Effects of Cation on the Morphology of Boehmite Precipitated from

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Effects of cation on the morphology of boehmite precipitated from alkaline solutions by adding gibbsite as seed Zheng Li, Guihua Liu, Xiaobin Li, Tiangui Qi, Zhihong Peng, and Qiusheng Zhou Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01756 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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Crystal Growth & Design

Effects of cation on the morphology of boehmite precipitated from alkaline solutions by adding gibbsite as seed Zheng Li,†,‡ Guihua Liu,*,† Xiaobin Li,† Tiangui Qi,† Zhihong Peng,† Qiusheng Zhou† †School

of Metallurgy and Environment, Central South University, Changsha, Hunan Province, 410083, PR China

‡School

of Materials Science and Engineering, Central South University, Changsha, Hunan Province, 410083, PR China

Abstract: To regulate boehmite morphology is important for improving its performance in application. In this paper, dependence of boehmite morphology on cation ions was studied by XRD, SEM, TEM, FTIR, and Zeta-potential, together with ion-association-degree calculation. Boehmite was respectively precipitated from sodium, potassium, lithium, or barium alkaline solution by adding gibbsite as seed at 180 °C for 2h. Cation concentration and species determined the boehmite morphology. Increase in cation concentration or precipitation in K+, Na+, Li+ and Ba2+

alkaline

solution correspondingly precipitated rhombic, hexagonal, elipse-like morphology for boehmite. Al(OH)4− was predominantly found in filtrate solution by Raman spectra, whereas red shift of FTIR was observed in alkaline solution containing Ba2+, Li+, Na+ and K+ cations. The association degree of M+Al(OH)4− increased in the following order: Ba2+[Al(OH)4−]2 > Li+Al(OH)4− > K+Al(OH)4− > Na+Al(OH)4− in the same concentration of cation solution. This finding well agreed with the variations in FTIR

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change and zeta potential. Meanwhile, increasing cation and Al(OH)4− concentration, or precipitation in lithium, barium alkaline solution caused the (202) facet of boehmite to be gradually disappeared and the (200) facet to be exposed. Therefore, the selective adsorption of ion pairs on the (202) and (200) facet of boehmite mainly accounted for the boehmite morphology. 1 Introduction Boehmite with diverse morphologies provides various applications in filler,1 flame retardant,2 flexibilizer,3 and biomedical applications.4 Moreover, as boehmite is an important precursor in preparing active alumina (γ-Al2O3) and corundum (α-Al2O3),5 boehmite morphology also determines application of fore-mentioned alumina in refractory material,6 catalyst,7 adsorbent,8,9 abrasive10 and ceramics.11 Thus, to control boehmite morphology has been extremely interesting. In acidic solution or weak alkaline solution (pH11.5),30 and increasing the alkaline concentration promoted the ≡AlONa amount and the zeta potential. Rhombic boehmite is often precipitated from sodium aluminate solution within the temperature range of 140~300 °C.31-33 However, dependence of boehmite morphology on aluminate structure is not discussed clearly in sodium aluminate solution. In addition, a large amount of inorganic impurities coexists in sodium aluminate solution. K+, Li+ and Ba2+ in silicate mineral are readily leached into sodium



aluminate solution during bauxite digestion. 10~100 g/L K2O, 0.07~0.1 g/L Li2O and Ba2+ have been found in sodium aluminate solution in practice. Effect of K+ Cs+ or Ca2+ impurities on the precipitation rate of gibbsite was studied in precipitation of gibbsite.

34

Results showed that a large amount of the fine particle or secondary

nucleation occurred in sodium aluminate solution. We also observed that Li+ in alkaline solution generated the fine gibbsite and reduced the strength of the sandy alumina. However, relative to gibbsite precipitation for producing sandy alumina in alumina refineries, few papers were published on boehmite morphology affected by the above impurities mainly owing to no sandy alumina prepared from boehmite. Therefore, the influence and mechanism of these alkali metal ions on the boehmite should be discussed so that boehmite morphology can be regulated in alkaline solution. In this study, sodium, potassium, lithium and barium alkaline solution were used to prepare boehmite by adding gibbsite as seed at 180 °C. The effect of cations on boehmite morphology was then investigated. And precipitation mechanism was also discussed. The results provide a new process to understand boehmite precipitation and to prepare boehmite with various morphologies in alkaline solution. 2 Experimental 2.1 Materials The alkaline solutions were prepared by adding and LiOH, NaOH, KOH, or Ba(OH)2 (Tianjin Kermel Chemical Reagent Co., Ltd.,Tianjin China) into deionized water. The medium diameter (d(0.5)) of gibbsite seed was 2.5 μm. As shown in Fig. 1,


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the hexagonal or approximate hexagonal flake-like gibbsite seed existed in uniform morphology.

Fig. 1 Scanning electron microscopy (SEM) image of gibbsite seed

2.2 Experimental Procedure Boehmites were precipitated from 100mL LiOH, NaOH, KOH or Ba(OH)2 solutions by adding gibbsite seed (100 g/L) in 150 mL autoclaves at 180 °C for 2 h. Afterwards, the slurry was filtered, followed by cooling of autoclaves with water. The filtrate was used to determine Na2O and Al2O3 concentrations, whereas boehmite in filter cake was washed by boiling water and dried 100 °C for 24 h. 2.3 Characterization The concentrations of Li+, Na+, K+, Ba2+ and Al2O3 in liquid were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES, ICAP7400, Thermo Scientific). Fourier transform infrared spectroscopy (FTIR) spectra of alkaline solution were recorded with the NICOLET6700 (Thermo Fisher Scientific, USA) by scanning in the range of 4000–400 cm−1 at a spectral resolution of 4 cm−1. The Raman spectra of solution were obtained using the RM1000 Laser Confocal Raman Microspectroscopy (Renishaw) at 514.5 nm and a resolution of 4 cm−1. The



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boehmite zeta potential was measured with Zeta-potential meter (Zetasize, Malvern, UK) at 25 °C. X-ray diffraction (XRD) measurements were performed with an X-ray power diffractometer (D/max2500, Rigaku, Japan) by using Cu Kα radiation (λ=1.54178 Å). Scanning electron microscope (SEM, JSM-6360LV, JEOL, Japan) was used to analyze the morphology change. The structure was examined using a High-resolution Transmission Electron Microscope (HRTEM, Tecnai F20, FEI, American) equipped with Selected Area Electron Diffraction (SAED). 2.4 Method and data processing Al(OH)4− is the predominant anion in sodium aluminate solution,35-37 which its concentration increases with the temperature.38 Previous works have proved that ion pairs of Na+Al(OH)4− and Na+OH− exist in concentrated sodium aluminate solution.39,40 The association degree of Mn+(Al(OH)4)nn− in various alkaline solution was then calculated based on the following Bjerrum (Equ. 1) and modified Debye-Hückel (Equ. 3) equation: 3 4𝜋𝑁 |𝑍𝑖𝑍𝑗|𝑒2

𝜃 (1 ― 𝜃)

b

2

(

= 𝑓2± 𝑐1000

𝜀𝑘𝑇

) 𝑄(𝑏),

(1)

Zi Z j e2

 kTa ,

(2) 𝐴 (1 ― 𝜃)𝑐

𝑙𝑔𝑓 ± = ― 1 + 𝐵𝑞

(1 ― 𝜃)𝑐

,

(3) 𝐴= 𝐵=

1.8248 × 106𝜌1/2 (𝜀𝑇)3/2

,

(4)

50.291 × 108𝜌1/2 (𝜀𝑇)1/2

,

(5)

where 𝜃 is the association degree for ion pair. 𝑓 ± stands for the mean activity coefficient for electrolyte. c is the ionic concentration, mol/L. N is Avogadro's


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constant. Zi, Zj are the charge of the anion and cation. ε is the water dielectric constant (78.36 F/m), and k is the Boltzmann constant(1.38 × 10−23 J/K). T is the absolute temperature, K. a is the distance, nearly equal to distance between the cation and anion, generally substituted by the ion radius (Li+=0.059 nm, Na+=0.099 nm, K+=0.137 nm, Ba2+=0.135 nm, Al(OH)4−=0.1685 nm, OH−=0.14 nm).41 ρ is the water density (g/cm3). 3 Results and discussion 3.1 Phase and morphology of boehmite 3.1.1 XRD During precipitation of coarse boehmite in sodium aluminate solution by adding gibbsite, there existed gibbsite dissolution and boehmite precipitation.26 Fig. 2 shows the influence of cation on the XRD patterns of boehmite precipitated at 180 °C for 2

(202) (222)

(200) (220)

a

(002) (022)

(130)

(021)

(020)

h.

b

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c d Boehmite(JCPDS: 74-1895) 0

20

40

2 

60

80

Fig. 2 Effect of cation in solution on the XRD patterns of boehmite



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a. C(LiOH) = 1.00 mol/L, b. C(NaOH) = 1.00 mol/L, c. C(KOH) = 1.00 mol/L, d. C(Ba(OH)2) = 1.00 mol/L, reaction time = 2 h，temperature = 180 °C, gibbsite seed = 100 g/L

As showed in Fig. 2, the diffraction peaks of samples were in well agreement with boehmite (space group: Cmcm, JCPDS:74-1895), and no peak assigned to gibbsite can be observed. Furthermore, the (020) facet in boehmite was prior formed. Based on Debye-Scherrer equation, the boehmite diameters in various hydroxide solutions along the [020] direction (D(020)) were calculated as follows: 27.8 nm (K+), 37.2 nm (Na+), 41.1 nm (Li+) and 44.4 nm (Ba2+). The difference in boehmite sizes may be related to cation in alkaline solution. 3.1.2 Morphology Fig. 3 presents the influence of cations and concentrations on the morphology of boehmite formed at 180 °C for 2 h. Cationic concentration and species

K+

Na+

0.1 mol/L

1.0 mol/L


Li+

Ba2+

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3.0 mol/L

Fig. 3 Effect of cationic concentration and species on boehmite morphology Reaction time = 2 h，temperature = 180 °C, gibbsite seed = 100 g/L

The results in Fig. 3 indicated that the morphologies of boehmite depended on cationic concentration and species. Boehmite exhibited a flake rhombic morphology in dilute alkaline solution containing K+, Na+ or Li+, whereas boehmite morphology changed into hexagonal-flake, olive-like, and ellipse-like morphology in barium alkaline solution as well as in the concentrated alkaline solution containing K+, Na+ or Li+. Moreover, cation played a remarkable role in boehmite morphology. Flake-like rhombic boehmite was formed in Na+ and K+ alkaline solutions at 0.1 mol/L, and defect in flake hexagonal boehmite was found in Ba2+ alkaline solution at 0.1 mol/L. Meanwhile, hexagonal-flake boehmite was observed in Na+, K+ alkaline solution of 3.0 mol/L, which was similar to gibbsite morphology and was thinner compared with gibbsite precipitated from sodium aluminate solution in the production of smelter grade alumina by Bayer process. In addition, boehmite morphology almost changed into olive-like, ellipse-like morphology in alkaline solution containing 3.0 mol/L Li+ or Ba2+. Therefore, cationic concentrations and species in alkaline solution had considerable influence on the boehmite morphologies. 3.2 Al(OH)4− concentration and aluminate structure in solution Temperature and solution structure have significant influence on the solid-liquid



interface, which determines the morphology and particle size of boehmite.22,42,43 Al(OH)4− concentration and aluminate structure in solution were investigated to reveal precipitation mechanism and morphological evolution of boehmite in alkaline solution at 180 °C. 3.2.1 Al(OH)4− concentration in precipitation Variation for Al(OH)4− concentration in precipitation boehmite by adding gibbsite as seed can be seen in Fig. 4. 1.0 LiOH4 NaOH4

0.8

KOH4 Ba(OH)2

Al(OH)4, mol/L

0.6



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.2

0.0 0

20

40

60

Time, min

80

100

Fig. 4 Al(OH)4− concentration in various alkaline solutions a. C(LiOH) = 1.00 mol/L, b. C(NaOH) = 1.00 mol/L; c. C(KOH) = 1.00 mol/L, d. C(Ba(OH)2) = 1.00 mol/L, temperature = 180 °C, gibbsite seed = 100 g/L

As shown in Fig. 4, Al(OH)4− concentration in various alkaline solutions sharply increased in 20 min owing to gibbsite dissolution and then decreased within the time range of 20-60 min for the remarkable boehmite precipitation. Finally, it nearly remained constant after 60 min because the precipitation reached apparent equilibrium.


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Results also indicated that cationic species had a minimal effect on Al(OH)4− concentration during boehmite precipitation although Al(OH)4− concentration in barium alkaline solution was slightly higher than that in Na+, K+, Li+ alkaline solution.

3.2.2 Raman spectra Various aluminate anions existed in concentrated sodium aluminate although Al(OH)4− is the predominate anion in solution. To reveal relationship between cationic species on aluminate structure, Fig. 5 shows effect of cation on aluminate structure in filtrate solution by Raman spectra.

-1

620 cm

a b Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c d

200

400

600 800 -1 Raman shift, cm

1000

Fig. 5 Raman spectrum for various alkaline solution a. C(K+) = 1.00 mol/L, b. C(Na+) = 1.00 mol/L, c. C(Li+) = 1.00 mol/L, d. C(Ba2+) = 1.00 mol/L, temperature = 25 °C, gibbsite seed = 100 g/L

A peak at 620 cm−1 was observed in Fig. 5 in various alkaline solutions, which can be assigned to the Al-OH vibration of Al(OH)4−.40 The peak height decreased in



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the following order: K+> Na+> Li+> Ba2+. However, no peaks for Al2O(OH)62− was found owing to Al(OH)4− concentration less than that in Bayer liquor.36 This result suggests that cation rarely affects the aluminate structure in solution with cation concentration of 1.0 mol/L.44 Therefore, boehmite precipitation can be written as follows: Al(OH)4− =AlOOH+OH−+H2O

(6)

Based on our previous work,26 gibbsite seed dissolved to saturated solution for precipitation of the flake boehmite, and it also provided a nucleation site for boehmite. However, same anion (Al(OH)4−) in various alkaline solutions cannot precipitate different morphology of boehmite on basis of Raman spectrum. This phenomenon implies that cation instead of anion in solution determine the boehmite morphology.

3.2.3 Association degree of ion pair Al(OH)4− mainly exists in dilute sodium aluminate solution, whereas the ion pair occurs in the concentration solution (Reaction equation 7).45 Na+ +Al(OH)4− = Na+Al(OH)4−

(7)

Given that alkaline solution was considered as a pure solution, the association degree of Mn+(Al(OH)4)nn− was calculated based on cation (Mn+) concentration. Fig. 6 plots dependence of cationic concentration on ionic association degree.


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0.7 0.6 LiAl(OH)4 0.5

NaAl(OH)4 KAl(OH)4

0.4

Ba(Al(OH)4)2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3 0.2 0.1 0.0 0.0

0.5

1.0

1.5

n+

2.0

Concentration of M , mol/L

2.5

3.0

Fig. 6 Association degree of aluminate for various cation Temperature = 180 °C

In Fig. 6, increase in cation concentration benefitted the formation of ion pair due to the increase in association degree. Meanwhile, all association degrees increased rapidly within the concentration range of 0~1.0 mol/L and then almost level off in the concentrated solution. In addition, the association degree significantly depended on cation species in alkaline solution. Ba2+ in alkaline solution remarkably favored in the formation of ion pair. By contrast, K+ in alkaline solution had less effect on the formation of ion pair compared with Li+, Na+ in alkaline solution. The association degree order was written as follows: θ(Ba2+) > θ(Li+) > θ(Na+) > θ(K+). This condition was attributed to the strong electrostatic interaction between Ba2+ and Al(OH)4−, while the small radius for Li+ also strengthened the electrostatic interaction compared with K+ and Na+. The results can provide a reasonable explanation for Raman spectra in Fig. 5. Many of ion pair occurred in barium alkaline solution, reducing concentration of Al(OH)4−, and peak height at 620 cm−1 subsequently decreased.



3.2.4 Zeta potential of boehmite During gibbsite precipitation, the Al(OH)4− concentration was maintained constant in the seed surface,46 and zeta potential of gibbsite was often negative in sodium alkaline solution.47 Fig. 7 shows the effect of OH− concentration on zeta potential of boehmite in various alkaline solutions. 5 0 Zeta Potential, mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5

KOH NaOH LiOH Ba(OH)2

-10 -15 -20 -25 -30 0.0

0.2

0.4 0.6 0.8 Concentration of OH, mol/L

1.0

Fig. 7 Zeta potential of boehmite in various alkaline solutions Temperature = 25 °C Similar to that of gibbsite precipitated from sodium aluminate solution, the zeta potentials of boehmite in Fig. 7 were all negative in various alkaline solutions.47 Meanwhile, increasing OH− concentration remarkably improved the zeta potential in dilution solution because the boehmite surface was negatively charged in alkaline solution,30 and more cations were adsorbed on the boehmite surface after concentration increased.48 The zeta potential of boehmite slightly raised or nearly remained constant in the concentrated solution for the amount of ion pair (Mn+(Al(OH)4)nn−) formed in concentrated solution and adsorbed on boehmite, leading to almost constant zeta potential due to the neutral charge in ion pair. Moreover, cation significantly affected the zeta potential in the same OH−


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concentration, and the zeta potential of boehmite varied according to the following order: Ba2+> Li+ > Na+ > K+ in alkaline solution. This result proves that cation, related to the zeta potential of boehmite, determines liquid−solid interface, and subsequently affects the precipitation of boehmite. 3.2.5 FTIR spectra Formation of ion pair will change the vibration of Al–O bond in Al(OH)4− owing to the occurrence of ion pair of Mn+(Al(OH)4)nn− in various concentrated alkaline solutions caused by electrostatic interaction. Those change may be found by FTIR spectrum. Fig. 8 presents FTIR spectra of various alkaline solutions.

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a b c d

500

-1

725.9cm

-1

717.8cm

-1

703.4cm -1

684.0cm

600

700

800

900

-1

Wavenumber ,cm

1000

1100

1200

Fig. 8 FTIR of alkaline solution for various cations a. C(KOH) = 1.00 mol/L, b. C(NaOH) = 1.00 mol/L, c. C(LiOH) = 1.00 mol/L, d. C(Ba(OH)2) = 1.00 mol/L, C(Al(OH)4−) = 0.50 mol/L

In Fig. 8, peak at 720 cm−1 was generally assigned to vas Al–OH of Al(OH)4− in alkaline solution.46 This peak shifted from 725.9, 717.8, 703.4 to 684.0 cm−1 in alkaline solution containing K+, Na+, Li+ and Ba2+, respectively. The decreases in wavenumber from K+ to Ba2+ alkaline solution may result from increase in association



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degrees of ion pair as shown in Fig.6.49 The reason is explained that the association between cation and Al(OH)4− affects the force constant (k) of Al–OH on basis of Eq. (8) at constant μ ( as shown in Fig. 9). 1

𝜈 = 2𝜋

𝑘 𝜇

,

(8)

where k is the force constant, μ is the reduced mass. The frequency (ν) is Al–OH bond. In Fig. 9, the positive charge in cation changed the electron distribution of O in Al–O bond of Al(OH)4– by electrostatic interaction and then changed the Al–O bond length, which decreased k in Al–OH bond (Equation 8). Moreover, Ba2+ had the greatest effect on the bond length of Al–OH for the largest association degree proven in FTIR spectra. Meanwhile, a large amount of ion pair occurred in alkaline solution, reducing the concentration of Al(OH)4− ion structure. This results was in well agreement with that in Raman spectra in Fig. 5. Therefore, the association degree at the same concentration of cation adhered to the following order: K+ < Na+ < Li+ < Ba2+, and the occurrence of ionic pair Mn+(Al(OH)4)nn− was verified by calculation and FTIR spectra.

Fig. 9 Effect of cation and Al(OH)4− association on Al–OH bond


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(Arrow represents the electron in O atom is attracted by cation. This leads to change the electronic distribution in O atom and subsequently change the bond length in Al–OH) 3.2.6 TEM and SAED To further illustrate variation in morphology of boehmite, Fig. 10 displays TEM images and SAED patterns of boehmite.

(a)

(b)

(c)

(d)

Fig. 10 TEM and SAED of boehmite precipitation from various alkaline solutions



(a) KOH = 1.00 mol/L，(b) NaOH = 1.00 mol/L，(c) LiOH = 1.00 mol/L，(d) Ba(OH)2 = 1.00 mol/L Association degree: a>b>c>d As shown in Fig. 10, all boehmites were in flake morphology, and (020) facet was their exposed surface. Rhombic-like (Fig. 10a), hexagonal-like (Fig. 10b and Fig. 10c) and olive-like boehmite were observed (Fig. 10d) in potassium, sodium, lithium and barium alkaline solutions, respectively. This finding was consistent with that in SEM image (Fig. 3). Results also showed the exposed area of (200) facet grew remarkably, and the flake-like rhombic boehmite correspondingly transformed into flake-like hexagonal boehmite. Furthermore, an olive-like boehmite was grown in Ba2+ alkaline solution owing to the rapid growth of (200) facet. Above growth of (200) facet was attributed to the high surface energy, negative zeta potential, and occurrence of ion pair in concentrated solution. The fact suggests that ion pair contributes to the boehmite morphology precipitated from alkaline solution by adding gibbsite seed. 3.3 Precipitation mechanism of boehmite Various surface energies for boehmite were calculated as the following order, (010)

Effects of Cation on the Morphology of Boehmite Precipitated from

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