Effects of V₂O₅-Al₂O₃ sintering aid on properties of low temperature sintered Li-Zn microwave ferrites

Abstract

With the progress of modern wireless communication technology, microwave communication devices are developing toward miniaturization and integration, and low temperature co-firing ceramic/ferrite technology is the key to meet the above demands. In this paper, V₂O₅-Al₂O₃ (VA) sintering aid is used to realize low temperature sintering (below melting point of silver that is 950 ℃) of Li-Zn microwave ferrite, which is suitable for radar phase shifter. In this paper, firstly, Li-Zn ferrite pre-firing material is prepared by mechanochemical ball milling method, and then VA composite is selected as sintering aid to prepare Li_0.42Zn_0.27Ti_0.11Mn_0.1Fe_2.1O₄ ferrite at low temperatures. The effects of adding amount of VA sintering aid and sintering temperature on crystal structure, microstructure and magnetic properties (saturation induction, coercivity, linewidth of ferromagnetic resonance, et al.) of the materials are also studied. The VA sintering aid can reduce the sintering temperature and maintain the spinel crystal structure of Li-Zn microwave ferrite, the diffraction peaks of V₂O₅ and Al₂O₃ involved in the sintering process are not observed for all the samples sintered at different temperatures, because the mass ratio of V₂O₅ and Al₂O₃ in VA sintering aid is very low compared with Li-Zn ferrite, so there are no corresponding impurity peaks. The introduction of VA sintering aid can promote the grain growth, with the average grain size increasing from 0.92 mm to 9.74 mm. In the sintering process of Li-Zn ferrite, V₂O₅ in VA sintering aid will melt first to form liquid phase due to its low melting point, which promotes the growth of grains. At the same time, Al₂O₃ with higher melting point can inhibit the grain growth and make the grain uniform. It is worth noting that when excessive VA sintering aid is added, the grain size of ferrite will decrease instead, because too much VA sintering aid will form a large number of liquid phases around the grains, thus splitting the grains and hindering the further growth of the Li-Zn grains. Under the condition of optimal VA sintering aid addition (0.18 <em>wt. </em>%), the saturation induction of the sample increased from 144 mT to 281 mT; The rectangular ratio increased from 0.57 to 0.78; The coercivity decreased from 705 A/m to 208 A/m; The linewidth of ferromagnetic resonance decreased from 648 Oe to 247 Oe. In summary, VA sintering aid can effectively improve the properties of Li-Zn microwave ferrite, which has a positive significance for the development of low temperature co-firing ceramic/ferrite technology.