Na-β-Al2O3 ceramics is a kind of polyaluminate, which is a series of compounds composed of aluminum oxide composite ions and alkali metals, alkaline earth metals or certain monovalent and divalent cations. Its general formula is nAO3·Me2O, which is a non-stoichiometric compound. Where A represents trivalent metal ions such as Al3+, Ga3+, Fe3+, etc. Me represents monovalent ions such as Na+, Li+, K+, H3O+, etc. Na-β-Al2O3 ceramics refer to solid solution electrolyte ceramics whose main crystal phase is β-Al2O3, mainly Na-β-Al2O3 (Na2O-11Al2O3), β"-Al2O3[Na2O-(0.5~7)Al2O3] etc.
1.Conductive Mechanism of Na-β-Al2O3 Ceramics
First of all, we must understand the crystal structure of Na-β-Al2O3. Na-β-Al2O3 is actually formed by overlapping aluminum oxide blocks and sodium oxide layers. Oxygen ions are packed into a dense layer in the most densely packed manner in ABCA levels, and Al3+ ions are located in the tetrahedral voids composed of oxygen ions. There are [NaO]-layers between the dense layers, and the [NaO]-layers are connected by Al-O-Al chains. The atomic configuration of the dense layer in the figure is similar to the spinel structure, so it is also called "spinel" Base block". A Na-β-Al2O3 unit cell consists of two "spinel base blocks", and a relatively loose sodium oxide layer is arranged between the base block and the base block. There is an aluminum atom above and below the sodium-oxygen layer to form an aluminum-oxygen bridge O3-Al-O-Al-O3, which serves as a connection between the base blocks.
Due to the above-mentioned structural characteristics of Na-β-Al2O3, it is difficult for ions to move in the densely packed base block, and in the loose sodium-oxygen layer in the loose layer, sodium ions can move, diffuse, and exchange ions. This sodium ion can only move in the [NaO]-diffusion layer between the two spinel bases. Therefore, it can be considered that the conductivity of β-Al2O3 is determined by Na+ in the [NaO]-layer plane perpendicular to the c axis. Within the movement. At a temperature of 300℃, the Na+ diffusion coefficient can reach 1x10^(-5)cm2/s, and the conductivity can reach 0.03Ω-1·cm-1. The conductivity of β"-Al2O3 is 0.2Ω-1·cm-1, which is more than 6 times that of β-Al2O3. In fact, the conductivity of β-Al2O3 conductivity is affected by many factors such as temperature, cations, applied voltage, sodium chemical potential energy, and sodium concentration.
2.Manufacturing Process of Na-β-Al2O3 Ceramics
The manufacturing process of Na-β-Al2O3 ceramics is similar to that of general ceramics. The most important thing is to pay attention to the purity of the raw materials. Pure Al2O3 powder should be used, and pure Na2O (usually in the form of NaCO3) should be added according to a certain ratio. For β"-Al2O3 ceramics, some additives (such as MgO, Li2O, etc.) can be appropriately added to make its performance stable.
There are two methods for powder preparation; solid-phase reaction method and chemical reaction method. The solid-phase reaction method is preliminarily performed at a temperature of about 1600℃.
Synthesize Na-β-Al2O3 powder; the chemical reaction method is to use the soluble salt of the oxide to form a solution in proportion, spray, freeze-dry and then thermally decompose, or make a gel from the solution, and then thermally decompose into a powder after vacuum dehydration. The former method is simple, more economical, and easy for industrial production. The powder obtained by the latter has a uniform composition and a lower synthesis temperature. In addition, there is a so-called "slurry solution spray drying method" between the above two methods, that is, soluble salts of sodium and lithium are dissolved in a suspension of α-Al2O3 and then spray-dried.
Under the premise of not affecting the performance of Na-β-Al2O3, some additives can be appropriately added to reduce its sintering temperature and change good performance. Since Na is easy to evaporate at high temperature, certain measures must be taken to maintain the ratio of Na2O/Al2O3 during the batching process to prevent the evaporation of Na.
The raw material of β-Al2O3 can be prepared by using fused β-Al2O3 through decarburization, ball milling, pickling, cleaning, drying and other processes.
Since the sodium content in the blank is lower than the chemical formula of Na-β-Al2O3, Na2O·11Al2O3, it is necessary in the above powder,then add a certain amount of Na2CO3 to increase the sodium content in the composition to 7.5wt%. In order to reduce the sintering temperature and improve the sodium ion for electrical conductivity, an additional 2. 5wt% MgO is required.
3.Properties and uses of Na-β-Al2O3 ceramics
Na-β-Al2O3 ceramics have high electrical conductivity. Na-β-Al2O3 ceramics are mainly used as diaphragm materials for sodium-sulfur batteries and sodium-bromine batteries. These two types of batteries are widely used in electronic watches, electronic cameras, stethoscopes and Pacemakers, etc.
Sodium-sulfur battery is a new type of solid electrolyte battery. Its theoretical specific energy is 760Wh/kg, which is 10% of that of lead-acid batteries times. Moreover, it has high charging effect, no pollution, and abundant sources of raw materials.
The structure of sodium-sulfur battery is: Na∣β-Al2O3∣NaxSx·S(C)
Na is the negative electrode, β-Al2O3 is the ion channel, and S is the positive electrode. In order to reduce the resistance of the sulfur electrode, porous graphite, carbon felt, etc. are added to the sulfur electrode (C means carbon).
Sodium-sulfur batteries can be used continuously like lead-acid batteries, can be recycled up to 1,000 times, and their life span is 5 times longer than that of lead-acid batteries.