Higazy, A A

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Indian J  Phys. 69A (2), 213 -  222 (1995)IJP A- an international |oumalE la s t i c  c o n s t a n t s  a n d  e l e c t r i c a l  p r o p e r t i e s  o f  th e  s e m ic o n d u c t i n g  M 11O 2- P 2O 5 g la s s e sA A HigazyDepartment of Physics, Faculty of Science, Qatar University, Doha, Qatar Received 6 October 1994, accepted 8 November 1994Abstract : The elastic moduli and DC-eleclncal conductivity arc studied as a function of composition for the entire vitieous range ol the system MnOj^O^ that can be prepared by melting MnO? and P7O5 oxides in open crucibles The ultrasonic wave velocities (longitudinal and shear), the elastic moduli. Poisson's latio and the DC-clecirical conductivity are found to be sensitive to the glass composition It is found fiom these data that the present semiconducting glass system can be divided into three compositional regions The results are nitci preted in terms of changes in the interatomic lorce constant and the cross-link densities of network bonds.Keywords : Acoustic properties, electrical conductivity, semiconducting phosphate glasses PACS Nos. : 62 20 Dc, 62 65 +k, 72 80,NgI . IntroductionThe sludy of the variation of ultrasonic properties with gradual and wide ranging changes in glass composition provides essential information about atomic and molecular configurations in glass. The dependence of elastic moduli on glass compositions of a number of phosphate glasses has been studied by Farley and Saunders [1 ], Filed [2], Bridge and Moridi |3], Patel and Bridge |4 |, Higazy and Bridge [5], Bridge and Higazy [6], Higazy etal  [7] and Ewaida etal [8], The results showed discontinuities in elastic constants with composition. This behaviour is qualitatively interpreted in terms of metal ion coordination numbers, stretching lorce constants and cross link densities of network bonds.The electrical conduction processes in the amorphous semiconductors have been an interesting subject. Several investigators [9-15] have paid considerable attention to the study of the electrical properties of phosphate glasses. It has been reported that the electrical conduction in these glasses takes place as a result of electrons jumping from metal ions with a low valency state to others with a higher valency. However, this conduction process is difficult to interpret quantitatively since it is affected by many factors, such as the type and© 1995IACS214 A A Htgazyconcentration of the metal ions, its proportions in the two valency states, the preparation conditions and the existence of microstructures within the glass matrix.The present work forms part of a programme to explore what information can be obtained about atomic and molecular configurations in glass, from studies of the compositional dependence of the elastic constants of M n C ^ ^ O j glass system. Furthermore, the compositional dependence of DC-electrical conductivity for the same glass series is also highlighted2. Experimental techniqueGlass preparation: \Manganese-phosphate glasses were prepared from laboratory reagent grades of Analar manganese oxide (M n02) and Analar phosphorus pentoxide (P20 5), using alumna crucibles heated in an electric furnace, open to the atmosphere. The weighed quantities of these chemicals in appropriate proportions were thoroughly mixed and placed in an electric lurnace, held at 350°C for one hour. This allows the PzQs to decompose and react with M n02 beloic melting would ordinary occur. After this heal treatment the mixture was transferred to the second furnace which was already held at a temperature range from 850°C to 980°C for 40 minutes (the highest temperature being applicable to the mixes richest ip M n02) The glass melts were stirred occasionally with an alumina rod to ensure homogeneous melts Each melt was cast into two mild-steel moulds to form glass rods 2 cm long by 1.5 cm diameter Alici casting each glass was immediately transferred to an annealing furnace held at 3 0 0 ^  lor one hour. After this time, the furnace was switched off and the glass were allowed to cool to room temperature gradually. This procedure was employed to prepare glasses with a glass formation range from 5 to 60 mole % M n 0 2 (starting compositions).Specimens used for ultrasonic and electrical conductivity measurements were in the form of cylindrical rods of 1.5 cm diameter and 0.5 cm thickness with parallel faces.The densities of the glasses were measured by the Archimedes method using toluene as the immersion liquid and for comparison of the different glasses only they arc accurate to + 0.001 g cm '3.3. Ultrasonic m easurem entsThe ultrasonic compressional and shear wave velocities were made by the pulse echo technique, using commercial transducers at a frequency of 4 MHz, actuated by an ultrasonic flaw detector (ultrasonoscope ML 32). Details of the techniques arc presented elsewhere 15J.The elastic constants of the studied glasses were calculated at room temperature using the measured densities, p , and the velocities of longitudinal, Vh and shear, V s, waves using the following expressions :longitudinal modulus L = p v f ,G = p V lshear modulusElastic constants and electrical properties etc 215bulk modulus K  = L -  (4/3) G,Poisson's ratio a  = ( v f  -  2 V j  2{vJ -Young's modulus E = (1 + CT) 2G,The total maximum error in the measurements of clastic moduli due to changes in specimen thickness (0.02%), velocity (0.05%) and density (0.001%) is therefore about0 08% .4. DC-electrical conductivity measurements1 oi ihe measurements of DC-electrical conductivity a( room temperature, electrodes were lormed by brush painting silver paste. In the present measurements, the current was measured by means of a Kcithley electrometer model 616, with a smoothing adjustable power supply (0-1 KV). The fixed voltage of 300 volts w«1s applied.The DC-electrical conductivity, of each glass sample was calculated by using theexpressiona  = L jR A ,where L is the thickness of the sample, A is the area of the electrode and R is the resistance.5, Results and discussionTabic 1 contains the compositions, the densities, the molar volumes and the elastic constants o! MnOr P20^ glasses. The data of this table has shown that there is a change in behaviour of the compositional dependence of all the properties examined in this work around 15 and 35 nmle % M n02 contentThe plot of density versus M n 0 2 content (see Figure 1(a)) shows an increase with increasing M n02 content up to 15 mole %», which is probably attributable to the effect of adding manganese cations into the vitreous structure of P 20 5. The sole elfect of the entry of manganese into the glass in the first composition region (0-15 mole % MnO 2 oxides) was to rupture P= 0 bonds and produce P-O —Mn cross-links. This leads to a compaction of the structure which causes a decrease in the glass molar volume (see Figure 1(b)) and an increase ,n the glass density.Upon further increase in M n02 content, the variation of the density is seen to display a decrease up to 35 mole % M n02 content (see Figure 1(a)). This decrease in density indicates a structural change in the glass network, which is accompanied by an increase in the molar volume (see Figure 1(b)).As M n 0 2 oxide increases beyond 35 mole %, substantial increase in density occurs, which is probably attributable to a change in the compaction of the glass structure.Table 1. Composition, density, molar volume, longitudinal and shear ultrasound velocities and elastic moduli of MnOi-PjOj glasses at room temperature2 1 6 A A H ig a z yJoO .  —  c*-i ^  w-i «( S c F s s s s s soo f isjiu/NjOl =«DMElastic constants and electrical properties etc 217Introduction o f  M n 0 2 units in this co m p o sitio n a l region  (3 5 - 6 0  m o le  % ),  lead s to a com paction o f  the structure i.e. causes a decrease in the m olar volum e (see  Figure 1(b)).Figure 1. Variations o f (a) density and (b) molar volume with MnCb mole %T he ultrasonic w ave velocities m easured in this work are found to be sensitive  to the  glass com position , as sh ow n in F igures 2(a) and (b). T he addition o f  M n 0 2 to the vitreous P A s  structure in creases both the longitudinal and the shear w ave ve loc ities up to 15 m ole  %  M n02 o x id e . B ey o n d  15 m o le  %, there is a decrease in the ultrasonic w ave ve loc ities w ith  further addition  o f  m anganese o x id e  until about 35 m ole  % M n 0 2 o x id e  content. For high  Mn02 percentages i.e. >  35 m o le  %, the v e lo c itie s  increase again (see  F igures 2(a) and (b)). All the e lastic  m oduli, v iz .  longitud inal, shear, bulk and Y oung's m odulus sh ow  the sam e  trend as the acoustic w ave  velocities (see  F igures 3(a) and (b )), i.e. they exhib it the sam e 3- com position -reg ions’ behaviour.It is generally  considered that pure vitreous P A  is built up o f  an in fin ite  netw ork o f  P 04 tetrahedra w ith each tetrahedron being jo in ed  at three corners to other tetrahedra and the len iaining unshared o x y g en  atom s being linked to the phosphorus atom s by a dou b le  bonds (sec F igure 4 (a)). T he addition o f  M n 0 4 tetrahedra to P A  network w ill transform som e o fWA(2)-4the Ps= 0  double bonds into cross-link ing (bridging) bonds o f  the type P -O -M n  and P -O -p  (see  Figure 4(b)). This increases the average cross-link density o f  g la sses from  1 for the218 A A HigazyFigure 2. Dependence of (a) longitudinal wave velocity, V/ and (b) shear wave  velocity, Vs , on the composition of M 11O 2-P2O5 glassesvitreous P 2O 5 to-about 2  for the g la sses having 3 5  m o le  % M n O v  H ow ever , the m anganese  ions resided in this g lass com position  range (0 —3 5  m olc% ) entirely  in the tetrahedral form with its low er cross-lin k  density  o f  2 (see  F igure 4 (b )). So  w e m ay interpret our results according to the m odels put forward by B ridge and H igazy  | 6 ] w h ich is sim ply  expressed  in the formK  =  constant. F /(/)n,w here K  is the bulk m odulus o f  a structure co n sistin g  o f  a 3 -d im cn sio n a l netw ork o f  A - 0  bonds, F  is the bond stretching force constant, / is the d iam eter o f  the atom ic rings i.e. the sm allest c lo sed  circuit o f  A —O bonds w hich is depend en t on  the cro ss-lin k in g  d en sities ol g la sse s  and n is typically  high =  4. From  the a b ove  e x p ress io n , o n e  can a ssu m e that the e lastic  m oduli tend to increase w ith both cro ss-lin k  d en sity  and the bond stretch in g  force constants. S o  the increased amount o f  cross-linking w ith increasing content o f  M n 0 2  up to 15Elastic constants and electrical properties etc 219m ole % ca u ses the average atom ic ring size  and P oisson's ratio (see  Figure 5(a)) to decrease  and this leads to increase the elastic  m oduli o f  the glasses (see  Figure 3).F ig u re  3. Compositional dependence o f (a) hulk modulus, K, and shciii modulus, G, and (b) longitudinal modulus, L, and Young's modulus, /*, m Mn02-P2^5 glaresIn the reg io n  1 5 -3 5  m o le  % M n 0 2 , tw o  o p p o sin g  p ro cesses  arc taking p lace  sim ultaneously. A n increase in cross-link density  tor the vitreous P 20^  from I to 2 and a lso  increase the num ber o f  w eaker M n—O  bonds com pared w ith P—O bonds 116J. At the grcatci fraction o f  M n - 0 4 content, the effect o f  cross-link ing is overridden by that o f  weaker M n -O  bonds and for this reason, the elastic  m oduli decreases with increasing M n 0 2 con ien l in this glass com positional range (see' F igure 3).A s the MnC>2 o x id e  increases beyond 35 m ole  %. an increase in the clastic  m oduli is o b se r v e d -T h e  in crea se  in  e la s tic  m odu li m ay be  attributed to the gradual transition o f  tetrahedral M n - 0 4 (cross-link density =  2) to octahedral M i>-0 6 (cross-lin k  density  =  4 ) (see  Figure 4 (c )) .  H ow ever , P o isson 's ratio data sh ow s an increase with increasing M n 0 2 content beyond 35  m o le  %  in sp ite  o f  the fact that the cross-link  density  increases. T his behaviour  might indicate that the m anganese ion in octahedral coordination is usually w eak directionally.220 A A Higazywhich produces a low ratio of F J F  (where F b and F  are the bending and stretching force constants, respectively) followed by an increase in Poisson's ratio values (see Figure 5(a)).— o -— o—o-r-  0  -  P - -  0  — P ■0II0  — P - -o —I I I0 0 0I ' I- P — 0  0  — P — O 'I I0 o1 II I I-  P — 0  —  Mo — 0  — P — 0  -  I I I0 0 0I I I-Mn — 0  — P — 0  — M o - 0 —I I I0  0  0I I I(a )(b)V  I I \ /- 0 — M o —0 - P — 0  — P - 0  — M n - 0  —/ \  I I / \0 0 0 0 o '  01 I \ /  I I- P - 0  — P  -  0  -  Mn  — 0  — P  — 0  — P  —I I / \  I I0 0 0 0 0 0\  /  I I \ /------ Mn  —0  — P - 0 — P - 0  — M o - 0  —\  J  / \I I v  I I( C )F igu re  4. Schematic two-dim ensional representation o f  the effect o f  the network modifying Mn0 2  oxide on the P 2O 5 network (a) the P 2Q5 network structure, (b) the structure at 35 mole % MnO 2 oxide and (c) the structure o f  manganese phosphate glasses when MnO 2 mole % > 35.The variation o f D ebye temperature w ith MnC> 2 con ten t { se e  F igu re 5 (b )) sh ow ed  the sam e three com positional regions w hich had been found in the c o m p o sitio n a l d epend en ce  of the clastic m oduli.The general condition for sem iconducting behaviour in transition m etal o x id e  glasses, is that the transition m etal ion should ex is t in m ore than on e  va len cy  state so  that conduction can lake p lace by the transfer o f  e lectrons from  lo w  to h ig h  va len cy  states. In the present investigation, the conduction  may take p lace by the hop ping  o f  an e lectron  d irectly  between occupied M n 2+ (3d'15) and unoccupied M n3+(3d4) sites, a lso , it is generally  con sid ered  that the conduction-changes in transition m etal o x id e  g la sses are not on ly  d u e  to the concentration of free change carriers but are also due to their m obility.Figure 6 shows that the DC-electrical conductivity is composition dependent, where it decreases with the increase in theMn02 content up to 15 mole %. This decrease in DC- conductivity may be attributed to the increase in the average cross-link density of glassesElastic constants and electrical properties etcFigure 5 . V  arm  lio n  o f  (a ) P o isso n 's  ra lio  an/' (b ) D e b y e  tem p eratu re w ith  MnC>2 m o le  %Figure  6. Compositional d e p e n d e n c e  o f  D C -e le c tr ic a l c o n d u c t iv ity2 2 2which causes the mobilily to decrease. However, in the glass compositional region 1 5 - l s  mole % an increase in the DC-conductivity is observed (see Figure 6). As MrO, is increased in this compositional region, the effect of cross-linking is overridden by that of the free charge carriers concentrations (free electron density for Mn = 16.5 x 1022 a n  3).A s  M n O ?  i n c r e a s e s  b e y o n d  3 5  m o l e  %, D C - c o n d u c t i v i l y  d e c r e a s e  w i t h  t h e  in c r e a s e  in t h c M n O : c o n t e n t .  In th e  g l a s s  c o m p o s i t i o n a l  r e g i o n  ( 3 5 - 6 0  m o l e  % ) ,  w e  b e l i e v e  that  tlx- m a n g a n e s e  i o n s  are g r a d u a l ly  c o o r d i n a t e d  w i t h  s ix  o x y g e n  a n i o n s .  T h e r e ! o r e ,  w e  a s c r ib e  t lx  d e c r e a s e  in D C - c o n d u c t i v i t y  w i t h  M n 0 2 o x i d e  c o n t e n t  to  a g r a d u a l  t r a n s i t i o n  f r o m  M r x ( ) 4 t e trah ed ra  ( c r o s s - l i n k  d e n s i t y  =  2 )  to  M n - 0 6 o c t a h e d r a l  ( c r o s s - l i n k  d e n s i t y  =  4 )  i.e. d u e  to ilu* d e c r e a s e  in the  free  carr ier s  c h a r g e  m o b i l i t y .  \6. ConclusionF r o m  t h e  i o r e g o i n g  a n a l y s i s ,  it i s  f o u n d  th a t  a l l  t h e  p r o p e r t i e s  s t u d i e d  in  t h e  pieseni  i n v e s t i g a t i o n s  are f o u n d  to  b e  s e n s i t i v e  to  the  g l a s s  c o m p o s i t i o n .  O n  th is  b a s i s ,  it is  found lluii th e  p r e s e n t  g l a s s  s y s t e m  c a n  b e  d i v i d e d  in to  th r ee  d i s t i n c t  c o m p o s i t i o n a l  r e g i o n s .R e fere n cesf 11 J M Farley and C A Suundeis /7 m  Star Sol A28 170 (1075) »[2] M B Field J Appl. Phys 40 2628 (1060)[3| H Bridge and G R Moridi Institute of Acousm . Spnnt> Con/eient e ami exhibition p 6 (1077)(4J N I) Paid and B Bridge /7 m  ('hem Glasses 24(5) 1 3 0 (1 9 8 ^ )|5 | A A H iga/y and B Bridge V NoihCryst Solids 72 81 (1085)| 6 | B Bridge and A A H iga/v /7 m  Cliem Glasses 21 1 (1086)[7] A A llig a /y , B Bridge, A M Hussein and M A E w aida./ Atoustit Sot A m  86 14 S  ^ (1080)[8 ] M A Ewaida, A A H iga/y, A A Arnmar, A H Khafagy, B Bridge and A A El-Adawy Intel notional Cool <>" Condensed Matter Physus and Applications' Bahrain p 150(1002)10] K W Hansen J Uectrochem. Sot 112 994 (1965)110] C F Drake, I F Scanlan and A Engel Phys Stat Sol 32 193 (1969)[II] J G Vaughan, C H Perry and D E Kmsci Phys. Chem. Glasses 18 87 (1977)[J2J C A Hogarth and M J Basha J Phvs I) Appl Phvs 16 860 (1983)113] P Gray and L C Klein J Non-Cryst Solids 68 75 (1084)[14] A A Higazy, M A Ewaida, A M Hussein and R M El-Bahnasawy Indian J Phys. 63A 124 (1989)[15] A Abdel-Kader, A A Higazy, M M Elkholy and R M El-B ahnasaw y,./ Mater St i 26  4298 1 19 9 1)116] W B Pearson Crystal Chemistry and Physu s of Metals and Alloys (New Yoik . W iley) (1972)A A Higazy

Elastic Constants and Electrical Properties of the Semiconducting MnO2-P2O5 Glasses

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Elastic Constants and Electrical Properties of the Semiconducting MnO2-P2O5 Glasses

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