1. SYNTHESIS AND INVESTIGATION
OF THERMOELECTRIC PROPERTIES OF Na-DOPED V2O5
Monika Marciniak1
, Shiho Iwanaga2
, and Fumio S. Ohuchi1
1
Department of Materials Science and Engineering
2
Department of Electrical Engineering
University of Washington
Seattle, WA 98195
Keywords: vanadium-oxide, electrical conductivity, Seebeck coefficient, doping
Abstract
Vanadium pentoxide (V2O5) is a material of interest for potential thermoelectric
applications. To extend its utility for practical applications, Na was systematically introduced as
a dopant to study how the thermoelectric properties were influenced. The melt-quench technique
was implemented to dope various concentrations of sodium ions into the V2O5 host lattice. X-ray
diffraction showed that the doped Na samples dominantly formed the crystalline β- NaxV2O5. It
is shown that an increase in Na concentration causes an increase in the electrical conductivity by
a factor of ~104
but decreases the Seebeck coefficient only by a half. The direct measurement of
the power output verified that those changes resulted in an overall improvement of the power
factor up to ~150 times, as compared to that observed from pure V2O5.
Introduction
Thermoelectric effect is a vital transduction mechanism in heat and temperature sensor
applications, as well as a power source for small power generation applications [1]. Over the past
decade, researchers have been investigating thermoelectric materials in thin film forms, as the
semiconductor industries have pushed their technology forward, creating high quality thin film-
based devices. Thus, thermoelectric thin films find various applications, for example, on-chip
temperature sensors or chip cooling.
29
2007 Nanomaterials: Fabrication, Properties and Applications
Edited by: Wonbong Choi, Ashutosh Tiwari, and Seung Kang
TMS (The Minerals, Metals & Materials Society), 2007

2. Oxides are inherently advantageous in their use at high temperatures, and a number of
oxide systems have been investigated recently [2, 3]. Here we report on the properties of sodium
doped-vanadium pentaoxide (NaxV2O5) as a thin film thermoelectric device material.
Electronically, pure V2O5 is a semiconductor with a band gap of ~2.2eV [11]. Electrical
conduction occurs via hopping between V5+
and V4+
impurity center [12], and specific
conductivities up to ~1 Ω-1
cm-1
have been reported [9]. Our preliminary experiments have shown
that a pure V2O5 thin film has a very high Seebeck coefficient of >500 μV/K (n-type). However,
it has a low electrical conductivity of ~ 4 x 10-4
Ω-1
cm-1
. For practical thermoelectric
applications, high electrical conductivity (σ) and high Seebeck coefficient (S) are desirable since
the measure of the efficiency is described by the power factor (PF), which is σ·S2
.
V2O5 has a lamellar, distorted orthorhombic structure, and this deformation creates its
sheet formation [9]. Due to its layered structure, V2O5 can take various ions as intercalants. With
alkali metal ions, such as Na+
, it forms several stable phases of the composition MxV2O5, such as
α- NaxV2O5 for x ≤ 0.02 [5], β- NaxV2O5 for 0.22 ≤ x ≤ 0.40 [5] and α’- NaxV2O5 for x = 1 [6],
among which β- NaxV2O5 is of particular interest. β-NaxV2O5 crystallizes into a monoclinic,
quasi-1-dimensional structure with comparably high electrical conductivity in the b-direction [7].
The present paper describes synthesis and characterization of electrical conductivity and Seebeck
coefficients of Na-doped V2O5 thin films that were fabricated by a newly developed melt-quench
method. Analysis including x-ray diffraction (XRD), power measurements for determining
Power Factor, electrical conductivity and Seebeck measurements will be discussed to illustrate
optimum processing conditions and to explain how doping influences these thermoelectric
properties.
Experimental Procedures
Materials Synthesis
To prepare sodium-doped V2O5 solutions, a new melt-quench technique was developed.
A fixed amount, 1.1 grams, of V2O5 powder was placed in a ceramic crucible, inserted into a
furnace heated to 900°C, and kept for 30 minutes. To melt the powder, the temperature in the
furnace was maintained above the melting point of V2O5, which is 670°C – 685°C [8]. A number
of literature references report polymerized V2O5 particles with ribbon-like structures dispersed in
water. This fibrous structure corresponds to double chains consisting of orthorhombic V2O5,
which then develops into a ribbon-like structure. The solution prepared in this manner has a long
gelation time: it is stable at room temperature for over a year. To enable introducing Na ions
into V2O5 structure, NaCl-containing water was prepared by mixing de-ionized (DI) water with
NaCl crystals. For every 1 gram of powder used to make a solution, 80mL of DI water was
prepared. Molten V2O5 was quickly poured into NaCl-containing water and stirred vigorously for
1 hour. Initially, a yellow liquid and large, black precipitates formed, which later changed into
yellow/brown liquid and smaller precipitates. By the end of the stirring process, the solution
changed its color to dark red-brown, and the Na-doped V2O5 solution fully or partially
gelatinized, depending on the amount of Na present. The solution was then transferred into a
plastic bottle and stored for further processing.
In order to investigate the effect of varying dopant concentration in doped V2O5 solution,
different amounts of NaCl crystals were prepared with DI water. Solutions with V2O5 : NaCl
30

3. ratios of 1:1, 1:0.8, 1:0.5, 1:0.04, 1:0.38, 1:0.2, 1:0.028, and 1:0 were systematically synthesized.
The amount of molten V2O5 left on the walls of the crucible was considered as “lost V2O5” and it
was calculated from the difference in weight between an empty crucible and the crucible with
lost V2O5.
Sample Preparation
The solutions that precipitated were first filtered and thick, gel-like mixture was used for
thin film deposition. Solutions with no precipitants were deposited directly on a quartz substrate
via a small pipette. All films were annealed at 400°C for 10 minutes after air-drying.
Measurements
Phase composition of thin films was determined by a Philips PW 1830 powder x-ray
diffractometer (PANalytical Inc., Natick MA) using a Cu-Kα x-ray source. Seebeck coefficients
were measured by a homemade system built in a vacuum chamber (~10-2
mTorr). Using vacuum
was to prevent heat convection through the air. Each sample was bridged between two copper
blocks; one end worked as heat source and the other end as the heat sink allowing for
establishing temperature gradient necessary to generate the Seebeck effect. An electrometer
(Keithley model 6512) was used to measure voltage between the two ends. Two thermocouples
at both ends of the sample measured the temperature difference ∆T. These were spring-loaded
and were pushing against the sample’s surface. The Seebeck coefficient was determined by
measuring the potential difference ∆V as a function of temperature gradient ∆T. Since ∆V =
S∆T, Seebeck coefficient could be found from the slope of ∆V vs. ∆T plot. For the measurement
of electrical resistance at room temperature, a homemade 4-point probe station was assembled.
To determine the electrical power factor of the material, the output power, P was obtained by
first measuring the voltage and current generated by the Seebeck effect. Adjustable current
source was connected to the sample in parallel to provide a current with the direction opposite to
the current generated by the Seebeck effect. By adjusting the current source, the operating point
swept along the load line. The I-V characteristics at different ΔT were obtained, from which the
output power (load characteristic, P) was calculated.
Results and Discussion
Ability to deposit a uniform thin film from a given solution is an important factor in
determining whether material will find applications as a thin film device or not. We observed
that at molar ratios 1:1, 1:0.8, 1:0.5, and 1:0.4 of V2O5 : NaCl, precipitants accumulated at the
bottom of the container and the melt was not polymerized. Systematic reduction of the NaCl
content resulted in forming gels without precipitants. Samples with ratios below 1:0.4 were
gelatinized and we were able to analyze them in a thin film form. Thin film sample 007, created
from a solution with a V2O5 to NaCl ratio of 1:0.38, contains the highest concentration of Na
doping in V2O5. Among samples below 40% Na, 007 was the only sample that initially had
formed precipitates and as a result, required aging (~1 month) to enable thin film deposition.
Sample 008 is a medium-doped film, and 010 is the lightest-doped film. Table I summarizes
these results. After examining the crystal structure of all the solutions through XRD tests, a total
of four films were used for further investigation: three films with various Na doping levels, and a
pure V2O5 film as a reference. The typical film thickness was about 1µm.
31

4. XRD Spectra
The XRD results for all synthesized solutions are shown in Fig. 1. They indicate that Na
ions were successfully incorporated into the V2O5 structure. Four samples with the most Na
concentration (100%, 80%, 50%, 40%) contained two phases; phase Na0.33 V2O5 and/or β-Na
xV2O5 (marked as a star) and phase γ-NaV3O8 (marked as a circle). As the amount of Na
concentration was decreased below 40%, the γ-NaV3O8 phase has diminished and it disappeared
in samples with 20% Na and below. Samples 010 and 008 (2.8% and 20%, respectively) formed
mixed phases; with x in the range 0.22 <x< 0.4, β- NaxV2O5 is present in addition to the original
α-V2O5 phase, demonstrating that the amount of Na incorporated in the whole structure was
insufficient to form a uniform phase.
Figure 1. XRD of Na-doped V2O5 solutions and an undoped V2O5. Samples contain
various Na% concentration: (a) 100%, (b) 80%, (c) 50%, (d) 40%, (e) 38%, (f) 20%, (g)
2.8%, (h) 0%. Pure α-V2O5 phase is represented by ( ), Na0.33V2O5 and/or β- NaxV2O5
phase is marked as ( ), and γ-NaV3O8 is indicated by ( ). Crystallographic orientations
are shown for β- NaxV2O5 and α-V2O5 phases.
SampleName
(a) 003
(b) 004
(c) 005
(d) 006
(e) 007
(f) 008
(g) 010
(h) V2O5
(002)
(102)
(-104)
(004)
(202)
(107)
(106)
(206)
(001)
(002)
32

5. Table I. Samples Prepared with different NaCl concentrations
Sample Name V2O5: NaCl ratio Precipitated?
V2O5 1:0
010 1:0.028
008 1:0.2
007 1:0.38
Successfully formed a gel
without precipitants. Made
into thin films.
006 1:0.4
005 1:0.5
004 1:0.8
003 1:1
Precipitated.
Seebeck Coefficient and Electrical Conductivity
Variation of the Seebeck coefficient and the electrical conductivity measured at room
temperature are shown in Fig. 2 (a) and (b), respectively. The pure V2O5 thin film sample had a
high Seebeck coefficient of -540 μV/K and an electrical conductivity of 4 x 10-4
Ω-1
cm-1
. As a
result of systematic doping, Seebeck coefficient continuously decreased by a factor of two for
sample 007 to S = -213.5μV/K, whereas the electrical conductivity dramatically increased with
Na doping. With the value of 3.15 Ω-1
cm-1
, electrical conductivity of sample 007 increased by a
factor of ~104
. Similar results were obtained for sample 008, where S = -245μV/K and σ = 2.7 Ω-
1
cm-1
. This dramatic change in electrical conductivity, accompanied by decrease in Seebeck
coefficient by two, lead to a significant improvement of the materials power factor, which is
described by S2
σ. To further assess this effect, the power factor values were studied.
Figure 2. (a) Seebeck, and (b) conductivity as a function of Na concentration.
007
007008
008
010
010
V2O5
V2O5
33

6. Power Factor
The results of the power factor (PF) measurements, shown in Fig. 3 confirmed that
increasing the amount of sodium dopant increases the values of the PF of a Na-doped V2O5. The
highest values were obtained for samples containing 20% Na (PF = 1.62*10-5
W/K2
*m) and 38%
Na (PF = 1.44*10-5
W/K2
*m). These values are 150 times higher than PF values for an undoped
V2O5 or for a slightly doped V2O5.
Figure 3. Maximum power factor as a function of Na concentration.
Phase Relation
Thin films obtained with melt-quench technique consist of several phases present in the
Na-doped V2O5 structure. To better understand phase changes phenomena of the selected
samples, a phase diagram of the V2O5 - NaVO3 [26] is used (Fig. 4), since a diagram for V2O5 -
NaV2O5 is currently not available. The corresponding molecular percentage of Na, V, and O on
V2O5 - NaVO3 plot is determined by calculating how much of these elements are present in the
glassy form of the thin film sample. While the melt-quench method is a unique way to
incorporate the ions into glassy V2O5 structures, care must be taken to use the phase diagram
since the system under investigation is not in thermal equilibrium. Annealing of our samples,
however, was made under nearly equilibrium conditions.
Based on the amount of NaCl used during solution preparation, we estimated that the
concentration of Na in 007 that corresponds to the concentration of Na on the phase diagram is
32.9 mol %. It is indicated as location (1) on a horizontal axis of Fig. 4. At an annealing
temperature of 400°C, this composition lies in the two-phase region consisting of β- NaxV2O5
and γ- NaxV3O8-y. This result relates to the XRD spectrum, discussed earlier, in which β- phase
dominates. Sample 008 (17.3 mol %) located at (2) in Fig. 4 is placed in the two-phase region; α-
V2O5 and β- NaxV2O5. This result agrees with the 008 XRD spectrum, in which β-phase
continues to dominate, γ-phase is entirely removed, and α-phase begins to appear at its
characteristic peak at 21.7°. It is expected that as the amount of NaCl is systematically reduced in
the synthesis experiments, α- V2O5 will begin to form due to lower NaCl to V2O5 molar ratio.
XRD tests revealed that this important phase transformation occurs when molar ratios of Na
V2O5
010
008 007
34

7. dopant vary between 2.8% and 20%. Phase diagram confirms that significant decrease of Na in
the V2O5, makes the α- V2O5 phase the only dominating phase.
Figure 4. The phase diagram of V2O5 – NaVO3 location (1) indicate where 007
sample would be placed in this diagram. (2) 008, and (3) V2O5.
Conclusions
We have developed a new melt-quench method that resulted in successful doping of
V2O5 with sodium. We were then able to alter the properties of Na-doped V2O5 by changing the
mole concentration of the dopant. As a result, Na-doped V2O5 samples - with the potential for
applications were identified. Particularly, solutions containing 20%-38% of NaCl became of the
great interest due to their significant improvement of the electrical conductivity 10,000 times,
where the Seebeck coefficient decreased only by half, resulting in the 150 times improvement of
the electrical power factor. This outcome may contribute to development of the improved
materials for thermoelectric applications.
Acknowledgments
This work was financially supported in part by the FY2004 International Joint Research Program
of the New Energy Development Organization (NEDO).
Na0.33V2O5
007008
T(°C)
(2)
(3)
(1)
35

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