Obtaining and conductive properties of a vanadate-borate-phosphate glass

Vanadate glasses exhibit semiconducting property at certain temperatures. This work demonstrates the conductivity of the composition 45V2O5–25B2O3–30P2O5, which is a new glass in the vanadium-boron-phosphorus ternary system that expands the glass forming area reported in literature data. The glass was obtained through a classical melt-quenching technique. The structural composition of the obtained glass was revealed with Raman spectroscopy and the amorphous characteristic has been highlighted with X-ray diffraction. The characteristic temperatures and the thermal expansion coefficient were determined by dilatometry. Based on the experimental measurements of electrical resistance, mathematical calculations were performed, resulting in a conductivity of 2.04·10−6 S/cm at 125 °C, and an activation energy of 42.91 kJ/mol for this glass. Impedance spectroscopy in DC and AC at 100 V and 100 Hz to 2 MHz, respectively, showed a lower activation energy of about 0.166 eV and transition temperatures of 24 °C and 11 °C, respectively. These results were compared with those from the literature considering the temperatures at which the reported conductivities were measured. This glass has potential applications in electronic devices and temperature sensors.

The precursors were weighed on an analytical balance, homogenized in an agate mortar and then transferred to a 100 ml sintered alumina crucible.The crucible was introduced into a MoSi 2 resistive elements-equipped electrical oven.
When the mixture reached the melting temperature of 950 °C, a plateau was maintained for 1 hour.The melt was then cast into a prismatic shape (for dilatometry analysis) and a disc shape for XRD and resistivity measurements (Fig. 1).The next step consisted of annealing the glass at 300 °C for 1h to reduce the thermal stresses of the glass.The resulting glass was cut, ground and polished to provide samples for analysis.
The sample was investigated by X-ray diffraction to demonstrate its amorphous characteristics.The diffractometer was an Empyrean diffractometer from Panalytical (Malvern, UK) operating with a generator power of 45 kV and 40 mA in a parallel beam geometry through a parabolic X-ray mirror for Cu Kα radiation and a 1/8° slit in the incident beam side and a parallel beam collimator of 0.27° in front of an X' celerator detector in the diffracted beam side.For Raman measurements, a LabRam HR Evolution HORIBA (Palaiseau, France) spectrometer was used.The Raman spectrometer has a 514 nm laser, and the acquisition time was 2 seconds.The hole diameter is 100 µm, the objective 50×, the grating of 600 gr/mm, and the range is between 100-16,000 cm −1 , with a measurement error of ±0.5 cm −1 .The characteristic temperatures and thermal expansion coefficient ( α 200 20 ) of the obtained glass were determined using a horizontal Netzsch DIL 400 PC dilatometer, NETZSCH Holding, Selb, Germany, equipped with a Proteus software for characteristic temperatures and thermal expansion coefficient calculation.The resistivity was measured using a Fluke 115 True RMS Multimeter with an accuracy of ±0.9%, coupled to a small furnace equipped with a thermocouple and two 1.6 cm diameter electrodes that are in contact with the two opposite faces of the sample, also 1.6 cm in diameter.The dielectric spectroscopy measurements were carried in vacuum, at temperatures between 50 and 495 K (− 223 °C, 221.8 °C), with a 1V amplitude a.c.signal by using a HIOKI IM3536 impedance analyzer (Z ±0.05% rdg.θ: ±0.03°).The sample was heated up to 293 K afterwards performing measurements during cooling down to 50K, with a constant cooling rate of 1K/min.The direct current measurements were carried in vacuum, at temperatures between 30 and 480 K (− 243 °C, 206.8 °C) using a Keithley 6517 (10*10 −18 A current measurement resolution).The sample was cooled down to 30K under an applied bias of − 100 V afterwards measuring during heating up to 480K with a constant heating rate of 1 K/min.

Results and discussion
Figure 2 presents the ternary diagram of glasses in the V 2 O 5 -B 2 O 3 -P 2 O 5 system, along with our composition (marked by a triangle) that extends the vitrification zone, as denoted by other researchers in the literature.

The X-Ray diffraction
Figure 3 shows the X-ray diffraction (XRD) pattern of the 45V 2 O 5 -25B 2 O 3 -30P 2 O 5 glass (the sample from Fig. 1b).As the pattern contains no sharp peaks, we concluded that a non-crystalline material was obtained, as intended and that no free crystalline oxides are present in the system.Their presence, if signaled by specific peaks, would have required the reprocessing of the sample.Therefore, the 45V 2 O 5 -25B 2 O 3 -30P 2 O 5 glass was correctly prepared and annealed and ready to be used for the analyses that investigate the structure and properties of interest.

The dilatometry analysis
This analysis method is the best method for determining the behavior of glass against thermal stresses.Thermal expansion is characterized by the linear expansion coefficient that depends on the composition of the glass (the energy of the chemical bonds) and directly influences the thermal shock resistance.The expansion coefficient depends on the sample shape (that must have parallel plane faces) and the thickness of the glass sample.That is the reason the analysis was carried out on the sample presented in Fig. 1a.This method also determines the specific temperatures of a vitreous material: strain temperature (T IR ); glass transition temperature (T g ) (above this temperature the viscosity of glass decreases, and atoms tend to rearrange); annealing temperature (T SR ); dilatometric softening temperature (T D ).These temperatures are determined by the dilatometer software from the plot of sample elongation (dL/L 0 ) versus temperature (where L 0 is the initial sample length) graph 18,26 .
The thermal expansion graph for the 45V 2 O 5 -25B 2 O 3 -30P 2 O 5 glass is represented in Fig. 4, together with the specific temperatures, as indicated by the Proteus software of the dilatometer, and the linear expansion coefficient.From the inflection of the curve, the value of glass transition temperature (T g ) is obtained.Up to this value, the glass behaves like vitreous solid, while above this temperature the structural mobility of the glass is high enough to allow structural transformations.Therefore, T g is an indicator of the structural stability of vitreous state.The higher the T g , the greater the range of temperatures where the glass can be used, whilst a glass with lower T g and conductivity at low temperatures can be used in temperature sensors 18,26 . 3,14,19,20,26,30,32.
The annealing temperature must be between T IR and T SR to remove the stresses in the glass 18,33 .The 45V 2 O 5 -25B 2 O 3 -30P 2 O 5 glass was annealed at 300 °C, which is between the strain and annealing temperatures (T IR and T SR -according to Fig. 4), which is the temperature range for annealing as presented by Balta 18 .Prolonged annealing increases the density and, in some cases, the electrical conductivity of glasses 34,35 .The sample didn't show any cracks, which means that the annealing reduced the internal thermal stresses enough for further processing.

Raman analysis
The Raman spectra was obtained at room temperature, in the range from 200 to 4,000 cm -1 and is presented in Fig. 5.
The peaks from 246, 306, and 408 cm −1 correspond to O-P-O bending vibrations in PO 2 groups 36,37 .The band at 246 cm −1 can also be assigned to the bending vibrations of O-VO 3 38 .The Raman band at 300 cm −1 is attributed to the bending vibrations of V-O-V bonds 39 .The low intensity bands between 576 and 664 cm −1 (the peaks are: 576 cm −1 , 601 cm −1 , 632 cm −1 , and 664 cm −1 ) are assigned as follows: at 576 and 601 cm -1 the bending modes of the orthophosphate PO 4  3− unit (O-P-O vibrations) are found 36,40 ; at 632 cm −1 has been ascribed to the vibrations of P-O-B 2 and V-O-P 31 bridges; the band from 664 cm −1 shows the presence of B-O-B units 25 and V-O stretching vibration 28,39 .
The bands around 880 cm −1 correspond to the vibrations of B-O in B(OH) 3 , to the stretching vibrations of P-O-P bonds, and to the pyroborate groups 36 .The band at 887 cm -1 is attributed to the V-O stretching vibration modes or to BO 4 unit 2,28,41 .Hejda and co-authors demonstrated in their paper 31 that the peaks from 800 to 1100 cm −1 are especially due to the V 2 O 5 content that creates the glass lattice because these peaks occur only when vanadium becomes the network former, a statement supported by other studies 15,27,31 .The bands between 800 and 1100 cm −1 can be also attributed to the symmetrical and asymmetrical vibrations of the PO 4 and VO 4 groups 27 .The Raman sharp band at about 1020 cm −1 is attributed to V 5+ =O stretching vibration of tetragonal pyramid VO 5 39,41 The band can be attributed also to stretching vibrations of BO 4 , to the pyrophosphate units due to P 2 O 7 4− ions and orthophosphate units 27,36,42 .The P=O stretching mode band is assigned at 1317 cm −1 , but due to the boron oxide presence, the band shifts to 1300 cm −125, 36 .At 1300 cm −1 and 1721 cm −1 there are vibrations belonging to the boron structure: BO 3 unit, B-O-B stretching, and B-O − stretching vibration between non-bridging oxygen and B in the BO 3 structure 28,42 .The low intensity band at 2250 cm -1 is attributed to the bending vibrations of residual hydroxyl groups in B(OH) 3 43 .

Electrical resistivity and conductivity-direct measurements
The glass (from Fig. 1b) was mechanically processed to produce a sample 1.6 cm in diameter and 3 mm thick, with polished parallel planar surfaces for the best contact between the sample and the electrodes of the apparatus used.In our experiments, the electrodes of the apparatus used had the same diameter as the sample, 1.6 cm.During the experiment, the dependence of the electrical resistance of the sample (Re) as a function of temperature is measured using direct current (DC).
The volume electrical resistivity (ρ) is deduced from the relation 18 : where Re is the resistance in volume of the glass measured with the multimeter, S is the surface area of the sample and l is the thickness of the glass sample.The volume electrical conductivity (σ) of glass is the reciprocal of the volume resistivity 18 : The electrical resistance was measured in DC at 0.2 V, at different temperatures, resulting the data in Table 1, together with the calculated resistivity, conductivity, log σ and 10 3 /T.
To determine the temperature at which the glass becomes semiconductor and to calculate the activation energy, the Arrhenius plot of the logarithm of the conductivity as a function of 10 3 /T is presented in Fig. 6, using the data from Table 1.From the graph in Fig. 6, the slope of the graph, m, is obtained from the equation of the regression line.
The calculus of the activation energy of the electrical conduction is carried out as the logarithm of equation 4,10 : (1)  www.nature.com/scientificreports/ that leads to the next equation 12,18 : So, the slope of the plot, m, is 26 : where R is the universal gas constant (R=k•N, where k is Boltzmann's constant of 1.3804×10 −23 J/K and N is Avogadro's number of 6.023×10 23 mol −1 ).From Eq. ( 5) it results 26 : Ec is the activation energy for electrical conductivity, expressed in J/mol.Considering the articles of other authors 4,10,[44][45][46][47] , some vanadate and phosphate-tellurite glasses can exhibit also ionic conductivity in addition to the much more important electronic conductivity.This ionic conductivity is due to the migration of non-bridging oxygen along the network-former chains 45,46 or to the ion hopping of oxygen vacancies 47 .In their rigid state, at room temperature, common glasses, such as flat glass or glassware, have a conductivity of about 10 -11 S/cm, which puts them in the category of insulators, but there are also semiconducting glasses with a conductivity of up to 10 −5 S/cm 48 .To compare the glasses in terms of electrical conductivity, the temperature at which the conductivity value is 10 -8 S/cm (10 -6 S/m) can be used as an indicator of the conductivity limit between dielectrics and semiconductors [18, https:// spark.iop.org/ condu ctivi ty-elect rical].
According to Table 1 data, the obtained glass has a resistance of only 73•10 3 Ω at 125 °C and became semiconductor at a lower temperature, since at 125 °C the conductivity value is 2.04 • 10 -6 S/cm, much higher than the considered limit value of 10 -8 S/cm.
From the extrapolation of the graph (Fig. 6) the value for 10 3 /T and, implicitly, for T, corresponding to log σ = − 8 (for 10 −8 S/cm), is obtained.This gives a value for 10 3 /T of 3.54 and, consequently, a temperature of 9 °C at which the glass becomes a semiconductor , meaning that the developed glass has electrical properties suitable for the temperature sensing field at room temperature.The activation energy value (E c ) of the glass is calculated using Equation 6and has a value of 42.91 kJ/mol, which corresponds to an activation energy of 0.44 eV, making this glass suitable for use in photoelectronic applications.This result is comparable to the work of Khan 32 , Saiko 8 and Saetova 16   www.nature.com/scientificreports/

Electrical properties from impedance measurements
From impedance spectroscopy measurements, the real and imaginary part of impedance and dielectric permittivity, together with the dielectric loss and conductivity, and with the activation energy E c , were measured and calculated for AC and DC, on a large scale of temperatures, between 50 to 495 K and 30 to 480 K, respectively.For AC the frequency was varied between 100 Hz and 2 MHz.The equations used are well known and indicated in many papers 44,49 : where Z = impedance, Z' the real part and Z" the imaginary part of the impedance: The dielectric permittivity ε is expressed: here ε' is the real part and ε" is the imaginary part of the dielectric permittivity: where: l = thickness of sample S = area of sample ε 0 = permittivity of vacuum~8.85*10−12 F/m C is the capacitance: with: f = frequency.Dielectric loss, tan δ, are calculated as: The variation of conductivity with temperature in the case of DC, 100 V, is shown in Fig. 7a.It can be seen that as the temperature increases, the conductivity increases for all temperature ranges up to 480 K.
Since the entire temperature range is below T g , the Arrhenius law of variation is applicable, and the Arrhenius plot is shown in Fig. 7b.From the slope of the curve as seen in Fig. 7b, we calculated the activation energy in a similar way as in the case of resistivity measurements below 1 V and obtained a similar value of 0.166 eV and a temperature for the dielectric-semiconductor transition of 24 °C.
For AC at frequencies between 100 Hz and 2 MHz, the variation of the real and imaginary part of the dielectric permittivity for the temperature range between 50 and 495 K as shown in Fig. 8. www.nature.com/scientificreports/ As can be seen in Fig. 8, the real and imaginary parts of the permittivity decrease with frequency for all temperatures.This decrease become sharp for temperatures close to room temperature.It is also visible that both the real and imaginary parts of the dielectric permittivity increase with temperature for all domains.
The dielectric loss is shown in Fig. 9 for the same temperatures and frequency ranges.The dielectric loss maximum increases slightly with frequency and moves to higher frequency with increasing temperature.This type of evolution of dielectric loss is close to that observed by Barde 4,10 .Conductivity versus frequency and temperature, together with the Arrhenius plot of logarithmic conductivity with 10 3 /T, are shown in Fig. 10.
AC conductivity increases with both temperature and frequency.The increase with frequency decreases and the variation becomes nearly linear at temperatures around 100 °C.The increase with temperature becomes sharp at temperatures around 100 °C and frequencies above 100 Hz.The increase of conductivity with temperature is signaled also by Barde 10 for vanadium-boron-phosphate glasses with 60 % vanadium oxide.The linear variation of the conductivity with temperature can indicate the presence of an ionic charge transfer mechanism, due to the migration of non-bridging oxygen, via an activated hopping mechanism [44][45][46][47] .
From the slope of the Arrhenius plot shown in Fig. 10c, we calculated the activation energy in a similar way as in the case of resistivity measurements below 1 V and obtained a similar value of 0.167 eV and a temperature for the dielectric-semiconductor transition of 11 °C.
The transition temperature is close to that obtained for DC and to that obtained from resistivity measurements.The activation energy in both DC and AC impedance measurements is significantly lower than that

Figure 1 .
Figure 1.The obtained samples after glass casting in (a) prismatic and (b) disc shapes. www.nature.com/scientificreports/

Figure 8 .
Figure 8.(a) The real part of dielectric permittivity variation with frequency and temperature; (b) The imaginary part of dielectric permittivity variation with frequency and temperature.The temperature increases from blue to red in figure.

Figure 9 .
Figure 9.The dielectric loss versus frequency and temperature.The temperature increases from green to red.

Table 1 .
The results of the calculated electrical conductivity (σ).