Materials Research Bulletin 46 (2011) 428–431

Sub-micrometer sized yttrium oxide fibers prepared throughhydrothermal reaction

Nan Li a,*, Kazumichi Yanagisawa b

a College of Material Science and Engineering, Key Laboratory of Automobile Material of Ministry of Education, Jilin University, 2699 Qianjin Street, Changchun 130012, PR Chinab Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University, 2-5-1 Akbono-cho, Kochi 780-8520, Japan

A R T I C L E I N F O

Article history:

Received 27 October 2010

Received in revised form 18 November 2010

Accepted 29 November 2010

Available online 8 December 2010

Keywords:

A. Oxides

B. Chemical synthesis

C. Thermogravimetric analysis (TGA)

C. X-ray diffraction

A B S T R A C T

Yttrium oxide fibers have been synthesized via hydrothermal reaction and subsequent thermal

treatment using yttrium chloride as precursor. The products before and after the thermal treatment

were characterized by powder X-ray diffractions (XRD), scanning electron microscopy (SEM), ion-

chromatograph analysis, and thermogravimetry and differential thermal analysis (TG-DTA). The fiber

diameter ranged from 100 to 300 nm, while the length was up to tens of microns. It was found that the

chemical composition and morphology of the products were closely related to the pH value of reaction

solution, and fibrous products could be obtained at pH 9.5–10.25. These oxide fibers exhibited

outstanding high-temperature stability, which maintained their morphology at temperature up to

1400 8C.

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1. Introduction

As an important class of structural materials, oxide ceramicfibers are attractive as reinforcements for high-temperaturematerials due to the combination of high strength and excellenthigh-temperature stability in air. Bulk oxide fibers can also bedirectly used as high-temperature fill and packing material in avariety of high-temperature applications. The most widely knownoxide fiber is alumina, which normally contains silica to maintainthe transitional forms of alumina and inhibit a-alumina graingrowth [1]. Other familiar oxide fibers include zirconia, magnesia,yttrium aluminum garnet (YAG), etc.

Yttrium oxide is one of the most important rare-earthcompounds and has been used as functional material in a broadrange of fields such as optics, optoelectronics, and catalyticreactions. With respect to the physical properties, yttrium oxidehas a high melting temperature (Tm = 2430 8C), which is higherthan that of a number of other well-known oxides, such as alumina,zirconia, YAG, and spinel. Yttrium oxide normally exists as a cubicphase and is stable up to melting point without phase transfor-mation. Furthermore, with the C-type rare-earth sesquioxidecrystal structure, it has a very large unit cell, which results in largeunit slip distances. Hence, it is expected that plastic deformationin yttrium oxide by dislocation motion would be difficult [2].

* Corresponding author. Tel.: +86 431 85094856; fax: +86 431 85094856.

E-mail addresses: lin@jlu.edu.cn, linanyy2003@yahoo.com.cn (N. Li).

0025-5408/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.11.033

These properties endow it potential usefulness in fiber form as hightemperature material. Several reports have described the hydro-thermal synthesis of 1D yttrium hydroxide and correspondingyttrium oxide nanostructures, like nanowires [3–6], nanotubes[3,4,6–12], and nanobelts [13,14]. In this paper, we demonstratethe hydrothermal reaction of yttrium chloride in neutral tobasic conditions, yielding yttrium chloride hydroxide with fibrousshape, which lead to yttrium oxide fibers on thermal decomposi-tion. The high-temperature stability of these fibers was alsoevaluated for the purpose of potential high-temperature applica-tions.

2. Experimental

In a typical synthesis process, 0.45 g of yttrium oxide wasdissolved in 6 mL of 3.0 mol/L HCl solution under heating. Thenammonia solution was added to adjust the solution to neutral orbasic condition. The as-obtained colloidal suspension was trans-ferred into a 25 mL Teflon-lined autoclave with filling degree of60%, followed by hydrothermal treatment at 200 8C for 12 h. Aftercooling down, the precipitate was collected by centrifuge, washedwith distilled water and dried in air. Finally, it was calcined at800 8C for 4 h in air.

Powder X-ray diffractions (XRD) were measured on a RigakuRTP-300RC diffractometer operating at 40 kV and 100 mA usingCu Ka radiation (l = 1.54056 A). The patterns were collected inthe range of 5–708 in 2u/u scanning mode with a 0.028 step andscanning speed of 48/min. Micrographs of scanning electron

[()TD$FIG]N. Li, K. Yanagisawa / Materials Research Bulletin 46 (2011) 428–431 429

microscopy (SEM) were obtained using Hitachi S-530 electronmicroscope operating at 25 kV. A SSC5200 thermal analysis systemwith Seiko TG/DTA 320 module was used for the thermogravi-metric analysis. Samples were heated in air with a ramp rate of5 8C/min. The content of Cl�was measured using a Dionex DX-120ion-chromatograph analyzer. The samples were dissolved in 5 mLof 1 mol/L H2SO4 then diluted to 50 mL.

3. Results and discussion

The hydrothermal reactions were conducted at pH value from6.5 to 11.25. The crystal structure and morphology of productwere studied by powder X-ray diffraction and scanning electronmicroscopy, respectively. XRD results indicated that all theproducts received in this pH range could be converted intoyttrium oxide upon calcination at 800 8C. Fig. 1 shows a typicalXRD pattern of the product synthesized at pH 9.50. All of the peakswere in agreement with a pure cubic phase of Y2O3 with latticeconstant a = 10.604 A (JCPDS 41-1105). SEM observation revealedthat during the pH value from 9.50 to 10.25, fibrous products weresynthesized, which was made up of sub-micrometer fibers withdiameter ranging from 100 to 300 nm and length up to tens ofmicrons, as shown in Fig. 2a and b.

In order to understand the reaction mechanism of the formationof Y2O3 fibers, the products received from the hydrothermalreaction, i.e. before calcination, were investigated. Fig. 3 shows theXRD patterns of the yttrium compounds prepared at pH 6.5–11.25.During this pH range, the chemical composition of the productsvaried remarkably with increased pH value. When the pH valuewas lower than 9.0, highly crystallized yttrium chloride hydroxidehydrate, Y2(OH)4.86Cl1.14�1.07H2O (JCPDS 30-1445) was received,which was in prolonged hexagonal plate shape, as shown in Fig. 4a.The length, width and thickness of the plates were around 10 mm,1.5 mm and 0.5 mm, respectively. As the pH value increased to 9.15,another highly crystallized phase was obtained. However, theXRD peaks of it cannot be indexed to any known phase. SEMobservation revealed that it was composed of microrods withdiameter of�3 mm and length of�500 mm, as shown in Fig. 4b. Itscrystallinity decreases significantly with the slight increase in pHvalue. The compound received at pH 9.25 consists of individualrods with diameter ranging from 500 nm to 1 mm, as shown inFig. 4c. With further increasing pH value, the rigid rods becomeflexible, accompanied by the decrease in diameter. During therange from 9.5 to 10.25 in pH value, the obtained products were allin fiber shape. When the pH value was further increased to 10.5,the intensity of the XRD peak for the product decreasedcontinuously, while the products turned from fibers into irregular[()TD$FIG]

Fig. 1. Typical XRD pattern of Y2O3 fibers. The sample was hydrothermally

synthesized at pH 9.5 and calcined at 800 8C for 4 h in air.

Fig. 2. SEM images of Y2O3 fibers at (a) low and (b) high magnification. The sample

was synthesized at pH 9.5 and calcined at 800 8C for 4 h in air. (c) The SEM image of

this sample after further calcination at 1400 8C for 2 h.

lumps. In order to investigate the hydrothermal reaction understrong basic condition, 5 mol/L NaOH solution was used to replaceammonium solution and adjust the solution to pH 12. Afterhydrothermal reaction, hexagonal yttrium hydroxide with highcrystallinity was obtained (not shown). All of these yttriumcompounds could be converted into oxide through calcination, andthe morphologies were maintained except for the slight sizeshrinkage by comparison of those prior to heat treatment, resultingfrom the higher density of yttrium oxide.

The reaction between aqueous solutions of yttrium salts(chloride and nitrate) and alkaline solutions normally resultsin basic salts. For example, Y(OH)3(NO3)3�x�yH2O was obtainedwhen yttrium nitrate was used as precursor [5,15]. Our experi-ment also proved that when the hydrothermal reaction was

[()TD$FIG]

Fig. 3. XRD patterns of yttrium compounds hydrothermally synthesized at different pH value.

N. Li, K. Yanagisawa / Materials Research Bulletin 46 (2011) 428–431430

carried out at pH 6.5–9.0, yttrium chloride hydroxide hydrate,Y2(OH)4.86Cl1.14�1.07H2O was received. It is possible that the highlycrystallized compound synthesized at pH 9.15 belongs to chloridehydroxide hydrate, Y(OH)xCl3�x�yH2O. Ion chromatograph analysishas determined the Cl content of 8.60–8.91 wt% in this compound.[()TD$FIG]

Fig. 4. Typical SEM images of yttrium compounds hydrothermally synthesized at var

The calcining behavior of the compound was investigated by TG-DTA, as shown in Fig. 5, which exhibits an overall weight loss of23.2%. Combined with the ion-chromatograph analysis and TGresult, the approximate molecular formula of this compound isY(OH)2.63Cl0.37. During the calcinations, the sample undergoes

ious pH value. The pH value for (a)–(d) is 8.0, 9.15, 9.25, and 10.50, respectively.

[()TD$FIG]

Fig. 5. TG-DTA curve of yttrium compound hydrothermally synthesized at pH 9.15.

N. Li, K. Yanagisawa / Materials Research Bulletin 46 (2011) 428–431 431

two decomposition procedures. The first one below 400 8C gaveweight loss of 14.0%, which is consistent with the release ofwater (theoretical weight loss: 13.86%), and the weight loss of9.2% during the second step corresponds well to the release ofHCl (theoretical weight loss: 9.2%). Although all the fibrouscompounds synthesized at pH 9.5–10.25 were poorly crystallized,the XRD patterns of which show some characteristic peaksconsistent with that of Y(OH)2.63Cl0.37, indicating the similarityin structure. It can be deduced that with increasing pH value,the Cl� in the structure was substituted by OH� step by step,resulting in indefinite OH� to Cl� ratio. Finally, hexagonal yttriumhydroxide, without chlorine in its composition, was thereforereceived at highly basic conditions. These results are similar asour previous study using yttrium nitrate as precursor, where

Y2(OH)5.14(NO3)0.86�H2O, Y4O(OH)9(NO3) and hexagonal Y(OH)3

was prepared in turn along with the increasing basicity [5].To study their stability at high temperature, the oxide fibers

were further calcined at 1400 8C for 2 h. It can be seen in Fig. 2cthat the morphological characteristic was maintained afterthe calcination, indicating their outstanding high-temperaturestability.

4. Conclusion

In summary, Y2O3 fibers with diameter of 100–300 nm andlength up to tens of microns have been successfully synthesized byhydrothermal reaction and subsequent calcination. These fibersshow intriguing high-temperature stability, which are promisingmaterial for refractory applications, such as refractory insulationand high temperature gas filtration.

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