Effect of Lithium Nitrate on the Structure and Property of α-Al2O3Platelets Prepared via Solid-state Reaction

Miao Zhuang; Shi Jiangong; Miao Qingyuan; Hao Jianwei; Zhang Yi; Zhang Wenping

(1. SINOPEC Catalyst Beijing Co. Ltd., Beijing 102500;2.National Laboratory of Flame Retardant Materials, National Engineering and Technology Research Center of Flame Retardant Materials, Beijing Institute of Technology, Beijing 100081;3.Production and Technology Department, SINOPEC Catalyst Co., Ltd., Beijing 100029)

Effect of Lithium Nitrate on the Structure and Property of α-Al2O3Platelets Prepared via Solid-state Reaction

Miao Zhuang1; Shi Jiangong1; Miao Qingyuan1; Hao Jianwei2; Zhang Yi3; Zhang Wenping1

(1. SINOPEC Catalyst Beijing Co. Ltd., Beijing 102500;2.National Laboratory of Flame Retardant Materials, National Engineering and Technology Research Center of Flame Retardant Materials, Beijing Institute of Technology, Beijing 100081;3.Production and Technology Department, SINOPEC Catalyst Co., Ltd., Beijing 100029)

The α-Al2O3platelets were prepared via solid-state reactions and the effect of the amount of lithium nitrate additive on the property of the platelets was investigated. The ICP results indicated that the high temperature calcination process resulted in a large loss of lithium species because of volatilization, but there was still a small amount of residual lithium species in the α-Al2O3platelets. The SEM micrographs showed that lithium nitrate led to decrease in the thickness of α-Al2O3platelets and irregular morphology of aggregates. Pore structures results exhibited that addition of lithium nitrate led to decrease in the pore size and increase in the specifc surface area of aggregates of α-Al2O3platelets. The XRD and IR patterns suggested that the residual lithium and aluminum oxide formed LiAl5O8. The existence of LiAl5O8was the basic reason for the changed performance of α-Al2O3platelets.

solid-state reaction, α-Al2O3platelets, LiNO3, property

1 Introduction

α-Al2O3(corundum) is one of the most important functional materials, thanks to its high mechanical hardness and strength, chemical inertness, and good thermal conductivity. The α-Al2O3platelet not only possesses the excellent physicochemical properties of α-Al2O3, but also the unique plate-like structure endowed with good adhesion and light-reflecting capacity. These platelets are widely used in the field of pearlescent pigment[1], refractory[2], ceramic[3-4], fller[5-7], and catalyst support[8-9]. The property of these materials can directly impact on their performance in various applications, and it is very essential to study the factors influencing the performance of these platelets. The solid-state reaction is one of the most important techniques used to prepare α-Al2O3platelets. This high-output method is simple, and low-cost, which can result in only small amounts of pollution, and is well-suited for use in the industry[10-13]. The platelets may also be prepared via the molten-salt synthesis. However, this method demands complex preparation processes, and the materials must be subsequently washed with deionized water (in order to remove inorganic salts), and repeatedly dried[14-21].

Lithium, an active alkali metal, exists in aluminum-silicon associated minerals, with its average content equating to 20 g/t in crust, and 30 g/t in bauxite[22]. The property of α-Al2O3platelet may be infuenced by the trace amount of lithium species in the hydrated-alumina originating from lithium-bauxite.

In the previous study[10], α-Al2O3platelets were obtained via the solid-state synthesis using gibbsite as the raw material, pseudo-boehmite as the binder, and NH4F as the mineralizer, in order to delve into how nano-SiO2and nano-TiO2can affect the property of α-Al2O3platelets. The mechanism of reaction between nano-SiO2and NH4F in alumina was studied in the reference[23]. In this project, the effect of the amount of LiNO3used on the structure and property of the platelets was mainly investigated.

2 Experimental

2.1 Sample preparation

The α-Al2O3platelet samples were prepared accordingto the following steps: Gibbsite (with a BET surface area of 212 m2/g, and a particle size d90of 133 μm) and the pseudo-boehmite (with a peptizing index of ≥95%) were pre-mixed according to a mass ratio of 1:2. Then, 1.5% of NH4F (A. R.) and various amount of LiNO3(A. R.) were added to the mixture, which was mixed for 15 min in a kneader, at a speed of 30 r/min. 20.0% of dilute nitric acid solution were then added to the mixture, which was then kneaded for another 30 min. During the process, the mixture gradually became damped, thereby leading to the rapid formation of a paste. An extrusion device was then used to shape the plastic feedstock into strips, which were subsequently placed in a drier at 120oC for complete removal of the moisture contained therein. The α-Al2O3particles were obtained by calcining the resulting strips for 4 h at 1 250oC in air.

2.2 Characterization

The lithium content of each sample was determined by Inductively Coupled Plasma (ICP, Optima7300, Perkin Elmer Co., USA). Before the examination, the samples were dissolved by microwave digestion. The morphology of the particles was examined by using a scanning electron microscope (SEM, Quanta 200, FEI. Co., USA). Prior to this examination, the samples were placed in copper holders (using conductive carbon tape) and subsequently sputtered with thin conductive layers of gold. In addition, the specifc surface area of each sample was determined via N2adsorption (BET method) at -196oC, by using a surface area analyzer (NOVA4000e, Quantachrome Instruments). The samples were outgassed at 300oC for 1 h, prior to the analysis. The pore structure of the samples was tested by the mercury porosimetry (MP, PoreMaster 60GT, Quantachrome Co., USA), in accordance with Chinese standard GB/T 21650.1—2008. The phase composition of the sample was determined via X-ray diffraction (XRD, X’ Pert MPD, Philips Co., Netherlands) using Ni-filtered CuKα radiation (λ=0.154 nm), and the diffraction patterns were collected at a scanning rate of 4(°)/min and over a 2θrange of 5°—80°, respectively. The structure of each sample was investigated through IR spectrometry (IR, Quanta560, Perkin Elmer Co., USA). Before investigation, a tiny amount of sample was mixed with KBr, and then the mixture was pressed into a transparent thin fake. The IR spectra were collected at a scanning range of between 400—4 000 cm-1.

3 Results and Discussion

3.1 ICP characterization

The actual measurable content and theoretical calculated content of lithium in the α-Al2O3platelet samples are summarized in Table 1.

Table 1 Actual measurable content and theoretical calculated content of lithium in α-Al2O3platelet with various additive amount of LiNO3

It can be seen from the data listed in Table 1 that the actual lithium content in the sample gradually increases with an increasing additive amount of LiNO3. It is worth noting that the actual measurable content of lithium was lower than that of theoretical calculated content by one order of magnitude in each sample. High heating temperature (1 250oC) resulted in decomposition of LiNO3to Li2O, due to its low melting point of 255oC and low thermal decomposition temperature of 600oC[24]. The reaction proceeds as follows:

Li2O is an ionic compound, which possesses high melting point and boiling point of 1 567oC and 2 600oC, respectively. However, when the calcination temperature increases up to 1 000oC, Li2O begins to sublime. These facts indicate that the actual measurable content of lithium in the samples is signifcantly lower than that of additive content because of the sublimation of Li2O above 1 000oC. It must be pointed out that a loose powder was formed when the pulpy sample with 4.03% of LiNO3was dried at 120oC for 2 hours. The pulpy sample is a mixture composed of hydrated alumina, LiNO3, dilute nitric acidand NH4F. When a surplus of LiNO3was added in the sample, a rapid evaporation of mixed gases of HNO3, NOx, HF, NH3and water vapor from the samples takes place in the process of drying, leading to collapse of the sample skeleton. Yang[25]obtained the same experimental results in preparation of γ-LiAlO2by a sol-gel combustion synthesis.

3.2 SEM analysis

The effect of the LiNO3content on the morphology of α-Al2O3platelets was investigated via SEM observation (Figure 1 (a-f)).

Addition of LiNO3led to reduction in the thickness and irregular shape of α-Al2O3platelets. The pure α-Al2O3platelets measuring ～3.1 μm (diameter) ×0.66 μm (thickness) (Figure 1 (a)) are disk-shaped and hexagonshaped partly. Figure 1 (b) shows the α-Al2O3platelets mixed with 0.32% of LiNO3. This mixing resulted in a morphology which was similar to the pure α-Al2O3platelet, because a tiny amount of LiNO3had no prominent effect on the morphology of α-Al2O3particles. Addition of 0.53% of LiNO3resulted in platelets with a slightly enlarged diameter and an average thickness of 0.42 μm (Figure 1 (c)). Figure 1 (d) and (e) exhibited severely irregular shapes of the platelets with a LiNO3content of 0.85% and 1.60%, respectively. The α-Al2O3platelets with a LiNO3content of 4.03% were obtained via roasting the dried loose alumina powder at 1 250oC for 4 hours, with their micro-morphology shown in Figure 1 (f). The SEM micrograph exhibits the irregular platelets as a crossing growth based on a spherical aggregate. It can be observed that the shape and diameter of the aggregate are similar to those of the gibbsite particle (Figure 1 (g)).

Figure 1 SEM micrographs of α-Al2O3platelets and a gibbsite particle, (a)-(f) with 0%, 0.32%, 0.53%, 0.85%, 1.60% and 4.03% of LiNO3, respectively, (g) a gibbsite particle

3.3 BET and MP analyses

The specific surface areas of aggregates of α-Al2O3platelets with various additive amount of LiNO3are summarized in Table 2. It can be seen from the data listed in Table 2 that the specific surface area increased with an increasing addition of LiNO3, which was ascribed to the change of pore structure in the samples.

Table 2 Specific surface area of aggregates of α-Al2O3platelets with various additive amount of LiNO3

The pore size distribution curves of α-Al2O3samples are shown in Figure 2. The pore diameter of α-alumina decreased with an increasing addition of LiNO3. All these results were attributed to the fractured thin α-Al2O3platelets resulted from the introduction of LiNO3.

Figure 2 Pore size distribution curves of aggregates of α-Al2O3platelets, (a)-(e) with 0%, 0.32%, 0.53%, 0.85%, and 1.60% of LiNO3, respectively

3.4 XRD analysis

α-Al2O3belongs to the rhombohedral space group, with its space structure of O2-and Al3+ions shown in Figure 3[26]. If the structure is chosen at hexagonal close packing, the configuration of O2-ions is organized at ABAB…, while Al3+ions only fll out the gaps at a 2/3 volume of octahedron[27]. In the structure of the AlO6octahedron, since the average bond length of three Al-O is 0.189 nm and the average distance of other three Al-O bonds is 0.193 nm, hence the average bond length of Al-O is 0.191 nm[28].

Figure 3 Structure of α-Al2O3

The XRD patterns of α-Al2O3platelets with different content of LiNO3are shown in Figure 4.

Figure 4 XRD patterns of α-Al2O3platelets, (a) -(e) with 0%, 0.32%, 0.85%, 1.60% and 4.03% of LiNO3, respectively

All samples displayed the main crystalline phase of α-Al2O3, and there was no other transition phase of alumina in the matrix. It illustrated that although lithium existed in the samples, it had no obvious impact on the phase transition to α-Al2O3. However, the XRD peak intensitiy of α-Al2O3decreased gradually, and an increasing LiAl5O8crystalline phase was formed with the increase in additive content of LiNO3. It indicated that some of Li2O species were lost due to sublimation and the other Li2O species entered into a solid-state reaction with alumina to form LiAl5O8during the calcination process. The reaction proceeded as follows:

Table 3 illustrates that the relative crystallinity of the α-Al2O3platelets was decreased by addition of LiNO3, which indicated that the structure of α-Al2O3was infuenced by LiAl5O8via the reaction between Li2O and Al2O3.

Table 3 Relative crystallinity of α-Al2O3platelets with various content of LiNO3

3.5 IR spectroscopic analysis

The IR spectra of α-Al2O3samples with various contents of LiNO3are illustrated in Figure 5.

Figure 5 IR spectra of α-Al2O3platelets, (a)-(f) with 0%, 0.32%, 0.53%, 0.85%, 1.60% and 4.03% of LiNO3, respectively

The peaks at 801, 639, 588, 504 and 450 cm-1are the typical characteristic absorption peaks of Al-O, and the same results had been reported by the reference[29]. However, there was a little difference with the reference[30]and the peaks of which appeared at 645, 751, 635, 583, 569, 578, 400, 442, 451, 432 and 418 cm-1. The difference may possibly stem from the diverse preparation technology of α-Al2O3or different resource of raw alumina. The spectrum of the sample with 0.85% LiNO3was similar to that of the pure sample, because the existence of inadequate lithium would have no significant effect on the properties of α-Al2O3platelets. It is worth noting that the peak at 801 cm-1became boarder and shifted towards the shorter wavelength region with an increasing content of added LiNO3, which indicated that the involved lithium could affect the Al-O vibration in the structure of α-Al2O3. The spectra of the samples containing more than 0.85% of LiNO3displayed new absorption peaks at 727, 679, 534 and 487 cm-1, indicating that a new formation originating from lithium and alumina disturbed the Al-O vibration in the structure of α-Al2O3. LiAl5O8, testified by the XRD patterns, is a kind of spinel with a regular array, which was formed via the solid-state synthesis between Li2O and Al2O3[31-32]. Therefore, the regular growing direction of α-Al2O3was destroyed by the existence of LiAl5O8, leading to a twisted micromorphology, a transformed pore structure and a modifed surface area of α-Al2O3platelets.

4 Conclusions

A solid-state reaction route to synthesize α-Al2O3platelets was successfully developed, using gibbsite and pseudoboehmite as the starting materials, while the additive LiNO3was used as a modifier during the reaction. Most of added lithium in the alumina was lost owing to sublimation during the high temperature calcination, but there still was residual lithium in the α-Al2O3, the content of which gradually increased with an increasing amount of LiNO3added. LiNO3led to a decreased thickness of α-Al2O3platelets, while their shape changed from hexagonal to irregular. Due to the change in platelets features, the pore diameter of aggregates of α-Al2O3platelets decreased and their surface area increased. LiAl5O8was formed by the solid-state reaction of Li2O with Al2O3during the calcination process, and it was the main reason ascribed to the changes of morphology and pore structure of α-Al2O3platelets.

Acknowledgment: This work was supported by the Technology Development (Commission) Project of SINOPEC Catalyst Co. Ltd. (Grant No. 14-05-01). The authors would like to express their gratitude to the reviewers whose valuable suggestions improved the quality of the article.

[1] Sigrid T, Gerhard P, Katsuhisa N. New effect pigments using innovative substrates[J]. European Coating Journal, 1999, 4: 90-96

[2] Yoshitaka N, Shinobu H, Sawao H, et al. Dielectric breakdown and thermal conductivity of textured alumina from platelets[J]. Journal of Ceramic Society Japan, 2010, 118 (1383): 1032-1037

[3] Shinobu H, Nishimura Y, Hirano H, et al. Microtexture control of alumina using anisotropic alumina particles[J]. Advances in Science and Technology, 2010, 70: 82-90

[4] Hashimoto S, Horita S, Ito Y, et al. Synthesis and mechanical properties of porous alumina from anisotropic alumina particles[J]. Journal of European Ceramic Society, 2010, 30 (3): 635-639

[5] Dharmendra S K, Rachit S K. Effect of alumina platelet reinforcement on dynamic mechanical properties of epoxy[C]//Proceedings of the World Congress on Engineering, London, 2011-03

[6] Dharmendra S K, Venkitanarayanan P. Epoxy composites with 200 nm thick alumina platelets as reinforcements[J].Journal of Materials Science, 2007, 42 (15): 5964-5972

[7] Lorenz B J, Andre S R, Jorg W, et al. Strong and ductile platelet-reinforced polymer films inspired by nature: microstructure and mechanical properties[J]. Journal of Materials Research, 2009, 24 (9): 2741-2754

[8] Sten W, Juliana S G, Madan B M, et al. Shaped porous bodies of alpha-alumina and methods for the preparation thereof: the United States, US20100056816[P]. 2010

[9] Li Jinbing, Li Xianfeng, Lin Wei, et al. Carrier for silver catalyst, its preparation, a silver catalyst made from the same and its use: the United States, US 20120172608[P]. 2012

[10] Miao Zhuang, Shi Jiangong, Hao Jianwei, et al. Effect of nano-TiO2and nano-SiO2addition on the morphological control of α-Al2O3platelets via solid-state reaction[J]. Ceramics International, 2016, 42 (1): 1183-1190

[11] Shaklee C A, Messing G L. Growth of α-Al2O3platelets in the HF-γ-Al2O3system[J]. Journal of the American Ceramic Society, 1994, 77 (11): 2877-2984

[12] Fu Gaofeng, Wang Jing, Kang Jian. Infuence of AlF3and hydrothermal conditions on morphologies of α-Al2O3[J]. Transactions of Nonferrous Metals Society of China, 2008, 18 (3): 743-748

[13] Hill R F, Robert D, Robert P T. Synthesis of aluminum oxide platelets[J]. Journal of the American Ceramic Society, 2001, 84 (3): 514-520

[14] Tan H. Influence of calcium oxide on alumina phase transition and morphology development in NaCl-KCl fux[J]. Materials Technology, 2012, 27 (2): 165-168

[15] Zhu Lihui, Huang Qingwei, Liu Wei. Synthesis of platelike α-Al2O3single-crystal particles in NaCl-KCl flux using Al(OH)3powders as starting materials[J]. Ceramics International, 2008, 34: 1729-1733

[16] Zhu Lihui, Huang Qingwei. Morphology control of α-Al2O3platelets by molten salt synthesis[J]. Ceramics International, 2011, 37: 249-255

[17] Jin Xihai, Lian Gao. Size control of α-Al2O3platelets synthesized in molten Na2SO4flux[J]. Journal of the American Ceramic Society, 2004, 87 (4): 533-540

[18] Su Xinghua, Li Jiangong. Low temperature synthesis of single-crystal alpha alumina platelets by calcining bayerite and potassium sulfate[J]. Journal of Materials Science and Technology, 2011, 27(11): 1011-1015

[19] Shinobu H, Akira Y. Formation of porous aggregation composed of Al2O3platelets using potassium sulfate fux[J]. Journal of the European Ceramic Society, 1999, 19 (3): 335-339

[20] Xu G G, Cui H Z, Ruan G Z, et al. Synthesis of highly dispersed α-Al2O3platelets in KCl-K2SO4-Na2SO4composite fux[J]. Materials Research Innovations, 2013, 17: 224-227

[21] Shinobu H, Akira Y. Synthesis of α-Al2O3platelets using sodium sulfate flux[J]. Journal of Materials Research, 1999, 14 (12): 4667-4672

[22] Li Chunchao, Huang Jian. The existing state of lithium in the process of alumina production[J]. Light Metals, 2005 (6): 17-19

[23] Miao Zhuang, Shi Jiangong, Zhang Wenping, et al. Effect of NH4F and nano-SiO2on the morphological control of α-Al2O3platelets via solid-state reaction[J]. China Petroleum Processing and Petrochemicals, 2016, 18 (4): 88-93

[24] Chen Wensheng, Cao Xifang. The discussion on caloric stability in nitrate of alkaline metals[J]. Journal of Hubei Institute of Education. 2005, 22 (2): 1-3

[25] Yang Xiping, Qiu Zumin, Liu Yanfeng, et al. Preparation of γ-LiAlO2powders by a sol-gel combustion synthesis process[J]. Materials Review, 2008, 22 (7): 213-215

[26] Chen Wei. Study on the microstructure evolution of α-Al2O3in the formation process and it’s controlling[D]. Changsha: Central South University, 2010

[27] Zhou Gongdu. Inorganic Structural Chemistry[M]. Beijing: Science Press, 1982: 328

[28] Gu Xuemin. Be—Alkaline Earth Metals[M]. Beijing: Science Press, 1990: 25

[29] Jin Riya, Hu Shuangqin, Gu Qiumin. Disposal and recycling techniques of aluminum industrial wastewater[J]. Mater Science and Technology, 2008, 16 (4): 519-522

[30] Zhang Changshuan, Zhao Feng, Zhang Jijun, et al. Infrared spectroscopic studies of alumina on a nanometer scale[J]. Acta Chimica Sinica, 1999, 57: 275-280

[31] Xie Limei, Yang Dingming, Wei Le, et al. Synthesis and luminescence properties of plate-like europium ion doped LiAl5O8[J]. Journal of Synthetic Crystals, 2011, 40 (3): 684-688

[32] Xie Limei, Yang Dingming, Wei Le, et al. Synthesis and luminescence properties of LiAl5O8: Tb3+green phosphor[J]. Chinese Journal of Luminescence, 2012, 33: 36-40

date: 2017-01-03; Accepted date: 2017-02-20.

Shi Jiangong, Telephone/Fax: +86-10-69346355; E-mail: shijiangong@126.com.