Post on 02-Jul-2015
description
transcript
101
Preparation of Glass-like Materials by Sol-Gel Method Using
Alkoxide Precursor
Dr. Adam Hassan Elhaj Yousif, Department of chemistry, Faculty of
Education, University of Alfashir, Al Fashir , Sudan, E-mail :
Adamhassan43@gmail.com
جل باستخدام -تحضير المواد التي تشبه الزجاج بطريقة الصول
مادة أولية الكوكسيدية
: املستخلص
وتتلخص . يثوكسي السيالن لتكوين مادة اجللتشمل هذه الدراسة حلمأة رباعي إ
يثوكسي سيالن من تفاعل رباعي كلورو السيالن ذا الدراسة يف تصنيع مادة رباعي إه
ثو . يثوكسوي السويالن يف حالوة ة يوة مون النوات يفصول ربواعي إ و. مع الكحوو اييثيلوي
ين ى ويضاف إليه قليل من املاء يف وجوود عامول مسواعد ضوي او قاعود حتوى ينت وي
م 000ويوو ا اجلوول ليثووحير ثوو دووة عنوود درجووة حووةارة . التفاعوول بتكوووين مووادة اجلوول
ضوةة باسوتخدام ا و ة ومن ث حتلل النووات ام . للحصو على املادة اليت تشبه الزجاج
حيووأ اكودت ةتوواه هوذه التحاليوول تكوون مووادة . حتو اممووةاء وحيوود ا وو ة السوينية
. اجلل واملادة الصلبة اليت تشبه الزجاج على درجة عالية من الن اء واجلودة
Abstract:
This study involves simple treatment of tetraethoxysilane with
water which results into formation of gel. This work shows the
conversion of tetrachlorosilane to tetraethoxysilane. The hydrolysis
and poly-condensation reactions of this product, the backbone of this
study which result in formation of the gel. The aging process of
formed gel followed by drying and firing at 6000 C gave a solid glass-
like material. The obtained products were analyzed by IR and XRD.
101
The results of these analyses confirmed the formation of gel and high
purity glass-like material.
Keywords : sol-gel , tetraethoxysilane, Glass-like material, Gelation
time.
1- Introduction:
Interest in sol-gel processing of inorganic ceramics and glass
materials began as early as the mid 80s with (Paul 1990) Ebelman and
Graham’s studies on silica gel. These early investigators concluded
that the hydrolysis of tetraethoxysilane (TEOS), Si(OC2H5)4 , under
acidic condition yielded SiO2 in the form of “ Glass-like material”,
essentially, this study centers round glasses or – materials derived
from the sol gel process. The use of this process in the preparation of
silicate gel, glasses, and ceramics has been an active area of research
for the past three decades (Bryans et. al 2000)
Sol- gel process used liquid alkoxide precursor such as
tetraethoxysilane to react with water in the presence of acid or base
catalyst at room temperature to produce gel. (Chrusciei & Slusarski
2003)
Si(OC2H5)4 + 2H2O SiO2 + 4C4H5OH 1
The gel product is then aged and dried to the formation of bulk
glasses and ceramics, ultra porous materials (aerogels and Xerogles),
and thin films. The stages of sol – gel process using tetraethoxysilane
precursor are presented in the following scheme 1 (khimich 2004):
101
Scheme1: the stages of sol-gel process using TEOS precursor
The synthetic routes to glass formation by sol gel processes can be
divided into two distinct areas, namely the alkoxide route and the
aqueous route (Hench & West 1990). The latter route rely on
generating a stable dispersion of colloidal sized particles in a solvent
(in this case water) and destabilizing this sol in a controlled manner to
give a solid gel from which the solvent can be removed and then
sintering to give a dense amorphous mass.
The schematic representation of aqueous route to silica glass is shown
below (Bradley 1960)
Sodium silicate solution Na2 SiO3
Silicic acid Si(HO)4 - NaO - resin
(HO)3Si – O – Si(OH)3 condensation
H2O
Dense silicate particle with hydroxylated surface for the condensation and agglomeration
Silica gel
Scheme 2 : preparation of silica glass by aqueous route.
101
The alkoxide route has attracted more attention in recent years.
This route is also divided into two types. These types are aqueous and
nonaqueous (Fig. 3) (Kichelbick 2008):
Sol – gel process
aqueous: step 1 step 2
Hydrolysis condensation between
M(OR)4 H
2O / (cat)
(RO)3M-OH active species
(RO)3M-O-
M(OR)3+ROH
Precursor active species R= organic rest or H
Metal – Alkoxide
Nonaqueous:
M(OR)4 (RO)
3M – O – R
condensation
(RO)3M-O-
M(OR)3+ROR
Precursor (RO)3
M – O – R
Metal – Alkoxide
Fig. 3 Similarities and differences of sol – gel process using metal
Alkoxide precursor:
The following equations describe the fundamental reactions
which allow the conversion of monomeric organ metallic precursor
into gels and ultimately glasses or Ceramics (Lukowiak & Strek
2009):
101
Hydrolysis
M(OR)X + H2O M(OR)n-1 OH + ROH
H
2O
M(OR)n-2 (OH)2 + ROH
Condensation
M-OH + HO-M M-O-M + H2O
M-OH + RO-M M-O-M + ROH
M-OR + OR-M M-O-M + ROR
[M is a metal chosen from (Al, In, Si, Ti, Zr, Sn, Pb, Ta, Cr, Fe, Ni,
Co) and others, R represents alkyl group].
In general the sol-gel process is a low temperature glass
formation method that consists of hydrolysis and condensation
reactions (see scheme 4) of metal alkoxide, such as tetramethoxysilane
(TMOS) or Tetraethoxysilane (TEOS) (Bryans et. al 2000)
≡SiOR + H2O
≡ SiOH + ROH (1)
2≡SiOH
≡Si – O – Si≡ + H2O (2)
≡SiOH + ROSi≡
≡Si – O – Si≡ + ROH (3)
Scheme4: Representation of the sol –gel process.
In the hydrolysis reaction, (Eq.1), the addition of water replaces
alkoxide group, (OR), with hydroxyl Group, (OH). Subsequent
condensation reactions (Eq.2 and 3) involving the silanol groups (Si-
100
OH) produce siloxane bonds (Si – O – Si) Plus the byproducts water
or alcohol (keefer 1990)
The gelation point of any system, including sol-gel silica, is
easy to observe qualitatively and easy to define in abstract terms but
extremely difficult to measure analytically. As the sol particles grow
and collide, condensation occurs and macro particles form. The sol
becomes agel when it can support a stress elastically. This is typically
defined as the gelation point or gelation t, (t gel). Gelation time (t gel)
of hydrolyzed tetraethoxysilane depends on some experimental
parameters. These parameters include catalyst nature, PH value of
solution, (Klein et. al 2010), the ratio of water to alkoxysilane, Alkoxy
group (OR), (Rakhimov et. al 2009) Temperature and solvent
(chrusciei & Slusarski 2003). When a gel maintained in its pore liquid,
its structure and properties continue to change long after the gel point.
This process called aging. The gel is dried by one of a number of
methods.
The method used here had been indicated by the characteristics
and composition of the material. Drying techniques which have been
employed include filtration, oven drying, vacuum drying, spray
drying, microwave drying, liquid drying and freeze drying (Aravind
et. al 2010). Drying time has not been qualified. Firing of the hydrated
species is converted to oxides and clear glass-like materials are
obtained. Thus the glass can be made by sol-gel process (Voronkov et.
al 2008). This process of disordered ceramics from liquid chemical
solution, (referred to as sol - gel processing), followed by
101
densification, convert into glass (Chiang et.al 1997). Despite sol-gel is
widely applied, the chemistry utilizing metal alkoxide precursor is
hardly understood (kesslar et. al 2006). Therefore, the aim of the
present work is to prepare glass-like materials using tetraethoxysilane
as alkoxide precursor by sol-gel method and study the reactions
parameters involved in this process.
2. Experimental
2.1 Chemicals
All chemical used were of analytical grade type.
2.2 Preparation of tetraethoxysilane by Halosilane Route.
50 ml – 150 ml of absolute ethanol were placed in the reaction
flask , 6-18 cm3 of tetrachlorosilane were then added drop wise from a
separatery funnel. The mixture was then brought to the programmed
temperature range of 60 – 70 Co. the reaction was allowed to proceed
under reflection was stopped by rapid cooling. A colorless liquid was
obtained. The resulting product was analyzed by IR. For results see
Figure 0 & Table1.
2.3Preparation of Gels by Acid or Base catalyzed hydrolysis of
tetraethoxysilane and condensation of their hydrolyzed species:
2.3.1 Preparation of Gel-1 by Acid catalyzed hydrolysis of
tetraethoxysilane and condensation of their hydrolyszed species
tetraethoxysilane (6 ml, 5.556 g/cm3) was placed in a test tube, also
hydrochloric acid ( 1 ml, ph 1) was added. 1 ml of absolute ethanol
used as solvent dropwise and 1 ml of double distillated water. The
mixture began to form gel. After 144 hrs there was a finger of gel in
101
the test tube at room temperature, after that test tube was wormed to
take out figure of the gel and weight. The gel was analyzed by IR
spectroscopy. For results see Table (2), Figure 1 & Table 3.
2.3.2 Preparation of gel-2 by base catalyzed hydrolysis reaction of
TEOS and condensation of its hydrolyzed species.
6 ml of tetraethoxysilane was placed in a test tube and ammonium
hydroxide ( 1 ml , PH = 13) was also added 1 ml of absolute ethanol
used as solvent & drop wise 1 ml of double distilled water. The
mixture began to form a gel. After 261 hrs there was a finger of gel in
the test tube. After that test tube was wormed to take out finger of the
gel and weigh it. The gel was analyzed by IR for results see Table 4,
Figure 1.
2.4 Preparation of Glass-like materials from gels prepared by
Acid or base catalyzed hydrolysis reaction of TEOS and
condensation of their hydrolyzed species.
Glass–like materials have been prepared by drying and firing of
the prepared gels according to programmed temperature as follows:
1- Heat at 650 C for 3 – 4 hours
2- Heat at 950 C
for 2 – 3 hours
3- Heat at 1200 C for 3 hours
After that the dried gel heat at 180 Co for 2 hours. Then the obtained
gel placed in porcelain curricible and fired at 600o C.
2.4.1 Preparation of Glass-like material – 1 by Drying & firing of
gel-1 prepared from TEOS precursor.
Glass-like material -1 has been prepared by firing the gel-1
prepared in experiment (2.3.1) using programmed temperature to give
101
dried gel. Dried gel was heated at 180 Co for 2 hours. Then the
resulting gel was placed in porcelain curricible and fired at 600 Co for
2 hours. The final product of glass-like material was analyzed by IR
and XRD. For results see Fig. 1 & Fig. (11).
2.4.2 Preparation of glass-like material – 2 by Drying & firing of
gel-2 prepared from TEOS precursor.
Glass-like material -2 has been prepared by firing of the gel-2
prepared in experiment (2.3.2) using programmed temperature to give
dried gel. Dried gel was heated at 180 Co for 2 hours. Then the
obtained gel was placed in porcelain curricible and fired at 600 Co for
2 hours. The ultimate product of glass-like material was analyzed by
IR and XDR. For results see Figure 10 and Figure 11.
2.5 Instrumentation
The prepared compounds such as TEOS, gels, & Glass-like
materials were analyzed by IR spectroscopy. Infrared spectra were
recorded on a 800 – pc FT IR schimadzu spectrophotometer using K
Br pellets in the mid infrared region 4000 – 400 Cm-1
. Also infrared
spectra were recorded on a 4100 – FT IR Jacso spectrophotometer
using K Br pellets in region 4000 – 400 Cm -1
.
The obtained glass-like materials were also characterized by X-ray
diffraction (XDR) analysis using AD8 advanced X-ray from Bruker
Analytical X-ray system model.
3. Results & Discussion:
Two glass-like materials were prepared by three stages. The first
stage is the preparation of tetraethoxysilane (TEOS) by halosilane
110
route. The second stage includes the synthesis of gels by hydrolysis of
tetraethoxysilane (TESO) in the presence of acid or base catalyst and
condensation of their hydrolyzed species. The third stage involves
drying of the prepared gels followed by firing at 600 Co to convert to
glass-like material. These stages will be illustrated as following.
3.1 Synthesis of TEOS by the reaction tetrachlorosilane and
absolute ethanol:
Tetraethoxysilane was prepared by the reaction of
tetrachlorosilane and absolute ethanol (2-2) according to the following
equation:
SiCl4 + 4EtOH Si(OEt)4 + 4HCl
The obtained colourless liquid analyzed by FT IR analysis
showed the presence of the bands 88 1 cm-1
, 1090 cm-1
, 1050 cm-1
&
804 cm-1
. These results indicate the presence of TEOS, which is in
good agreement with the reported values (Zaman & Bhuiyan 2009)
for tetraethoxysilane.
111
Fig. 6 In
frared sp
ectra of th
e pro
du
ct ob
tained
form
the reactio
n o
f tetrach
loro
silane an
d ab
solu
te ethan
ol
111
. Table:(1) Assignments of IR absorption peaks for TEOS prepared by
the reaction of tetrachlorosilane and absolute ethanol.
Frequency cm-1
Assignment
3345 B O-H stretch
2974 M C-H asymmetric stretch
2256 W
1925 W
C-H asymmetric stretch
1381 S
1332 W
C-H bend
1275 W CH3 rocking
1090 M
1050 S
C-O stretch
881 S C – C stretch
804 W SiO4 asymmetric stretch
B= broad, S = strong, W = weak, M = moderate
3.2 Characterization of gels prepared by hydrolysis of
tetraethoxysilane
Gels were prepared by hydrolysis of tetraethoxysilane using acid
or base catalyst. These products were characterized by FT IR
spectroscopy and their results will be discussed in the following
sections (3.2.1, 3.2.2 & 3.2.3)
111
3.2.1 Characterization of gel-1 prepared by acid catalyzed
hydrolysis of tetraethoxysilane.
Gel-1 has been prepared by hydrochloric acid catalyzed
hydrolysis of TEOS (2.3.1). The results of this experiment were
summarized in Table 2.
The final product was analyzed by FT IR spectroscopy. FT IR
analysis of obtained product was given in Fig.1 & Table 3.
Table (2): Summary of gel-1 prepared by acid catalyzed
hydrolysis reaction of TEOS and its hydrolyzed species of
condensation.
TEOS 6 ml
HCl 1 ml, PH = 1
H2O 1 ml
C2H5OH 1 ml
Gelation time / hrs 144
Product weight / g 1.937
111
Fig. 7 In
frared sp
ectra of th
e gel-1 p
repared
by acid
catalyzed h
ydro
lysis reaction
of TEO
S and
con
den
sation
of its
hyd
rolyzed
species.
111
Table 3: Reported FT IR characteristic bands for gel-1 prepared
by acid catalyzed hydrolysis of TEOS and condensation of its
hydrolyzed species.
Assignments Frequency / Cm-1
References
Obtained Literature
O-H
stretching
3433 3450 – 3400 (Al-owein & El-Rasy
2009; Nadargi et, al
2009)
O-H
deformation
1634 1653 – 1634 (Socrtaes 2001)
Si-O
stretching
1082 1200 – 100 (Estella et al 2007;
Dobryanska et. al
2006; Smith 1974)
Si-O –Si
stretching
795 800 (Al-owein & El-Rasy
2009; Nadargi et, al
2009)
Si-O –Si
bending
467 467 (Al-owein & El-Rasy
2009; Nadargi et, al
2009)
Fig. 10 & Table 3 show the results of FT IR analysis for the
prepared gel-1. The FT IR absorption band near 3450 cm-1 is
attributed to stretching modes of residual Si-OH and some adsorbed
water. This band goes often accompanied by another at 1634 cm-1
,
which is attributed to – OH vibration of molecular water that is
physically adsorbed in the net work. Because the low intensity of 1634
Cm-1
band, the 3433 Cm-1
band would reflect the contribution of
silanol groups. The smaller the amount of residual Si-OH, the larger
degree of condensation. The most intense peak was observed at 1082
cm-1
due to the asymmetric stretching vibration of (Si-O) bond.
Oxygen atoms play the role of bridges between each two silicon sites
110
in these silicon- oxygen covalent bonds. On the other hand, the
symmetric stretching vibration of (Si –O– Si) appear at 550 cm-1 and
its bending mode appear at 467cm1. The low energy band at a round 5
Cm-1
is assigned to (Si – O – Si) stretching of the (SiO2 ) network
defects.
3.2.2 Characterization of gel – 2 prepared by base catalyzed
hydrolysis reaction of TEOS and condensation of its hydrolyzed
species:
Gel-2 has been prepared by hydrolysis reaction between TEOS
and water using ammonium hydroxide as catalyst and absolute ethanol
as solvent (2.3.2) the results of this experiment were presented in
Table 4. The obtained gel-2 product was analyzed by FT IR
spectroscopy. FT IR analysis of gel-2 product was shown in Fig. 11
below.
Table 4: Summary of gel-2 prepared by base catalyzed hydrolysis
reaction of TEOS and condensation of its hydrolyzed species:
TEOS 6 ml
NH4OH 1 ml, PH = 13
H2O 1 ml
C2H5OH 1 ml
Gelation time / hrs 216
Product weight / g 0.941
111
Fig. 8 In
frared sp
ectra of gel-2
prep
ared b
y base catalyzed
hyd
rolysis reactio
n o
f TEOS &
con
den
sation
of its h
ydro
lyzed
species.
111
The prepared gel-2 was characterized by FT IR spectroscopy. Fig.
11 illustrates the results of FT IR analysis for gel-2. These results
were found to be similar to the results of FT IR analysis of gel –
1prepared by acid catalyzed hydrolysis of TEOS and water. The
results are in good agreement compared with the literature values (Al-
Owein & El-Rasy 2009) indicating the formation of the gel-2.
3.2.3 Gelation time
In the current work the effect of the PH value (type of catalyst) on
gelation time for hydrolysis reaction of TEOS has been carried out in
a similar experimental condition. Gelation time for base catalyzed
hydrolysis reaction of TEOS is longer than the gelation time of acid
catalyzed hydrolysis of TEOS see Table 2 & Table 4. So that the
acidic catalysis results in much quicker hydrolysis than the basic one
at the same concentration of a protolytic catalyst, indicating that
tetraethoxysilane is stronger Lewis base than Lewis acid.
3.3Characterization of Glass-like materials synthesized from the
prepared Gels.
Two glass-like materials such as glass-like material-1 and glass-
like material-2 were analyzed according to sections (2.4.1 & 2.4.2)
respectively. The ultimate glass-like materials were characterized by
FT IR & XDR for results see Fig.1, Fig. 10, Fig. 11 & Fig. 11.
111
Fig 9 in
frared sp
ectra of glass-like
material-1
prep
ared fo
rm G
el-1
.
110
Fig 10
infrared
spectra o
f glass-like m
aterial-2 p
repared
from
Ge
l-2.
111
Fig.1 & fig. 10 show infrared spectra of glass-like material-1&
glass-like material-2 respectively. The FT IR absorption band at
1107cm-1
due to asymmetric stretching vibration of the Si-O bond
constitutes asketel SiO2 network. This band goes together with two
bands a round 800 Cm-1
and 471 Cm-1
which are assigned,
respectively, to bond bending and bound rocking vibration of Si-O
bonds in the three dimensional net work. The absent of the absorption
bands in the FT IR region from 4000 – 1200 cm-1
indicate that the
obtained products have not any organic groups in their structures.
Because all carbon and hydrogen atoms burned off at 600 C during the
firing leaving glass-like materials as high purity products.
111
Fig 11
x-ray po
wd
er diffractio
n o
f Glass-like
material-1
prep
ared fro
m G
el – 1
111
Fig 12
x-ray po
wd
er diffractio
n o
f Glass-like
material-2
prep
ared fro
m G
el – 2
111
X-ray diffraction analysis of the obtained glass-like materials was
carried out and the results were presented in Fig.11 & Fig.11. It was
found that the prepared glass-like materials were amorphous solid,
(Ameen et. al 2008), because no crystalline phase was detected by
conventional powder XDR.
4- Conclusion:
The glass-like materials were prepared from the densed gels
which are the hydrolytic products of metal alkoxides or semi metal
alkoxides such as tetraethoxysilane. Kinetic studies of gelation for
hydrolysis reaction of TEOS in the acid or base catalyst were
investigated in the present work using similar experimental
conditions. It was found that gelation or rate of hydrolysis reaction of
TEOS using acid catalyst is higher than using base catalyst because,
gelation depends on PH value. Sol – gel process has been described to
be the technological back up for the production of these glasses with
better purity and homogeneity than high temperature conventional
method or process.
111
References:
1. Al- Oweini, R., El-Rassy, H., (2009), Jounral of Molecular
Structure, 919, 140-145.
2. Ameen, k.B., Ragiasekar, K., Rajasekharan, T.,
Rajasekharan, M.V., (2008), J Sol-Gel SCi Techanl, 45, 9-15.
3. Aravind, P.R., Shajesh,P., Soraru, G.D., (2010), J Sol-Gel Sci
Technol, 54, 105-117.
4. Bradley, D.C., (1960), Progress in Inorganic Chemistry, inter-
Science, New York, vol.2 P. 314.
5. Bryans, T.R., Brawner, V.L., Quitevis, E.L., (2000), Journal of
Sol-Gel Science and Technology, 17, 211- 217.
6. Chiang, Y.M., Birnie, D.P., Kingery, W.D., (1997) Principles
for Ceramic Science and Engineering, John Wiley & Sons, Inc.,
P.80.
7. Chrusciei, J., Slusarski, (2003), Material Science, Vol. 12, No.4,
PP. 461 – 469.
8. Dobryanska , H.1., Melnyk, 1.v., Zub, Yu.L., Chuiko, A.A.,
Dabrowski, A., (2006), Russian Journal of Physical Chemistry,
Vol.80, No.6, PP. 939- 944.
9. Estella, J., Echeverria, J.C., Laguna, M., Garrido, J.J.,
(2007), Journal of Non-crystalline Solids, 353, 286-294.
10. Hench, L.L. and West, J.K., (1990), Chemical Review, Vol.90,
No. 1, P. 51.
11. Iler R.K., (1979), the Chemistry of Silica, Wiley, New York.
110
12. Keefer, K.D.,(1990), J. American Chemical Society, Washington,
PP. 227-240.
13. Khimich, N.N., (2004), Glass Physics and Chemistry, Vol. 30,
No. 5, PP.430-442.
14. Kicheldick, G.,(2008), J Sol-Gel Sci Technol, 46, 281-290.
15. Klwin, L.C. & Jitianu A., (2010), J Sol-Gel Sci Technol, 55, 86
– 93.
16. Lukowaik, A., Strek, W., (2009), J Sol-Gel Sci Technol, 50,
201-212.
17. Nadargi, D.Y., Kalech, R.R, Rao,A.V, (2009), Journal of Alloys
and compounds, 480, 689 – 695.
18. Paul, A., (1990), Chemistry of Glasses, 2nd
ed., Chapman and
Hall, London, PP.52-81.
19. Rakhimov, V.1., Rakhimova, O.V., Syomov, M.p., (2009),
Glass Physics and Chemistry, Vol. 35, NO. 2, PP. 163- 169.
20. Smith, A.L., (1964), Analysis of Silicones, Wiley & Sons,
London, Vol.41, PP.217-271.
21. Socrates, G., (2001), Infrared and Raman Characteristic Group
Frequencies: Tables and Charts, Third ed., Wiley, P.245.
22. Voronkov,M.G., Khimich,E.N.,Khimich, N.N., (2008), Russian
Journal of Applied Chemistry, Vol.81, No.4, PP. 732-735.
23. Zaman, Z., Bhuiyan, A.H., (2009), Thin Solid Flims, 517, 5431-
5434.
111
Acknowledgement
The editorial board would like to thank the reviewers, who have
reviewed the manuscripts submitted to the journal. Their efforts
have been indispensible for the publication of this issue. They
are:
1. Prof Mukhtar Ahmed Mustafa Uof K
2. Prof Kamal Fadl Elseid Elkhalifa U of K.
3. Prof.Sanaa Osman YagoubEl Neelain University
4. Prof. Mohamed Ali Hussain. Africa International University
5. Prof.Yousif Rizgalla Sulieman Bahri University
6. Prof.Omer Yousif Alhussain U of K
7. Prof. Aisha Zohair Almagboul Medicinal and Aromatic Plants
Research Institute
8. Prof. Osman Taha Elzaki Institute of Technological Research
9. Dr. Dr. Zaynab Abdallah Yosuf Bahri University
10. Dr. Ahmed Mohamed Adam Eldoma Sudan University of
Technology
11. Dr.Eltom Elsadig Ali Desertification and Desert cultivation
Institute.U of K
111
12. Dr.Eltayeb Elhag Ali Ahmed UNESCO Chair of Desertification
U of K
13. Dr.Elmahdi Ahmed Haroun Bahri University