The phosphate network gets de-polymerized due to the addition of Li2O and consequently glassy network re-polymerized due to the cross-linking of alkali ion polyhedral connecting neighboring phosphate groups
Zinc borophosphate glasses showed high physical and chemical durability when the two-dimensional phosphate networks were converted into highly cross-linked structures by the addition of B2O3
Zinc borophosphate glasses were investigated for thermal, chemical, and structural properties and results showed that they have good thermal stability and chemical durability. Zinc Oxide belongs to intermediate oxides, and therefore it enters the structural network and forms covalent Zn-O-P bonds
Analysis of dc conductivity of ZnO-B2O5-P2O5 revealed that electrical conductivity is affected by structural change. The electronic conduction was reported to be dominant in the higher concentration range
Centikaya Colak et al. (2016) have investigated Zinc-borate glasses and reported that ZnO plays a dual role as a modifier and a network former. If ZnO behaves as a modifier, the number of NBOs will increase, and this will cause the expansion of the glass network
For the 0.45Li2O - 0.05ZnO - 0.20B2O3 - 0.30P2O5 glass composition, the dynamics of lithium ions in glass has been investigated by S Kabi
Dielectric properties, AC conductivity and activation energies were investigated for Vanado- Zinc- Boro-Phosphate glasses at different frequencies
The previous researchers Koudelka et al.
The composition of glasses is defined as (P2O5)0.65-x (B2O3)0.1 (ZnO)0.25 (Li2O) x where x=0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40 was prepared by standard melt-quenching technique and labeled as BPZL1, BPZL2, BPZL3, BPZL4, BPZL5, BPZL6, BPZL7 respectively.
The AR grade chemicals, ammonium dehydrogenate phosphate (NH4H2PO4), Boric Acid (H3BO3), Lithium Oxide (Li2CO3), and Zinc Oxide (ZnO) were used as starting materials. The appropriate molar ratio of chemicals was weighed in precision weighing balance sensitive to accuracy of 0.1 mg and mixture was grounded into fine powder in an agate mortar, taken in silica crucible and melted in a muffle furnace at 1473K.
The melt was quenched to room temperature between stainless steel plates. To relieve the mechanical stresses, the samples were annealed at 450K.
The details of the preparation of glasses have been described in references
Powder XRD studies were carried out for finding structural phase confirmation. This experiment was carried out in a Rigaku make diffractometer with Cu-Kα radiation in the Bragg's angle range 10°-80°.
The samples of 3.5 mm thickness and cross-sectional area in the range 65 mm2 to 100 mm2 size were shaped, silver coated on both major surfaces to form electrode contact, and measured conductivity using two probe methods by using Chromel (Cr)-Alumel(Al) type- K thermocouple for temperature measurement in the range 300-620K and applying a constant voltage across the sample and measuring the current through the sample. Conductivity was determined as σ =1/ρ, ρ = RA/t with R being resistance, A cross-sectional area, and t the thickness of the glass.
The glass samples BPZL1, BPZL2 & BPZL3 shows no sharp peaks but a broad hump appears between 2θ angle from 20° to 30°.Samples BPZL4 & BPZL7 shows no bumps and peaks. XRD pattern showing no sharp peaks and bumps like peak are the indication of amorphous nature. Samples BPZL5, BPZL6 and BPZL8 shows few sharp peaks. So, they may be treated as semi-crystalline in nature. The semi-crystallinity can be due to any or all the constituent oxides. All samples show small peak centered at 43.33° is observed in all samples and crystallite size is found to be 27.47 nm determined by Debye-Scherrer’s formulae. In sample BPZL5 and BPZL7 peaks centered at 2
The conductivity measured in the temperature range from 320 K to 610K varied in the range from 0.5x10-4(Ωm)-1 to 7.64x10-3 (Ωm)-1as shown in
As per the glass compositional formulae at a 0.25-mole fraction, the glass forming and modifiers are in the ratio of P2O5 (0.40), B2O3(0.1) and ZnO (0.25) and Li2O (0.25) mole fractions, ZnO acts as a modifier and former role ascertained on the basis of conductivity and activation energy variations with composition and its effect on structural reorganization of the glass system.
At 0.35 mole fraction of Li2O, ZnO acts as an intermediate network former along with other two glass former P2O5 and B2O3 resulting more BO's and is predicted to cause a decrease in the conductivity. This trend clearly reflects that ZnO's role in structural reorganization is due to its properties as an intermediate network former. From 0.25 to 0.35-mole fraction of Li2O, the conductivity and activation energy behaves perfectly monotonically at temperature 553K as compared to variation at 503K. The electrical properties show that maximum appears at the mole% ratio of Li2O/ZnO equal to unity and at the mole% ratio of (P2O5+B2O3)/(Li2O+ZnO) equal to unity.
For the Non-Crystalline or amorphous solids or glass systems which are amorphous in nature they can exhibit temperature dependent conductivity and show thermally activated process of conductivity. To investigate the conduction mechanism in the present samples, we employed Mott's Small Polaron Hopping (SPH) model and variable range hopping (VRH) model
Present sample was investigated in the temperature range 300K to 620K, the temperature where the data deviated from Mott’s SPH model is termed as lower temperature region in comparison with higher temperature region of our investigating temperature range.
The conductivity in the non-adiabatic regime as per the SPH model is given as
Where σ0 is the pre-exponential factor and W is the activation energy.
The data is best linear fits to Mott's (SPH) model expression as per equation (1), and the plots drawn are shown in
It is observed that the activation energy for Mott's (SPH) model lies in the range of 58 – 409 meV
The SPH model showed activation energy in the high-temperature regime, which is found to decrease with Li2O gradually. Activation energy values suggest that it suits the high-temperature region with improved lower activation energy values.
The data that deviated from Mott's (SPH) fit has been considered for analysis using Mott's variable range hopping (VRH) model. In 1968, Mott put forward the idea that T-1/4 hopping was due to variable range. The low-temperature mode of transport occurs in the vicinity of the Fermi energy. At very low temperatures, Mott has proposed that carriers having sufficient energy to hop to nearest neighbors will hop further to find sites of comparable energy. As per the model, the conductivity is given by,
where A=[e2/2(8π)1/2] ��0[N(EF)/α kBT]1/2, B=4[2α3/9πkBN(EF)]1/4
N(EF) is the density of states at the Fermi level.
In the low-temperature regime, the plots of Log(σ) versus T-1/4 fits to straight lines.
As per the Eqn. (2) the VRH model plots have been drawn and best linear fits within the limit of ± (0.2-0.5%) of error to slopes are shown in
From the slopes of VRH plots, the density of states at Fermi level N(EF) has been determined and tabulated in
Glass |
Li2O (Mole fraction) |
Density of states (1023) x N(EF |
|
0.05 |
128.86 |
|
0.10 |
32.27 |
|
0.15 |
4.45 |
|
0.20 |
9.54 |
|
0.25 |
5.10 |
|
0.35 |
8.65 |
Chatterjee and Ghosh (2018) have studied ion transport of xLi2O - (1-x) P2O5 glasses, where 0.30 ≤ x ≤ 0.55, on increasing Li2O content, the dc conductivity increases, and the activation decreases along with the increase of the concentration of NBOs in the glassy network. Present results agree with these
J.S. Ashwajeet et al. (2015) reported conductivity values in the range1.088 x10-3-3.378x10-3 and activation energy in the range of 0.502 eV - 0.741 eV for borophosphate glasses doped with CoO and Li2O. The conductivity values agreed with present glass at 0.25,0.35 mole fraction of Li2O doped glasses, whereas activation energy values are well agreed with obtained values for all glass samples
The conductivity for semiconducting cobalt-phosphate glasses has been measured at temperature range 213-530 K conductivity values reported were lower compared to our room temperature conductivity at 303K. From Mott’s VRH conduction determined density of states N(EF) at Fermi level is comparable to our results
V. K. Deshpande et al. (2020) have been reported conductivity was in the range 10-2 (Ω m)-1 - 10-1 (Ω m)-1 is comparable to present glasses at a higher temperature, concentrations of Li2O
Money & Hariharan (2008) have reported conductivity range 8.4x10-7 (Ωm)-1- 2.15x10-5 (Ωm)-1, activation values lie in the range 0.47 (eV) - 0.77 (eV). These results agree with the experimental results of our BPZL glasses
DC conductivity of the 50Li2O- 50P2O5 glass with CuO it was reported that an increase in CuO contents and temperature increases the conductivity. For Li2O contents and temperature increases, similar conductivity behaviour was observed in our glasses up to 25% mole fraction of Li2O
Shruthi and Madhu (2021) have reported that from plot log(σac) versus log(ω) showed that no dc plateau observed at low frequency region and conductivity was found to be completely frequency dependent as compared to our present glasses exhibited dc conductivity
The conductivity of present glasses for a 0.25-mole fractions of Li2O at room temperature is obtained to be 0.6x10-3 ((Ω m)-1, and at 608K it is 6.74x10-3 (Ω m)-1. In our previous work dielectric properties and AC conductivity were reported for Vanado - Zinc - Boro - Phosphate glasses. These glasses were of different composition compared to present glasses
For Li3PO4 single phase sample was investigated and reported by Norikazu et al. (2021) that particles of size 20-30 nm suitable for the fabrication of dense solid electrolyte in all-solid-state lithium battery applications
From
Glasses of composition (P2O5)0.65-x (B2O3)0.1 (ZnO)0.25 (Li2O) x where x=0.10, 0.15, 0.20, 0.25, 0.35 and 0.40 were prepared by novel melt quenching method. Their amorphous nature was estimated from XRD studies. The electrical transport mechanism has been understood by analyzing conductivity variation with temperature using Mott's Polaron hopping mechanisms. SPH model fits gave activation energy for conduction, and VRH model fits gave density of states at Fermi level. Activation energy and density of states agree with literature values reported for other similar glasses. Changes in conductivity and activation energy with Li2O content infer that Li2O also contributes to total conductivity. Conductivity ranges obtained are better than earlier proposed glasses for battery application. So, present glasses can be considered for battery usage. Present glasses further need to be studied for structural, thermal stability and frequency dependent conductivity.