NOVEL PROCESSING COATING TECHNIQUES TOWARD IMPROVED ELECTRO-CERAMICS

Open Access
Author:
Sohrabi baba heidary, Damoon
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 05, 2016
Committee Members:
  • Clive A Randall, Dissertation Advisor
  • Clive A Randall, Committee Chair
  • Susan E Trolier-Mckinstry, Committee Member
  • Suzanne E Mohney, Committee Member
  • Michael T Lanagan, Outside Member
  • Nasim Alem, Committee Member
Keywords:
  • ALD
  • MLCC
  • Nickel
  • Hydrogen
  • Degradation
  • Conductivity Preservation
Abstract:
Novel coating processing techniques were utilized to improve the electrical performance of electro-ceramics. In the first part of this work, atomic layer deposition (ALD) coatings were used to encapsulate the BaTiO3 multilayer ceramic capacitors and ZnO varistors against hydrogen and humidity exposure. In the second part, the Ni particles used in the manufacturing of MLCCs were coated by lithium carbonate to preserve their conductivity in oxidizing atmospheres. This method together with fast firing enabled us to sinter the MLCC in higher partial pressure of oxygen and, consequently, decrease the concentration of oxygen vacancies in them. In both parts, the applied methodologies were demonstrated to be effective in the improvement of electroceramic performance. In following, a brief description will be provided for every chapter in this dissertation for both the first and the second parts to provide a roadmap for readers. Encapsulating of BaTiO3 MLCCs and ZnO varistors against humidity and hydrogen exposure by ALD coatings Hydrogen gas creates a highly damaging environment that degrades electrical properties in oxide based dielectrics and piezoelectrics. In the second chapter, the degradation resistivity due to hydrogen gas in a barium titanate X7R dielectric is studied for base metal electrode capacitors. The present paper is devoted to I-V measurements and the loss of resistivity in the electrode Schottky barriers. The DC degradation and asymmetries noted in I-V forward and reverse biasing conditions were assumed to be due to hydrogen ion interstitials, locally creating donor substitutions. Thermionic and field emission conductivity mechanisms are applied to model the I-V data; the conductivity is controlled by the Schottky barrier heights and hydrogen ions localizing at the interfaces. Finally, a mechanism was proposed for resistivity degradation due to exposure to hydrogen gas. The proposed mechanism predicts the degradation should be reversible, and its validity was examined by recovery tests. In the third chapter, the contributions of electrode interface, grain boundary, and grain to the total resistivity was differentiated by impedance spectroscopy (IS) tests and a 3RC model. It turned out that the largest contribution to total resistivity comes from electrode Schottky barriers, which control the major part of the degradation. Based on the IS analysis, the hydrogen diffusion coefficients of those three components were successfully calculated and compared with the diffusion coefficient in other systems. Determination of the hydrogen diffusion in grains and grain boundaries is important in understanding how hydrogen penetrates capacitors and can also be useful for applications that involve extreme environments. In this study, we also considered the kinetics and role of the metal electrode chemistry (Ag, Au, and Pt) and the thickness of active layers on the hydrogen degradation. In chapters two and three, the degradative effect of hydrogen exposure was studied and documented. Next, ALD coatings were introduced as a solution to make a physical barrier between the electroceramic device and the degrading environment. In the chapter one, the encapsulating power of ALD coating has been reviewed. In addition, it was shown that the ALD techniques can coat the nanostructures (i.e. nanowires and nanoparticles) for both encapsulation or core-shell proposes. In the fourth chapter under the title of “Evaluating the merit of ALD coating as a barrier against hydrogen degradation in capacitor components”, the encapsulating power of ALD coating was evaluated. Three ALD chemistries of ZnO, Al2O3, and HfO2 with different thicknesses were coated onto BaTiO3 capacitors, and their merit as hydrogen gas barriers at high temperatures was evaluated by I–V and impedance spectroscopy which could monitor the degradation of resistivity. These experimental investigations provide the temperature of merit (T0) and the proton (H-ion) diffusion coefficients of the ALD layers, which can be used to evaluate their barrier effectiveness. Transmission electron microscopy (TEM) analysis was applied to examine the ALD layers before and after the I–V tests and find out the physical dimensions, conformity, and structure (amorphous and crystalline) of the ALD layers. We determine that the failure of the barrier characteristics at elevated temperatures is due to crystallization. The diffusion coefficient associated with protons before and after crystallizations in ALD layers was determined. Within the chemistries investigated here, the most effective ALD layers are made of HfO2 with an amorphous structure. In the fifth chapter, the idea of using ALD coatings to protect electroceramics against hydrogen and humidity were tested in other electroceramics. Effectiveness of HfO2 Atomic Layer Deposition coatings has been studied on ZnO varistors by I–V tests, impedance spectroscopy, and highly accelerated life test. Based on impedance spectroscopy analyses, the proton diffusion coefficient was measured to be 400 K times less in the coating. Transmission electron microscopy analysis shows that Atomic Layer Deposition films are continuous and conformal. After exposure to high temperature, partial crystallization was detected in the coating and increases proton diffusion coefficient by 150 times. Preserving the electrical conductivity of nickel nanoparticle by Li2CO3 coating to decrease the number of oxygen vacancies in MLCCs In the first try to preserve the electrical conductivity of Ni particles during sintering, the ALD coatings was coated on the Ni foils to study the behavior of these coatings at the sintering conditions. Although many techniques have been applied to protect nickel (Ni) alloys from oxidation at intermediate and high temperatures, the potential of atomic layer deposition (ALD) coatings has not been fully explored. In the sixth chapter, the application of ALD coatings (HfO2, Al2O3, SnO2, and ZnO) on Ni foils has been evaluated by electrical characterization and transmission electron microscopy analyses in order to assess their merit to increase Ni oxidation resistance; particular consideration was given to preserving Ni electrical conductivity at high temperatures. The results suggested that as long as the temperature was below 850 °C, the ALD coatings provided a physical barrier between outside oxygen and Ni metal and hindered the oxygen diffusion. It was illustrated that the barrier power of ALD coatings depends on their robustness, thicknesses, and heating rate. Among the tested ALD coatings, Al2O3 showed the maximum protection below 900 °C. However, above that temperature, the ALD coatings dissolved in the Ni substrate. As a result, they could not offer any physical barrier. The dissolution of ALD coatings doped on the NiO film, formed on the top of the Ni foils. As found by the electron energy loss spectroscopy (EELS), this doping affected the electronic transport process, through manipulating the Ni3+/Ni2+ ratio in the NiO films and the chance of polaron hopping. It was demonstrated that by using the ZnO coating, one would be able to decrease the electrical resistance of Ni foils by two orders of magnitude after exposure to 1020 °C for 4 min. In contrast, the Al2O3 coating increased the resistance of the uncoated foil by one order of magnitude, mainly due to the decrease in the ratio of Ni3+/Ni2+. Based on the previous chapter, Li+ can be potential candidate to increase the ratio of Ni3+/Ni2+ and increase effectively the chance of electron hoping. In chapter seven, a new Ni electrode was manufactured from Ni nanoparticles, which coated by Li salts, namely Li2CO3, LiOH, and LiF with an innovative coating method. After the confirmation of a successful coating on Ni particles by SEM (Scanning Electron Microcopy) imaging, the Ni particles with Li2CO3 coating demonstrated the greatest preservation in conductivity among the other salts. Thus they were selected for further investigation; the effect of Li2CO3 percentage, sintering program, and pO2 on electrode resistance were systematically studied. The results suggest that the coating can reduce the resistance by five orders of magnitude at oxidizing atmosphere (e.g. pO2=2×10-4 atm). The reduced resistances were as low as 1 Ω, suitable to be used as an electrode. SEM and FIB (Focused Ion Beam) cross section imaging were used to document the Ni oxidation, the sintering process of Ni particles, and the Li2CO3 decomposition. In chapter eight, a systematic study of the influence of sintering atmosphere and temperature on the morphology of Li2CO3-coated Ni particles was investigated via TEM (transmission electron microscopy), SEM-FIB (scanning electron microscopy - focused ion beam) imaging, and TGA (thermogravimetric analysis). The results suggested that the Li2CO3 decomposes and produces CO in the presence of residual carbon; CO, as a reducing agent, decreases the amount of NiO phases formed during sintering. At the same time, the NiO phase was doped with Li+, resulting in a decrease in its resistivity. The combination of these two processes decreased the resistance of the final Ni electrodes from 140 kΩ to 1 Ω. The effect and role of these two mechanisms were separated, and it is shown that both of them are equally useful in the preservation of electrical conductivity. In chapter nine, by utilizing the novel coating together with fast firing techniques, the MLCCs were sintered in oxidizing atmosphere without losing their electrode conductivity. It demonstrated that the oxygen vacancy concentration decreased in the new MLCCs relative to conventional MLCCs, as illustrated by EELS analysis, as well as the color change of the samples. Due to the decline in oxygen vacancies, the dissipation factor was decreased by 60%. In addition, the distribution of Li was mapped with TOF-SIMS (time of flight–secondary ion mass spectrometry). Li was localized at the interface of electrode and dielectric and, as demonstrated with impedance spectroscopy, caused the energy activation of interfaces to increase. This rise in the activation energy improved both dielectric constant and the leakage current of the MLCCs. In chapter ten, the effect of Li+ doping by using HALT tests in both dried and humid environments as well as TSDC (thermally stimulated discharging current) was evaluated. The results suggested that Li doping does not affect the electrical properties of multilayer ceramic capacitors, and the reliability is not limited by the strategies of Li2CO3 coating. So we see that the main goal to establish an approach to customize multilayer BaTiO3-Ni structures at high oxygen activities and H2-free atmospheres were successful, with no unforeseen limitations with Li-addition.