Piezoelectric Ceramics: Science Meets Pottery

Feb. 14, 2008
These small and strong materials offer a wide frequency range, suiting a variety of applications and specific design needs without sacrificing performance.

Electronic engineers are no strangers to odd-sounding and difficult-to-pronounce methods and mater ial s . However, the words piezoelectric ceramics still trip up even the most experienced designers. And why not? These materials are relatively new to the world of electronics.

Many engineers are still learning about the piezoelectric effect or have little exposure to ceramic material advances. But when they’re combined, ceramics and piezoelectric elements can lead to incredible improvements in component design and function. So where did it all begin?

Discovered in 1880, the piezoelectric effect causes crystal materials (like quartz) to generate an electric charge when the crystal material is compressed, twisted, or pulled (Fig. 1). The reverse also is true, as the crystal material compresses or expands when an electric voltage is applied.

Later, scientists also found that applying an electrical oscillation to a piezoelectric material produces a mechanical oscillation. Thus, the materials have natural resonant frequencies and, at these frequencies, very little electrical energy is required to make the material vibrate mechanically. The resonant frequency could be adjusted by changing the shape and size of the piezoelectric material.

The piezoelectric effect couldn’t be used in practical product applications until 1921, when William Cady developed a quartz-crystal frequency-control element for a radio transmitter. Yet by the end of World War II, most of the good “electronic quality” quartz had been mined. The lack of naturally occurring quartz crystal fueled the development of cultured or laboratory-grown quartz crystals, but it was still relatively expensive, fragile, and difficult to mass produce.

Advent of Piezoelectric Ceramics Quartz’s small supply and high cost fueled efforts to find alternative materials that could demonstrate the piezoelectric effect. Researchers later discovered that certain ceramic formulations could be made to exhibit and permanently retain piezoelectric properties.

While these ceramics don’t naturally exhibit the piezoelectric effect, an application of a strong polarization voltage to the ceramic material could achieve it even after removing the voltage. This was an important find, as ceramic materials are relatively easy to manufacture and their raw materials are readily available.

In the 1950s, barium titanate was the first piezoelectric ceramic material to be adopted, mainly for use as ultrasonic transducer elements in fish finders. By the late 1950s, lead zirconate titanate (PZT) became the prevalent piezoelectric ceramic material. PZT exhibited more desirable characteristics, like improved temperature stability, than barium titanate.

Quartz Versus Ceramic Crystals are naturally piezoelectric. They have a nearly perfect internal structure that results in a very stiff material, offering the potential for excellent material repeatability. Ceramics, on the other hand, aren’t naturally piezoelectric and must be imparted with the piezoelectric characteristics through a polarization process in manufacturing. Also, ceramic materials don’t have a perfect internal structure, which results in more flexible material (relative to crystals) and more variation in material variability.

These differences greatly determine where and how the parts are used. PZT ceramic demand has steadily increased mainly due to size, shock reliability, and cost benefits. But it can’t be used in all of the places that employ crystal materials.

To illustrate the amazing scope of PZT ceramic usefulness, Figure 2 categorizes the different ceramic vibration modes and the typical electrical component. Forming the ceramic material into different shapes and thicknesses produces different vibrating modes, allowing for different ranges of resonant frequencies inside the material. For example, designers can adapt PZT ceramic materials to suit buzzers, filters, or resonators.

The vibration mode needed for the desired frequency range dictates the general shape of the piezoelectric ceramic part. Within a vibration mode, frequency is selected by changing the thickness of the material. In practical production, ceramic material can only be made so thin or thick before it breaks in production or the thick material size results in high cost. When these limits are reached, use of the next vibration is needed. This explains why ceramic devices come in different sizes and shapes, depending on the frequency range of operation.

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Ceramic Filters Ceramic filters, mainly bandpass types, started the mainstream commercialization of piezoelectric ceramic components. First used in AM radios, the 455-kHz ceramic bandpass ladder filter was replaced when FM radios became popular (Fig. 3). FM radios needed an IF filter at 10.7 MHz and a slightly wider bandwidth.

By 1971, leaded ceramic filters even appeared in American OEM car radios. Soon afterward, a 4.5-MHz version of the 10.7-MHz filter was created for the booming television market. By the 1980s, analog cellular phones and pagers needed narrow-bandwidth 455-kHz filters, and demand continued to grow. The first surfacemount filters also began to emerge by the 1980s, further driving the trend toward reduced sizes in consumer electronics.

By 1996, the demand for narrow-band ceramic kilohertz filters started to decline as new radios (cellular and paging) moved toward wider-bandwidth technologies and higher-IF frequencies, or to radio designs that didn’t need IF filters. Even AM radios started moving away from kilohertz filter technologies.

The ceramic, 10.7-MHz, bandpass filters have been better able to adapt to the market changes. The wider bandwidth of the 10.7-MHz ceramic filter (bandwidths from 25 kHz up to 1 MHz are possible today) fits better with the wider bandwidth requirements of digital technology. It also offers smaller size, low cost, and robust packaging. Common modern applications, such as remote keyless entry, tire-pressure monitoring, wireless remote controls, and FM and HD radios, use 10.7-MHZ ceramic filters today in their smallest surface- mount packages.

The main feature that continues to drive this portion of the high-tech ceramics filter business is the extreme degree of miniaturization that can be achieved (compared to the “discrete” component approach) without sacrificing performance. Indeed, the superior properties of piezoelectric ceramics allow for the extreme miniaturization of high-performance filters, to the point where it would take dozens of discrete inductors and capacitors, over a large amount of valuable board space, to come close to duplicating ceramic filter performance.

Ceramic Resonators Perhaps no other component has been as positively impacted by the science of PZT ceramics as resonators. Resonators are closely related to timekeeping, even though their specifications are stated in terms of frequency (kHz or MHz) and are rarely mentioned in the customary units of time, such as seconds, minutes, or hours.

For a long time, quartz was the premier material for timekeeping. From Cady’s frequency-control element through today, quartz crystals are synonymous with this function. In fact, most designers became so accustomed to using quartz crystal resonators for any type of “clock” that they frequently overlooked the disadvantages of quartz, or at least tolerated them.

Quartz crystals are notoriously fragile (stiff material) and relatively large (hard to make small without breaking in production). They’re also prone to damage from electrical “overdrive,” as the stiff material is over-flexed electrically and breaks. And, it’s difficult to inexpensively mass-produce them in small package sizes.

In 1977, the first leaded ceramic resonators appeared on the market, offered in frequencies ranging from hundreds of kilohertz up to a few megahertz. One interesting new feature of megahertz ceramic resonators was the inclusion of load capacitors inside the resonator package itself. These load capacitors control the oscillation circuit and prevent it from stopping. With crystal resonators, the load capacitors could not be included in the package. Even today, this holds true.

By 1986, the first surface-mount resonators were offered in the lower-megahertz range. This doesn’t seem like a major milestone, but high temperatures can damage piezoelectric ceramic. In surface-mounting processes, a ceramic resonator body can be exposed up to 240°C. At this temperature, existing leaded ceramic resonators become damaged.

To survive such high temperatures, resonator packaging was changed. And more importantly, the ceramic material was improved to resist the effects of high temperatures. While this was a great technical achievement for the industry, the more robust ceramic had a steeper price tag and is the main reason today why leaded piezoelectric components cost less than surfacemount components.

Nowadays, ceramic resonators are found mainly in surface-mount packages, and they’re available up to 70 MHz. While leaded megahertz ceramic resonators and kilohertz resonators still exist, most new products use surface-mount megahertz resonators.

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The explosive growth in car electronics (airbag, car networks/CAN bus, antilock brakes, etc.) has fueled the growth of the more mechanically robust ceramic resonators (Fig. 4). In addition, the rising popularity of Universal Serial Bus (USB) timing has triggered an increase in ceramic resonators.

The big trend in ceramic resonators is in regards to improved accuracy. Crystal resonators have part-to-part accuracies of ±100 ppm or lower, but ceramic resonators are typically ±2500 ppm and higher. While many applications need the tolerance of crystals (like time of day clocks or radios), just as many applications do not, but they could really benefit from the cost, size, and ruggedness of ceramics.

In 2004, the first ±500-ppm resonator was commercially released, targeting the new USB 2.0 high-speed, 480-Mbit/s connections. This was a significant breakthrough, as it was the first time a sub- 1000-ppm ceramic resonator was readily available. Besides all of the traditional benefits (size, cost, reliability), ceramic resonators are truly starting to gain on the tolerance advantage of crystal resonators.

Buzzers Buzzers are low-cost noisemakers that reproduce a single loud tone. They allow many types of consumer products (such as smoke and burglar alarms) to inexpensively add an audible sound without resorting to mechanical bells or chimes.

The leaded buzzer can be mounted on the circuit board with the rest of the electronics, allowing for easier assembly (Fig. 5). Most piezoelectric buzzers are available in the 800-Hz to 6-kHz range, with 1, 2, and 4 kHz being the most common. (3.4 kHz is very popular for smoke and gas alarms, but the frequency generally isn’t used outside of these markets.)

Compared to buzzers (like electromechanical buzzers), PZT ceramic buzzers are a great way to make “noise,” not only because of their low cost, but also because of their ability to conserve battery power and eliminate interference. Piezoelectric ceramic buzzers are driven by voltage, not current, so they’re ideal for use in batterypowered products where high current draw could prevent long battery life.

The applied voltage controls the piezoelectric buzzer’s volume. The higher the drive voltage, the louder the buzzer sound. Traditional electromagnetic buzzers comprise wire coils and moving magnets. They’re current hogs by design, draining precious battery power. If you’re developing a product like a cell phone, portable music device, or even a smoke detector where battery power is critical, you need a buzzer that won’t drain energy.

Another advantage closely related to the “voltage-driven” issue is electromagnetic interference (EMI). An electromagnetic speaker, with its necessary magnetic field used to (indirectly) drive the speaker cone, will generate EMI. This unavoidable side effect can range from annoying to illegal, depending on the application. Piezoelectric ceramic audio devices neatly eliminate EMI because they generate sound without wire coils and magnets.

Most buzzers, whether they’re leaded or surface-mount, have two terminals in one of two formats. The first format is a buzzer with a built-in drive circuit. The internal drive circuit creates an alternating ac waveform at a specific frequency from an external dc source to drive the piezoelectric element inside the buzzer. While more expensive than buzzers without internal drive circuits, the parts are easy to use and can run directly from a dc battery. They are the quickest and easiest way to add sound to an application.

In the second format, external ac signals drive the buzzer. These buzzers need to be driven with an ac voltage at a specific frequency. (Actually, you can drive the buzzer at any frequency and get sound out of it, but maximum volume only occurs at the rated output frequency.) These are the least expensive type of buzzers (again, the higher the frequency, the smaller the part and lower the cost) and fit well in highvolume and low-cost applications.

With the proliferation of portable devices that require battery power conservation, piezoelectric buzzers should be in strong demand for the foreseeable future. And with their small size, low cost, and durability, PZT ceramic buzzers are already having a positive impact on the market.

It’s Piezoelectric! Despite being a mouthful to pronounce, piezoelectric ceramic components offer design engineers a variety of quality options through an impressively wide frequency range. PZT ceramics are smaller and stronger than their counterparts, and they can be tailored to fit very specific design needs without sacrificing performance. In addition, their ability to be easily mass-produced makes them a cost-effective alternative to quartz and other materials. When a stone meets science, the effect is truly piezoelectric.

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