Synthesis and characterization of some thiadiazole compounds as new corrosion inhibitors for mild steel in cooling water

Article abstract of DOI:10.14233/ajchem.2017.20658

In present work, five 1,3,4-thiadiazole compounds were synthesized. The prepared compounds were identified by CHNS analysis, FT-IR and 1H NMR spectroscopy. The corrosion rates in the presence of thiadiazole as a steel corrosion inhibitor in the cooling water system which taken from South Fertilizer Company, Basra, Iraq were measured by the weight loss method and potentiodynamic polarization measurements. The weight loss method was studied in different times (1-5 h) and in 303 to 333 K temperature range. Results obtained revealed that thiadiazole compounds performed as a corrosion inhibitor for mild steel in this medium and its efficiency attains to 86.55 % at 5 × 10-3 M at 303 K and by potentiodynamic polarization measurements its efficiency attains to 85.35 % in the same conditions. The apparent activation energies, enthalpies and entropies of the dissolution process and the free energies were determined and discussed.

Full text of DOI:10.14233/ajchem.2017.20658

Asian Journal of Chemistry; Vol. 29, No. 11 (2017), 2361-2365
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2017.20658
Synthesis and Characterization of Some Thiadiazole Compounds as
New Corrosion Inhibitors for Mild Steel in Cooling Water
1
2
EKHLAS QANBER JASIM , MUNTHER ABDULJALEEL M.A.1,* and RAJAA HUSSEIN FAYADH
1Pharmaceutical Chemistry Department, College of Pharmacy, Basrah University, Basrah, Iraq
2Medical Technical Institute, Foundation of Technical Education, Basrah, Iraq
*Corresponding author: E-mail: muntheralamery@yahoo.com
Received: 18 March 2017;
Accepted: 5 June 2017;
Published online: 29 September 2017;
AJC-18550
In present work, five 1,3,4-thiadiazole compounds were synthesized. The prepared compounds were identified by CHNS analysis, FT-IR
and 1H NMR spectroscopy. The corrosion rates in the presence of thiadiazole as a steel corrosion inhibitor in the cooling water system
which taken from South Fertilizer Company, Basra, Iraq were measured by the weight loss method and potentiodynamic polarization
measurements. The weight loss method was studied in different times (1-5 h) and in 303 to 333 K temperature range. Results obtained
revealed that thiadiazole compounds performed as a corrosion inhibitor for mild steel in this medium and its efficiency attains to 86.55 %
at 5 × 10-3 M at 303 K and by potentiodynamic polarization measurements its efficiency attains to 85.35 % in the same conditions. The
apparent activation energies, enthalpies and entropies of the dissolution process and the free energies were determined and discussed.
Keywords: Mild steel, Corrosion, Thiadiazoles, Thermodynamics.
extensive studies have been performed to look for new
environmental friendly organic inhibitors in recent decades,
including triazole derivatives [8], thiazolidin [9], oxadiazole
derivatives [10], isoxazolidines [11,12], imidazole derivatives
[13], etc.
INTRODUCTION
Open recirculation cooling water systems are commonly
used for industrial cooling purposes to efficiently dissipate
unwanted process heat. The essence of cooling water system
consists of plant heat exchange equipment and the water that
passes through it to reduce heat from process fluids. Water is
a universal solvent and thus becomes a potential medium to
result into cooling water problems. It carries minerals, suspen-
ded colloidal and biological impurities [1].
The recent work includes synthesis and characterization
of some thiadiazole derivatives, and used them as corrosion
inhibitors in cooling water system of South Fertilizer Company,
Basra, Iraq, using weight loss and galvanostatic polarization
techniques. Moreover, the effect of temperature on the dissolution
carbon steel, as well as, on the inhibition efficiency of the studied
compounds was also investigated and some thermodynamic
parameters were computed.
The main problems associated with this system are scaling,
corrosion, fouling and microbiological growth which if left
untreated can lead to various problems like reduced operating
efficiency, increased maintenance cost, loss in heat transfer
efficiency and energy and ultimate shut down. Three basic
types of corrosion, which occur in the cooling water system,
are uniform, pitting and galvanic corrosion. The use of organic
derivatives as inhibitors in the protection of metals and their
alloys, cooling water corrosion is one of significantly important
strategies [2,3] due to the predominance in their chemical
structures and properties, such as containing polar groups,
conjugated double bonds or various heteroatoms-sulphur,
nitrogen and oxygen [4,5]. For instance, benzotriazole (BTA)
and its derivatives are often employed to protect copper and
its alloys from corrosion. While owing to the toxicity of
benzotriazole and its derivatives to environments [6,7], the
EXPERIMENTAL
Melting points were determined by open capillary and
are uncorrected. The CHNS analysis measurements for the
synthesized compounds were performed using EuroVector
model EA3000A (Italy) and 1H NMR spectra performed using
Bruker model ultra shield 300 MHz (Switzerland), at the
analytical Laboratory of AL-ALBAYET University, Jordan.
DMSO-d6 was used as a solvent andTMS as an internal standard.
IR spectra were recorded using KBr disc on Shimadzu FT-IR
model 8400 Spectrophotometer at Chemistry Department,
College of Education of Pure Sciences, Basrah University.

2362 Jasim et al.
Asian J. Chem.
Synthesis of thiadiazole 1 (1-phenyl-1,3,4-thiadiazole-
compounds inhibitor after different time (1-5 h), the strips were
taken out washed, dried and weighed accurately. Duplicate
experiments were performed in each and the mean value of
the weight loss was reported. Inhibition efficiency E %, surface
coverage (θ) and corrosion rate were determined.
Tafel extrapolation method: Polarization studies were
carried out using Bank Eleiktronkik Intellgent Controls Model
MLab 200, Chemistry Department, Education College of pure
Science, Basrah University, Iraq. Tafel polarization obtained
by changing the electrode potential automatically from (+250
mV to -250 mV) at open circuit potential with a scan rate 0.5
mV S-1 to study the effect of inhibitor on mild steel corrosion
[17]. The linear Tafel segment of cathodic and anodic curves
were extrapolated to corrosion potential to obtain the corrosion
current densities (Icorr).
5-thiol) [14]: To a solution containing 95 % ethanol and 0.1
mol of sodium hydroxide (dissolved in the least amount of
water), 0.1 mol of thiobenzoic hydrazide was added, followed
by 0.15 mol of carbon disulfide. The reaction mixture was
heated under reflux for 3 h till all the evolution of hydrogen
sulfide ceased. The resulting mixture was diluted with water
and acidified with diluted hydrochloric acid containing ice.
The reaction mixture was allowed to stand at the ice bath for
60 min, filtered, washed with water and recrystallized from
methanol (Scheme-I). The characterizations of the synthesized
compounds are listed in Table-1.
N
N
S
KOH + CS2
EtOH
SH
NHNH
2
S
RESULTS AND DISCUSSION
Scheme-I: Synthesis of thiadiazole
FT-IR spectra: FT-IR spectra of the synthesized compounds
were carried out using KBr disc method. The IR data of the
parent thiadiazole compound showed a band at 2766 cm-1
which is characteristic of the S-H stretching [18,19] (Fig. 1).
This band was lacked in the S-substituted derivatives S-R
(Table-2).
Preparation of thiadiazoles derivatives:The compounds,
5-ethylthio-1-phenyl-1,3,4-thiadiazole (2a), 5-butylthio-1-
phenyl-1,3,4-thiadiazole (2b), 5-pentylthio-1-phenyl-1,3,4-
thiadiazole (2c) and 5-benzylthio-1-phenyl-1,3,4-thiadiazole
(2d), were prepared by the same method [15].
All IR spectra of thiadiazole compounds showed strong-
weak bands at 1623-1590 and 1551-1487 cm-1, which are
characteristic of the C=N and C=C ring stretching, respectively.
Strong-medium bands at 1271-1245 and 1126-1032 cm-1
which are characteristic for C-N stretching of thiadiazole ring
[20]. Strong-weak absorption band at 3048-3024 and 784-746
cm-1, which are characteristic of aromatic C-H stretching and
bending, respectively.
A mixture of 0.015 mol (2.67 g) of compound 1, 0.018
mol of alkyl bromides or benzyl chloride and 0.02 mol (1.64g)
of sodium acetate in 50 mL of ethanol was heated under reflux
for 4 h and allowed to cool and poured into 100 mL of cold
water containing ice. The solid product was collected and
recrystallized from ethanol (Scheme-II). The characterizations
of the products are summarized in Table-1.
N
N
1
1H NMR: H NMR spectra of the prepared thiadiazole
N
N
R-X
+ CH COONa
3
R
compounds were performed in deuterated dimethyl sulfoxide
solutions with tetramethylsaline as an internal standard. All
these spectra showed signals at 2.5 ppm, which was due to
DMSO solvent.
Ethanol, Ref. 4 hrs
SH
S
S
S
R = CH3CH2- = 2a; CH3CH2CH2CH2- = 2b;
CH3CH2CH2CH2CH2- = 2c; Ph-CH2- = 2d
The parent thiadiazole (1) has a characteristic singlet
signal at 2.821 ppm due to proton of thiol group [19] and this
signal is not appeared in the thiadiazole derivative compounds.
The compounds 2a-d exhibited a characteristic aliphatic
system which gave signals in the high field range between
0.912-4.528 ppm.All thiadiazole compounds exhibited multi-
plet signals in the downfield range between 7.272-8.168 ppm
due to the protons of the aromatic systems (Table-3).
Scheme-II: Synthesis of thiadiazole derivatives
Weight loss measurements [16]: The mild steel sheets
used in this present work have the composition presented in
Table-1 and strip of 3.5 cm × 2.5 cm × 0.4 cm size. Before
measurements, the mild steel coupons were mechanically
polished with series of emery paper of variable grades starting
with the coarsest and proceeding in steps to the finest (600)
grade, degreased with absolute ethanol, dipped into acetone
and washed with deionized water. The coupons were dried
and kept in desiccators. After weighing accurately, the speci-
mens were immersed in 50 mL of cooling water and without
and with addition of different concentrations of thiadiazole
Gravimetric measurements
Inhibition efficiency at different times: Inhibition effi-
ciency (E %), surface coverage (θ) and corrosion rate was deter-
mined by using following equations [21]:
TABLE-1
PHYSICAL CHARACTERISTICS OF SYNTHESIZED THIADIAZOLE COMPOUNDS
Elemental analysis (%): Found (calcd.)
Compd.
No.
m.w.
(g/mol)
m.p.
(°C)
Yield
(%)
m.f.
Colour
C
H
N
S
C8H6N2S2
194.28 White needle crystal
222.33 White needle crystal
250.38 White crystal
209-211
156-158
147-149
140-142
122-124
88
74
83
84
80
48.76 (49.46)
54.42 (54.02)
56.13 (57.56)
59.11 (59.05)
63.10 (63.35)
3.33 (3.11)
4.69 (4.53)
5.55 (5.64)
6.22 (6.10)
4.34 (4.25)
13.87 (14.42) 33.87 (33.01)
13.02 (12.60) 28.26 (28.84)
11.47 (11.19) 26.02 (25.61)
1
C10H10N2S2
C12H14N2S2
C13H16N2S2
C15H12N2S2
2a
2b
2c
2d
264.40 White crystal
10.24(10.59)
10.02 (9.85)
24.65(24.25)
22.19(22.55)
284.39 White needle crystal

Vol. 29, No. 11 (2017)
Synthesis of Some Thiadiazole Compounds as New Corrosion Inhibitors for Mild Steel in Cooling Water 2363
TABLE-2
KEY IR BAND OF SYNTHESIZED THIADIAZOLE COMPOUNDS
Assignment
1
2a
2b
2c
2d
3040 w
3040 w
2924 s
3032 w
2927 s
3048 w
2945 m
3024 w
2940 s
C-H stretching aromatic
C-H stretching aliphatic
S-H stretching
2766 m
1590 m
1576 s
1548 w, 1487 m
1455 m
1571 m
1487 m
1623 s
1551 m, 1478 m
1448 m
1601 m
1481 s
C=N stretching of thiadiazole ring
C=C stretching of aromatic rings
N=N stretching of thiadiazole ring
C-H bending aliphatic
1550 m, 1482 s
1464 w
1467 m
1465 m
1365 s
1346 s
1326 m
1373 m
1267 m, 1032 m
784 m
1267 s, 1064 s
766 s
1271 m, 1089 s
762 s
1245 m, 1126 m
768 m
1242 m, 1112 s
746 s
C-N stretching of thiadiazole ring
C-H bending aromatic
677 m
662 m
678 s
573 m
597 m
C-S stretching
TABLE-3
1H NMR DATA OF THIADIAZOLE COMPOUNDS
δ ppm
Compd. No.
Aromatic
-SH
-CH3
CH3-CH2-
-CH2-CH2-S-
-CH2-CH2-CH2-S-
1.816(p)
-CH2-S-
7.265-8.254 (m, 5H)
7.425-7.921 (m, 5H)
7.395-8.201 (m, 5H)
7.409-8.211 (m, 5H)
7.333-7.762 (m, 10H)
2.821 (s)
1
1.416(t)
0.912(t)
0.921(t)
3.456(q)
3.449(t)
3.441(t)
4.528(s)
2a
2b
2c
2d
1.345(sx)
1.255(sx)
1.684(q)
1.371(p)
TABLE-4
W
corr − W



corr(inh) 
ACTIVATION ENERGY, ARRHENIUS FACTOR, THE
ENTHALPY ∆H OF ACTIVATION AND THE ENTROPY
OF ACTIVATION OF THIADIAZOLE INHIBITORS
E (%) =
×100


(1)
W
corr
Ea
∆H
-∆S
Conc.
(M)
Wcorr − Wcorr(inh)
Inhibitor
A × 1010
(KJ mol–1)
40.90
52.98
55.53
56.89
57.86
52.52
53.84
54.36
56.12
56.46
58.13
59.87
60.98
53.88
55.79
57.02
59.11
54.63
56.54
57.79
58.71
θ =
(2)
W
corr
Blank
0.269
20.0
34.7
36.3
43.7
11.5
14.1
14.5
18.6
46.8
79.4
117
43.52
56.77
59.03
59.52
60.48
55.16
56.48
56.98
58.76
59.10
60.77
62.51
63.62
56.52
58.43
59.66
61.74
57.27
59.18
60.43
61.33
-0.073
-0.041
-0.033
-0.032
-0.031
-0.042
-0.040
-0.040
-0.038
-0.030
-0.026
-0.023
-0.022
-0.039
-0.034
-0.032
-0.029
-0.038
-0.034
-0.033
-0.031
1 × 10-4
5 × 10-4
1 × 10-3
5 × 10-3
1 × 10-4
5 × 10-4
1 × 10-3
5 × 10-3
1 × 10-4
5 × 10-4
1 × 10-3
5 × 10-3
1 × 10-4
5 × 10-4
1 × 10-3
5 × 10-3
1 × 10-4
5 × 10-4
1 × 10-3
5 × 10-3
where Wcorr(inh) and Wcorr are the weight loss values in the
presence and in the absence of inhibitor, respectively.
1
∆W×K
Rcorr
=
(3)
A×D×T
where ∆W = weight losses of metal (g), K = constant (5.34 ×
105), A = sample area (cm2), D = metal density (g/cm3) and T
= exposed time (h).
2a
2b
2c
2d
The results of weight loss measurements show that by
increasing the time the efficiency increases and there is good
inhibition process for these thiadiazole compounds where the
efficiency was in the range 80.12-86.55 % at the best time of
5 h. There is proportional relationship between the inhibitor
concentration and the efficiency. The best efficiency is recorded
at the higher concentration of 10-3 M.
126
16.2
29.5
39.8
55.0
18.2
30.2
35.5
40.7
Kinetic parameters: In order to obtain the effect of inhibitors
on the kinetic parameters, gravimetric weight loss experiments
were conducted at 30, 40, 50 and 60 °C at 5 h of immersion in
the absence and presence of thiadiazole at 5 × 10-3 M. The
activation parameters for the system were calculated (Table-4)
from Arrhenius-type plot (4) and transition state eqn. 5 [22].
I
corr − Icorr(inh)






E (%) =
×100
(8)
Icorr
−Eact
log(Rcorr)
=
+ logA
(4)
where Icorr and Icorr(inh) are the corrosion current in the absence
and in the presence of inhibitor, respectively.
2.303RT
RT
∆S
∆H




The polarization curves of thiadiazole compounds in cooling
water at different temperature are represented in Fig. 1. It is
clear that the inhibition decreases as the temperature increases.
It is evident from the Tafel plots that the inhibitor adsorption
shifted the corrosion potential (Ecorr) in the negative direction
Rcorr
=
exp
exp −

(5)



Nh
R
RT




Potentiodynamic polarization measurements: The inhi-
bition efficiency was evaluated from the calculated Icorr values
using the relationship [23]:

2364 Jasim et al.
Asian J. Chem.
2
1.5
1.0
(a)
(b)
1
0
0.5
0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-3.5
-1
Blank
Blank
1
2a
2b
2c
2d
1
2a
2b
2c
2d
-2
-3
-4
0
-100
-200
-300
-400
-500
-600
-700
0
-50 -100 -150 -200 -250 -300 -350 -400 -450
E (mV)
E (mV)
3
2
1
(c)
(d)
2
1
0
0
-1
-2
-3
-4
Blank
1
2a
2b
2c
Blank
1
2a
-1
-2
-3
2b
2c
2d
2d
0
-100
-200
-300
-400
-500
-600
0
-100
-200
-300
-400
-500
-600
X Axis Title
X Axis Title
Fig. 1. Anodic and cathodic polarization curves (Tafel curves) of mild steel cooling water in the absence and presence of 5 × 10-3M of
inhibitors at different temperatures (a) 30 °C, (b) 40 °C, (c) 50 °C and (d) 60 °C
TABLE-5
ELECTROCHEMICAL PARAMETERS FOR CORROSION OF MILD STEEL IN COOLING
WATER IN THE PRESENCE OF DIFFERENT INHIBITORS AT VARIOUS TEMPERATURES
E (%) Elec.
method
E % Weight
loss method
Inhibitor
Blank
Temp. (°C)
Icorr (µA/cm2)
Ecorr (mV)
βC (mV/dm)
βa (mV/dm)
30
40
50
60
30
40
50
60
30
40
50
60
30
40
50
60
30
40
50
60
30
40
50
60
198
349
470
565
40
-241.2
-273.2
-279.8
-289.6
-294.2
-307.8
-314.1
-384.5
-290.4
-340.4
-365.9
-424.4
-291.3
-329.1
-350
-113.1
-99.2
95.2
110.7
107.5
103.2
95.5
108.3
88.2
88.9
96
–
–
–
–
–
–
–
–
-120
-107.3
-120.8
-155.1
-142.1
-143.5
-133.1
-191.6
-194.7
-134.8
-143.3
-178.5
-129.8
-161.9
-155.9
-211.4
-175.7
-152.5
-116.5
-161.8
-185.7
-117
79.79
77.36
71.91
64.77
83.33
81.94
77.02
71.32
82.82
78.22
72.55
67.25
84.34
80.22
78.93
70.97
85.35
82.52
78.51
73.98
80.12
76.38
70.27
63.83
83.04
80.06
77.61
70.02
84.80
76.69
71.62
68.57
–
79
1
132
199
33
63
83.5
80.9
81.6
101.8
91.5
102.8
75.7
94
2a
2b
2c
2d
108
162
34
76
129
185
31
-394.2
-306.4
-350.3
-360.5
-402.1
-312.3
-320.4
-354.4
-414.1
69
71.1
100.1
75.3
124.8
85.4
94.3
79.1
–
99
–
164
29
–
–
61
–
101
147
–
–

Vol. 29, No. 11 (2017)
Synthesis of Some Thiadiazole Compounds as New Corrosion Inhibitors for Mild Steel in Cooling Water 2365
7. Z. Chen, L. Huang, G. Zhang, Y. Qiu and X. Guo, Corros. Sci., 65, 214
(2012);
https://doi.org/10.1016/j.corsci.2012.08.019.
with reference to the blank, signifying that suppression of the
cathodic reaction is the main effect of these corrosion inhibitors
(Table-5).
8. M. Mihit, S. El Issami, M. Bouklah, L. Bazzi, B. Hammouti, E.A. Addi,
R. Salghi and S. Kertit, Appl. Surf. Sci., 252, 2389 (2006);
https://doi.org/10.1016/j.apsusc.2005.04.009.
Whereas, we found that by increasing the temperature,
the current density increases, which led to decrease the corrosion
efficiency. This may be attributed to decrease the adsorption
process of inhibitor on the metal surface.
9. A. Doner, E.A. Sahin, G. Kardas and O. Serindag, Corros. Sci., 66, 278
(2013);
https://doi.org/10.1016/j.corsci.2012.09.030.
10. M. Palomar-Pardavé, M. Romero-Romo, H. Herrera-Hernandez, M.A.
Abreu-Quijano, N.V. Likhanova, J. Uruchurtu and J.M. Juárez-García,
Corros. Sci., 54, 231 (2012);
Conclusion
The corrosion behaviour of mild steel in cooling water in
the absence and presence of thiadiazole compounds was investi-
gated using weight loss and galvanostatic polarization tech-
niques. From the results obtained the following conclusions
could be drawn:
https://doi.org/10.1016/j.corsci.2011.09.020.
11. S.A. Ali, M.T. Saeed and S.U. Rahman, Corros. Sci., 45, 253 (2003);
https://doi.org/10.1016/S0010-938X(02)00099-9.
12. S.A. Ali, H.A. Al-Muallem, S.U. Rahman and M.T. Saeed, Corros. Sci.,
50, 3070 (2008);
https://doi.org/10.1016/j.corsci.2008.08.011.
13. M.W. Jawich, G.A. Oweimreen and S.A.Ali, Corros. Sci., 65, 104 (2012);
https://doi.org/10.1016/j.corsci.2012.08.001.
• Thiadiazoles exhibited good inhibition efficiency reaches
to 86.55 %. The results showed that its inhibition efficiencies
increased by increasing the concentration of inhibitors, but
decreased with increase temperature.
14. M.K. Bharty,A. Bharti, R.K. Dani, S.K. Kushawaha, R. Dulare and N.K.
Singh, Polyhedron, 41, 52 (2012);
https://doi.org/10.1016/j.poly.2012.04.025.
• By increasing the side chain length of alkyl group, the
inhibition efficiency was found to be increases.
• There are good agreements between weight loss and
galvanostatic polarization techniques to determine the effi-
ciency of corrosion inhibitor.
15. M.A. MohammedAli, Ph.D. Thesis, Basrah University, Basrah, Iraq (2008).
16. S. Banerjee, V. Srivastava and M.M. Singh, Corros. Sci., 59, 35 (2012);
https://doi.org/10.1016/j.corsci.2012.02.009.
17. A.S.Abdul-Nabi andA.A. Hussain, J. Basrah Res. (Sci.), 38, 125 (2012).
18. A.E.-A. Gaber, M.S. El-Gaby, A.M. El-Dean, H.A. Eyada and A.S. Al-
Kamali, J. Chin. Chem. Soc. (Taipei), 51, 1325 (2004);
https://doi.org/10.1002/jccs.200400192.
REFERENCES
19. R.M. Silverstein and F.X. Webster, Spectroscopic Identification of
Organic Compounds, John Wiley & Sons, New York, edn 6 (1998).
20. M.S. Khan, G. Chawla and M.A. Mueed, Indian J. Chem., 43B, 1302
(2004).
1. M.M. Antonijevic, S.M. Milic and M.B. Petrovic, Corros. Sci., 51, 1228
(2009);
https://doi.org/10.1016/j.corsci.2009.03.026.
21. A. Singh, V.K. Singh and M.A. Quraishi, J. Mater. Environ. Sci., 1, 162
(2010).
2. F.M. Al-Kharafi, B.G. Ateya and R.M.A. Allah, J. Appl. Electrochem.,
34, 47 (2004);
22. A. Singh,V.K. Singh and M.A. Quraishi, Int. J. Corros., Article ID 275983
(2010);
http://dx.doi.org/10.1155/2010/275983.
https://doi.org/10.1023/B:JACH.0000005616.41240.d0.
3. M.M. Antonijevic, S.M. Milic, M.B. Radovanovic, M.B. Petrovic and
A.T. Stamenkovic, Int. J. Electrochem. Sci., 4, 1719 (2009).
4. R. Ravichandran and N. Rajendran, Appl. Surf. Sci., 239, 182 (2005);
https://doi.org/10.1016/j.apsusc.2004.05.145.
23. M.D. Shah,A.S. Patel, G.V. Mudaliar and N.K. Shah, Portug. Electrochem.
Acta, 29, 101 (2011);
https://doi.org/10.4152/pea.201102101.
5. T. Murakawa, S. Nagaura and N. Hackerman, Corros. Sci., 7, 79 (1967);
https://doi.org/10.1016/S0010-938X(67)80105-7.
6. X. Liu, X. Fang, Y. Tang, C. Sun and C. Yao, Mater. Sci. Appl., 2, 1268
(2011);
https://doi.org/10.4236/msa.2011.29171.