TECHNICAL PAPER

Smart metal sheets by direct functional integrationof piezoceramic fibers in microformed structures

A. Schubert • V. Wittstock • H.-J. Koriath •

S. F. Jahn • S. Peter • B. Muller • M. Muller

Received: 4 April 2013 / Accepted: 30 May 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract Microassembly of piezoceramic fibers in micro

cavities at the surface of sheet metal is a novel approach for

high volume production of smart adaptronic structural

metal parts. In this paper a technology, manufacturing

processes and characterization results of metal sheets with

an active piezo-metal substructure based on directly inte-

grated piezoceramic fibers are described. The processes

include micro-milling of cavities in sheet metal, precision

grinding of piezoceramic fibers, PECVD insulating layers

with high dielectric strength, microassembly and force-

locked joining by forming. In experiments, measurements

of the surface roughness and geometric parameters of the

piezoceramic fibers and micro cavities were performed.

Further, the electrical properties of the insulation coatings

were measured. The sensor function of the piezo-metal

substructure was proven by a mechanical excitation

resulting in a proportional measured sensor signal.

1 Introduction

Smart metal structures production is a key for numerous

future products aiming for increased resource efficiency,

safety and performance. Smart structures are passive

structural parts which are enlarged by functional integra-

tion of distributed sensors and actuators in combination

with power electronics and innovative control strategies. A

high-level integration of miniaturized sensors and actuators

leads to smart structures with no significant change of mass

and structural stiffness compared to their passive equiva-

lents (Monner and Wierach 2005). In most smart structure

applications, lead zirconate titanate (PZT) piezoceramic is

used as active material. This is due to its high stiffness on

the order of the stiffness of light weight structures, for

example made from aluminum alloys and the high stresses

that can be generated in actuator mode. Moreover, vibra-

tions in a wide range of frequencies from a few Hz to

several MHz can be measured using the direct piezoelectric

effect or can be generated using the inverse piezoelectric

effect (Williams et al. 2002). Piezoelectric transducers can

further be used to harvest energy from structural vibrations

(Sodano et al. 2006) in order to supply power in autono-

mous sensor and actuator networks.

Major applications of smart structures are active vibra-

tion control and structural health monitoring (SHM).

Reduced sheet metal thicknesses for light weight con-

struction lead to problems due to structural vibrations of

the parts which can be overcome by distributed sensor-

actuator networks within the structure (Freymann 2001). In

automotive applications, adaptive car roofs for active

acoustic design and active vibration control in convertibles

have already been investigated (Monner and Wierach

2005). Further, there is interest for smart structures in the

field of non-destructive evaluation (NDE) (Bowen et al.

2008) and SHM (Mayer et al. 2008) of non-planar parts.

The broad application of SHM is still limited by the lack of

appropriate permanently integrated distributed micro sen-

sors and actuators which are able to generate and detect

A. Schubert � V. Wittstock � S. F. Jahn � B. Muller �M. Muller (&)

Institute for Machine Tools and Production Processes,

Technische Universitat Chemnitz, 09107 Chemnitz, Germany

e-mail: wzm@mb.tu-chemnitz.de

A. Schubert � H.-J. Koriath

Fraunhofer Institute for Machine Tools and Forming Technology

IWU, Reichenhainer Str. 88, 09126 Chemnitz, Germany

S. Peter

Institute of Physics, Technische Universitat Chemnitz,

09107 Chemnitz, Germany

123

Microsyst Technol

DOI 10.1007/s00542-013-1836-6

vibrations with high frequency and low amplitude, e.g.

lamb waves (Achenbach 2009).

The functional integration of active materials in sheet

metal provides direct coupling of the sensors and actuators

to the structure. Distributed sensor and actuator networks

enable the characterization and actuation on the level of

both small areas and the whole structure. By means of

functional integration a reduction of product weight and

cost along with superior sensor and actuator performance

compared to current products can be achieved.

In the following the state of the art production tech-

nologies for piezo-metal composites are discussed. After

that, the basic concept of the piezo-metal composites with

directly integrated piezoceramic fibers is described.

Finally, the results of the experimental investigations, the

setup of the prototypes and the test of the prototypes are

presented.

2 State of the art

2.1 Piezoelectric effects

State of the art technologies for functional integration of

piezoceramic transducers in smart structures are based

upon pre-packaged transducers. Piezoelectric transducers

for function integration in sheet metal structures are com-

monly foil-like transducers characterized by high flexibility

for attachment to curved structures. By pre-packaging, the

piezoelectric elements are embedded between foils in an

epoxy resin. Thereby, the piezoelectric elements are either

monolithic piezoceramic wafers (GmbH 2007) or com-

posites made of piezoceramic fibers embedded in an epoxy

resin. Well-established types of composite actuators are

macro-fiber-composites (MFC) with rectangular fibers cut

from wafers (Williams et al. 2002) and active-fiber-com-

posites (AFC) with round fibers extruded from slurry

(Rosetti et al. 2000). Due to limited crack propagation

within the composite and higher flexibility of fibers com-

pared to monolithic wafers, fiber composites generally

offer superior properties in terms of achievable bending

radii and fatigue behavior compared to monolithic wafers.

In fact, pre-packaged transducers offer advantages

compared to bare monolithic piezoceramic elements. The

transducers are completely electrically insulated and can

easily be interconnected by large contact pads (Monner and

Wierach 2005). However, due to softening of the embedding

polymers well below the glass transition temperature, there

are significant changes in the properties of pre-packaged

transducers at temperatures above 80 �C (Kobayashi et al.

2006). Further, delamination is a general problem in terms of

long term stability (Bowen et al. 2008).

There are three piezoelectric effects that can be applied

in flexible piezoelectric transducers as depicted in Fig. 1.

The transverse effect is characterized by contraction of the

transducer in actuator mode. Flexible transducers made

from monolithic wafers commonly use this principle. The

full area electrodes on top and bottom of the wafer are easy

to manufacture. A high capacity and therefore high

impedance can be achieved as this is controlled by the

thickness of the piezoelectric wafers (Sodano et al. 2006).

Transverse effect transducers are well suited for energy

harvesting, sensors and low strain actuators (Monner and

Wierach 2005).

In contrast, longitudinal effect transducers elongate in

actuator mode. They require electrode patterns to achieve

an electric field in the direction of the elongation. Com-

monly, interdigitated electrodes (IDE) are applied to the

surface of a piezoceramic fiber composite. The spacing of

the IDE determines the required driving voltage for large

signal actuation. Due to the resulting highly inhomoge-

neous electric field, the minimal electrode spacing is lim-

ited by high local electric fields and as a result driving

voltages larger than 1,000 V are typically required for large

signal actuation. As these transducers have low impedances

they are less qualified for sensor mode and energy

Transverse effect (d31) Longitudinal effect (d33) Shear effect (d15)

3

2

1

Δl −U

l0

P

electrodes

F

S 1,T1

1

3

2

Δll0

U

electrodes

S3,T3

PF

1

3

2

U

electrodes

FS5,T5

Δll0

P

b h h h

b b

++ −

+

−

d31 < d33 < d15

Fig. 1 Piezoelectric effects used in pre-packaged flexible piezoelectric transducers

Microsyst Technol

123

harvesting but they enable high strain actuation. It should

be noted that the piezoelectric charge constant d33 is

commonly twice as large as d31 for the transverse effect

leading to higher maximum forces in actuator mode and in

principle also higher charges to be generated in sensor

mode of longitudinal effect transducers compared to

transverse effect transducers.

The third type is the less established shear effect

transducer. These transducers are primarily applicable as

actuators for smart structures, whereby the top of the pie-

zoelectric element is shifted with regard to the bottom of it.

Due to very large charge constants d15, particularly in large

signal mode, the achievable shear strains exceed the ones

achievable in longitudinal and transverse mode.

2.2 Application and limitations

Both transverse and longitudinal effect transducers are

commonly placed on the surface of sheet metal structures

by means of manual or automated adhesive bonding. The

use of partly cured adhesive even allows forming of the

structure after bonding the piezoelectric transducer on top

or between two layers of sheet metal (Drossel et al. 2009).

Shear actuators are practically placed between double

layers as a matter of the shearing principle (Edery-Azulay

and Abramovich 2006; Benjeddou and Deu 2005).

Numerical investigations state that the bonding layers can

be neglected for large signal actuation (Nguyen and

Kornman 2006). However, it is obvious that elastic poly-

mer layers are damping low amplitude and high-frequency

vibrations as required for example in SHM applications.

In conclusion, state of the art transducers are pre-pack-

aged and therefore limited with regard to the miniaturiza-

tion of the transducers. The coupling between the

transducer and the structure is through elastic interlayers

leading to damping of transmitted vibrations and to the

risk of delamination. To overcome these limitations, this

paper proposes the use of bare piezoceramic fibers instead

of pre-packaged transducers. This will enable further

miniaturization of piezoelectric transducers without dete-

riorating the sensor and actuator performance.

3 Direct integration of piezoceramic fibers

The direct functional integration of piezoceramic fibers is a

fundamentally different approach compared to the adhesive

bonding of pre-packaged transducers, which is illustrated

in Fig. 2. Piezoceramic fibers are placed into micro cavities

at the surface of a sheet metal without the need for prior

packaging.

Instead of bonding the pre-packaged transducer on top

of the metal, the piezoceramic fibers are within the sheet

and coupled to the metal without elastic interlayers. The

piezoceramic fibers have a Young’s modulus of about

50 GPa, which is only about 30 % less than the aluminum

sheet metal and one order of magnitude larger than for

most polymers, e.g. epoxy resin. Therefore, the coupling is

much stiffer when compared to adhesively bonded piezo-

electric transducers.

Figure 3 illustrates the assembly of the piezoceramic

fibers. Practical dimensions of the piezoceramic fibers are

wfiber = 260 lm, dfiber = 270 lm and lfiber = 10 mm.

Given nominal cavity dimensions in the sheet metal of

wcav = 300 lm, dcav = 300 lm and lcav = 10 mm, an

assembly clearance of 20 lm on each left and right side of

the fibers results. The web between two cavities is 200 lm

wide and the pitch between the piezo-fibers in the structure

follows as 500 lm. In a piezo-metal substructure within a

1.5 mm thick metal sheet, the resulting volume fraction of

the active piezoceramic material is as large as 9 % of the

structure. The active material is thereby located far-off the

neutral axis for efficient actuation and sensing. Mechanical

and electrical coupling and therefore functional integration

of the piezo-fibers in the sheet metal is achieved by a

subsequent forming process recently described in (Schubert

et al. 2010). Furthermore, the joining by forming process

provides the required pretension of the piezoceramic fibers.

piezoceramic fibers

shaped sheet metal

interdigitated electrodes (IDE)

adhesive layers

center interface electrodes (CIE)

polymer foil

micro-cavitiesin sheet metal

Fig. 2 Principles of adhesive

bonding of MFC on top of a

metal sheet (left) and direct

functional integration of

piezoceramic fibers in the

surface of the sheet metal (right)

Microsyst Technol

123

Similar as for the pre-packaged transducers, three basic

configurations of the directly integrated piezo-fibers are

possible. Figure 4 (left) shows the transverse effect con-

figuration applicable for simple sensor-mode transducers or

energy harvesters. Electrodes are placed on top and bottom

of the fiber. Though this type is easy to manufacture,

performance would be limited due to the low piezoelectric

charge constants d31 and in actuator mode the fibers would

contract and loose the electromechanical contact.

Shear effect transducers can also be set up by means of

direct integration as shown in Fig. 4 (right). The basic

configuration is similar to the one of the transverse effect

but poling is transverse to the thickness direction.

The center of Fig. 4 illustrates the longitudinal effect

configuration, which is preferred in this paper. The piezo-

electric d33-effect provides for both high charges generated

by strains resulting from deformations of the sheet metal

and high strains generated in actuator mode. Comparable to

piezoelectric stack actuators, the piezoceramic fibers are

arranged in an electrical parallel circuit and mechanical

series connection. Thereby, the ground electrode is formed

by the basis material and the signal is generated at the

center interface electrode (CIE). Short circuits between the

center electrode and the carrier sheet are prevented by

the application of an insulating layer on the bottom of the

cavity. This paper focusses on the manufacturing and

experimental investigation of the longitudinal configura-

tion in a prototype with ten piezoceramic fibers.

4 Experimental

The processes used for the fabrication of piezo-metal

substructures are illustrated in Fig. 5. Piezoceramic plates

with sputter electrodes and aluminum sheet metal are the

raw material and functional piezo-metal substructures in

sheet metal are the final product.

4.1 Micro cavities in an aluminum sheet carrier

The micro structuring of the sheet metal with micro cavi-

ties as required for the direct integration of piezoceramic

fibers can be performed either by subtractive technologies

like micro-milling or non-subtractive technologies like

micro forming. For the first prototypes, micro-milling was

chosen because of the flexibility in terms of the dimen-

sions. The process achieves cavities with steep flanks and

sufficient dimensional and form accuracy of the structures.

Micro cavities were micro-milled in an aluminum sheet

metal made of EN AW5182 (AlMg4.5Mn0.4), which is a

common car body alloy.

Sample sheets were prepared with each ten parallel

micro cavities and two perpendicular areas for electrical

interconnection of the fibers. Figure 6 shows a scanning

electron microscope (SEM) image of the structure with

micro cavities in the surface of the metal sheet. The

geometry of the cavities was measured using a Nikon MM

400 optical measuring microscope. As an example,

wfiber

l fiber

d fib

er

x

yz

microassembly

piezo

ceram

ic

fiber

micro-cavitiesin sheet metal

d she

et

d cav

wcav

l cav

electro-mechanicalinterface betweenmetal and piezo

sheet metalthin film electrodeor insulation layerpiezoceramic fiber

Fig. 3 Principle of microassembly of piezoceramic fibers in micro-

structured sheet metal

Transverse effect (d31) Longitudinal effect (d33) Shear effect (d15)

12

3S1,T1

S 2,T2

P +−

PP

32

1S3

32

1

S5,T5

P

5T3 +−

+−

PZTPZT

aluminu

malu

minum

CIE

d31 < d33 < d15

Fig. 4 Configurations of

directly integrated piezoceramic

fibers for piezoelectric

transverse, longitudinal and

shear effect (from left to right)

Microsyst Technol

123

dimensions of micro cavities according to the nomencla-

ture in Fig. 3 within one sample sheet were wcav =

301 lm, dcav = 312 lm, and lcav = 10 mm, whereby the

range of the width was 5 lm and the range of the depth was

7 lm.

Figure 7 shows a close-up SEM image of the micro

cavities, in particular the top of the webs, the cavity side-

walls and the bottom of the cavities. The different surface

roughness of these areas is visible. Between the cavity

sidewalls and the piezoceramic fibers a large contact area is

desired to set up a stiff mechanical coupling during joining

by forming. A prerequisite to achieve that close contact is a

minimum surface roughness at the cavity walls. An average

surface roughness of Ra = 0.36 lm and Rz = 2.20 lm has

been measured on the cavity walls using a laser micro-

scope. The surface roughness of the bottom of the cavity

influences the properties of the subsequent insulating

coating for preventing shorts and is therefore crucial for the

function of the piezoelectric sensor or actuator module.

The roughness of the bottom of the cavities was determined

to be in the range of Ra = 0.27 lm and Rz = 0.35 lm,

which is expected to be sufficiently smooth for electrical

insulation.

4.2 Insulation of the aluminum sheet

The electrical insulation of the CIEs of the piezoceramic

fibers against the micro-structured sheet metal is one of the

most critical issues for the direct integration of piezoce-

ramic fibers in sheet metal. The electrical insulation layer is

applied in a plasma enhanced chemical vapor deposition

(PECVD) process either on the piezoceramic fibers or the

sheet metal. Because the handling of the single piezoce-

ramic fibers is challenging, the metal sheet was coated for

the fabrication of prototypes. The effectiveness of the

insulation depends on the chosen material, the deposition

parameters and the above described roughness of the bot-

tom of the cavities.

Insulation layers were fabricated using a vacuum

deposition system MicroSys400 (Roth & Rau). A hydro-

genated silicon carbonitride film SiCN:H was deposited in

a PECVD process from trimethylsilane (SiH(CH3)3;

‘‘3MS’’) in mixture with nitrogen, hydrogen and argon

using 13.56 MHz RF discharge. PECVD of hydrogenated

silicon carbonitride (SiCN:H) films is capable of coating

joining byforming

microassembly ofpiezoceramic fibers

dicing of piezo-ceramic fibers

aluminumsheets

functional piezo-metal substructure

milling ofmicro-cavities

PECVDSiCN:H

piezoceramic plates

bonding ofdouble-layer plates

p, ϑ, t

p, t

Fig. 5 Process chain for direct integration of piezoceramic fibers in micro cavities at the surface of aluminum sheet metal parts

Fig. 6 SEM picture of micro cavities milled in aluminum sheet metal Fig. 7 Magnified SEM picture of micro cavities illustrating the

surface roughness of the cavity sidewalls, bottom of cavities and top

of webs

Microsyst Technol

123

cavities with vertical sidewalls. A layer thickness of

2.7 lm was earlier achieved on trenches in silicon, which

are comparable to the micro cavities in the sheet metal with

regard to size and shape. The ratio of the layer thickness on

the walls to the layer thickness on the bottom can be

controlled by process parameters (Peter et al. 2010, 2013).

For these experiments, minimal coating of the sidewalls

is required in order to generate an electrical contact of the

piezoceramic fiber and the metal. Thereby, significant

differences with regard to the layer thickness on top of the

webs and on the bottom of the cavities have been

observed. On flat areas outside the micro cavities the

insulator thickness was 4.5 lm and hence about 35 %

larger than the 3 lm layer thickness at the bottom of the

micro cavities.

The performance of the insulating layer is particularly

defined by the dielectric strength. Typical breakdown field

strengths for SiCN:H are above 100 kV/mm. Therefore, the

layer thickness of 3 lm theoretically allows an operation

voltage of the piezoceramic fiber of 300 V, which would be

sufficient for sensor and actuator operation of piezo-metal

substructures. Breakdown voltage measurements were

conducted for numerous samples by positioning a

V-shaped electrode at the center of the bottom of the

cavities and applying a variable voltage ranging from

0 to 1,100 V in 5 V increments (voltage source Keithley

2410). Breakdown voltages in the range of 600 V have

been measured at the bottom of the cavities, which is even

higher than expected. The breakdown voltage of the

SiCN:H films on top of the webs even exceeded values of

1.1 kV.

4.3 Piezoceramic fibers with center interface electrodes

In order to manufacture piezoceramic fibers with a CIE,

piezoceramic plates (M1100, Johnson Matthey Catalysts)

were bonded by eutectic soldering using a Sn42Bi58 alloy

to prepare double-layer plates. The interface layer is

required for driving the fibers in 3-direction and further

decreases the brittleness of the plates and fibers.

The plates were machined by lapping to achieve the

desired thickness of 260 lm, which represents the width of

the fiber wfiber. Figure 8 compares the surfaces of the

machined piezoceramic. The grinded sample (as received

from manufacturer) shows a significant roughness and small

grains are visible. The lapped piezoceramic is much

smoother, though grain pullout is visible. The best surface is

achieved by CMP polishing. Thereby, surface roughness

values as of Sa \ 0.02 lm and Sq \ 0.03 lm were observed.

Piezoceramic fibers with dimensions of 0.26 mm 9

0.27 mm 9 10 mm and CIE were cut from the piezoceramic

plates using a peripheral wafer dicing saw (Logitech APD12).

In order to achieve rectangular shaped fibers without burr, the

plates were bonded to a lapped glass substrate using acetone

soluble glycophilic wax instead of wafer tape. Cutting was

performed with a 0.070 mm thick dicing blade (D46) at a

cutting speed of 26 m/s and feed rate of 0.1 mm/s.

4.4 Assembly and joining by forming

of the piezo-fibers in metal

The piezoceramic fibers with CIE finally need to be

assembled to the insulator coated micro cavities

Fig. 8 Surfaces of piezoceramic after machining by grinding, lapping and CMP polishing

excessheight

electrical andmechanical interface

assemblyclearance F F

Fig. 9 Principle of joining by

forming for functional

integration of the piezoceramic

fibers

Microsyst Technol

123

sequentially. Due to the size and the brittle nature of the

piezoceramic fibers, gripping with tweezers-type grippers

and vacuum grippers is not feasible. Automatic handling

and visually servoed assembly of single piezoceramic

fibers was proven to be successful by the use of transparent

planar electrostatic grippers (Neugebauer et al. 2010) and

soft polyurethane van-der-Waals force grippers (Matope

et al. 2013). The actual assembly clearances required for

inserting the fibers are influenced by the clearance fit and

the deviations of the width of the piezoceramic fibers and

the micro cavities.

Figure 9 illustrates the joining by forming process. The

webs are plastically deformed using a planar die. This

results in an electrical and mechanical interface between

the piezoceramic and the sheet metal. Thereby, the ratio of

the excess height to the assembly clearance is important in

order to guarantee that the assembly clearance is filled

properly (Schubert et al. 2010). The excess height is

calculated from the difference of the cavity depth

dcav = 312 lm and the height of the fiber dfiber = 270 lm

resulting in 42 lm for this experiment. The assembly

clearance is the difference of the cavity width

wcav = 301 lm and the width of the fiber wfiber = 260 lm

and resulted in 41 lm for the prototype.

The joining by forming process was carried out in a

hydraulic force-controlled press applying a maximum load

of 12 kN to the planar die. This corresponds to an average

stress of 545 N/mm2 on the top of the webs between the

cavities. Figure 10 shows a box-whisker plot of the mea-

sured results of the cavity width before and after joining by

forming. The assembly clearance is closed and a mean

width of the cavities of wcav,joined = 260 lm eventuated.

This width is determined by the width of the piezoceramic

fibers. The larger variation of the cavity width after joining

compared to the initial cavity width can be explained by the

variation of the thickness of the piezoceramic fibers. It is

expected that this effect can be regarded as an averaging of

dimensional errors since the variations of the width of the

piezoceramic fibers are reflected in the joined cavity width.

Figure 11 shows SEM images of joined piezoceramic

fibers in the microstructured sheet metal. The magnification

on the right image shows that the high stress during the

forming process caused small fractures (circles) within the

piezoceramic fibers, which have been described earlier in

(Schubert et al. 2010). It should be noted that eccentricities

of the CIE result from inaccuracies due to the manual

machining of the piezoceramic double-layer plates. Fur-

ther, it can be seen from Fig. 11 that piezoceramic fibers

are clamped by the deformed webs. The close contact

between the piezoceramic fibers and the cavity sidewalls is

more clearly observable from Fig. 12, which shows a

micrograph of a polished section depicting the interface

between piezoceramic and metal.

4.5 Setup of piezo-metal sensors

After joining by forming, the piezoceramic fibers with CIE

were conformal coated with an insulating varnish (RS 199

310

300

290

280

270

260

250

240after millingand PECVD

after joiningby forming

cavi

tyw

idth

wca

v/µm

meanassemblyclearance

Fig. 10 Principle of joining by forming for functional integration of

the piezoceramic fibers

Fig. 11 SEM images joined piezoceramic fibers in the micro cavities, left overview, right micro-cracks

Microsyst Technol

123

1480, RS Components) and electrically interconnected in

parallel by use of a silver-filled conductive paint (RS

186-3593, RS Components). The vias through the confor-

mal coating were set up by manually removing the con-

formal coating so that a connection of the CIE was

achieved by the conductive paint. Figure 13 shows the

setup of the whole piezo-metal substructure and two fibers

in detail.

The breakdown voltage was measured for the piezo-

metal sensors in order to justify the dielectric strength of

the insulation layer at the bottom of the cavities. Break-

down voltages ranging from 170 to 280 V have been

measured for the modules with integrated piezoelectric

fibers. This deteriorated performance is likely to be caused

by pinholes generated either due to roughness peaks in the

micro cavities, inhomogeneous deposition of the insulating

layer or defects resulting from the plastic deformation

during the joining of the piezoceramic fibers and the micro

structured sheet metal. However, this dielectric strength is

sufficient for sensor mode operation and would also be

sufficient for small signal actuation.

For function tests, the piezo-metal sensor was connected

to a charge amplifier (Type 2635, Bruel & Kjaer). The

metal sheet was clamped on one side and mechanically

deflected by a piezoelectric shaker (PI P-611:3S Nano

Cube) driven by an amplified sinusoidal signal (see

Fig. 14). The deflection u causes stress and strain within

the sheet metal and the piezoceramic fibers. For a 1.5 mm

thick sheet the average stress within the piezo-fibers is

0.24 N/mm2 per 10 lm deflection at the tip.

The frequency of the periodic deflection was swept

from f = 0.1 Hz to f = 280 Hz for the experiments.

The first mechanical resonance of the system was found at

f = 257 Hz. In Fig. 15 the charges generated by the piezo-

metal sensor module are plotted in comparison to the

Fig. 12 Interface between piezoceramic and metal

parallel electricalinterconnection

joined piezo-ceramic fibers

CIE

CIE

PZT

PZT

PZT

PZT

aluminum webwith SiCN:H layer 250 µm

2 mm

conformalpolymer coating

aluminum webwith SiCN:H layer

Fig. 13 Photograph of setup of prototype piezo-metal sensor

+−

V

PP

ua

GND

Charge Amplifier Type 2635 (Bruel & Kjaer)

u(t)=u0 · sin(2πft + φ0)

clampingsheet metal withmicro-cavities

piezoceramicfibers with CIE

Fig. 14 Experimental setup for the test of the piezo-metal sensor

Microsyst Technol

123

exciting sinusoidal deflection measured by a high resolu-

tion laser triangulation sensor. It can be seen from the

graphs that the sensor signal is proportional to the

mechanical excitation with high repeatability for both

frequencies of 10 and 250 Hz. This proves the effective

electromechanical coupling of piezoceramic and sheet

metal.

In fact, the charges generated by the module are small.

This can be explained by the partial fracture of the piez-

oceramic fibers and therefore loss of active material of the

sensor. Furthermore, the CIE was partly inhomogeneous

due to the manual manufacturing process.

5 Conclusion

The feasibility of the direct functional integration of

piezo-fibers in micro-structured sheet metal has been

demonstrated. It was shown that a configuration consisting

of piezoceramic fibers with CIEs, where the sheet metal

is used as ground electrode, allows for operation of

piezo-metal substructures as sensors integrated within

structures. The mechanical and electrical connection of the

metal and the piezoceramic fiber is solely achieved by a

force-locked connection generated by a forming process.

This process is obviously capable of averaging dimensional

errors of piezoceramic fibers and micro cavities which are

present due to tolerances in manufacturing. Piezoceramic

fibers with center electrodes can bear the mechanical stress

induced by the forming process, though small fractures

occur. It was shown that the insulating coatings applied are

sufficiently strong to endure assembly and joining by

forming. The sensors were tested and generated charges for

excitation frequencies in the range of 0–280 Hz.

Further research aims for high-volume capable produc-

tion technologies particularly for the manufacturing of

micro cavities, the assembly and joining by forming pro-

cesses. A significant reduction of the cycle times and

increased repeatability and therefore reliability is expected

by the use of micro impact extrusion to produce micro-

formed cavities and automated micro-assembly of parallel

batches of piezoceramic fibers.

Further efforts are taken to increase the strength of the

insulating coatings in order to increase the reliability of the

piezo-metal sensor module, to enable in situ polarization

techniques, and use of the module as actuator.

Acknowledgments This research is supported by the Deutsche

Forschungsgemeinschaft (DFG) in context of the Collaborative

Research Centre/Transregio 39 PT-PIESA, subproject A2.

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0

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0

10

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0 5 10 15 20

deflection charge

-1500

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deflection charge

defl

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char

geQ

/pC

time t/ms100 200 300 400

defl

ecti

onu/

µm

char

geQ

/pC

time t/ms

10 Hz

250 Hz

Fig. 15 Deflection u (bending) of the sheet metal with piezo-metal

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