702 (2017) 442e448, https://doi.org/10.1016/j.jallcom.2017.01.262. X. Chen, T. Li, Q. Ren, X. Wu, H. Li, A. Dang, T. Zhao, Y. Shang, Y. Zhang, Mullite whisker network reinforced ceramic with high strength and lightweight, J. Alloys Compd. 700 (2017) 37e42, https://doi.org/10.1016/ j.jallcom.2017.01.075. X. Chen, T. Li, Q. Ren, X. Wu, A. Dang, H. Li, T. Zhao, Fabrication and morphology control of high strength lightweight mullite whisker network,
154
[25]
[26]
[27]
[28]
Q. Ren et al. / Journal of Alloys and Compounds 793 (2019) 146e154 J. Alloys Compd. 729 (2017) 285e292, https://doi.org/10.1016/ j.jallcom.2017.09.150. S. Lahiri, S. Sinhamahapatra, H.S. Tripathi, K. Dana, Rationalizing the role of magnesia and titania on sintering of a-alumina, Ceram. Int. 42 (2016) 15405e15413, https://doi.org/10.1016/j.ceramint.2016.06.189. I. Ahmad, M. Islam, M.A. Dar, F. Xu, S.I. Shah, Y. Zhu, Magnesia tuned multiwalled carbon nanotubes-reinforced alumina nanocomposites, Mater. Char. 99 (2015) 210e219, https://doi.org/10.1016/j.matchar.2014.12.002. R.L. Michalowski, S.S. Nadukuru, Static fatigue, time effects, and delayed increase in penetration resistance after dynamic compaction of sands, J. Geotech. Geoenviron. Eng. 138 (2012) 564e574, https://doi.org/10.1061/ (ASCE)GT.1943-5606.0000611. R.H. Brzesowsky, S.J.T. Hangx, N. Brantut, C.J. Spiers, Compaction creep of sands due to time-dependent grain failure: effects of chemical environment,
[29]
[30]
[31]
[32]
applied stress, and grain size, J. Geophys. Res. Solid Earth 119 (2014) 7521e7541, https://doi.org/10.1002/2014JB011277. S. Man, R. Chik-Kwong Wong, Compression and crushing behavior of ceramic proppants and sand under high stresses, J. Pet. Sci. Eng. 158 (2017) 268e283, https://doi.org/10.1016/j.petrol.2017.08.052. Q. Ren, H. Li, X. Wu, Z. Huo, O. Hai, F. Lin, Effect of the calcining temperatures of low-grade bauxite on the mechanical property of mullite ceramics, Int. J. Appl. Ceram. Technol. 15 (2018) 554e562, https://doi.org/10.1111/ijac.12815. C.J. Xiao, H.T. Zhang, L.Y. Zhu, Z.X. Li, Effect of BaTiO3 addition on mechanical properties of BaTiO3/Al2O3 composite, Appl. Mech. Mater. 422 (2013) 57e61, https://doi.org/10.4028/www.scientific.net/AMM.422.57. H. Li, X. Xi, J. Ma, K. Hua, A. Shui, Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength, Ceram. Int. 43 (2017) 5108e5114, https://doi.org/10.1016/j.ceramint.2017.01.024.