One-Step Mechanochemical Synthesis and Enhanced Ionic Conductivity of AgI-Al₂O₃ Composite Materials

1 Introduction

Silver iodide, AgI, is a well-known ionic conductor that has been extensively studied for decades due to its high ionic conductivity compared with other monovalent cation conductors. At room temperature, AgI has β- or γ-phase structures with poor ionic conductivity in the range of 10^–6–10^–7 S/cm. A phase transition to the superionic α-phase takes place when heating the sample to 147 °C. The α-phase shows the highest ionic conductivity of 1 S/cm. Therefore, extensive efforts have been made in order to stabilise the high-temperature phase to a lower temperature or to enhance the ionic conductivity of the room temperature phases. For this purpose, the researchers worked in two directions. In the first, researchers try to synthesise AgI nanostructures such as AgI nanorods, nanoplates, or nanowires with considerable enhancement of the ionic conductivity by three to four orders of magnitude, i.e., a room temperature conductivity of 10^–3–10^–4 S/cm is obtained [1–7]. In the second direction, researchers aim to fabricate two-phase composite materials consisting of AgI and insulating oxides such as Al₂O₃, SiO₂, MgO, Fe₂O₃, ZrO₂, WO₃, etc. [8–17]. In these composite materials, different degrees of conductivity enhancement is usually observed compared with pure AgI material.

There are different forms of AgI-Al₂O₃ composites that have been reported in the literature. Liu et al., for example, had synthesised AgI nanowires/Al₂O₃ composites using nanoporous anodic aluminum oxide (AAO) as a template [6]. AgI NWs/AAO composites were prepared by melt impregnation of solid AgI into AAO membranes [6]. Moreover, AgI nanowires could be synthesised by ion exchange method through the pores in polycarbonate membrane as template [18]. However, the most popular way to prepare AgI/Al₂O₃ composites is through a multistage high-temperature processing that includes mixing, grinding, firing, re-grinding, pressing into pellets, and sintering at high temperatures [8–17]. In all of the above-mentioned works, different degrees of conductivity enhancements have been reported.

From the above, discussion we notice that the AgI/Al₂O₃ composites are usually synthesised through complex multistage processes. Therefore, it is interesting to find a nonconventional, cost-effective, environmentally friendly, easily accessible, and productive route for the fabrication of different kinds of composite materials. For this regards, mechanical milling (mechanochemical synthesis) is considered a complete general method of producing all forms of materials just at room temperature. In this technique, AgI/Al₂O₃ composites can be synthesised directly by the one-step mechanical milling of AgI and Al₂O₃ for few hours at ambient conditions. This technique is a clear advantage compared with the multistage processing techniques that are currently used.

The aim of the current work is to optimise the ionic conductivity of AgI in the AgI-Al₂O₃ composite system prepared by one-step mechanical milling process. More important is the generalisation of the mechanochemical milling technique for the preparation of different composite materials.

2 Experiment

Composite materials of (1-x)AgI-(x)Al₂O₃ (x = 0.2, 0.3, 0.4, 0.5) were prepared by the high-energy mechanical milling technique (Pulverisette 6, Fritcsh, Idar-Oberstein, Germany) using vials and balls made from zirconia with a ball-to-powder weight ratio of 20:1. The milling time and speed were fixed at 6 h and 20 g, respectively. In the synthesis process, high-purity starting materials of AgI (99%, Kanto Chemical Co., Tokyo, Japan) and γ-Al₂O₃ (99.9%, Soekawa Chemicals, Tokyo, Japan) were used. The product materials were then characterised by X-ray powder diffraction (XRD), using Bruker D8 Advance (Bruker AXS GmbH, Karlsruhe, Germany) diffractometor with CuKα₁ radiation of 0.1540562 nm in the 10–60 2θ range. Moreover, the microstructure of the investigated materials is studied by scanning electron microscope (SEM, Joel, Tokyo, Japan). Complex impedance measurements were performed using compressed pellets of ∼13 mm diameter and ∼1 mm thickness. Carbon paint was evenly applied on both sides of the pellets for better electrical contact, and the sample was then held between two spring-loaded electrodes. The impedance ∣Z*∣ and phase angle θ were measured in the 50-Hz to 1-MHz frequency range using a HIOKI 3532 LCR (Hioki, Nagano, Japan) meter with a heating/cooling rate of 0.5 K/min. The impedance measurements were performed in two heating–cooling cycles in the 300- to 470-K temperature range. For comparison purposes and to understand the effect of the mechanical treatment on the conduction process, similar processes and measurements were applied for pure AgI.

3 Results and Discussion

Figure 1 shows the XRD patterns of pure AgI before and after mechanical milling for 6 h. Unmilled AgI exhibits the standard pattern of the β-AgI phase with hexagonal wurtzite structure [12]. This phase transforms easily during grinding to the metastable γ-AgI phase with fcc-cubic structure [12]. This is clearly observed in Figure 1 where mechanical milling for 6 h leads to the formation of the γ-AgI phase. Similar results were also obtained when milling AgI for a shorter period of only 1 h. The metastable γ-phase is suggested to transform irreversibly to the thermodynamically stable β-phase again when the sample is heated above 100°C [12]. Similarly, XRD patterns for the (1-x)AgI-(x)Al₂O₃ compositions, with x = 0.3 and 0.5, prepared by mechanical milling for 6 h are shown in Figure 2, where the γ-AgI phase is formed. The microstructure and the grain size of the studied composite materials are determined by SEM as shown in Figure 3. The SEM micrographs of Figure 3 show the formation of fine grains with grain size smaller than 100 nm. However, these nano-sized grains agglomerate to form larger particles in the sub-micrometer range.

Figure 1

XRD patterns for AgI (a) before and (b) after mechanical milling for 6 h.

Figure 2

XRD patterns for AgI-Al₂O₃ compositions with (a) x = 0.3 and (b) x = 0.5 compositions.

Figure 3

SEM micrographs of (1-x)AgI-(x)Al₂O₃ composite materials.

The temperature dependence of the ionic conductivity for pure-milled AgI is shown in Figure 4. The first heating process was performed from RT to 413 K. We notice that the conductivity is considerably high compared with unmilled sample by more than three orders of magnitudes with a conductivity value at RT of ∼3.7 ⋅ 10^–4 S/cm. Above 400 K, the conductivity started to decrease with heating. After first cooling from 413 K, the conductivity drops by one order of magnitude, which may indicate partial transformation to the β-AgI phase. In the second heating process, the conductivity increases steadily with increasing temperature and the superionic phase transition is manifested by an abrupt jump of the conductivity value at 425 K. In the second cooling process, the conductivity decreases considerably, and its value of 2.2 · 10^–7 S/cm at room temperature agrees with the conductivity of β-AgI, as can be seen in the figure. This behaviour suggests that complete transformation from γ-AgI to β-AgI takes place after heating the milled AgI sample above the phase transition temperature. Moreover, the hysteresis of the phase transition in the heating and cooling processes was narrow within 10 K. The enhanced conductivity in milled AgI sample could be due to the increased density of crystal defects due to the mechanical milling process.

Figure 4

The temperature dependence of the ionic conductivity of milled AgI in two heating–cooling cycles. The solid curve shows the conductivity of β-AgI phase from [15].

The above assumptions could be justified by the values of the activation energy of the conduction process during the different heating–cooling cycles. The activation energy (ΔE) calculated from the straight line fits of the curves in Figure 4 have values of 0.24–0.27 eV in the 1st heating–1st cooling–2nd heating processes. These values of ΔE agree with the reported activation energy for γ-AgI phase [12–14]. During the second cooling process from 459 K, the activation energy has a value of 0.54 eV, which corresponds to the conduction behaviour of β-AgI phase [12–14].

The ionic conductivity as a function of temperature is shown in Figure 5 for the AgI-Al₂O₃ composites. In this figure the conductivity increases with increasing temperature in the whole temperature range with a jump of the conductivity at the phase transition temperature to the superionic α-AgI phase. The conductivity of x = 0.2 composition is a little lower than other compositions. The conductivity of x = 0.3, 0.4, and 0.5 compositions are very close with x = 0.3, showing the highest conductivity value of 8 ⋅ 10^–4 S/cm at RT. The activation energy for Ag⁺ cationic conduction is 0.29, 0.27, 0.30 and 0.32 eV for x = 0.2, 0.3, 0.4 and 0.5 compositions, respectively. It is important to check the stability of the conductivity behaviour of the AgI-Al₂O₃ composites. In Figure 6, we show the temperature dependence of the ionic conductivity during heating–cooling–heating cycles for x = 0.3 composition. We notice from this figure that the conductivity drops after the first heating/cooling cycle to a RT value of 1.5 ⋅ 10^–4 S/cm compared with the 8 ⋅ 10^–4-S/cm value during the first heating. No further decrease of the conductivity was observed. Clearly, the stabilised conductivity value of 0.7AgI-0.3Al₂O₃ composite is three orders of magnitude higher than the conductivity of β-AgI phase, which suggests that the preparation of AgI-Al₂O₃ composites by mechanical milling stabilised a highly conducting state. Figure 7 shows the composition dependence of the ionic conductivity at room temperatures.

Figure 5

The temperature dependence of the ionic conductivity of AgI-Al₂O₃ composites during the first heating process.

Figure 6

The temperature dependence of the ionic conductivity of 0.7AgI-0.3Al₂O₃ composite materials in heating–cooling–heating process.

Figure 7

The composition dependence of the ionic conductivity of AgI-Al₂O₃ composites at RT in the first heating. The open square symbol is the RT conductivity of x = 0.3 sample in the second heating process.

It is interesting to mention that the phase transition to the superionic phase takes place in a two-step process, as can be seen in Figure 5 and the first heating of Figure 6. Similar effects were reported in [14]. This behaviour could be explained as follows: The mechanical milling of AgI and Al₂O₃ leads to the formation of two constituents (i) the AgI-Al₂O₃ composite materials and (ii) part of the β-AgI transforms to γ-AgI phase. When the sample is heated, the γ-AgI proportion transforms first to β-AgI, then to α-AgI phase at higher temperatures, leading to the two-step phase transformation. This could also explain the reduced conductivity after the first heating process of 0.7AgI-0.3Al₂O₃ composition.

4 Conclusions

Composite materials of AgI-Al₂O₃ were successfully synthesised by the one-step mechanical milling process at room temperature. The inclusion of the insulating Al₂O₃ leads to a considerable increase in ionic conductivity, with the composition of x = 0.3 showing the highest stable conductivity of 1.5 ⋅ 10^–4 S/cm at RT, which is three orders of magnitude higher than the conductivity of the pristine β-AgI phase. The activation energy of the conduction process ranges between 0.27 and 0.32 eV for the studied compositions. The enhanced conductivity could be due to the formation of two-phase mixtures and the increased defect density of the studied materials.