Preparation of polycrystalline NH+4 β″-alumina

Solid State Ionics 37 ( 1989 ) 11-16 North-Holland, Amsterdam

PREPARATION OF POLYCRYSTALLINE NHg I$"-ALUMINA N o b u o IYI *, Alicja G R Z Y M E K a n d Patrick S. N I C H O L S O N Ceramic Engineering Research Group, Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L 7 Received 18 November 1988; in revised form 22 June 1989; accepted for publication 7 July 1989

Polycrystalline NH~-IY'-alumina ceramics were successfully prepared by NH~- exchange of precursor Rb+-IY'-alumina ceramics derived from Na+-13-aluminaby ion exchange. The obtained ceramics were crack-free and translucent. The electric conductivity by ac impedance spectroscopy was trgrain= 1.7 × 10--4f~--l cm- l and atotaj= 3.0 × 10-5 f~- l cm- ~.The lattice parameters expressed in the hexagonal system were a = 5.6231 (2) A and c= 35.354 (2) A. The chemical formula of the polycrystalline NH~ lY'-alumina was [ (NH4)o.Ta (H30)0.22 ] 1.66Mgo.49Al10.siO17.085on the basis of TGA/DTA data and chemical analysis.

1. Introduction A m m o n i u m 13"-alumina is one o f the IV-alumina family o f compounds. It was reported stable to 250 oC and to exhibit high p r o t o n c o n d u c t i v i t y (10 -4 f ~ c m - ~ at r o o m t e m p e r a t u r e ) [ 1 ]. M o s t solid state p r o t o n c o n d u c t o r s d e c o m p o s e < 100°C with water loss [2]. N H + [~"-alumina is therefore a p r o m i s i n g m e d i u m t e m p e r a t u r e p r o t o n conductor. This comp o u n d is usually p r o d u c e d by the ion-exchange o f N a +- or K+-[3"-alumina in m o l t e n a m m o n i u m nitrate at 200°C. This ion exchange is successful for single crystals [ 3 ] but causes severe cracking o f sintered material. This is due to the a n i s o t r o p i c expansion o f the c-axis p a r a m e t e r when the larger N H + ion replaces the smaller N a + o r K + ions. The resuiting stresses destroy the ceramics. T h e p u r p o s e o f the present study was to synthesize crack-free NH~--13"-alumina polycrystalline ceramics. One way to avoid (or reduce) ion exchange stresses is to use high temperatures. T h e latter facilitates annealing during ion-exchange. This m e t h o d was developed to p r o d u c e crack-free K+-IY'-AIzO3 [5,6 ]. T e m p e r a t u r e s > 2 5 0 ° C cannot be e m p l o y e d for NH~- ion exchange as a m m o n i u m nitrate a n d NH~-[~ " - a l u m i n a decompose. O n e avenue to N H ~ * Present address: National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba-shi, Ibaraki 305 Japan.

[l"-alumina ceramics is to p r e p a r e a 13"-alumina containing a m o n o v a l e n t cation o f similar ionic size to N H + as a precursor m a t e r i a l for N H + exchange. The ionic radii o f the c o m m o n m o n o v a l e n t cations, N a +, K +, R b + a n d N H + are 0.95, 1.33, 1.48, a n d 1.48 A respectively [ 7 ]. As R b + a n d N H + have similar ionic radii, R b + [3"-alumina was chosen as the precursor ceramic in the present study. R b + precursors h a d been used to prepare NH+-lY'-gallate by Ikawa et al. [8] a n d T s u r u m i et al. [9]. Rb+-[3"-alu m i n a ceramics were prepared by ion exchange rather than by direct synthesis. The ion exchange sequence e m p l o y e d was: Na+I~"-A12 O3 ~ K + [~"-AI2 O3 __, R b + 13"-A1203 ~ NH~- 13"-A12 0 3 .

2. Experimental 2. I. Precursor materials and ion exchange

The precursor Na+-~i"-alumina ceramics were m a d e by the c o n v e n t i o n a l sintering method. The p r e p a r a t i o n c o n d i t i o n s d e v e l o p e d by Sheng et al. [ 10 ] were employed. A mixture o f Na2CO3, M g O a n d A1203 ( 2 . 2 3 : 0 . 4 4 : 1 1 . 9 7 by weight) was vibromilled for two days in acetone. After drying, the

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N. lyi et al. / Polycrystalline NH~ fl"-alumina

powder was calcined for three hours at 1150°C. The [3-alumina content (f(13) [ 11 ] ), was ~20% at this stage. After crushing and sieving, the powder was uniaxially and isostatically pressed at 35000 psi. The resulting pellets were immersed in a powder bed of a Na+-13"-alumina and sintered in an MoSi2 furnace between 1655 and 1670°C for ~ 5 min. The maximum temperature was achieved in 40 min, The sintered pellets had a density of 3.20-3.25 gcm -3 (theoretical density: 3.26 gem -3) and dimensions - 11 mm diameter by 1 mm thick. The weight loss was 0.5 wt%. The sintered pellets were translucent with f(13) ---0.15. A small amount of NaA102 was detected by the powder XRD. After polishing on 400 grit paper, the Na+-13"-al umina pellets were K + ion-exchanged. This process involved two stages: The Na+-13"-alumina pellets were embedded in a K2COa-AI203 mixture ( 1 : 9 by weight) in an alumina crucible and held at 1350°C for 24 h. Weight change revealed = 40% exchange. The NaAIO2 XRD peak disappeared at this stage. Next the partially exchanged samples were placed above molten KC1 at 1200 °C for two days to bring the exchange to 80-90% by weight change. The exchange rate depended on the weight ratio of the samples and KC1. 15 g of KC1 was used for 4 pellets (ca. I g in total). After exchange, the samples were washed briefly with water and ultrasonically vibrated in acetone. The K ÷ exchange process was slightly modified from that developed by Tennenhouse [ 5 ] and Crosbie and Tennenhouse [6 ]. K + ~ R b + ion-exchange was conducted under conditions similar to those for K ÷ exchange. Specimens were held above an RbC1 melt at 1200°C for two days. After washing and drying, weight change indicated 80-90% replacement. Direct immersion in molten RbCI exchanged the K ÷ samples to 100% Rb ÷. Pellets were immersed in excess of RbC1 and heated to 800°C for two days. The Rb ÷ samples obtained were crack-free and = 100% exchanged. The Rb+-I~"-AIzO3 pellets were immersed in molten NHaNO3 at 200°C and the exchange rate was monitored via weight change. 60% replacement took place after 24 h. Full exchange took 7-10 days. The exchange rate differed from specimen to specimen. In a few cases, complete exchange took three days. The reason for this variation is not known. It is possibly related to sintering conditions and the grain

boundary phases present in the Na-~"-Al203 precursor ceramic. 2.2. Characterization o f the N H ~ -fl" alumina ceramics SEM 1 fractography was conducted at an operating voltage of 3.0 kV. Surfaces were neither washed nor coated. Lattice parameters were calculated by least-square analysis of XRD data 2 using CuKct radiation. The XRD chamber was kept dry with silica gel desiccant and the pellets used were polished fiat on one side. Pure Si powder was used as the external standard. The range of 20 utilized was 50-100 ° and ---20 peaks (CuKa~ diffraction, 2=1.5416 ~,) were involved in the analysis. Thermal analysis was carried out on a T G A - D T A apparatus 3. The heating rate was 10°C/min and an air atmosphere was used. The NH3 content of the samples was determined by the Kjeldahl method. The crushed samples were digested in concentrated n 2 s o 4 for 16 h under pressure. The recovery of ammonium after this treatment was estimated to be 99.5% based on the standard, (NH4)2504. Na ÷, K ÷, Rb ÷ and Mg 2÷ were analysed by atomic absorption spectrometry. The AI3÷ content was determined by the chelate titration method. The electric conductivity was determined by impedance analysis 4 at room temperature in the frequency range, 5 Hz to 13 MHz. Gold was sputtered onto faces of the discs as blocking electrodes. The Cole-Cole impedance plot analysis was employed to determine grain- and total conductivity.

3. R e s u l t s and d i s c u s s i o n

The resultant NH4÷ -13t! -alumina ceramics were visually free of cracks and maintained translucency (fig. 1 ). In some cases, edge and surface cracks developed during the NH~- exchange. These defects are recognized by their brown colour. The fracture surfaces o f R b ÷ and NH~- -13-alumina were observed by SEM (fig. 2). The microstructure 1 IS1-DIS30(AkashiCo. Ltd). 2 PW1700*Phillips Co. Ltd). 3 RigakuCo. Ltd. 4 HP 4192A (Hewlett-Packard).

iV. Iyi et al. / Polycrystalline N H + fl"-alumina

13

O

Fig. 1. The crack-freeNH~ -I~"-aluminaceramics.

of Rb+-13"-alumina ceramics (fig. 2a) indicates transgranular fracture. The fracture of N H +-~l'-alumina (fig. 2b) is intergranular. This observation implies a reduction of grain boundary strength during NH~- exchange. Minor amounts of a porous substance and tiny particles ( -~ 1 g m ) were detected between grains in parts of the NH+-~l"-alumina ceramics (fig. 3). Secondary phases such as NaA102 may change composition during the ion exchange sequence developing porosity and initiating small surface cracks. This identifies that further improvements of the final product could be achieved by optimization of the precursor Na+-fl"-alumina ceramics. The NH3 content in the N H +-13"-alumina pellets was 3.62_+0.04 wt% by chemical analysis. The remaining Rb + and K + were both 0.08 wt%. Na + was not detected and estimated <0.01 wt%. T G A / D T A was carried out on another portion of the same specimen from room temperature to 1000°C (fig. 4). The weight loss can be divided into 4 stages: (1)0.21 wt%-50-100°C (2)0.95 wt%-140-300°C, (3)3.80 wt%-300-400°C, and (4)2.43 wt%-430-860°C. The first loss is attributed to adsorbed water. DTA showed a large exothermic peak between 300-400 ° C, corresponding to the third stage of weight loss. This is interpreted as the liberation of NH3 known to give an exothermic peak in air [ 12] and because the weight loss is consistent with the NH3 content de-

b Fig. 2. (a) The fracture surfaceof Rb+-~"-alumina before NH4+ exchange; (b) the fracture surface of NH~-I]"-alumina derived from the Rb+-lY'-aluminaby NH~ exchange.

Fig. 3. The porous phase observed at grain boundaries in NH+ 13"-aluminaceramics.

14

N. lyi et al. / Polycrystalline N H + fl"-alumina

0.0

terms of atomic ratio. Accordingly, the ideal formula for the present NH~--l~"-alumina is

2.0

[ (NH4)0.7s (H30)0.22 ] 1.66Mgo.49Allo.51 O17.085 •

The lattice parameters of fully exchanged Rb+- and

o 4.O

NH~--6"-alumina pellets were measured. The crystal

7= ._m 6.0

8.0

200

400

600

Temperature

800

1000

('C)

Fig. 4. Thermal analysis of NH + -~ "-alumina.

termined by wet chemical analysis. The second loss is thought to be due to the water incorporated as H3 O+ on the conduction plane of the 13"-alumina structure. Incorporation of the H30 + into NH~--I~"alumina has been reported [ 1,3,13 ], although there are discrepancies as to the content. Farrington and Briant [ 1 ] pointed the H 3 0 + may be sourced by partial decomposition of the ion exchange medium, NHaNO3. The fourth weight loss stage corresponds to the liberation of H20 from the remaining protons and structural oxygen. The chemical formula of the NH +-13"-alumina was estimated from these data. Assuming that the Mg/ AI ratio did not change and the 0.21 wt% loss of the first stage is due to adsorbed water, the chemical formula is: (NH4) 1.277(H3 O)0.366K0.012 Rbo.o05 Mgo.49Allo.51 -O17.o85.0.07H20

wherein Mg+A1 is adjusted to 11.0. The calculated weight loss at each stage is 0.21, 1.10, 3.62, and 2.46 wt%, respectively, based on this formula. The weight change due to ion exchange from Na+-I~ "-alumina to Rb+-~"-alumina (0.10824 g to 0.12675 g) suggests the formula: K1.685 Nao.o06 Mgo.49 A110.51O17.10,

where (Mg+AI) was fixed at 11.0. These three formulae for the different stages were derived from different methods and are consistent with each other in

system is trigonal and the parameters expressed in the hexagonal unit cell system are: a = 5.6237 (2) A and c=34.350(2) A for Rb+-13"-alumina, and a=5.6231 (2) A and c=34.354/~ for NH+-~"-alu mina. Full exchange was confirmed by weight change in the case of NH~-I~"-alumina and by chemical analysis in the case of Rb+-13"-alumina. In the latter case, the remaining Na and K were below 0.1 wt% and 0.36 wt%, respectively. These data show there is very little difference of lattice parameters, so fullyexchanged Rb+-I~"-alumina is a suitable precursor for NH~- -13"-A1203 from the viewpoint of stress development. However, considering that good NH + IY'-alumina is produced even from 80% Rb+-ex changed samples, there appears to be some tolerance of stress in the polycrystal. The lattice parameters determined for NH + -6"-alumina are somewhat different from those of ref. [3] (a single-crystal analyzed by neutron diffraction). This may be due to water content and cation population on the conduction plane. The electrical conductivity was measured by the ac impedance method (ref. [ 14] ) at room temperature. Fig. 5 shows the complex impedance diagram expected for the equivalent circuit illustrated. A portion of the impedance plot for the present sinters is shown in fig. 6. The intercept on the real axis and the area/length factor ( = 2 . 8 cm), result in a conductivity of Crgrain( = 1/Rg) = 1.7 X 10 -4 r - ~ cm-1 and a t o t a t ( = l / R g + R g h ) = 3 . 0 X 1 0 - 5 f l - i c m - l . The conductivity within the g a i n is consistent with the reported value ( 10 -4 ~~-1 cm-1 [ 1 ] ). Colomban et al. [ 15 ] reported a lower value ( 10- 6 ~-~- I c m - 1 for NH~--I~"-alumina. There is a possibility that composition differences cause these different conductivities and further research is needed to clarify the discrepancy. On the other hand, an extremely high g a i n boundary resistivity for NH~- I~"-gallate was reported [ 8 ]. Such was not observed in the present NH+ -I~"-A1203.

N. lyi et al. I Polycrystalline NH~ #"-alumina

Cgll

out extension o f the surface cracks. T h e second m e t h o d involved direct R b + - v a p o u r exchange o f the Na+-13"-alumina ceramics. The resultant samples were translucent a n d ---95% exchange was achieved after two days without breakage. T i n y surface cracks were however, observed. These samples were also exchanged to NH~- -13"-alumina without p r o p a g a t i o n o f these cracks. These results suggest that i m p r o v e d v a p o u r exchange c o n d i t i o n s a n d quality o f starting N a + - ~ - a l u m i n a , m a y result in higher quality NH~-13"-alumina ceramics.

(A) Rel

Rgb

O

X I

Rg

15

Rg+Rgb

R (ohm)

(B) Fig. 5. The assumed equivalent circuit for the NH + -[~"-A1203POlycrystal system (A), and the corresponding impedance diagram (B) (resistance (R) versus reactance (X)). P~ is the resistance of the grains; R~, the resistance of the grain boundaries; Re,, the resistance of the electrode. Wis the Warburg impedance and Cg, the capacitance of the specimen).

2

Acknowledgement The authors acknowledge the help o f Dr. Ying Sheng o f Beijing University, C h i n a for his help in preparing the N a + and K+-13"-alumina ceramics, Dr. P. Sarkar o f M c M a s t e r University, C a n a d a is also acknowledged for his suggestions on the ac i m p e d ance m e a s u r e m e n t . T h a n k s are also due Mr. Y. Yaj i m a at N a t i o n a l Institute for Research in Inorganic Materials ( N I R I M ) in J a p a n for chemical analysis, a n d Mr. M. T s u t s u m i at N I R I M for his help on the SEM a n d useful suggestions.

v

X 1 1

10 6

References

••

~,,~"

10 7

I

2 a ( k ohm)

Fig. 6. Part of the ac impedance diagram for NH + -13"-alumina showing the Re intercept on the real axis. (Frequency range 1.6X 103 Hz to 1.3× 107 n z ) .

3.2. Other ion exchange parameters Two other routes to Rb+-l~"-alumina were explored. The first p r o d u c e d K+-13"-alumina b y direct K - v a p o u r exchange o f the N+-13"-alumina. Na+-13 "a l u m i n a pellets were exposed t o KC1 v a p o u r at 1200 ° C for two days. Several tiny surface cracks were observed after this t r e a t m e n t but the samples d i d not break. These specimens were successfully exchanged to Rb+-I3"-A1203 a n d then N H + - ~ " - a l u m i n a with-

[ 1] G.C. Farrington and J.L. Briant, in: Fast ion transport in solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, Amsterdam, 1979) p. 395. [2] M.G. Shilton and A.T. Howe, Mat. Res. Bull. 12 (1977) 701. [3] J.O. Thomas and G.C. Farrington, Acta CrystaUogr. Sect. B39 (1983) 227. [4 ] J.B. Goodenough in: Solid state protonic conductors II, eds. J.B. Goodenough, J. Jensen and M. Kleitz (Odense University Press, Odense, 1980) p. 123. [ 5] G.J. Tennenhouse, U.S. Patent 3 (1969) 446, 677. [6] G.M. Crosbie and G.J. Tennenhouse, J. Am. Ceram. Soc. 65 (1982) 187. [7 ] L. Pauling, The nature of chemical bond, 3rd Ed. (CorneU University Press, Ithaca, NY 1960). [8] H. Ikawa, T. Ohashi, M. Ishimori, T. Tsurumi, K. Urabe and S. Udagawa, in" High tech ceramics, ed. P. Vincenzini (Elsevier, Amsterdam, 1987 ). [9]T. Tsurumi, H. Ikawa, M. Ishimori, K. Urabe and S. Udagawa, Solid State Ionics 21 (1986) 31. [ 10] Yin Sheng, B. Cobbledick and P.S. Nicholson, Solid State Ionics 27 (1988) 233.

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N. lyi et al. / Polycrystalline NHg p"-alumina

[ 11 ] A. Pekarsky and P.S. Nicholson, Mat. Res. Bull. 15 (1980) 1517. [ 12 ] H. Ikawa, T. Tsurumi, K. Urabe and S. Udagawa, Solid State Ionics 20 (1986) 1. [13]K.G. Frase, J.O. Thomas, A.R. McGhie and G.C. Farrington, J. Solid State Chem. 62 (1986) 297.

[ 14] A. Hooper, NATO ASI Ser. Ser. E 101 (1985) 261. [ 15] Ph. Colomban and A. Novak in: Solid state protonic conductors I, eds. J.B. Goodenough, J. Jensen and M. Kleitz (Odense University Press, Odense, 1980 ) p. 153.