Dieter Sporn

Sol-gel processing of functional and structural ceramic oxide fibers

Structural ceramic oxide fibers like α-Al2O3, MgAl2O4 (spinel), Y3Al5O12 (YAG) and eutectic Al2O3−Y3Al5O12 as well as the functional Pb(Zr1−xTix)O3 (PZT) fibers were successfully prepared by sol-gel processing. All precursors are based on metal oxohydroxopropionates. A comparative study of sol-gel routes leading to spinnable sols demonstrates the key role of propionic acid as an excellent agent for controlling hydrolysis and condensation reactions.

Properties of fine scale piezoelectric PZT fibers with different Zr content

Abstract Smart materials containing piezoelectric fibers have a large potential in sensoric, actuatoric and ultrasonic transducer applications. Fine scale PZT fibers with five systematically adjusted Zr/Ti ratios ranging from x= 58 42 to 48 52 have been prepared using a sol–gel route. The fibers were embedded in a polymer matrix to produce 1-3 composites and the effective piezoelectric and dielectric coefficients were measured. An analytical model was used to extract the corresponding values of the fibers from the effective material properties. The dependence of the coefficients on the Zr/Ti ratio shows a pronounced maximum at x= 53 47 . This correlates with an analysis of the phase content. For x= 53 47 the fibers consist of 31% rhombohedral and 69% tetragonal phase, whereas the content of rhombohedral phase is 100 and 7% for x= 58 42 and 48 52 , respectively. For x= 53 47 maximum values of e33T=1200 and d33=127 pm/V were found. The latter one is about 50% of the value of corresponding bulk materials.

A New Method for the Determination of Elastic Properties of Thin Piezoelectric PZT Fibers

For the optimization of thin piezoelectric fibers it is necessary to determine their dielectric, elastic and piezoelectric properties. We developed analytical approximations to calculate the elastic coefficient S E 33,f of the fibers from the effective coefficients C E 33,eff and S E 33,eff of 1-3 composites containing such fibers. The analytic solutions have been checked by modeling with the Finite-Element-Method and compared with experimental results of model structures containing rods of soft PZT ceramic with known material properties. The elastic coefficients of the 1-3 composites were determined using the resonance method with different sample geometries for different vibration modes. The new method was applied to characterize the elastic properties of undoped PZT fibers with a Zr-content from 48 to 58 mol%. The elastic coefficient S E 33 of the fibers shows a maximum value of 15*10 m 12 m 2 /N at a Zr/Ti ratio of 51/49.

Lead Zirconate-Titanate Films Prepared from Soluble Powders

Precursor powders for Pb(Zr1 − xTix) O3 (PZT) thin films were produced by the reaction of zirconium-and titanium-n-propoxides with acetylacetone and lead acetate trihydrate. The subsequent complete removal of volatile components yielded powders that can be handled in air. The powders are indefinitely stable under ambient conditions.High molarity (>2m) coating sols were prepared by dissolution of the precursor powders in mixtures of 1,3-propanediol, triethanolamine (TEA) and water. Excess amounts of lead to compensate lead loss during firing were easily introduced into these solutions.The deposition of these sols on steel substrates and firing at 600°C yielded PZT films. Many physical parameters like film thickness, morphology and electrical performance could be influenced by choice of the solvent mixture composition and oxide content of the sols. Depending on the preparation dielectric permittivities, εr of up to 840 were measured at 1 kHz. By hysteresis measurements at 50 Hz and a field amplitude of 50 V/μm a remanent polarization of about 40 μC/cm2 and coercivity of about 8 V/μm was obtained. The films were stable against dielectric breakdown up to 70V/μm.

Inorganic Thin Films Prepared from Soluble Powders and Their Applications

Sol-gel processing has proved to be a useful method for the preparation of a wide range of inorganic thin films. However, gelation or precipitation of coating solutions and thus limited shelf-life have been drawbacks for the industrial application of this technology.Soluble powders of various compositions can be prepared from modified metal alkoxides commonly employed in sol-gel processing. Even though these powders contain a considerable amount of organic moieties they can be stored indefinitely under ambient conditions, thus diminishing shelf life problems. Redissolution in polar organic solvents, solvent mixtures or even water yields ready-to-use coating solutions. Coating solutions for the preparation of titania (TiO2), zirconia (ZrO2) and lead zirconate titanate (PbTixZr1−xO3, PZT) films have been synthesized by this method. Thickness, microstructure and properties of the resulting films can be varied by modification of the solvent, the coating procedure and thermal treatment. Inorganic thin films for various applications have been prepared.

Piezoelectric properties of PZT thin films on metallic substrates

Abstract The piezoelectric properties of PZT thin films on metallic substrates prepared by sol-gel processing were investigated. The films show a high remanent polarization of about 40 μC/cm2 and a coercive field strength of about 10 V/μm. The direct piezoelectric effect was used for measuring the electrical charge generated by an applied strain. The generated charge depends linearly on the strain up to a strain of about 0.1 %. A charge per area of about 0.1 μC/cm2 was measured for a strain of 0.1 %. From these measurements the piezomodulus d31 was calculated. The values of d31 were about 29 pC/N and have been compared with former measurements of the piezomodulus d33. The time dependence and the mechanical fatigue of the piezomodulus were investigated. The piezomoduli remain nearly constant up to 105 mechanical or electrical cycles, after this the piezomoduli decrease.

Mechanical and electric fatigue of PZT(53/47) films on metallic substrates

Abstract We have investigated the fatigue of electromechanical and dielectric properties of sol-gel derived PZT(53/47) thin films deposited on metallic substrates by means of electric and mechanical cycling. For the mechanical cycling a two point bending method was used to apply transversal stress to the samples. During mechanical cycling the piezoelectric coefficient d31 remained constant up to about 105 cycles, for a higher number of cycles a strong decrease was observed. During electric cycling no significant changes in the ferroelectric and electromechanical hysteresis loops could be found up to about 3×105 cycles. Above this number the coercive field increases, the maximum strain and the remanent polarization decrease. Obviously each electric cycling of the investigated films is accompanied by a mechanical cycling. It is assumed, that microcracks induced by mechanical stress are the main reason for the deterioration of the physical properties films during electric and mechanical cycling both.

A new way to spinnable sols derived from modified aluminumalkoxides

The preparation of spinnable sols has been developed to obtain a new type of sol-gel derived pure alumina fiber. The starting material was aluminum sec-butoxide, which was modified in a first step with glycolether, e.g., isopropoxyethanol. This leads to a partial replacement of alkoxy groups via alcoholysis and a change of the precursor structure from a trimeric to a tetrameric one. In the second step, carboxylation, the transformation to six-coordinated aluminum could be observed. Due to the formation of a sol with Newtonian flow behavior after hydrolysis, gel fibers up to several kilometers long could be drawn directly from the clear colorless sol with spinning velocities up to 200 m/min. The addition of an organic filament-forming polymer is not necessary. The sol is spinnable for periods up to one year. The transformation of the gel fiber to α-Al2O3 occurs at 1150° C.

Poled composites with Nd-Mn-doped PZTfibers under bipolar electrical load

Using the sol-gel process, Nd-Mn-doped PZT fibers were produced. The PZT was doped with 2 mol% neodymium and 1.1 mol% manganese. For characterization, the fibers were embedded in a polymer. The resulting 1-3 composites were poled with constant electric field. Strain and polarization were measured by applying a bipolar sinusoidal voltage of high amplitude. Instead of the expected shifted butterfly-shaped strain hysteresis, an asymmetric strain-field relation was observed. It is characterized by a rather linear region in direction of the poling field and an inflated region without strain switching for reversed polarity. Within the temperature range from room temperature to 80degC, the strain switching seems to be suppressed. Measurements of the piezoelectric coefficient at superimposed electric field prove the blocking of strain switching. Cyclation experiments with sesquipolar load show a pronounced linearity of the strain loops that declines after more than 2 times 104 cycles.