Ewaida, M A

Ghoneim, M M

Hager, I Z

Khafagy, A H

IACS Institutional Repository

Indian J. Phys. 71A (1), 29-38 (1997)I J P  A- an international journalDTA and annealing investigations of some V2O5/P2Q5 glassesA H Khafagy, M A Ewaida, M M Ghoneim and I Z Hager Department of Physics, Faculty of Science, Menoufia University,Shebin El-Kom, EgyptReceived 19 December J995, accepted 16 May 1996Abstract : DTA thermograms were recorded for some V2O5/P2O5 glasses with different composition (9.33, 18, 29, 53, 33 and 65 mole% of V2O5) in the temperature range from 27 to 650°C. Two exothermic peaks were observed and referred to relaxation and crystallization processes in order of increasing temperature respectively. In addition to the above two peaks, an endothermic peak was observed between them and attributed to the glass transition of each sample. Also the effects of the heating rate as well as the annealing time on the observed peaks (relaxation and ordering) were investigated, where the activation energy of crystallization Ec was calculated according to Kissinger and Ozawa models. The dependences of both Ec and glass transition temperature Tg on glass composition were also investigated.Keywords : Vanadate phosphate glasses, phase transformation and annealing PACS Nos. : 8130, 8I40E and 8140G 11. IntroductionIt is known that alkali metavanadate crystals have a chain structure composed of the V 0 4 letrahedra and these VO4  tetrahedra were found in the skeleton of metavanadate glass [1]. Structure studies of the V2Qs crystal [2,3] revealed that each V5* ion is making a distorted VO5  tetragonal pyramid, which can also be.viewed as a distorted V Q  trigonal bipyramid, sharing edges and comers to form a layer structure. The coordination number of the V5* ion is always 5 in the case of the V2O5/P 2Q5 glasses and they are composed of the interconnected and distorted V 05 tetragonal pyramids [4], Also, structural studies of vanadate glasses containing Fe2Oj in iht V2Q5 matrix [5] showed that the Fe3* ions occupy substitutional position in all glass composition without affecting the V=0 bond vibration.DTA thermograms of the V2Q5/P 2Q5 glasses containing 5 mol% F^Oy as reported[6] consist of an endothermic peak due to glass transition and one or a few exothermic peak (s) due to crystallization. The saihe authors reported also that the distinct increase in the© 1997 LACS30 A H Khafagy, M A Ewaida, M M Ghoneim and / Z Hagerglass nan si non temperature TK of the V20^ /P 2Os glasses, observed with increasing the P20 5 content, is ascribed to a continuous change of the glass matrix having a layer structure composed of the V 05 tetragonal pyramids into a more complicated three dimensional structure composed of the V O 5 tetrahedra.Since the glass transition reflects a change in the coordination number o f the network forming atoms and a destruction of the network structure brought about by the formation of some non-bridging atoms, DTA studies on the structure of several glasses were performed and revealed that Tx is closely correlated with the change in the coordination number of glass forming atoms (network former) and with the fnrmnti™  r " ' * " * » *  ■ » » »  « « * «  MS] . . .  chlorine A 7 „  'bridging oxygen, fluorine and chlorine atoms in the glass is equivalent to devitrification ofthe glass matrix. 1 orv o , l T r M rr*' “, s “mieM ** ^A / P ’O, glasses and the eltecl of annealing on these transitions.2 , Experimental techniqueVanadium phosphate glasses of starting compositions expressed inby melt-quench technique. The final product are chemically analysed and I T T  7 ^are explained in details, picviously 1 1 7 1  A micro difOro r 1 T d ^ Cchn,<|UCs-  -  f r 1- "  *»“ 7 " °* “ " » «  •“ > « .  • Ihermally a.W, „ fetCTCt m, J ia| ^side in the furnace. The temperature variation AT«r h,i T  A  C PU‘ Sldc b>DTA runs were carried out from 27°C ud to 6 Sfi°r A n  2 0  and 30T/min, noting that at e-ich hnJ( P L° °  d,tTerenl heating rates, 1 0 , 15.'he tested glass and the reference specimen" T T ' T  differe"<* AT betweenor a valley. IS 6  ccted> ^Phfied and recorded to be a peak3.1 1 T al ™ “ D n .g'““  *i“> ,"P"'d W Wshown m Figures (la, h, c and d) respectively. T hem  ^ ^  "  investi«ated and were carried out in the temperature range from ~n easuremeMs of-the tested specimen 15. 20 and 30°C/min. In general each DTa ^  ,0 650<>C *  d'ffcrent heatin* rates* 10’ Peaks and an endothermic one between them j T *  ^  ^  reVC8,S tW0 exother7nic sample with 29 mol% 0f V ,0 , there is n owevcr’ on *he plots corresponding to the Peak observed in DTA thermograms mav Z T* ^  ^  first MothcrmicP" kW is/are * *  *0 crystallization as^  r e ^ V T ’ 0ther « 0themlic "Pon heating the glass. The last one wh h f0rmation o f s ° ™  ordered regionswhtch occurred between the above peaks is thejJ In m  >I, Ban c f ( klie jlie Js K,c0 toHethoc (|n the any o f ^awa31endothermic peak, attributed to glass transition. The DTA thermograms have been used to study the amorphous crystalline-transformation in chalcogenide glasses. This has been done hy deducing the order of reaction n and the apparent activation energy of the partial crystallization or ordering Ec [19]. The values of n were determined using the shape indexDTA and annealing investigations o f  some V20 s/P 20 5 glassesFigure 1. The effect of heating rate on the DTA thermograms of the V2O5/P2O5 glasses.method [20], which reports that n = 1.26 Vs where s is the ratio of the base parts of the peak on the base line of DTA thermogram. And the values of Ec could be determined by using any of the following equations (1) and/or (2) which were suggested by Kissinger [20] and | Ozawa [2 1 ] respectively.In (7^0) = (E J R T £  + constant, (1)In (0) = -  { E J R T ^  + constant, (2)Inhere 0 is the heating rate, Tc is the crystallization temperature and R is the universal gas constant. These equations are plotted for all investigated glasses and shown in Figures (2a32 A H  Khafagy. M A Ewaida. M M Ghoneim and I Z  Hager„„d [,). The calculated value* for the aclivalion clergy of c„st» lli» lion  (orderln*) and theFigure 1. (Continued)3.2. The effect of composition on DTA peaks observed for glasses :The dependence of the glass-transition temperature (7^) on the composition of the studied glasses are investigated at fixed heating rate (10, 15, 20 and 30°C/min). Figure 3(a) shows the variation of the glass-transition temperature with the V2O5 content for the operating different heating rates. The behaviour of the plots in Figure 3(a) revealed three composition regions. The first region shows a rapid decrease in Tg as the V20 5 content increases. While in the second region, Tg is slowly decreased as the V20 5 content increases. In the third one, Tg is decreased rapidly with increasing V20 5 content. It is known that the DTA measurements of the glass-transition temperature (Tg) are useful in suggesting structural changes that take place by compositional change. This is, because Tg is very sensitive to any change of the coordination number of networks-forming atoms (network-former) andLn(TcV<*> )D TA and  annealing investigations o f  som e glasses 33( K cal / mDlA H  Khafagy. M  A Ewaida, M  M  Ghoneim  and  /  Z  H a g e rTable 1. Th,; values o f r r , n and Et at different heating rales for the V 2 O5 /P 2 O5  glassesV ;0 SITlol e/(heating rale /  fX'/rmn)T  °C T( °C n £ f (k cal/inol according to Kissinger Ozawo1 0 495 545 1 09IK 15 482 589 0 85 15 6 8 15.652 0 505 615 1 2 230 515 650 1.191 0 452 475 1.0329 15 460 485 1.03 30.44 30.432 0 470 500 1.0130 475 505 1.031 0 420 500 1.0553 33 15 430 515 I 04 32.25 32 222 0 452 533 0 9330 450 540 1 321 0 365 405 1 1365 15 372 410 1 16 45 8 6 54 89 12 0 425 470 1 26 \30 420 470 1 19 \DTA and annealing investigations o f some V2O5/ P 20 5 glasses 35also to the formation of non-bridging oxygen [9-12], chlorine [8| and fluorine [16] respectively. On the other hand, an introduction of P20 5 into V2Q; matrix is concluded to ihrce-dimcnsional network structure composed of the V 04 [9-12] and also P 04 tetrahedral 161 respectively. In case of glasses, it is speculated that non-bridging oxygenatoms arc not formed in the V 04 tetrahedra [11]. In general, the decrease in the TK of the present V2CyP2Q5 glasses, which observed with increasing the V2O5 content, arc therefore, ascribed to some structural changes which take place in the glass matrix leading to layered structure instead of the three-directional complicated structure. This agrees with previous works [12,6]. The crystallization (ordering) temperature T( as obtained |6 | for some V2Q5/P2Q  glasses, was higher than TK by about 67°C. This observation has indicated the high stability ol the V20«/P2Q  glasses. In the present work, the crystallization temperature f\ is also higher than the glass transition temperature 7^  by about 85°C, which is consistent with the results reported previously.The compositional dependence of the activation energy of crystallization for some prepared V2Q5/P2Q  glasses, is shown in Figure 3(b). This Figure indicates three- tomposition-regions, which are in converse behaviour to that of glass-lransilion- icmperature. The first region shows a rapid increase in the activation energy of crystallization or ordering as the V20 5 content increased while in the second one, Ec is s lo w ly  increased up to 53 mol% of V20 5. Beyond 53 mol% of V20 5, the activation energy ol ciystalli/alion E( shows considerable increase with increasing V2Os content.Since the compositional dependence of both the glass-transition temperature (7^ ,) and activation energy of ordering (Ec) has revealed the three-compositional-regions, this is another more evidence which supports all our previous and similar data of density, molar volume, infrared [17] and ultrasonic attenuation of the V2Q5/P2Qi glasses |22].* 3 Effect o f annealing :In order to investigate the effect of annealing lime on the exothermic and endothermic peaks which are observed in all DTA thermograms of the present work; some more DTA and X-ray diffraction measurements were made on the sample which contain 9.33 mol% of V2Ov These measurements were carried out several times after annealing the tested specimen at 350°C for 1 ,2 , 3, 4, 8, 12, 48 and 96 hours, respectively.Figure 4(a) shows the X-ray diffraction pattern obtained from the conducted runs of ihe above investigations. Also, Figure 4(b) shows the results representing the corresponding DTA thermograms. However, X-ray plots [Figure 4(a)] indicate the amorphous nature of the tested specimen, since no diffracted peaks have been observed for all; measurements after the heat treatment at the different annealing times. Also, the DTA thermograms [Figure 4(b)] reveal two exothermic peaks and an endothermic one between them. The first exothermic peak is characterized by its broadening as occurred in the range °1 temperature from about 27 to 450°C. This peak is attributed to some sort of relaxation 118] which influences the atomic rearrangements leading to a short range order but noAitSNUN36 A H Khafagy, M A Ewaida. M M Ghoneim and /  Z HagerFigure 4. Effect of annealing time on X-ray diffraction and DTA glass with 9.33 mol% V20 5. (a) and (b) respectively.thermograms of V2O5/P2O5DTA and annealing investigations o f some V20 ^ /P 20^ glasses 37crystallization. It is observed that this relaxation peak observed here is affected by the annealing time; its area has changed with annealing time at 350°C as shown in Figure 5(a). Inspection of this Figure shows that as the specimen is annealed, the area of the relaxation peak rapidly decreases up to 10 hours time of annealing. Then, it becomes nearly constant.I*lOsFigure 5. Effect of annealing time on the area of both the relaxation and ordering peaks (a) and (b). respectivelyThis however, indicates that the quasi-equilibrium state of the tested specimen resulted due to relaxation during heating, can be reached when the specimen is annealed at 350°C for about 10 hours time. The above exothermic relaxation peak is followed by the endothermic one of the glass-transition as shown in Figure 4(b). It is assumed that at a glass-transition, the amorphous state with short range order changes into more random distribution of atoms, and alter that the crystallization (long range order) is facilitated. An obvious result for the atomic and the ionic rearrangements which could take place during heating is the observation of the second exothermic peak of crystallization (i.e.t long range ordering) after the glass-transition region. The magnitude of this peak is also showed to be dependent on the annealing lime, where its area is varied, in a converse sense to that of the relaxation peak [Figure 5(b)|, (>., the area of this peak is increased considerably with annealing time up to 10 hours, then it reached a constant value when the size of the ordered regions in the tested amorphous matrix has attained maximum value after annealing it for 10 hours at 350°C.4. ConclusionIn the present work, DTA thermograms were carried out in the temperature range 27- b50°C, at different heating rates for different composition of V2C>5/P20> glasses. These measurements revealed the different phase transitions (e.g. glass, crystallization and melting) as well as a relaxation process which occurred during the change in the state of the specimen as the temperature is increased.38Also the elfecis ol heating rale as well as the annealing time on the observed peaks(relaxation and ordering) were investigated. However, the results of the former effectsenabled us to determine values for activation energies for both glass and crystallization transitions and k t respectively.lUfvrriKTs| j | y Dimirnev, V Dimilruv, M Anuudov and D Topalov J Non-Crvsl. 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DTA and Annealing Investigations of Some V2O5/P2O5 Glasses

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DTA and Annealing Investigations of Some V2O5/P2O5 Glasses

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