T E M P E R A T U R E D E P E N D E N C E

E L A S T I C C O N S T A N T S O F T H E

C E R A M I C B a T i O a

OF D I E L E C T R I C A N D

C H E M I C A L L Y P U R E

A. I . C h e r n o b a b o v , A. V. T u r i k , UDC 537.226.33 a n d V . D . K o m a r o v

The aim of the present work is to investigate the tempera ture dependence of the dielectr ic and elastic constants of the chemical ly pure ce ramic (CPC) BaTiO a and to elucidate the effect on these constants of the size of the c rys ta l l i t es in parae lec t r ic and fe r roe lec t r i e phases of different symmetry . Chemically pure ce ramics with different mean crysta l l i te grain s izes (0.1-500 #) were prepared by the procedure described in [1, 2]. The samples used in the measurements were disks of d iameter 10 mm and thickness 1 ram, with im- planted s i lver e lect rodes . The permit t ivi ty e was measured using an E8-2 bridge at a f requency of I kHz with a measur ing field strength of about 2 V- cm -i. The elast ic pliancy all was determined by measur ing the r e so - nance frequencies of the radial oscil lat ion of disks [3, 41 to which a small constant field of 1.5-2 kV. cm -1 was applied. In view of the smal lness of the constant field and the weak polarization of the ceramic , it was a s - sumed that the measured values cor respond to the unpolarized mater ia l . Measurements were made in the tempera ture range f rom -180 to +160~C, reducing the tempera ture at a rate of 1.5-3.0 deg/min. All the r e - suits obtained were reduced to zero porosi ty using the Wagner formula [5] for e and the Mackenzie formula [61 for s.

Typical curves of the temperatui~e dependence of s for CPC with three average crysta l l i te s izes d are shown in Fig. 1. The tempera ture of the phase transi t ion f rom parae lec t r ic (cubic) to f e r roe t ec t r i c (tetragoaal) increased and the t empera tu re of the t ransi t ions between fe r roe lec t r i c phases decreased with increase i ad . Subsequent increase in tempera ture was accompanied by tempera ture hys teres is , amounting to 1-1.5~ for cubic - tetragonal (C - T) phase transi t ions, 3.5-4.5~ for t e t r a g o n a l - or thorhombic ( T - O) transit ions, and 12-15~ for or thorhombic - rhombic ( O - R) transit ions. Note also that on the curves of the temperature depen- dence of e for CPC with a crys ta l l i te size d ~ 10 p there is a change in the tempera ture coefficient of the per - mitt ivity (1/e)(de/dT) at a t empera ture between -130 and -140~

In cont ras t to [7], the CPC permit t ivi ty depends significantly on the crys ta l l i te grain size in all f e r r o - e lec t r ic phases; the maximum on the e(d) curves is shifted toward l a rge r d in low- tempera ture phases. In the T phase the maximum of e is at d ~ 0.7 p [2], while in the R phase the cor responding value is d ~ 6/~. The e(d) dependence is undoubtedly due to the presence of an additional low-frequency contribution to e. This contribution evidently has the same origin as the low-frequency contribution to the T phase, which d isperses at f requencies in the 1 - m range and is associated with charges screening the spontaneous polarization of 180 ~ domains and single-domain c rys ta l grains [8].

8

?00

T t

V, ..... ! V ~ - - _ _ L ~ - _ _

______WI___ l -t0o 0 18o r, "C

Fig. 1. Tempera ture dependence of per - mit t ivi ty of BaTiO 3 CPC with a mean crys ta l l i te size of 0.4 ~ (1), 5 # (2), and 400 ~ (3). The symbols C, T, O, and R denote the cubic, tetragonal, o r thorhom- bic, and rhombic phases, respectively.

Rostov State University. Transla ted f rom Izvest iya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 145-146, April, 1978. Original ar t ic le submitted July 12, 1977.

0038-5697/78/2104-0535 $07.50 �9 1978 Plenum I>ablishing Corporat ion 535

The tempera tu re dependence of the elast ic pliancy s n is charac te r i zed by a discontinuous increase in st1 at the t empera tu re of the C - T t ransi t ion and maxima at the t empera tures of the T - O and O - R phase t r ans i - tions. In the B phase, sit dec reases monotonically with reduction in tempera ture . After introducing the c o r - rect ion for porosity, the elast ic pliancy, unlike ~, is only different in the T and O phases, where stt increases monotonical ly with decrease in d, as in [4]. In the C and R phases, for all t empera tures and all the ce r amics investigated, sit is independent of the erysta l l i te size. This indicates that in the T and O phases there is a domain contribution to st~ due to revers ib le oscil lat ions of the 90 ~ (or non-180 ~ domain walls (180 ~ reor iea ta - tion does not change the s t ra in and elast ic pliancy of the ceramic) and to the absence of this contribution in the R phase.

It is important to note that the dependence sll(d) is terminated not on reaching a cer ta in sufficiently low tempera tu re but over the whole of the R phase. This may be indirect evidence of the total order ing of the R phase, in cont ras t to the O and T phases, which are par t ia l ly d isordered because of t r ans fe r of the Ti ion (or other ions of the lattice) between severa l equivalent minima of the one-par t ic le potential [9]. This ion t ransfer , together with some thermal contribution to the permitt ivity, may st imulate the revers ib le displacement of 90 ~ (non-180 ~ domain walls, giving a contribution to the elast ic and dielectr ic constants in the T and O phases.

1, 2. 3.

4. 5. 6. 7. 8. 9.

L I T E R A T U R E C I T E D

A. V. Turik and V. D. Komarov, Izv. Vyssh. Uchebn. Zaved., Fiz. , No. 9, 138 (1970). V. D. Komarov and A. V. Turik, Izv. Akad. Nauk SSSR, Neorg. Mater., 1_~1, 1831 (1975). D. Berl inkur, D. Kerran, and G. Jaffe, in: Physical Acoust ics [Russian translation], ed. W. Mason, Voh la , Mir, Moscow (1966), pp. 204-326. A V. Turik and V. D. Komarov, Izv. Akad. Nauk SSSR, S e r . Fiz., 3_.~4, 2623 (1970). J. A. Reynolds and J. M. Hough, Proc . Phys. Soc., 7_0.0, 769 (1957). J. K. Mackenzie, Proc . Phys. Soc., 63B, 2 (1950). K, Kinoshita and A. Yamaji, J. Apph Phys., 4_~7, 371 (1976). A. V. Turik, K. R. Cherayshov, and V. B. Komarov, Fe r roe l ec t r i c s , 6, 45 (1973). R. Com~s, M. Lambert , and A. Guinier, J. Phys. Soc. Jpm, 2_88, Suppl. ,195 (1970).

" E X C I T A T I O N O F A T O M I C A N D I O N I C S P E C T R A L L I N E S

IN C A T H O D E R E G I O N O F G L O W D I S C H A R G E

R. G. K a r i m o v UDC 537.525

In the cathode region of a glow discharge the mechanism of spectral line excitation of the basic gas and the impuri t ies depends s t rongly on the spatial s t ruc ture of that region of the discharge. This dependence may be used in many cases to facil i tate more sensitive spectral analysis of gas mixtures and to elucidate the exci-

tation mechanism of the investigated levels [1-4].

The present work gives experimental resul ts on the excitation of atoms and ions of inert gases and hydro- gen. The basic measurements were made in an a tmosphere of helium with impurit ies of neon, argon, xenon, and hydrogen at p r e s su re s of 1-10 t o r r and a total impuri ty content of 0.1% in the discharge tube shown in Fig. 1. The tube s t ruc ture was such that optical and probe measurements could be made simultaneously. Each of the parallel molybdenum elec t rodes (diameter 20 mm, thickness 0.3 mm, interelectrode distance 24 mm) could be made ei ther the cathode or the anode, s imply by revers ing the polar i ty of the supply potential, and so the e l ec t ron -ene rgy distribution function (EEDF) could be measured with a fixed probe at distance of 9 and 16 mm f rom the cathode. A cyl indrical molybdenum probe (diameter 0.04 mm, length 5 mm) was fixed parallel to the electrode plane and the optical axis. The back of the d ischarge- tube e lect rodes was not screened, so

as to hasten aging and degassing.

V. D. Kuznetsov Siber ianPhys ico techntca l Institute at Tomsk State Universi ty . Trans la ted f rom Izvest iya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 146-148, April, 1978. Original ar t ic le submitted July

15, 1977.

536 0038-5697/78/2104-0536 $07.50 �9 1978 Plenum Publishing Corporat ion