Optically-transparent and electrically-conductive AgI-AgPO3-WO3 glass fibers

Article abstract of DOI:10.1039/c5ra00681c

In this study, we report to our knowledge the first optically-transparent and electrically-conductive optical glass fiber belonging to the system AgI-AgPO₃-WO₃. The addition of tungsten oxide (WO₃) into the phosphate glassy network allowed the adjustment of the glass transition temperature, thermal expansion coefficient, refractive index, optical band edge, and electrical conductivity, which are all very important parameters in view of drawing glass fibers with a desired set of electrical and optical properties. Furthermore, the addition of WO₃ can improve considerably glass stability against water and humidity in the environment. AgI-AgPO₃-WO₃ glass fibers with 15 mol% WO₃ showed 2 dB m^-1 optical propagation loss from 800 to 950 nm wavelength range, and 10^-3 S cm^-1 electrical conductivity at 1 MHz AC frequency. Complex impedance spectra and thermal activation energies ranging from 0.15 to 0.30 eV are indicative of a dominant conductivity mechanism being ionic in nature within the range of AC frequencies from 1 Hz to 1 MHz. Fibers exhibited higher electrical conductivities than the bulk glasses. Glasses in the AgI-AgPO₃-WO₃ system can be used for fibers that require a set of adjustable properties pertaining to electrical conductivity, optical transparency, and environmental stability.

Full text of DOI:10.1039/c5ra00681c

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PAPER
Optically-transparent and electrically-conductive
AgI–AgPO –WO glass fibers†
3
3
Cite this: RSC Adv., 2015, 5, 40236
Maxime Rioux,*ab Yannick Ledemi, Jeff Viens, Steeve Morency,
b
b
b
b
b
Seyed Alireza Ghaffari and Younes Messaddeq
`
In this study, we report to our knowledge the first optically-transparent and electrically-conductive optical
glass fiber belonging to the system AgI–AgPO –WO . The addition of tungsten oxide (WO ) into the
3
3
3
phosphate glassy network allowed the adjustment of the glass transition temperature, thermal expansion
coefficient, refractive index, optical band edge, and electrical conductivity, which are all very important
parameters in view of drawing glass fibers with a desired set of electrical and optical properties.
Furthermore, the addition of WO
the environment. AgI–AgPO –WO
loss from 800 to 950 nm wavelength range, and 10
3
can improve considerably glass stability against water and humidity in
ꢀ1
3
3 3
glass fibers with 15 mol% WO showed 2 dB m optical propagation
ꢀ3
ꢀ1
S cm electrical conductivity at 1 MHz AC
frequency. Complex impedance spectra and thermal activation energies ranging from 0.15 to 0.30 eV
are indicative of a dominant conductivity mechanism being ionic in nature within the range of AC
frequencies from 1 Hz to 1 MHz. Fibers exhibited higher electrical conductivities than the bulk glasses.
Received 14th January 2015
Accepted 20th April 2015
DOI: 10.1039/c5ra00681c
3 3
Glasses in the AgI–AgPO –WO system can be used for fibers that require a set of adjustable properties
www.rsc.org/advances
pertaining to electrical conductivity, optical transparency, and environmental stability.
In recent years, extensive research has been conducted in the
eld of ion conductive glasses based on metal halides owing to
1
. Introduction

13–16
The development of multi-material optical bers that transmit their fast ion conduction properties.
The best known metal
both light and electrical current represents one of many chal- halide superionic conductive crystalline compounds are
1
–3
lenges in the eld of multifunctional bers. Phosphate-based rubidium silver iodide (RbAg
glasses, depending on the glass composition, exhibit high latter is used extensively for ion-conductive glasses because of
solubility to metallic ions, high chemical and mechanical its low cost and high conductivity compared to RbAg . The
I ) and silver iodide (AgI). The
4 5
I
4 5
ꢁ
stability, and high glass transition and crystallization temper- alpha phase of crystalline AgI (a-AgI) is stable above 147 C and
ꢀ1
4–7
atures.
In particular, the well-studied phosphate glasses can reach very high electrical conductivity (1 S cm ) owing to
belonging to the xAgI–(1 ꢀ x)AgPO pseudo-binary system its body-centered cubic structure (bcc), with the four iodide ions
3
ꢀ
provide high ionic AC conductivities at room temperature (I ) in the unit cell in closed-packed positions with a space
i.e. 10 $ s $ 10 S cm ). This set of attributes are of great group Im3m. The silver ions (Ag ) may be in either the octahe-
ꢀ
2
ꢀ3
ꢀ1
ꢀ
+
(
interest to science and technology, as these glasses may form dral sites, the tetrahedral sites, or between two adjacent iodide
the basis for electro-optics modulator for telecommunication ions. It has been demonstrated that this crystalline structure
17,18
applications where an electrical eld applied to the solid can includes 42 sites that provides mobility to the silver ions.
modulate the phase, the frequency, the amplitude, or the Moreover, the coordination around the ions is weak which
polarization of the incident light passing through the mate- enables the silver ions to move from one site to another at very
ꢀ
8
,9
rial. They may also be used as microprobes to measure the weak activation energies. The I anions are also highly polar-
10
electrical activity of nervous cells of the brain, or into smart izable, which enables the silver ions to move easily near the
textiles where electrically conductive bers can be integrated anions by the deformation of the electronic cloud. Below 147 C,
ꢁ
11,12
inside yarns for different uses in biomedicine.
silver iodide AgI is in the b-phase which presents a closed
wurtzite structure whereas its electrical conductivity decreases
ꢀ5
ꢀ6
ꢀ1
considerably to values ranging from 10
to 10
S cm .
a
Because of these remarkable electrical properties, many
Department of Chemistry, Laval University, Quebec, Canada. E-mail: maxime.rioux.
researches have been carried out to stabilize the a-phase at
2
b
@ulaval.ca
x y
Centre d'Optique, Photonique et Laser, Universit ´e Laval, 2375 rue la Terrasse, local room temperature, including the exploration of different M O
1
9–22
2
†
1
131, Qu ´e bec (Qc), G1V 0A6, Canada
glassy matrices (i.e. B
coated AgI nanoparticles. In M
O
, MoO
, GeO
, WO
)
and polymer-
2
3
3
2
3
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra00681c
23
x
O
y
glassy matrices, the
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a-phase has been stabilized by cooling the glass liquid very transmission properties. To the best of our knowledge, this is
rapidly into ake-like samples by using fast roller-quenching the rst report of light and electrical conduction in AgI–AgPO3–
methods used for the fabrication of metallic glasses. The cool- WO glass optical ber.
3
ing was so rapid that the transition between the a-phase and the
b-phase did not occur. Unfortunately, it is impractical to use
such as-prepared glasses due to their high mechanical fragility
2
. Experimental
related to the stress induced during fast quenching. However, 2.1 Bulk glass synthesis and optical ber fabrication
AgI can still be incorporated in many glassy matrices at rela-
tively high concentrations to produce low-to-medium ion
Materials. Tungsten(VI) oxide powder (99.9%) and ammo-
nium phosphate monobasic ($98.5%, ReagentPlus®) were both
obtained from Sigma Aldrich. Silver iodide (99.9%) and silver
nitrate (99.9%, ACS grade) were both obtained from Alfa Aesar.
All reagents were used as received.
conductive glasses such as the AgI–Ag O–M O and AgI–AgPO –
2
x
y
3
2
4–27
MO systems.
The reported highest conductivities for these
x
ꢀ3
ꢀ2
ꢀ1
glasses are in the range 10
temperature.
to 10
S cm
at room
Synthesis procedure. As a rst step, the silver meta-
Different mechanisms have been proposed over the last
thirty years to explain the ionic conductivity in AgI-containing
glasses, especially in AgI–AgPO glasses: the diffusion path
3
phosphate (AgPO
of silver nitrate (AgNO
NH H PO ) in a mass ratio AgNO /NH H PO of 1.48. First, the
3
) matrix has been synthesized using a mixture
3
) and ammonium phosphate monobasic
(
4
2
4
3
4
2
4
2
8
29–31
32,33
model, the cluster model,
the weak electrolyte model.
the cluster tissue model
or
ꢁ
mixture was loaded in an alumina crucible and heated at 100 C
during 20 h to allow the evaporation of residual water. The
34–36
In all these models, the
+
conduction process depends on the presence of Ag ions, and
the magnitude of ionic conductivity depends on the charge
carrier concentration. In the 90's, a Monte Carlo model was
ꢁ
ꢁ
ꢀ1
mixture was then heated to 350 C at a rate of 2 C min and
kept at this temperature during 24 h to allow the two
37
compounds to react completely together to form AgPO
3
. The
proposed to explain neutron and X-ray diffraction results on
ꢁ
AgPO
3
was then heated to 500 C and kept at that temperature
AgI–AgPO
3
glasses of different compositions. It was also
during 60 minutes to decrease its viscosity and facilitate its
casting into a stainless steel mold at room temperature. The
resulting glass was grinded into a ne powder for further use. In
a second step, the AgI–AgPO –WO glasses were prepared from
proposed that the presence of iodide in AgI decreases the acti-
vation energy required for the Ag ions to move from one vacant
site to another, thus improving carrier mobility and electrical
conductivity in AgI–AgPO3 glasses, as experimentally
3
3
3 3
high purity AgI, WO , and the as-prepared AgPO . The three
compounds were mixed together with the desired molar ratio
and loaded in a fused silica crucible according to the following
38
evidenced.
In the present work, we have investigated a new glass
composition belonging to the AgI–AgPO –WO pseudo-ternary
system. Vitreous AgPO silver phosphate was selected as the
3
3
composition law: 45AgI–(55 ꢀ x)AgPO
3
–xWO
3
with x ¼ 0, 5, 10,
3
1
5, 20 and 25 mol%. The mixture was then placed in a resistive
main glass former owing to the high metal ions solubility
characteristic to phosphate glassy network. Tungsten trioxide
ꢁ
furnace at a temperature of 1000 C. Aer 5 minutes, the melt
was manually agitated to assure its good homogeneity. Aer
another 5 minutes, the glass melt was cooled in a 10 mm
diameter cylindrical brass mold preheated at the glass transi-
WO was added to increase the glass mechanical and chemical
3
stability, particularly its stability against water degradation. We
report on the fabrication and characterization of electrically-
conductive bulk glasses, including electrical conductivity
measurements at room temperature. Finally, glass preforms of
determined composition were produced and successfully drawn
into bers with good electrical conduction and light
g
tion temperature (T ). The glass was then placed into a resistive
oven at the annealing temperature for 5 h and subsequently
cooled down to room temperature in approximately 10 hours to
remove any residual stress induced during the quenching.
Finally, the glass sample was polished using silicon carbide
3 3
Fig. 1 Photograph of 45AgI–(55 ꢀ x)AgPO –xWO glasses with x varying from 0 to 25 mol% (scaling bar: 10 mm).
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(SiC) polishing papers with decreasing grit size (400, 600, 800) thickness glass samples were disposed between two platinum
before using a 1/8 micron diamond colloid mixed with ethanol electrodes Probostat TM (Norecs) probe sample holder
to obtain sufficient surface quality for optical characterization embedded in a vertical resistive split furnace (Mellen) for
(Fig. 1).
temperature dependent measurements. The 2-point conductivity
To fabricate optical bers, cylindrical glass preforms of method with two outer platinum “hand” electrode contacts was
70 mm of length and 10 mm of diameter were prepared used with a 4-terminal measuring device that eliminated lead or
according the abovementioned procedure. The surface of the parasite resistances on the electrodes. To allow better contact
glass preforms were then polished to obtain a smooth surface between the platinum electrodes and the glass samples, silver
39,40
prior to ber drawing. This polishing step prevented the growth paint was applied on both contact surfaces of the glasses.
The
of defects at the preform surface during ber drawing, which silver paint used was colloidal silver from Pelco® with a sheet
otherwise resulted in poor mechanical resistance or even resistance of 0.02–0.05 ohms per square per mil and a service
ꢁ
ꢁ
breakage of the glass ber. The glass preforms were drawn into temperature range between ꢀ40 C and 260 C. Aer application

bers with controlled diameters of 125 and 250 mm in a 7 meter on the sample surface, the silver paint has been le overnight to
ꢁ
optical ber drawing tower set at a temperature of 300 C, giving dry at room temperature prior to further conductivity measure-
6
0 and 20 m long bers for the 125 and 250 mm diameters, ments. Electrical conductivity measurements were performed as
respectively. During the ber fabrication process, a UV-cured a function of temperature, from room temperature up to a
polymer coating (DeSolite® DS-2015) was applied to the ber temperature close to the T for each sample. For the electrical
g
surface to improve its mechanical resistance. All the glass conductivity characterization of the bers, we used the Solartron
fabrication steps and the characterizations have been per- impedance analyzer with a standard ber holder.
ꢁ
formed under ambient atmosphere at a temperature of 20 C
and relative humidity of 40%.
Density and XPS measurements. The glass samples density
was measured according to the Archimedes' principle by using a
Mirage Alfa-MS densimeter with a measurement precision of
ꢀ3
ꢂ0.002 g cm
For the XPS analyses, an Axis-Ultra system Kratos (UK) spec-
Elemental analysis and thermal characterization. The stoi- trometer equipped with an electrostatic analyzer, an 8-channels
.
2.2 Material characterizations
chiometry of the glasses has been determined using a CAMECA- detection system, a dual Al–Mg X-ray source without mono-
SX100 (Cameca, Gennevilliers, France) electron probe micro- chromator, and Al source at a power of 300 watts with a mono-
analyzer to demonstrate the reproducibility of the method used chromator was used. The system also included an electron gun
for the glass fabrication. The analysis was performed by of very low energy for neutralizing the electrostatic charges that
wavelength-dispersive spectroscopy (WDS) using an electron appeared on the electrically insulating samples upon exposure to
microprobe (EPMA) with an accelerating voltage of 15 kV and a monochromatic X-ray beam. This spectrometer was installed in
ꢀ8
current of 20 nA. Each glass composition has been analyzed on a vacuum system with a base pressure of 7 ꢃ 10 Pa.
three different samples with eight quantitative measurements
on each.
3
. Results and discussion
Differential scanning calorimetric (DSC) measurements were
performed using a Netzsch DSC Pegasus 404F3 apparatus on
3.1 Glass formation
ꢁ
small glass pieces into sealed Al pans at a heating rate of 10 C In Fig. 1, we can see that the coloration of the AgI–AgPO
3
–WO
3
ꢀ
1
ꢁ
min up to a temperature of 600 C. The thermal expansion glasses changes drastically from pale-yellow to orange and
coefficient (TEC) measurements were measured using a Netzsch nally to black with increasing WO concentration. According
3
TMA hyperion 402F1 apparatus on glass rods of 5 mm length at to the abovementioned synthesis procedure, it was not possible
ꢁ
ꢀ1
ꢁ
a heating rate of 5 C min up to 100 C and a load of 0.02 N. to obtain homogeneous bulk vitreous samples for concentra-
The TEC was determined in the temperature range from 20 to tion of WO
3
above 25 mol%. The glassy nature of the prepared
ꢁ
1
00 C.
samples was conrmed by X-ray diffraction (XRD) as shown in
–15WO glass composition. The
mission spectra were measured by using a UV-VIS-NIR Varian Cary XRD patterns recorded on the other glass compositions are
00 double beam spectrophotometer on polished 3 mm-thick presented in the ESI (Fig. S1 to S5†). Their respective glass
samples. Linear refractive index has been measured by using transition temperature (T ) was determined by DSC analysis.
Optical and electrical characterizations. The optical trans- Fig. 2a for the 45AgI–40AgPO
3
3
5
g
the prism coupling technique (M-lines Metricon 2010) at 633, 972, A typical DSC thermogram, recorded from the 45AgI–40AgPO
3
–
1
308 and 1538 nm with a precision of ꢂ0.001. To measure the 15WO
3
glass, is presented in Fig. 2b. The glass transition
optical ber propagation loss, we used a ber cut-back experi- temperature for the 15 mol% WO
mental set up comprising a SuperK COMPACT supercontinuum determined at T
white light laser from NKT photonics, a monochromator from
Brucker, and a 20ꢃ microscope objective from Olympus.
3
glass composition was
¼ 204 C (onset temperature).
ꢁ
g
The preparation method strongly inuences the thermal
properties of AgPO because of residual water affecting the
3
41
The electrical characterizations of the bulk glasses were per- AgPO glass. Depending on the method used, the glass tran-
3
formed using a 1260 Solartron impedance analyzer in the sition temperature Tg of AgPO3 may vary between 110 and
ꢁ
frequency range of 1 Hz to 1 MHz, with an applied voltage of 189 C, and the onset crystallization temperature T
1
x
between
ꢁ
00 mV and zero bias with an accuracy of 0.1%. The 5 mm 255 and 313 C. Therefore, in order to ensure reproducibility in
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Fig. 2 XRD pattern of the 45AgI–40AgPO
3
–15WO
3
3 3
glass (a) and DSC thermogram of the 45AgI–40AgPO –15WO glass (b).
ꢁ
ꢁ
the preparation of AgI–AgPO
3
–WO
3
glasses, each batch of determined at T
g
¼ 160 ꢂ 4 C and T
x
¼ 300 ꢂ 4 C. The DSC
AgPO was rst prepared following strictly similar conditions thermogram of the AgPO glass is presented in the ESI
3
3
and then characterized by DSC analysis prior to any utilization (Fig. S6†). It is worth noting that no crystallization peaks were
as starting material. Through the method used in the present detected for the studied AgI–AgPO –WO glasses under the
3
3
ꢁ
ꢀ1
study, the characteristic temperatures of AgPO
3
were conditions used for the DSC measurements (10 C min up to
ꢁ
6
00 C). Such absence of crystallization peaks on DSC curves
has already been reported in AgI–AgPO
3
based glasses with AgI
molar concentration above 20%. The addition of AgI to the
AgPO glass inuences substantially the thermal properties of
the glass, especially by decreasing its T as it has been
42
3
g
43
demonstrated by Novita et al. Originally, AgPO has a stressed
3
rigid glass network made of polymeric chains of PO tetrahedral
4
43,44
units.
ring-like structures are formed, decreasing its connectivity and
the glass transition temperature T
On the other hand, as can be seen in Fig. 3, addition of WO
3
By adding $10 mol% of AgI to the AgPO network,
g
.
3
results in a continuous increase of the glass transition
temperature T . Such behavior was expected due to the high
g
ꢁ
melting temperature of WO (1473 C). Tungsten (W) in WO
3
3
z+
has a high oxidation number (M , z ¼ 6). Consequently, the
2
electrostatic eld (z/a , with a representing the ionic radius) of
6
+
W
is high, which means that the interaction with the non-
bridging oxygens forming P–O–M links is quite strong,
strengthening the network cohesion, thus increasing the
viscosity and the T of the glass. Similar effects are observed
g
with the addition of WO in other phosphate glasses belonging
3
to the vitreous systems NaPO –WO , NaPO –Al O –WO and
3
3
3
2
3
3
45–47
K O–B O –P O –Nb O –WO .
As shown in Fig. 3, the T of
g
2
2
3
2
5
2
5
3
AgI–AgPO
3
–WO
3
glasses can be adjusted continuously and
. In addition,
reproducibly by varying the concentration of WO
3
the thermal expansion coefficient (TEC) determined from room
ꢁ
temperature up to 100 C on these glasses can be adjusted by
varying the concentration of WO
expansion coefficient is usually essential in optical ber fabri-
glasses as a cation, particularly when core/cladding bers have to be co-
3
. Knowledge of glass thermal
Fig. 3 Glass transition temperature (T
coefficient (TEC) of the 45AgI–(55 ꢀ x)AgPO
function of molar concentration of WO
g
) and thermal expansion of
3
–xWO
3
ꢁ
3
(a) and 60 C/48 h water soak
drawn from a same glass preform. The co-drawing of two
3 3 3
test on AgI–AgPO –WO glass samples, as a function of WO molar
glass materials of different compositions usually implies an
concentration (b).
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Table 1 Elemental micro-analysis of 45AgI–(55 ꢀ x)AgPO
3
–xWO
3
glasses, and weight percentages of every elements present in the glass
compositions
wt% Ag
Th
wt% I
Th
wt% P
Th
wt% W
Th
wt% O
Th
x
Exp
Exp
Exp
Exp
0.0
Exp
0
5
1
1
2
2
51.7
48.7
45.7
42.6
39.7
36.9
57.9 ꢂ 0.7
53.6 ꢂ 0.2
50.4 ꢂ 0.2
47.7 ꢂ 0.3
45.0 ꢂ 1.0
41.7 ꢂ 0.2
27.4
27.1
26.8
26.5
26.3
26.0
24.0 ꢂ 0.5
24.3 ꢂ 0.1
24.7 ꢂ 0.2
24.2 ꢂ 0.2
22.7 ꢂ 1.3
25.7 ꢂ 0.2
8.2
7.3
6.5
5.8
5.0
4.2
6.6 ꢂ 0.1
6.3 ꢂ 0.2
5.7 ꢂ 0.1
5.2 ꢂ 0.1
4.6 ꢂ 0.1
3.9 ꢂ 0.0
0.0
4.4
8.6
12.8
16.9
20.9
12.7
12.5
12.4
12.3
12.1
12.0
11.3 ꢂ 0.3
11.4 ꢂ 0.4
11.2 ꢂ 0.4
10.9 ꢂ 0.3
12.0 ꢂ 1.0
11.2 ꢂ 0.3
3.9 ꢂ 0.4
0
5
0
5
7.9 ꢂ 0.4
12.1 ꢂ 0.4
15.8 ꢂ 0.4
19.5 ꢂ 0.3
3
excellent matching of their thermal properties, including their characterize the effect of WO on the glass stability under wet
soening temperature, viscosity vs. temperature, and TEC. One conditions. To that end, the AgI–AgPO
can clearly observe in Fig. 3 a linear decrease of the TEC of the were soaked in 60 C temperature deionized water for a period
3
–WO
3
glass samples
ꢁ
ꢀ
6
ꢀ1
AgI–AgPO –WO3 glasses, from 29 to 24 ꢃ 10
K
, with of 48 hours, and inspected for weight loss per surface area. As
concentration from 0 to 25 mol%, respectively. shown in Fig. 3, signicant reduction of glass solubility into
This agrees with the reticulation effect of WO into the glass water occurred with increasing WO concentration. As WO is a
network. Furthermore, water soak tests were performed to Lewis acid, it interacts with the oxygen atoms into the
3
increasing WO
3
3
3
3
Fig. 4 Transmission spectra of 45AgI–(55 ꢀ x)AgPO
b) and normalized visible transmission spectra of pseudo-binary AgI–AgPO
xAgPO )–(xWO ) glasses (c). Samples thickness: 3 mm.
3
–xWO
3
glasses as a function of WO
3
content (a), normalized visible transmission spectra
than the (45AgI)–(55 ꢀ
(
3
glasses with the same ratios AgI/AgPO
3
3
3
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phosphate matrix and prevents hydrolysis reactions, thus tetraiodide (WI
4
), gray tungsten triiodide (WI
3
) or brown-
4
9
improving the glass resistance to water in the environment.
orange tungsten diiodide (WI2).
Furthermore, in the
6
+
Table 1 summarizes the experimental (Exp) and theoretical absence of oxygen during the melting, the W ions can be
5
+
(
Th) elemental compositions in wt% for each glass composition reduced into W by the non-bridging oxygens of the phos-
as a function of WO content. For each glass composition, three phate matrix, or by hydrogen absorption under heat treatment
samples have been investigated with eight quantitative in the presence of water vapor. In that case, the tungsten blue
3
5
+
measurements. The measurement error bars have been deter- oxide (TBO) can form, where the blue color is due to W ions
mined with a condence level of 99%. As can be seen, the as previously reported.
5
0,51
Usually, for glasses prepared under
concentration of the elements for each material showed small normal conditions, i.e. neither oxidant nor reductive condi-
6
+
5+
sample-to-sample variations demonstrating that the above- tions, such reduction of W to W can be observed for rela-
mentioned synthesis method provides good reproducibility. tively large concentration of tungsten oxide, typically above
Except for silver, all other elements showed some material loss 15 mol%. Therefore, we can assume the presence of TBO,
during the synthesis. The major loss comes from iodide, which could explain the observed greenish clouds inside the
because of its high vapor pressure, and phosphorus that can materials and opaque black solids at high concentration of
interact with environmental water to form phosphoric acid WO
PO ) which is volatile at high temperature. These could tungsten compounds can form in these glasses, but it is difficult
explain the higher experimental weight percentages obtained to know which one is formed and is responsible for the resulting
3
(i.e. x $ 25%). As mentioned previously, many different
(H
3
4
for silver.
color of the glasses. Moreover, as WO is substituted for AgPO3
3
according to the composition law used in this work, increasing
3
.2 Optical characterizations
The transmission spectra of the AgI–AgPO
presented in Fig. 4a and b as a function of WO
can observe that increasing the amount of WO
3
–WO
content. One
results in a
3
glasses are
3
3
concomitant shi of the glass optical band edge towards
longer wavelengths. Moreover, one can also note in Fig. 4b that
glasses with low concentrations of WO3 are optically trans-
parent in the visible range, with optical band edges near 500
nm wavelength corresponding to electronic transition ener-
gies of about 2.5 eV, except for the 45AgI–30AgPO –25WO
3 3
sample which is inhomogeneous with practically no trans-
mission in the visible spectrum. It is important to understand
the effect of addition of WO
3
on the light transmission of AgI–
AgPO glasses, which are pale yellow solids (Fig. 1). At high
3
temperature, tungsten can react with water or iodide to form
different compounds which change the optical band edge and
ꢁ
transmission spectrum. For instance, above 300 C and in
presence of atmospheric moisture, we may observe the
formation of ditungsten trioxide monohydrate (2WO
3
-H
which is an orange yellow colored solid. Then, tungsten-
iodide compounds may also be formed, like black tungsten glass composition. Applied voltage: 100 mV.
2
O)
Fig. 6 Electrical conductivity of the 45AgI–(55 ꢀ x)AgPO
3 3
–xWO
4
8
glasses at an AC frequency of 1 MHz, as function of temperature and
Fig. 5 Refractive index at 633, 972, 1308 and 1538 nm for 45AgI–(55 ꢀ x)AgPO
3
–xWO
content (b).
3 3
glasses as a function of WO content, measured with an
accuracy of ꢂ0.001 (a) and measured glass density as a function of WO
3
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ꢀ
the WO
3
content implies a decrease of AgPO
3
content, which explained by the large content of highly polarizable I anions
3
ꢀ24 52
may play a role in the glasses color change. Additional pseudo- (a (cm ) ¼ 7.1 ꢃ 10 ). Progressive addition of WO to the
3
binary AgI–AgPO glass samples were thus prepared according glass matrix resulted in a continuous increase of the glass
3
to the same fabrication method by maintaining the same refractive index, as shown in Fig. 5a, as well as an increase of the
AgI/AgPO
5/40, 45/35 and 45/30. First, homogeneous glass samples were can be ne-tuned in the range from 1.9 to 2.1 with the addition of
obtained only from the ratios AgI/AgPO to the glass, which opens many applications in ber
¼ 45/55, 45/50, 45/45, WO
5/40, illustrating therefore the role played by WO for the optics where adjustable numerical aperture may be highly
is considered as a glass desirable.
3
molar ratios, i.e. AgI/AgPO
3
¼ 45/55, 45/50, 45/45, glass density, as shown in Fig. 5b. Therefore, the refractive index
4
3
3
4
3
glass formation. Tungsten oxide WO
3
intermediary and can act as a glass-former and/or glass-modier
depending on the glass network. Then, from the visible trans- 3.3 Bulk glasses electrical characterizations
mission spectra of the prepared AgI–AgPO glasses presented in
3
Electrical conductivity measurements, under alternate current
AC) frequencies between 1 Hz and 1 MHz, have been carried
out for the AgI–AgPO –WO glasses as a function of temperature
and WO concentration, up to temperatures close to that of
3
glass transition for each glass samples. The electrical conduc-
tivity of the glasses was calculated using:
Fig. 4c, one can observe that the glass optical band edge shis
towards longer wavelengths when increasing the AgI/AgPO
ratio, or when decreasing the relative AgPO content or
(
3
3
3
3
increasing the relative iodine content. No signicant photocon-
ductive effects were measured when shinning blue light on the
glass samples.
l
The measured refractive index of the glass without WO is on
3
s ¼
(1)
ZS
the order of 1.9. Such high value of refractive index can be
Fig. 7 Nyquist complex impedance spectra of the 45AgI–(55 ꢀ x)AgPO
3
–xWO
3
glasses as a function of temperature for x ¼ 0 (a) and x ¼ 25 (b),
, C , CPE, C ) (c). Applied voltage: 100 mV.
and equivalent circuit consisting of electrode (R
1
, C
1
) and glass material contributions (R
2
2
3
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Fig. 8 Frequency–temperature dependent relative imaginary permittivity measurements of 45AgI–(55 ꢀ x)AgPO
3 3
–xWO glasses.
where s is the conductivity, l the sample thickness, Z the glasses did not show signicant photoconductive effects, it can
impedance magnitude and S the surface area of the sample. be inferred that valence-to-conduction electronic transitions are
Fig. 6 presents glass electrical conductivities at 1 MHz frequency not a dominant conductivity mechanism for these glasses, and
as a function of temperature; it can be seen that the electrical that the dominant conductivity mechanism is ionic in nature.
conductivities increase with temperature for all glass compo-
sitions, commensurate with thermal activation energies of 10 S cm for all the studied compositions, in agreement
The AC glass electrical conductivity at 1 MHz is on the order
ꢀ
3
ꢀ1
ranging from 0.15 to 0.30 eV, wherein increased WO concen- with previously published studies on 45AgI–55AgPO3
3
53–55
tration leads to higher AC conductivities and lower thermal glasses.
As a rst assumption, we propose that the
activation energies. The appearance of a dual slope in the increasing amount of WO in the glass increases the formation
3
3
Arrhenius plot for WO concentrations of x $ 20 may be of tungsten-iodide compounds, and consequently the silver
attributable to the presence of two different conductivity ions are less attracted by the iodide anions which facilitates
mechanisms in the glass within this temperature range, with their mobility inside the glass network. Also, it has been
thermal activation energies below 0.20 eV. Since the measured demonstrated by Raman spectroscopy that structural abnor-
thermal activation energies were well below the ꢄ2.5 eV elec- malities in AgI–AgPO
3
glasses could be the cause of variations as
tronic transition energies mentioned previously, and since the high as 35% of the ionic conductivity for the same molar
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We have also investigated the complex impedance of the
glasses to characterize the materials under AC current. Fig. 7
shows two examples of Nyquist complex impedance spectra
recorded at three different temperatures on samples x ¼ 0 and
25 mol%. The Nyquist complex impedance spectra obtained for
the other glass compositions are presented in the ESI (Fig. S7 to
S10†). The measured Nyquist proles are indicative of a resis-
tance–capacitance parallel circuit behavior that comprises a
phenomenological constant phase element (CPE). The CPE is a
component that models impedance elements exhibiting
distributed materials properties, and has been proposed to
explain the behavior of ionic charge carriers in ionic conduc-
56
tors. Equivalent circuits greatly aid in the process of tting
observed impedance data for elements with distributed prop-
erties. It has been shown in different studies that CPE behavior
could represent the ions mobility in Li-ions borate conductive
57,58
59
glasses
and yttria stabilized zirconia (YSZ). The CPE was an
essential element that provided accurate modelization (within
Fig. 9 Temperature dependent relative dielectric loss measurements
of 45AgI–(55 ꢀ x)AgPO
3
–xWO
3
glasses as a function of WO
3
ꢄ1% error) of the AgI–AgPO –WO glass complex impedance of
3
3
concentration. The inset is a magnification of the relative dielectric loss Fig. 7, for all range of compositions studied. The equivalent
ꢁ
between 20 and 80 C.
circuit that represents the conductivity of the glasses is sche-
matized in Fig. 7c. This simple electrical model could be
applied for all glass compositions investigated in the present
study. The model assigns resistances and capacitances related
concentration of AgI, and that water contamination could also
43
produce variations in the electric conductivity. The origin of
the effect of WO on the different glasses remains unclear, but
to the electrodes (R
Toward high frequencies, the glass-related capacitances
CPE–C –C ) are less dominant and the contribution of the
1 1 2 3 2
, C ) and to the glass (CPE–C –C –R ).
3
the study shows that it can promote the electrical conductivity
of the glass using the preparation method mentioned above.
(
2
3
electrodes is negligible, which mainly le the effect of the
Fig. 10 Photograph of the 45AgI–40AgPO
3
–15WO
3
polished glass preform (a), photograph of the drawn optical fiber of (125 ꢂ 5) mm diameter
with acrylate polymer coating (b), photograph of the optical fiber without acrylate polymer coating showing the efficient transmission of light;
scale bar: ¼ 10 mm (c), and SEM images of the fiber without acrylate polymer coating; scale bar: 50 mm and 200 mm, respectively (d).
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00
3 3
Table 2 Electrical conductivity of the 45AgI–40AgPO –15WO glass and the 3 is inversely proportional to frequency and corre-
fibers, as a function of the surface/volume ratio. Applied voltage:
00 mV; frequency: 1 MHz, accuracy: 0.1%
00
sponds to 1/f of the slope which indicates that 3 may be domi-
1
0
0
nated by the inuence of ionic conduction. Consequently, the 3
slope can provide information about the electrical conductivity
mechanism in the measured frequency range. Fig. 8 shows 3
curves typical of 1/f behavior within the range of AC frequencies
from 1 Hz to 1 MHz, with no other signicant polarization
mechanisms, for all compositions, and at room temperature and
Sample diameter
ꢂ5 mm)
Surface/volume
ratio
Conductivity
ꢀ1
00
(
(S cm
)
125 (ber)
250 (ber)
1000 (bulk)
320
120
7.50
0.428
0.140
0.0004
g
near T . Again, these results are indicative that the electrical
conductivity of these glasses is predominantly ionic in nature.
The relative dielectric loss, which is the ratio of the relative
00 0
2
). At low frequency, the imaginary permittivity (3 ) to the relative real permittivity (3 ),
conduction resistance of the glass (R
response due to the electrodes becomes more prominent.
We have also performed frequency dependent relative imag- temperature. 3 and 3 represent the material capacity to dissi-
0
0
0
noted as D ¼ 3 /3 , has also been investigated as a function of
L
0
0
0
0
0
inary permittivity (3 , also called dielectric loss factor) pate electric energy and to store electric energy, respectively.
6
0–62 00
measurements between 1 Hz and 1 MHz. When an electric eld Following the Debye theory for dielectric materials,
3 takes
is applied to a dielectric material, the charges become polarized into account electrical loss and can be expressed by the
to compensate for the electric eld. Four dielectric polarization following equation:
0
0
mechanisms may contribute to the dielectric loss factor 3 of the
sðTÞ
0
0
material: ionic, dipolar, atomic and electronic polarizations. At
low frequency (i.e. <10 Hz), the ionic polarization is very strong,
3 z
(2)
0
3 u
9
4 45
Fig. 11 XPS spectrum (a), high resolution Ag spectrum (b) and Ag-Auger M N N45 spectra (c) for the 125 mm fiber.
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where s(T) is the conductivity as a function of the temperature, bers with the bulk glass showed that drawing the glass led to
the vacuum permittivity and u the angular frequency. From an increase of its conductivity. During ber drawing, the
3
0
the results of Fig. 8 and 9, increasing temperature results in an surface-to-volume ratio (S/V) of the glass increases, which
increase of material electrical conductivity and relative dielec- means that surface conductivity may contribute to the overall
tric loss. The increase of dielectric loss with temperature may be electrical conductivity of the glass ber. To explore this
attributable to phonon-mediated carrier scattering in the glass, phenomenon further, we have fabricated bers with different
and to the generation of heat (i.e. friction) inside the glass. diameters to obtain different S/V ratios, and compared their
Furthermore, in Fig. 9, higher dielectric losses are observed for electric conductivity with that of the bulk. As can be seen in
glasses containing larger concentration of WO
mol%), which could be attributed to the higher glass density.
3
(i.e. x ¼ 20–25 Table 2, as the S/V ratio increased, the conductivity increased by
three orders of magnitude, thus providing a wide range of
electrical tunability for the bers.
In order to verify that the observed S/V conductivity increase
did not originate from any formation of metallic silver or silver
3.4 Optical bers characterizations
The 45AgI–40AgPO –15WO vitreous composition was selected oxide during the ber drawing, XPS analyses have been per-
3
3
3 3
for optical ber drawing as the best compromise among the formed on the 45AgI–40AgPO –15WO glass bulk and bers (of
studied glasses in terms of good thermal properties for ber 125 and 250 mm diameter). The glass bulk was set on a stain-
drawing (large difference between onset of crystallization and less steel sample holder and the bers were cut into small
glass transition temperatures), high optical transmission in pieces to be placed in a copper sample holder. The bers have
visible and short-infrared wavelengths, and high electrical been analyzed along the cylindrical surface. Fig. 11 presents
conductivity. First, this glass composition exhibits thermal the XPS spectra (Fig. 11a), the high resolution spectrum
4 45
stability with no apparent crystallization which is an important (Fig. 11b) and the Ag-Auger M N N45 spectra (Fig. 11c) for the
property for good drawing capability of the glass into a ber. 125 mm ber. The XPS spectra recorded on the bulk, the 250
Second, the transmission window covered by the glass is in the mm ber, and the Ag standard are presented in the ESI
range between 450 and 2800 nm. The glasses showing the best (Fig. S11 to S17†). The C1s spectrum was recorded in order to
transmission in visible light refer to x ¼ 0 and 5 mol% WO
3
correct the measured binding energies (BE). Indeed, the
concentrations, but these compositions are not sufficiently utilization of neutralizing canon tends to over compensate the
stable in normal environmental humidity and thus cannot be charges that are created on the samples surface under photon
used conveniently for ber optics application. Moreover, the bombardment. An internal standard or an element with
compositions with x ¼ 20 and 25 mol% show volume inho- known BE in the sample can be used for the correction. In the
6
+
mogeneities attributable to the reduction of the W , which can absence of a reliable internal reference, it is usually recom-
strongly degrade optical transmission. The compositions x ¼ 10 mended to use the contamination or aliphatic carbon which is
and 15 mol% provide high optical transmission between 500 imposed with BE of 285 eV. This correction was applied on all
and 2800 nm, and good chemical resistance against water. Also, the spectra of the same sample recorded under the same
the electrical conductivity of this glass is one of the best among conditions of neutralization. The decompositions of the
3 3
those measured in this study. The 45AgI–40AgPO –15WO
glass carbon C1s highlight, beside the aliphatic atoms, two types of
composition provided the best set of attributes for fabricating carbon–oxygen bonding: C–O and O]C–O. High-resolution
high-transparency, high-conductivity, and environmentally spectra of Ag provide, aer correction, a BE equal to 368.5–
stable optical bers.
368.6 eV for all the samples, which correspond to silver oxide
6
3,64
The 45AgI–40AgPO –15WO glass preforms were drawn to (Ag
2
O).
This is slightly higher than what is expected with
3
3
multi-mode bers, with a diameter of (125 ꢂ 5) mm, using a metallic silver, i.e. 368.25 eV. The absence of silver in its
standard ber drawing tower set at a furnace temperature of metallic form is conrmed by considering the Ag-Auger
ꢁ
3
00 C. The optical transparency of the glass bers was lowest in
M N N45 spectra. Although the Auger spectrum was not
4 45
the 700–1000 nm wavelength region, with a minimum propa- recorded on the glass bulk, the large intensity achieved in the
ꢀ
1
gation loss of about 2 dB m in the short-infrared wavelength XPS spectrum allowed the measurement of peak energy at
range from 800 to 950 nm. Following the ber drawing, Scan- 1128 eV with reasonable precision. Such value, coupled with
ning Electron Microscope (SEM) imaging was performed on the the Ag3d5/2 BE of 365.4 eV, allows us to calculate the modied

bers (Fig. 10d) to conrm that the implemented conditions Auger parameter, giving a value of 724 eV, which is attributed
permitted good control and reproducibility of the ber fabri- to the presence of Ag
cation process. The quality control related to the absence of metallic silver Ag .
2
63,64
O while a value of 726 eV is expected for
In the case of the 125 and 250 mm bers,
0
defects like crystallized particles or bubbles, and the absence of Auger spectra were recorded (Fig. 11c and S15†), leading to
signicant diameter variations over the whole ber length was Auger parameters of 724.1 eV and 724.7 eV, respectively. Again
evidenced. The glasses of composition 45AgI–40AgPO
3
–15WO
3
the measured values are still far from the expected value for
0
0
were easily drawn into bers without apparent crystallization in Ag . Immediately aer the sample analyses, the Ag standard
the volume or at the surface of the glass. Furthermore, the bers was analyzed aer argon ion etching to produce a clean metal
exhibited good mechanical strength even without the polymer surface. The Ag3d5/2 BE corresponds to 368.23 eV, as expected.
coating, which made them very easy to manipulate. Interest- The modied Auger parameter obtained was 726.3 eV, in
6
3,64
ingly, the comparison of the electrical conductivity of these perfect agreement with published data.
This last result
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conrms thus the previous ones: the Ag3d5/2 binding energy
Acknowledgements
on the glass bulk and the bers is denitely slightly higher
0
than that of Ag and not due to a calibration error of the This research was supported by the Canadian Excellence
instrument. According to these analyses, there is no evidence Research Chair program (CERC) in Photonics Innovations. The
0
of metallic silver Ag in the glass bulk neither in the bers. authors are also grateful to the Natural Sciences and Engi-
Very low relative atomic content of 5%, 1.5% and 3.5% (with neering Research Council of Canada (NSERC), the Fonds de
an accuracy of 10%) were indeed measured for these silver Recherche Qu ´e becois sur la Nature et les Technologies (FRQNT)
species at the surface of the bulk, the 125 and the 250 mm and the Canadian Foundation for Innovation (CFI) for the

bers, respectively. Interestingly, there is no increase and/or nancial support. The authors would like to thank Fr ´e d ´e ric
tendency in the concentration of these species from the bulk Lessard and Michel Jean for their technical support.
to the ber, which let us think that the stretching of the glass
into a ber could not affect signicantly their formation. We
conclude that such low concentrations of silver oxide
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