D13 Piezo Actuators 1. Introduction Physical basis - The piezoelectric effect in ceramic materials 3 3. Actuator properties...

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1 D13 4 Piezo Actuators 1. Introduction Physical basis - The piezoelectric effect in ceramic materials Piezoelectric materials Definition of Piezoelectric Coefficients and Directions Dielectric Hysteresis Piezoelectric constants Displacement of Piezo Actuators Creep (Drift) (open loop PZTs) Aging / Stability Depolarization Electrical depolarisation Mechanical Depolarisation Thermal Depolarisation Pyroelectric effect Actuator properties Low Voltage and High Voltage PZTs Resolution Open and Closed Loop Operation Dynamic Behaviour Mechanical Considerations Stiffness Load Capacity and Force Generation Protection from Mechanical Damage Maximum Applicable Forces Force Generation Displacement with external forces External force is constant External Force changes Dynamic Forces Resonant Frequency Response time Heat Generation in a PZT at Dynamic Operation Mounting Guidelines Bonding Techniques Clamping Soldering Gluing Environmental Considerations Application of PZTs in High Humidity Atmosphere Application of PZTs in Inert Gas Atmosphere Vacuum Application of PZTs

2 6. Lifetime of PZTs...21 Multilayer Actuators 7. Multilayer PZT The Manufacturing Process Mixing Milling Tape Casting # Stripping Tape Cutting Build Up # Stacking # Printing # Drying Edge Trimming Lamination Cutting # Firing # Sintering # Termination Polarisation Inspection Unique Production Advantages Noliac actuating products CMA - Ceramic Multilayer Actuator CMA - tilt SCMA - Stacked Ceramic Multilayer Actuator ASCMA - Amplified Stacked Ceramic Multilayer Actuator CMB - Ceramic Multilayer Bender CMB 2D - Ceramic Multilayer Bender 2 Dimensional CMB-R Ceramic Multilayer Bender, Ring Actuator Families Electrostrictive Actuators References

3 Piezo Actuators 1. Introduction The piezoelectric effect is often encountered in daily life. For example, in small butane cigarette or gas grill lighters, a lever applies pressure to a piezoelectric crystal creating an electric field strong enough to produce a spark to ignite the gas. Furthermore, alarm clocks often use a piezoelectric element. When AC voltage is applied, the piezoelectric material moves at the frequency of the applied voltage and the resulting sound is loud enough to wake even the strongest sleeper. The word "piezo" is derived from the Greek word for pressure. In 1880, Jacques and Pierre Curie discovered that pressure applied to a quartz crystal creates an electrical charge in the crystal; they called this phenomena the piezo effect. Later they also verified that an electrical field applied to the crystal would lead to a deformation of the material. This effect is referred to as the inverse piezo effect. After the discovery it took several decades to utilize the piezoelectric phenomenon. The first commercial applications were ultrasonic submarine detectors developed during World War I and in the 1940 s scientists discovered that barium titanate ceramics could be made piezoelectric in an electric field. Piezoelectric ceramics are hard, chemically inert and only slightly sensitive to humidity or other atmospheric influences. Their mechanical properties resemble those of the better known ceramic insulators and they are manufactured by much the same processes. 2. Physical basis - The piezoelectric effect in ceramic materials The piezoelectric effect is exhibited by a number of naturally-occurring crystals, for instance quartz, tourmaline and sodium potassium tartrate, and these have been used for many years as electromechanical transducers. For a crystal to exhibit the piezoelectric effect, its structure should have no center of symmetry. A stress (tensile or compressive) applied to such a crystal will alter the separation between the positive and negative charge sites in each elementary cell leading to a net polarization at the crystal surface. The effect is practically linear, i.e. the polarization varies directly with the applied stress, and direction-dependent, so that compressive and tensile stresses will generate electric fields and hence voltages of opposite polarity. It's also reciprocal, so that if the crystal is exposed to an electric field, it will experience an elastic strain causing its length to increase or decrease according to field polarity. As stated above, piezoelectric materials can be used to convert electrical energy into mechanical energy and vice versa. For nanopositioning, the precise motion which results when an electric field is applied to a piezoelectric material is of great value. Actuators using this effect first became available around 20 years ago and have changed the world of precision positioning. Piezoelectric actuators (PZTs) offer the user several benefits and advantages over other motion techniques: 3

4 Repeatable nanometer and sub-nanometer sized steps at high frequency can be achieved with PZTs because they derive their motion through solid state crystal effects. There are no moving parts (no "stick-slip" effect). PZTs can be designed to move heavy loads (several tons) or can be made to move lighter loads at frequencies of several 10 khz. PZTs act as capacitive loads and require very little power in static operation, simplifying power supply needs. PZTs require no maintenance because they are solid state and their motion is based on molecular effects within the ferroelectric crystals. With high-reliability PZT materials a strain on the order of 1/1000 (0.1%) can be achieved; this means that a 100 mm long PZT actuator can expand by 100 micrometers when the maximum allowable field is applied. 2.1 Piezoelectric materials Besides the crystals mentioned above, an important group of piezoelectric materials are the piezoelectric ceramics, of which PZT is an example. These are polycrystalline ferroelectric materials with the perovskite crystal structure - a tetragonal/rhombahedral structure very close to cubic. They have the general formula A 2+ B , in which A denotes a large divalent metal ion such as barium or lead, and B denotes a tetravalent metal ion such as titanium or zirconium. Materials such as PZT can be considered as a mass of minute crystallites. Above a temperature known as the Curie point, these crystallites exhibit simple cubic symmetry. This structure is centrosymmetric with positive and negative charge sites coinciding, so there are no dipoles present in the material (which is said to exhibit paraelectric behaviour). Below the Curie point, however, the crystallites take on tetragonal symmetry in which the positive and negative charge sites no longer coincide so each elementary cell then has a built-in electric dipole which may be reversed, and also switched to certain allowed directions by the application of an electric field. Such materials are termed ferroelectric because this electrical behaviour present a physical analogy with the magnetic behaviour of ferromagnetic materials. They don t necessarily contain iron as an important constituent. The analogy can, in fact, be carried further, since to some extent the polarization of ferroelectric materials exhibit hysteresis, and their dielectric constants are very high and temperature dependent (as are the permeabilities of ferromagnetic materials). The dipoles are not randomly oriented throughout the material. Neighboring dipoles align with each other to form regions of local alignment known as Weiss domains. Within a Weiss domain, therefore, all the dipoles are aligned, giving a net dipole moment to the domain and hence a net polarization (dipole moment per unit volume).the direction of polarization between neighboring Weiss domains can differ by 90 or 180, and owing to the random distribution of Weiss domains throughout the material, no overall polarization or piezoelectric effect is exhibited. The ceramic may be made piezoelectric in any chosen direction by applying a poling treatment, which involves exposing the ceramic to a strong electric field at a temperature slightly below the Curie point. Under the action of this field, domains most nearly align with the field. With the field applied, the material expands along the axis of the field and contracts perpendicular to that axis. When the field is taken off the dipoles remain locked in approximate alignment, giving the 4

5 ceramic a remaining polarization and a permanent deformation (i.e. making it anisotropic). The poling operation is usually the final treatment of PZT manufacture. Fig. 1 - Electric dipoles in Weiss domains; (1) unpoled ferroelectric ceramic, (2) during and (3) after poling Definition of Piezoelectric Coefficients and Directions Because of the anisotropic nature of PZT ceramics, effects are dependent on direction. To identify directions the axes, termed 1, 2, and 3, are introduced (analogous to X, Y, Z of the classical right hand orthogonal axial set). The axes 4, 5 and 6 identify rotations (shear). The direction of polarization (3 axis) is established during the poling process by a strong electrical field applied between two electrodes. For actuator applications the piezo properties along the poling axis are most essential (largest stroke). 2.2 Dielectric Hysteresis For standard Noliac PZTs, the allowable field strength ranges up to 3 kv/mm in the poling direction. The maximum voltages depend on the ceramic properties and the design of internal electrodes. Exceeding the maximum voltage may cause dielectric breakdown and irreversible damage of the PZT. 5

6 Fig. 2 - Response of a PZT actuator to a bipolar drive voltage. When a certain threshold voltage (negative to the polarization diretion) is exceeded, reversion of polarization can occur. With the inverse field, negative expansion (contraction) occurs to yield an additional 20 % of the nominal displacement. If both the regular and inverse electric field are used, a relative expansion (strain) up to 0.2 % is achievable with PZT stack actuators Hysteresis (open loop PZTs) Hysteresis can be eliminated by closed loop PZT actuators. Similar to electromagnetic devices, open loop piezo actuators exhibit hysteresis (they are also referred to as ferroelectric actuators). Hysteresis is based on crystalline polarization effects and molecular friction. The absolute displacement generated by an open loop PZT depends on the applied electric field and the piezo gain which is related to the remanent polarization. Since the remanent polarization and therefore the piezo gain is affected by the electric field applied to the piezo, its deflection depends on whether it was previously operated at a higher or a lower voltage. Hysteresis is typically in the order of 10 to 15 %. 6

7 Fig. 3 - Hysteresis curves of an open loop piezo actuator for various peak voltages. E.g. if the drive voltage of a 50 µm piezo actuator is changed by 10 %, (5 µm motion) the position repeatability is still on the order of only 1 % full travel or better than 1 µm. Classical motor driven leadscrew positioners will hardly beat this repeatability. Closed loop piezo actuator systems eliminate hysteresis. For positioning where the travel is controlled by an external servo loop (e.g. the eyes and hands of the operator or a sophisticated electronics system), hysteresis behaviour and linearity are of secondary importance since they can be compensated for by the external loop. 2.3 Piezoelectric constants It should be clearly understood that the piezoelectric coefficients described here are not independent constants. They vary with temperature, pressure, electric field, form factor, mechanical and electrical boundary conditions etc. The coefficients only describe material properties under small signal conditions. Compound components such as PZT stack actuators, let alone preloaded actuators or lever amplified systems cannot be described sufficiently by these material parameters. This is why each component or system manufactured by Noliac is characterized by specific data such as stiffness, load capacity, displacement, resonant frequency, etc., acquired by individual measurements. Piezoelectric materials are characterized by several coefficients: Examples are: 7

8 d ij : Strain coefficients [m/v]: strain developed (m/m) per electric field applied (V/m) g ij : Voltage coefficients or field output coefficients [Vm/N]: open circuit electric field developed (V/m) per applied mechanical stress (N/m²) k ij : Coupling coefficients [Dimensionless]. The coefficients are energy ratios describing the conversion from mechanical to electrical energy or vice versa. k² is the ratio of energy stored (mechanical or electrical) to energy (mechanical or electrical) applied. Other important parameters are the Young's modulus Y (describing the elastic properties of the material) and the relative dielectric coefficients (permitivity) (describing the capacitance of the material). To link electrical and mechanical quantities, double subscripts (e.g. d ij ) are introduced. The first subscript gives the direction of the excitation, the second describes the direction of the system response. Definition: S = strain = constant (mechanically clamped) T = stress = constant (not clamped) E = field = constant (short circuit) D = electrical displacement = constant (open circuit) The individual piezoelectric parameters are related by several equations that are not explained here because they are not important for the user of piezo actuators. 2.4 Displacement of Piezo Actuators Creep (Drift) (open loop PZTs) For periodic motion, creep does not affect repeatability Creep only occurs with open loop PZTs. Like hysteresis, creep is related to the effect of the applied voltage on the remanent polarization of the piezo ceramics. Creep decreases logarithmically with time. If the operating voltage of a (open loop) PZT is increased the remanent polarization (piezo gain) continues to increase, manifesting itself in a slow creep (positive or negative) after the voltage change is complete. Maximum creep (after a few hours) can add up to a few % of the commanded motion. 8

9 Fig. 4 - Creep of open loop PZT motion after a 60 µm change in length, as a function of time. Creep in the order of 1 % of the last commanded motion per time decade Aging / Stability Aging refers to reduced piezo gain as a result of the depolarisation process. Aging can be an issue for sensor or charge generation applications (direct piezo effect), but with actuators working at high electrical field it is negligible, because repolarisation occurs every time a higher electric field (in the poling direction) is applied to the element. 2.5 Depolarization Care must be taken during handling of the polarized PZT elements as they can lose polarity again, resulting in a partial or full loss of piezoelectric properties. The ceramic may be depolarised electrically, mechanically or thermally Electrical depolarisation Exposure to a strong electrical field of opposite polarity to the poling field will depolarise a piezoelectric element. The field strength required for marked depolarisation depends, among other things, on the material grade, the time the material is subjected to the depolarisation field and the temperature. An alternating field will also have a depolarising effect during the half cycles that it opposes the poling field Mechanical Depolarisation Mechanical depolarisation occurs when the mechanical stress on a piezoelectric element becomes sufficient high to disturb the domain orientation and hence destroy the alignment of dipoles. The safety limits for mechanical stress vary considerably with material grade. 9

10 2.5.3 Thermal Depolarisation If a piezoelectric element is heated to its Curie point, the domains become disordered and the elements becomes completely depolarised. A piezoelectric element can therefore function for a long period of time without significant depolarisation if operated a temperatures below Curie point Tc. 2.6 Pyroelectric effect The polarisation in a PZT material is temperature dependent. The orientation of the electric dipoles gradually disappear with increasing temperature. Within the safe operating temperature range, the changes are reversible. When exposed to excessive heat, depolarisation will take place, as already mentioned. Changes in the alignment of the dipoles results in a charge displacement and electric fields. In most applications this phenomenon is an unwanted side-effect, which causes measuring errors or even failure of sensitive electronic components like MOSFETs by over-voltage. In other applications the pyroelectric effect is very helpful e.g. in infrared detectors. Infrared radiation heats up the ceramic the resulting voltage or charge is a measure for the level of radiation. Primarily the pyroelectric effect is unwanted. The interference mainly occurs in very low frequency or quasi-static applications. To suppress it, parallel resistors are used to allow charge to flow away. 3. Actuator properties 3.1 Low Voltage and High Voltage PZTs Two main types of piezo actuators are available: low voltage (multilayer) devices requiring below 200 volts for full motion and high voltage devices requiring above 200 volts for full extension. Modern piezo ceramics capable of greater motion replace the natural material used by the Curies, in both types of devices. The maximum electrical field PZT ceramics can withstand is up to 3 kv/mm. In order to keep the operating voltage within practical limits, PZT actuators consist of thin layers of electroactive ceramic material electrically connected in parallel. The net positive displacement is the sum of the strain of the individual layers. The thickness of the individual layer determines the maximum operating voltage for the actuator. High voltage piezo actuators are constructed from approximately 0.5mm (and upwards) layers while low voltage piezo actuators are monolithic (diffusion bonded) multilayer designs constructed from 20 to approximately 100 µm layers. Both types of piezo actuators can be used for many applications: Low voltage actuators facilitate drive electronics design. Due to manufacturing technology high voltage ceramics can be designed with larger cross-sections for higher load applications (up to several tons) than low voltage ceramics. 10

11 3.2 Resolution Piezo actuators have no "stick slip" effect and therefore offer theoretically unlimited resolution. This feature is important since PZTs used in atomic force microscopes are often required to move distances less than one atomic diameter. In practice, actual resolution can be limited by a number of factors such as piezo amplifier (electric noise results in unwanted displacement), sensor & control electronics (noise and sensitivity to EMI affect the positional resolution and stability) and mechanical parameters (design and mounting precision of the sensor, actuator and preload influence micro-friction which limits resolution and accuracy). 3.3 Open and Closed Loop Operation PZT actuators can be operated in open and closed loop. In open loop, displacement roughly corresponds to the drive voltage. This mode is ideal when the absolute position accuracy is not critical or when the position is controlled by data provided by an external sensor (interferometer, CCD chip etc.). Open loop piezo actuators exhibit hysteresis and creep behaviour (like other open loop positioning systems). Closed loop actuators are ideal for applications requiring high linearity, long-term position stability, repeatability and accuracy. Closed loop PZT Actuators are equipped with position measuring systems providing sub-nanometer resolution and bandwidth up to 10 khz. A controller (digital or analogue) determines the output voltage to the PZT by comparing a reference signal (commanded position) to the actual sensor feed back position signal. 3.4 Dynamic Behaviour A piezo actuator can reach its nominal displacement in approximately 1/3 of the period of the resonant frequency. Rise times on the order of microseconds and accelerations of more than 10,000 g's are possible. This feature permits rapid switching applications. Injector nozzle valves, hydraulic valves, electrical relays, adaptive optics and optical switches are a few examples of fast-switching applications. Resonant frequencies of industrial reliability piezo actuators range from a few tens of khz for actuators with total travel of a few microns to a few khz for actuators with travel more than 100 microns. These figures are valid for the piezo itself; an additional load will decrease the resonant frequency. Piezo actuators are not designed to be driven at resonant frequency (with full stroke and load), as the resulting high dynamic forces might damage the internal structure of the ceramic material. 4 Mechanical Considerations 4.1 Stiffness In a first approximation, a piezo actuator can be regarded as a spring/mass system. The stiffness or spring constant of a piezo actuator depends on the Young's Modulus of the ceramic 11

12 (approximately 25 % that of steel), the cross section and length of the active material and a number of other non-linear parameters. When calculating force generation, resonant frequency, system response, etc., piezo stiffness is an important parameter. In solid bodies stiffness depends on the Young's Modulus which is the ratio of stress (force per unit area) to strain (change in length per unit length). It is generally described by the spring constant k T, relating the influence of an external force to the Dimensional change of the body. This narrow definition does not apply fully for PZT ceramics; large and small signal conditions, static and dynamic operation, open and shorted electrodes must be distinguished. The poling process of PZT ceramics leaves a remanent strain in the material which depends on the magnitude of polarization. The polarization is affected by both the drive voltage and external forces. When an external force is applied to poled PZT ceramics, the Dimensional change depends on the stiffness of the ceramic material and the change of the remanent strain (caused by the polarization change). The equation L N = F/k T is only valid for small forces and small signal conditions. For larger forces, an additional term describing the influence of the polarization changes, is superimposed on stiffness (k T ). Since piezo ceramics are active materials, they produce an electrical response (charge) when mechanically stressed (e.g. in dynamic operation). When the electric charge cannot be drained from the PZT, it generates a counter force to the mechanical stress. This is why a PZT element with open electrodes appears stiffer than one with shorted electrodes. Since stiffness values of PZT actuators are not constants they can only be used to estimate the behaviour under certain conditions and to compare different PZT actuators of one manufacturer. There is no international standard for measuring piezo actuator stiffness. Therefore stiffness data of different manufacturers cannot be compared without additional information. 4.2 Load Capacity and Force Generation PZT ceramics can withstand high pushing forces and carry loads to several tons. Even when fully loaded, the PZT will not lose any travel as long as the maximum load capacity is not exceeded. Load capacity and force generation must be distinguished. The maximum force (blocking force) a piezo can generate is determined by the product of the stiffness and the total travel. A piezo actuator (as most other actuators) pushing against a spring load will not reach its nominal displacement. The reduction in displacement is dependent on the ratio of the piezo stiffness to the spring stiffness. As the spring stiffness increases, the displacement decreases and the generated force increases. 4.3 Protection from Mechanical Damage Since PZT ceramics are brittle and cannot withstand high pulling or shear forces, the mechanical actuator design must isolate these undesirable forces from the ceramic. For example, spring 12

13 preloads can be integrated in the mechanical actuator assembly to compress the ceramic inside and increase the ceramic s pulling capabilities for dynamic push/pull applications. 4.4 Maximum Applicable Forces The mechanical strength values of PZT ceramic material is often confused with the practical long term load capacity of a PZT actuator. PZT ceramic material can withstand pressures up to approximately 250 MPa (2500 x 10 5 N/m²) before it breaks mechanically. For practical applications, this value must not be approached because depolarisation occurs at pressures on the order of 20 to 30 % of the mechanical limit. If the maximum compressive force for a PZT is exceeded, damage to the ceramics as well as depolarisation may occur. Tensile loads of non preloaded PZTs are limited to 5-10 % of the compressive load limit. Actuators with internal spring preload extends tensile load capacity. Preloaded elements are highly recommended for dynamic applications. Shear forces must be isolated from the PZT ceramics by external measures. 4.5 Force Generation In most applications, piezo actuators are used to produce displacement. If used in a restraint, they can generate forces. Force generation is always coupled with a reduction in displacement. The maximum force (blocked force) a piezo actuator can generate depends on its stiffness and maximum displacement: Fmax k PZT ΔL 0 At maximum force generation, displacement is zero. where ΔL 0 : Max nominal displacement without external force or restraint [m] k PZT : PZT actuator stiffness [N/m] In actual applications the load spring constant can be larger or smaller than the PZT spring constant. The effective force (F max eff ) a piezo actuator can generate in a yielding restraint is: k PZT Fmax eff k PZT ΔL 0 (1- ) k + k PZT spring where ΔL 0 = displacement (without external force or restraint) [m] k PZT = PZT actuator stiffness [N/m] k spring = spring stiffness [N/m]. 13

14 Fig.5 - Force Generation vs. Displacement The points where the dashed line (external spring curve) intersect the PZT force/displacement curves determine the force and displacement for a given setup with an external spring. Maximum work can be produced when the stiffness of the PZT actuator and external spring are identical. 4.6 Displacement with external forces When designing preloaded PZT systems, the stiffness of the preload spring should be less than 1/10 of the PZT stiffness. Otherwise too much of the unloaded displacement would be sacrificed. If the preload spring has the same stiffness as the PZT, displacement will be cut in half. Like any other actuator, a piezo actuator is compressed when a force is applied. Two cases must be considered when operating a PZT with a load: The load remains constant during the motion process The load changes during the motion process External force is constant A mass is installed on the PZT which applies a force F = M g (M: mass, g: acceleration due to gravity). The zero point will be offset by an amount ΔL N = F/k PZT. If this force is within the acceptable load limit, full displacement can be obtained at full operating voltage. 14

15 Fig. 6 - Zero points offset with constant force (mass). Δ L N F k PZT where ΔL N : Zero point offset [m] F: Force (generated by mass and gravity) [N] k PZT : PZT actuator stiffness [N/m] External Force changes Displacement of the PZT is reduced when subjecting it to an external force that changes during the motion process. For PZT operation with spring loads different rules apply. The "spring" could be an I- beam or a single fibre each with its characteristic stiffness or spring constant. Part of the displacement generated by the piezo effect is lost due to its elasticity of the piezo element. The total available displacement can be related to the spring stiffness by the following equations: k PZT ΔL max ΔLo k PZT + k Spring, which gives the maximum displacement of a piezo actuator acting against a spring load. 15

16 Maximum displacement loss due to external spring force is calculated by: Δ L R ΔL o (1 k PZT k PZT + k Spring ) In the case where the spring stiffness k spring is (infinitely rigid restraint), the PZT only acts as a force generator. Where, ΔL: ΔL 0 : ΔL R : k spring : k PZT : Displacement with external spring load [m] Max nominal displacement without external force or restraint [m] Lost displacement caused by the external spring [m] Spring stiffness [N/m] PZT actuator stiffness 4.7 Dynamic Forces Every time the PZT drive voltage changes, the piezo element changes its dimensions (if not blocked). Due to the inertia of the PZT mass (plus any additional mass), a rapid change will generate a force (pushing or pulling) acting on the piezo. The maximum force is equal to the blocked force described by: F max = ± k PZT ΔL 0 Maximum force available to accelerate the piezo mass plus any additional mass. where ΔL 0 : Max. nominal displacement without external force or restraint [m] k PZT : PZT actuator stiffness [N/m] Tensile forces must be compensated for by a mechanical preload in order to prevent damage to the ceramics. Preload should be around 20 % of the compressive load limit, with soft preload springs soft compared to the PZT stiffness (1/10 or less). In sinusoidal operation with frequency f and the amplitude ΔL/2, peak forces can be expressed as: F dyn 2 = ± 4π m eff ΔL f 2 2 Dynamic forces on a PZT in sinusoidal operation with frequency f. 16

17 Where, F dyn : Dynamic force [N] m eff : Effective mass [kg] ΔL: Peak to peak displacement [m] f: Frequency [Hz] The maximum permissible forces must be considered when choosing an operating frequency. 4.8 Resonant Frequency In general, the resonant frequency of any spring/mass system is a function of its stiffness and effective mass. The resonant frequency given in the technical data tables always refers to the unloaded actuators. Fig PZT stack without mass - 2. PZT stack with mass The effective mass of an actuator fixed on one end is calculated for the two cases: mpzt 1. meff 3 mpzt 2. meff + M 3 Resonant frequency of an ideal spring/mass system can be determined by: f 0 = ½π k m PZT eff where f 0 : Resonant frequency [Hz] k PZT : Actuator stiffness [N/m] m eff : Effective mass (about 1/3 of the mass of the ceramic plus any installed end pieces) [kg] 17

18 Note: Due to the non-ideal spring behaviour of PZT ceramics, the theoretical result of the above equation does not necessarily match the real-world behaviour of a PZT system. When adding a mass to the actuator the resonant frequency drops according to the following equation: m eff f 0 ' = f 0 m + M eff The above equations show that increasing the mass on the actuator by a factor of 4 will reduce the response (resonant frequency) by a factor of 2. Increasing the spring preload on the actuator does not significantly affect its resonant frequency. 4.9 Response time Fast response is one of the desirable features of piezo actuators. A rapid drive voltage change results in a rapid position change. This property is necessary in applications such as switching of valves/shutters, generation of shock-waves, vibration cancellation systems, etc. A PZT can reach its nominal displacement in approximately 1/3 of the period of the resonant frequency with significant overshoot. T min f res (requires an amplifier with sufficient output current and rise time). 3 For example, a piezo with a 10 khz resonant frequency can reach its nominal displacement within 30 µs Heat Generation in a PZT at Dynamic Operation As mentioned before, PZTs are reactive loads and therefore require charge and discharge currents that increase with operating frequency. The thermal heat, P, generated in the actuator can be estimated with the following equation: P tanδ f C 2 U p-p Where P: Power converted to heat [W] tan δ: Tangent of the loss angle (ratio of parallel resistance to parallel reactance) f: Operating frequency [Hz] 18

19 C: PZT actuator capacitance [Farad] U p-p : Peak-to-peak drive voltage [V] For standard actuator PZT ceramics the loss factor is on the order of 0.01 to 0.02 (large signal conditions, smaller for small signal conditions). This means that up to 2% of the electrical power pumped into the actuator is converted to heat. Therefore, the maximum operating temperature can limit the PZT dynamics. For large amplitude and high frequency, operation cooling measures may be necessary. A temperature sensor mounted on the ceramics is suggested for monitoring purposes Mounting Guidelines Piezo actuators must be handled with care because the internal ceramic materials are fragile. 1. PZT stack actuators must only be stressed axially. Tilting and shearing forces must be avoided (by use of ball tips, flexible tips, etc.) because they will damage the actuators. 2. PZTs without internal preload are sensitive to pulling forces. An external preload is recommended for applications requiring strong pulling forces (dynamic operation, heavy loads, etc.). 3. Maximum allowable torque for the top piece can be found in the technical data tables for all PZT stacks and must not be exceeded. 4. When PZTs are installed between plates, a ball tip is recommended to avoid bending and shear forces. Failure to observe these installation guidelines, can result in fracture of the PZT ceramics Bonding Techniques 3 common methods exist to fix a PZT element to some substrate. The three principles do so are: Clamping Soldering Gluing Clamping Clamping is often found to be an unreliable method. The function of the PZT element is of course to move it is very difficult to stop the minute vibrations that will sometimes allow the element to walk out of the clamp Soldering Soldering has the advantage of giving a conductive connection, however, a drawback is that the movement will cause fatigue in the bond. 19

20 Gluing Gluing is usually the optimum approach - modern epoxy- or acrylate glues, in particular, provide strong yet flexible joints between adjacent surfaces. Three is no fatigue and operating temperature up to 150 C are no problem for some of these glues. Most popular are the hot setting, however, if a given application prohibits this, or where fast curing is essential, alternatives such as general-purpose epoxies can be used with good results. Electrical connection between the substrate and the electrode on the PZT is nearly always required. One possibility is to use conductive glue, usually an epoxy filled with Ag particles, or silicones or BMI filled products. A drawback of these glues is that they are so heavily loaded with conductive particles that the glue line is often weak. Additionally, the fact that the glue has 3-dimensional conductivity can lead to short-circuits. Another way is to roughen one of the surfaces to be glued if the glue is cured under a pressure of approximately 10 5 Pa (10 5 N/m 2 ), many point contacts between the surfaces will be formed. This method, however, is not very reproducible. A better way is to mix nickel powder of a welldefined particle diameter of about 10µm into the glue ( 10% by weight). Again pressure during curing must be applied. Since the nickel particles are almost ideal spheres (carbonyl-nickel process) they tend to form a monolayer between the surfaces. Due to the pressure, thousands of point contacts will create a reliable electrical connect. Only around 2% of the surface area is occupied by the nickel spheres so the quality of the actual glue line is hardly influences. Moreover, the volume percentage of the nickel particles is so low that there is no risk of short circuit by excess of glue this is a real one dimensional conductive glue. 5. Environmental Considerations 5.1 Application of PZTs in High Humidity Atmosphere If insulation material (coating) is used for PZT stacks, this can create problems regarding sensitivity at elevated humidity atmospheres. Additionally, it is recommended that the PZTs are operated below the full field strength of 3kv/mm when humidity is above 75%RH to prevent dielectric breakdown, even though coating is not used. 5.2 Application of PZTs in Inert Gas Atmosphere Piezo actuators can be damaged if operated at maximum drive voltage in a helium or argon atmosphere. To reduce the risk of dielectric breakdown, the PZTs should be operated at minimum possible voltage. 20

21 5.3 Vacuum Application of PZTs When piezo actuators are used in a vacuum, two factors must be considered: 1) dielectric stability 2) outgassing The dielectric strength of a gas is a function of pressure. Air displays a high insulation capability at atmospheric pressure and below 10-2 Torr. However, in the range from 10 to 0.01 Torr (corona area) insulation properties are degraded. PZTs should not be operated in this range because an electric breakdown may occur. Outgassing (of the insulation material and the lead wires) may limit their use in applications where contamination or virtual leaks are an issue. 6. Lifetime of PZTs The lifetime of a PZT is not limited by wear and tear. Tests have shown that PZTs can perform BILLIONS of cycles without loss of performance if operated under suitable conditions. Generally, as with capacitors, the lifetime of a PZT is a function of the applied voltage. The average voltage should be kept as low as possible. There is no generic formula to determine the lifetime of a PZT because of the many parameters such as temperature, humidity, voltage, acceleration, load, operating frequency, insulation materials, etc. which have an (nonlinear) influence. Statistics show that most failures with piezo actuators occur because mechanical installation guidelines are not observed and mechanical stress, shear forces and torque exceed the permissible limits. Failures can also occur when humidity and conductive materials such as metal dust degrade the PZT's insulation strength, leading to dielectric breakdown. 21

22 Multilayer Actuators 7. Multilayer PZT Piezoelectric Materials Manufacturing Most commercial piezoelectric materials are based on Lead Zirconia Titanate - PZT - polycrystalline ceramics. These materials are produced in form of fine-grained powders and may be shaped into useful configurations by different processes like pressing, tape casting & laminating, screen printing, sputtering etc. These methods of shaping materials differ in complexity - some are very easy like pressing whereas tape casting & laminating and screenprinting are more complex to establish. Bulk PZT Manufacturing Bulk PZT materials are commonly manufactured by uniaxial pressing of a PZT powder. Pressed parts, typically plates, discs, rings or tubes are then fired to final density. Extrusion is also used for manufacturing of long tubes. Bulk PZT s has been manufactured for more than 30 years and several manufactures. Multilayer PZT Manufacturing During the 1980 ies a new generation of piezoelectric products was developed - the multilayer PZT. Here the PZT material is subdivided by a large number of internal conductive electrodes allowing substantially lower operating voltage for similar or better performance as compared with bulk PZT. The manufacturing process for multilayer PZT is based on mixing the PZT powder with solvent and additives creating a slurry for tape casting very thin sheets. Electrodes are then printed onto these thin ceramic sheets, which are then stacked, laminated and fired in order to create a laminated co-fired PZT material with internal electrodes. This process is far more complicated than the uniaxial pressing used for bulk PZT and only a handful of PZT manufacturers have developed production facilities for multilayer piezo. 7.1 The Manufacturing Process Noliac s multilayer piezo production consists of 18 different processes, which over the last 5 years have been optimised individually to the current advanced state of manufacturing accuracy and flexibility. Process Diagram A flow diagram of the manufacturing processes involved with piezo multilayer processing at Noliac is shown below. Following that each process is described. When a process is indicated by # it means that there is further information on Noliac unique Production Advantages in the subsequent section. 22

23 1 Mixing 10 Edge trimming 2 Milling 11 Laminating 3 Tape casting # 12 Cutting # 4 Stripping 13 Firing # 5 Tape cutting 14 Sintring # 6 Build up # 15 Termination # 7 Stacking # 16 Polarisation # 8 Printning # 17 Inspection # 9 Drying 18 Packing Mixing PZT powder is mixed with organic additives and solvents all accurately weighed to specific and well-defined ratios Milling The mixed materials are milled to form a very homogeneous suspension with a specific particle size distribution. After mixing, the viscosity is checked and the suspension is approved for tape casting Tape Casting # The suspension is tape casted on a carrier foil and dried. The casting machine can be adjusted to obtain specific ceramic tape thickness in the range 10μm - 40μm. The ceramic tape contains a large amount of organic binder materials making the tape flexible and easy to handle in the following processes. 23

24 7.1.4 Stripping The ceramic tape is removed from the carrier foil and wind up ready for cutting Tape Cutting The ceramic tape is cut into pieces of same size as the carrier blocks used in the subsequent processes Build Up # Build up covers the repetition of three processes, stacking, printing and drying. Each process is repeated a number of times according to the number of internal electrodes required in the product. A metal carrier block is used as support for the ceramic build up throughout the process. The output of the build up is an unfired green ceramic block with internal electrodes Stacking # Layers of ceramics are placed on the carrier blocks before and after each print. The carrier-block is placed in a press and each new ceramic layer is pressed gently against the underlying ceramic layers to adhere the layers to each other Printing # Internal electrodes having predetermined patterns are printed on the top of the ceramic layer of the carrier-block. A suitable organic paste mixed with metal particles, e.g. platinum, is used for printing Drying The printed electrode is dried in a tunnel oven. Upon completion of the drying process the entire build-up procedure is repeated, stacking, printing and drying, according to the specifications of the current production batch Edge Trimming After the build up procedure it is necessary to trim the edges of the blocks so that they fit precisely into the lamination tooling Lamination The ceramic blocks are laminated at well-defined pressure, temperature and time. At this process it is ensured that all layers (up to 200 layers) are firmly laminated and brought in appropriate 24

25 contact with each other. This process is very critical as both insufficient lamination or too much lamination has significant influence on the quality and accuracy of components Cutting # After lamination individual components are cut out of the ceramic blocks. Cutting needs to be accurately matched to the pattern f internal electrodes. Cutting might also be performed on fired parts rather than on green parts (green parts = parts still not fired) Firing # The organic compounds in the system, binder materials, additives and remaining solvents are removed by very slowly heating the green parts to a certain temperature, where the organic compounds decompose and evaporate from the ceramic Sintering # In order for the ceramic particles to grow and merge into a solid polycrystalline structure the fired parts need to be heated to a temperature well above 1000 C. The sintering is achieved in a protecting atmosphere in order to prevent lead loss from the components. Several parameters have to be optimised with this process, e.g. temperature ramp rates, holding time and the condition of protective atmosphere Termination External electrodes are applied to connect to the internal electrodes. External electrodes are commonly printed silver or gold, which after printing are fired onto the ceramic surface. Alternative methods of electrode deposition is brushing, evaporating, sputtering and electroless plating Polarisation In order to obtain the piezoelectric effect in the PZT material the dipoles in the grains must be aligned. This alignment is achieved in the poling process where a high electrical field, is applied to the external electrodes at elevated temperature for a certain period of time Inspection Parts undergo a final inspection that might, according to customer specifications, be based on statistical methods or 100% control. Various mechanical and electrical parameters might be tested. Mostly, dimensional tolerances, dielectric constant, dielectric loss, impedance spectrum and piezoelectric charge coefficients (d constant) are specified for inspection. 7.2 Unique Production Advantages 25

26 A number of the process steps involve specific experience and know-how in order to produce high accuracy components. This know-how is critical Noliac confidential information and certain aspects cannot be disclosed in detail. Tape Casting Equipment and processes have been adapted to be capable of producing homogeneous repeatable quality of thin ceramic tapes down to 10µm in thickness with a narrow thickness tolerance. Significant know-how is involved with the optimisation of binder systems and additives. Stacking - Printing Building homogeneous and very thin laminates down to µm with internal layers and customised external electrodes on top and bottom requires a set-up with a sequence of individually optimised processes and equipment. Noliac has developed special processes and equipment features enabling accurate manufacturing of complex thin structures including high repeatability between prints in each block and between blocks in a production batch. Cutting Separating the individual parts from the blocks must be carried out with very high accuracy in order to maintain tight tolerances of the produced parts - i.e. location of internal and external electrodes. Noliac has developed methods based on custom specified dicing equipment that allows for accurate dicing without damaging parts in the process. Firing & Sintering Firing of piezoelectric materials is carried out in a two-step firing process. First, very slow heating during which the organic materials decompose. Control of heating ramp rates and control of atmosphere is crucial for final product quality. Subsequent heating in a protective atmosphere is required to obtain final density of the components. Significant experience and know-how is required to obtain a high quality product as ramp rate, holding time & protective atmosphere all have significant influence on the electromechanical properties and geometrical tolerances as well as product quality and reliability. 26

27 8. Noliac actuating products At Noliac piezoelectric PZT and electrostrictive PMN ceramic powders are manufactured into several multilayer products. The advantage of the multilayer technology, when compared to the older bulk type single layer technology, is that substantially lower voltage is required to achieve useable piezoelectric activity. Another important advantage is that multiple functions may be integrated in one device, e.g. combination of actuator and sensor functions or combination of actuator and generator functions. The basic type of actuators developed at Noliac include: Abbreviation Full name CMA Ceramic Multilayer Actuator CMA-tilt Ceramic Multilayer Actuator tilt Linear mode SCMA Stacked Ceramic Multilayer Actuator displacement actuators ASCMA Amplified Stacked Ceramic Multilayer Actuator CMB-P Ceramic Multilayer Bender Plate Bending mode displacement actuators CMB-2D CMB-R Ceramic Multilayer Bender 2 dimensional Ceramic Multilayer Bender Ring Table 1. Noliac actuating products 8.1 CMA - Ceramic Multilayer Actuator CMA s are single cofired multilayer ceramics typically with a thickness up to 2-3mm and with up to 100 ceramic layers. CMA s can be made very small, e.g. 1,25mm x 1mm x 0,2mm, and in various geometries like squares, rectangles, rings and discs. Typical performance range is up to 3μm stroke and up to 20kN blocked force. CMA s are used in numerous applications within e.g. IT (hard disc drives), optics, telecommunication, instrumentation and nano positioning. 27

28 Fig. 8 - Examples of CMA s 8.2 CMA - tilt The design of the internal electrodes enables control of both sides individually, which allows the component to create movements on one side only creating an angle between each side. It is possible to design the CMA-T with 3 active sections in order to create a wave like movement or create adjustment in a fully three-dimensional spectrum. 8.3 SCMA - Stacked Ceramic Multilayer Actuator SCMA s are stacks of two or several CMA s glued together. The purpose of the stacking is to obtain an actuator with more displacement than can be achieved by a single CMA. As for CMA s, SCMA s may be manufactured in various geometries like squares, rectangles, rings and discs. A practical upper limitation of the length of SCMA s is 100mm. Typical performance range is up to 150μm stroke and up to 20kN blocked force. SCMA s are used in numerous applications within e.g. fuel injection, instrumentation and micro positioning. 28