Water-induced Degradation Research Articles

Degradation of SrTiO 3-based ceramic varistors induced by water and ac voltages

A comparison experiment was conducted in which some SrTiO 3-based ceramic varistors were simply immersed in a 0.01 M NaOH solution for some periods of time while other varistors were immersed in the solution with a 50 Hz ac voltage applied between the electrodes of the varistors and a counter electrode in the solution. Though the simple immersion showed no noticeable influence on the varistors, those varistors treated with the application of the ac voltage were greatly degraded in their properties. It was proposed that the degradation resulted from the collaborated reactions of atomic hydrogen and oxygen generated in the ac voltage-induced electrolysis of water with the insulating potential barriers of the varistors. It is important to prevent water-induced degradation in SrTiO 3-based ceramic varistors operating under ac voltages.

Effects of electrolysis of water on the properties of soft lead zirconate titanate piezoelectric ceramics

Water-induced degradation in soft lead zirconate titanate (PZT) piezoelectric ceramics has been studied using electrochemical hydrogen charging, in which the silver electrodes of the piezoelectric ceramics is made a cathode in a 0.01 M NaOH solution to evolve hydrogen by electrolysis of water. It is found that with increasing hydrogen charging time, the impedance at the resonance frequency Z r increases, and the difference between the resonance and anti-resonance frequency ( f a − f r) decreases. The dielectric permittivity and piezoelectric coefficient d 33 decreases while the dielectric loss increases. I– V measurement shows that the resistivity decreases after hydrogen charging. These degradation behaviors in the soft piezoelectric ceramics can be explained as hydrogen incorporating into the lattice and forming hydroxy (OH −) bonds in the perovskite structure, which restrains the Ti ions from switching. The soft piezoelectric ceramics is partially depolarized after hydrogen charging which is quite different from that of hard lead zirconate titanate piezoelectric ceramics.

Water-Induced Degradation in Lead Zinc Niobate–Lead ZirconateTitanate Soft Piezoelectric Ceramics

Water-induced degradation of lead zinc niobate–lead zirconate titanate(Pb(Zn1/3Nb2/3)O3–Pb(ZrTi)O3) softpiezoelectric ceramics is studied using electrochemical hydrogencharging, in which the silver electrodes of the piezoelectric ceramicsconstitute a cathode in 0.01-M NaOH solution to evolve hydrogen byelectrolysis of water. It is found that with the increasing hydrogencharging time, the resonance impedance increases, the differencebetween the resonance frequency and the anti-resonance frequencydecreases, the spontaneous polarization, the remanent polarization andthe piezoelectric coefficient d33 decrease. Thedegradation behaviour of the soft piezoelectric ceramics can beexplained to hydrogen incorporating into the lattice and forminghydroxy (OH−) bonds in the perovskite structure, which preventsthe Ti ions from switching and increases the coercive field Ec. Thedegradation characteristics of the soft piezoelectric ceramics are quitedifferent from that of lead zirconate titanate hard piezoelectric ceramics.

Water-induced degradation in 0.91Pb(Zn1/3Nb2/3)O3–0.09PbTiO3 single crystals

Water-induced degradation in 0.91Pb(Zn1/3Nb2/3)O3–0.09PbTiO3 (PZN–PT) single crystals was studied through an electrochemical hydrogen charging process, in which silver electrodes were fired onto the single crystals and hydrogen was deposited on the electrodes via electrolysis of water in 0.01 M NaOH solution. The degradation was characterized by dramatic increases in the leakage current and in the dielectric loss of the crystals. X-ray diffraction analysis showed that the crystals were decomposed into metallic lead, zinc, and a niobium-rich face-centered-cubic phase. Water-induced degradation limits the application of PZN–PT single crystals.

Degradation of ZnO Ceramic Varistors Induced by Water and AC Voltages

ZnO-based ceramic varistors were immersed in 0.01 M NaOH solution and 50 Hz AC voltages were applied between the electrodes of each varistor and a counter electrode in the solution. After the treatment, the leakage current of the varistors was increased by more than two orders of magnitude while the switch voltage remained unchanged, and the dielectric loss was markedly increased in the low-frequency region. The degradation induced by water and AC voltages was similar to that induced by water and DC voltages, indicating that water-induced degradation is important for ZnO ceramic varistors regardless of whether they operate under DC or AC voltages.

Water‐Induced Degradation of Barium Titanate Ceramics Studied by Electrochemical Hydrogen Charging

Water‐induced degradation of barium titanate (BTO) ceramics has been investigated using electrochemical hydrogen charging, in which the silver electrodes of BTO ceramic specimens are made cathodes in a 0.01M NaOH solution to evolve hydrogen by electrolysis of water. After 80 h of treatment, the resistance of BTO decreases by ∼3 orders of magnitude, and the dielectric loss obviously increases. The degradation can be explained by the reduction reaction of atomic hydrogen with BTO. Hydrogen acts as a donor in BTO, and electrons are formed by the reduction. It is proposed that the reduction reaction of atomic hydrogen at ambient temperature by electrolysis of water is an important origin for degradation of BTO‐based ceramic devices.

Water sorption and water-induced molecular mobility in dental composite resins.

Water sorption in two resin composites, Kulzer's Solitaire (S) and SDI's Wave (W), and in a polyacid-modified composite resin, 3M's F2000 (compomer F), was investigated by means of equilibrium sorption isotherms (ESI) and of dynamic sorption (DS) measurements. Molecular mobility in these materials was studied by means of dielectric relaxation spectroscopy (DRS) and of thermally stimulated depolarization currents (TSDC) measurements. The results of ESI measurements show that at equilibrium, water is molecularly distributed in the materials and the effects of hydrophilic sites and clustering are negligible. Hysteresis effects in sorption-desorption cycles are larger in the resin composites than in the compomer. Equilibrium water uptakes in both ESI and DS conditions are rather low, in the range 1-2%. Diffusion coefficients of water are about 1x10(-8) cm(2)/s in the resin composites and by a factor of about 2 smaller in the compomer. Molecular mobility increases with hydration, as suggested by preliminary DRS and TSDC measurements. Detailed dielectric measurements may give important information for understanding, at the molecular level, water-induced degradation in dental materials.

Water-Induced Degradation in Zinc Oxide-Based Ceramic Varistors

Water-induced degradation in ZnO-based ceramic varistors is reported. A comparison experiment was conducted in which some varistors were immersed in a 0.05M NaOH solution for some time while other varistors were placed in a 0.05M NaOH solution to evolve hydrogen on their silver electrode by electrolysis of water. No degradation occurred to those samples that were immersed in the NaOH solution; the leakage currents increased by orders of magnitude and the switch voltage remained unchanged in the samples treated by electrolysis of water. These results indicate that the degradation resulted from the reduction reaction of hydrogen atoms generated by electrolysis of water rather than the permeation of water into the ceramics. Because water can be formed on varistors by condensation of aqueous vapor in air and varistors are usually operated under electric fields, it is proposed that the reduction reaction of hydrogen atoms generated by electrolysis of water may be an important origin for the degradation in ZnO varistors.

Water-induced degradation in lead zirconate titanate piezoelectric ceramics

Water-induced degradation in lead zirconate titanate (PZT) piezoelectric ceramics was studied by a comparison experiment. In one treatment, PZT rings were immersed in a 0.01 M NaOH solution for long periods of time; in the other treatment, PZT rings were placed in a 0.01 M NaOH solution and hydrogen was evolved onto their silver electrodes by the electrolysis of water. No degradation occurred to samples immersed in the NaOH solution; but for the samples that were also treated by electrolysis of water, serious degradation occurred as their mechanical quality factor decreased dramatically and their dielectric loss increased considerably. I–V measurement showed that the resistance decreased greatly in the degraded samples, which could be explained by the formation of charge carriers due to the reduction reaction of atomic hydrogen from the electrolysis of water. It was proposed that the reduction reaction of atomic hydrogen at ambient temperature is an important origin for the degradation of piezoelectric ceramic devices.

Water adsorption and hydrolysis on molecular transition metal oxides and oxyhydroxides.

Addition of water to molecular transition metal oxides (TiO2(g) and CrO3(g)) and oxyhydroxides (ScO(OH)(g), VO2(OH)(g), and MnO3(OH)(g)) was studied by means of quantum chemistry. In the investigated reactions, each reaction step comprised the breaking of one M=O bond and the formation of two OH groups. Exothermicity was observed when the product had tetrahedral or lower oxygen coordination. The reactions were found to involve stable water complexes as intermediates. The stabilities of such complexes were accentuated in the addition reaction Sc(OH)3(g) + H2O(g), in which the formation of a tetrahedral complex was found exothermic. For VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g), water addition to the remaining M=O bonds was found endothermic, whereas the formation of water complexes, using hydrogen bonds and preserving the oxyhydroxide kernel, was preferred. Thus, the sequence of such kernels for water clustering in the investigated reactions was found to be Sc(OH)3.H2O(g), Ti(OH)4(g), VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g). These stability considerations are important, as CrO2(OH)2(g) is believed to be the product of water-induced degradation of the protective chromium oxide scale on stainless steel at elevated temperatures.

Water-induced degradation of chromium fluoride films

As an optical film, chromium fluoride is an important material because it has good transparency in short wavelength region. However, it was found that chromium fluoride film degrades at humid condition. Therefore, we investigated the degradation phenomena of chromium fluoride films under 100% humid condition. The degraded samples were analyzed by several thin film analyzing techniques. With these results, we proposed a possible degradation mechanism consisting of multi-step reactions. First, H 2O molecules are permitted to the porous structure in the films to percolate through numerous microchannels and microvoids. Then, the water molecules react with the chromium fluoride film to make hydroxide bonding producing HF gas. HF gas evolves and gathers at some spots, and dome structure was developed. Chromium oxide precipitates around HF gas gathered dome. After time passes, HF gas evolution occurs followed by explosion of the dome structure, and crater-like feature remains as a result around the dome. It was found that the chromium oxide ring is left around the feature. In the whole process, cracking also occurred due to the molecular volume difference between CrF x and degraded part.