Notation

Symbol Meaning Unit
C Capacitance F
F Applied Force N
V Voltage V
η Piezoelectric modulus CN-1

Introduction
When a load is applied to a piezoelectric crystal, the crystal becomes polarised (https://en.wikipedia.org/wiki/Piezoelectricity) . If the voltage generated is large enough and if the electric field between electrodes connected to opposite ends of the crystal exceeds the dielectric breakdown strength of air (≈3.24 x 106 V/m), a spark can be induced between the electrodes. This is exploited in the gas-lighter by allowing a squeezing force applied by the hand to generate a spark via the piezoelectric effect, which in turn would ignite the flow of gas. The aim of this exercise in reverse engineering is to establish how a Tefal gas-lighter has been designed to suit its purpose, including an evaluation of the materials and processes used to manufacture some of its component parts.
Essentially you will have to:
Make measurements on the piezoelectric crystals used in the gas-lighter.
Piezoelectric crystals have the property that when subjected to a mechanical force, an electric field (and hence a voltage) is generated between the ends of the crystal. In the gas lighter, the squeezing action on the handles causes the two PZT (lead-zirconate-titanate, Pb(ZrxTi1-x)O3) piezoelectric crystals to be compressed, generating a voltage which is sufficient enough to cause a spark to jump across the air-gap at the end of the lighter. For a piezoelectric crystal, an applied force, F, generates a voltage, V, given by:
V=ηF/C Eq [1]
Where C is the capacitance of the crystal, and  is the piezoelectric modulus (Units: CN-1) of the PZT material.
Identify some of the materials and processing techniques used in the manufacture of the metallic and ceramic components of the gas-lighter.
This includes studying the microstructure and surface coatings of the components as revealed using optical microscopy, as well as considering the components’ elemental compositions as revealed by X-ray spectra (produced in a Scanning Electron Microscope (SEM) when the electron beam impinges on the surface of a sample). Micro-hardness measurements of the near-surface region of some of the metallic components will also be considered. These analysis techniques are widely used in materials science and engineering.
At the start of the session you will be shown the gas-lighter and its components, and its operation will be explained. Make sure you understand how the gas-lighter is constructed and how it works.
You will be provided with optical microscope images and X-ray spectra elemental composition analyses from the scanning electron microscope, as well as hardness test results. These will all be discussed during the practical session. You will have to deduce the materials and processes which were used to manufacture the components, and why they were chosen.
The experimental measurements you will need to make during the lab will focus on measurements of the gas-lighter’s electromechanical properties.
Guidance on the format and content of your technical note is provided later in this lab script.
Aims and Objectives

The aim of this lab is to evaluate the design, materials and process selection underlying the manufacture of a Tefal piezoelectric gas-lighter.
Upon successful completion of this lab, you will be able to:
Judge whether the mechanical and electrical properties of a material make it appropriate for use
Describe and explain qualitative aspects of mechanical properties and microstructure
Record and present data, plot graphs, and improve your technical reporting skills
The technical objectives of this lab are to:
Determine the electromechanical properties of a piezoelectric crystal in the context of its use in a gas-lighter.
Understand how the materials selection and processing of the components in a gas lighter relate to engineering design and operation of the lighter.
Health and Safety
Students are reminded that they are required by law to comply with the Department’s basic rules of lab safety given to them at the start of the semester. The experimental equipment should only be used under supervision by the laboratory teaching assistant, and only after they have given you permission to do so.

Experimental Procedure
Part 1: The Piezoelectric Crystals’ property measurements
Mechanical advantage is the ratio of output force divided by input force for a simple machine, (https://en.wikipedia.org/wiki/Mechanical_advantage ). The mechanical advantage of the moving internal workings of the gas-lighter is about 300. The approximate gripping force of the human hand is 30-50N. Hence knowing the gripping force, you can determine the force that can be exerted on the PZT piezo-crystals as the gas-lighter handles are squeezed.
Use the piezo-loading equipment and an electrometer (a very sensitive voltage measurement device) to record the voltage generated by the PZT crystal as masses are loaded and unloaded at 1 kg intervals from 1 kg up to 6 kg. You must carefully follow the instructions of the demonstrator to prevent damage to the electrometer when loading and unloading. The electrometer measures the voltage generated by the crystal, but the voltage decays quite rapidly after the weight is applied, so make sure you record the highest voltage indicated by the electrometer immediately after loading. You should also record the electrometer voltage as you unload the crystal at each weight (after having zeroed the electrometer), as the same voltage should be generated when loading as when unloading. Repeat all your measurements at least 3 or 4 times to obtain an average voltage for each loading mass. Plot a graph of your measurements of the instantaneous voltage generated by the crystal vs. the applied force (10-60N). Using your graph, extrapolate the data linearly to provide an estimate of the voltage generated by the force that is applied to the piezo crystals when the gas-lighter handles are squeezed.
Estimate the capacitance of the PZT crystal using your voltage-force graph and equation [1], taking a value from the internet for the piezo-electric modulus () of your PZT crystals.
For the gas-lighter to operate successfully, the PZT crystals must generate sufficient voltage to cause a spark across the air-gap at the end of the gas-lighter. Measure the air-gap between the electrodes for your gas-lighter, using any method of your choice, and hence estimate the voltage needed to generate a spark by using the value for the electric-field breakdown strength of air given previously. Comment on whether your estimate of spark voltage is of the same order of magnitude as your estimate of the voltage generated by the gas-lighter piezo-crystals.
Part 2: Microstructure and Mechanical Properties of Gas Lighter components
The demonstrator will introduce you to the 12 main metallic components of the gas lighter. Most of the metallic components of the lighter have been sectioned and mounted as metallographic specimens and optical microscope images of their microstructures have been provided for you to examine. EDX spectra showing an elemental analysis of the components will also be provided. The demonstrator will spend some time discussing the components’ microstructures and compositions and how they relate to each component’s function in the gas lighter. Some of this information is summarised in Table 2 of the Appendix to this lab script. Vickers hardness measurements are also provided for many of the components.
For components 1 (Nozzle), 3 (Spring) and 11 (Small end-cap) little or no information is provided, except for the Hardness and EDX spectra (which the demonstrator will provide), and you must deduce the materials and manufacturing processes used.

For each of these three components you should certainly ensure you consider the following aspects of the optical microscope images in addition to general aspects of the microstructure:
Component 1 (Nozzle): the bulk and surface are both important, and the surface coating layer thickness needs to be measured.
Component 3 (Spring): the thickness of the spring should be measured.
Component 11 (Small end-cap): the difference between the modified near-surface region and the uniform bulk microstructure within the component – estimate the thickness of the surface-modified region below the surface.

Guidance as to what is required in the technical note is provided later in this lab script, including discussion questions which you must answer.
Also provided is some additional background information about piezoelectric crystals, hardness measurements, and explanations of terminology.

 
TECHNICAL NOTE: PRESENTATION, ANALYSIS OF RESULTS, AND DISCUSSION QUESTIONS:
Part 1: Piezo-electric measurements
Only include the following properly annotated and captioned graphs, tables and micrographs. No linking text is required.
The piezo-electric behaviour in Table 1 and Figure 1 (note that this graph must be your own excel graph, including an appropriate best straight-line fit to the data which goes through the origin).
Discuss whether the gripping force of your hand on the gas-lighter handles should be sufficient to generate a spark from your extrapolation of the data in Figure 1, and your estimate of the mechanical advantage of the device, the dielectric breakdown strength of air, and the electrode air-gap. Present your calculations and discussion clearly. Include your estimate of the capacitance of the PZT crystal using your voltage-force graph and equation [1], taking a value from the internet for the piezo-electric modulus () of your PZT crystals.

Part 2: Microstructure-property relationships
For components 1, 3 and 11, provide:
a completed version of Table 2 for only these three component rows
an optical microscope image from each of these three components with a scale-bar added to indicate the image magnification, and appropriately labelled arrows pointing to key features in the images (such as surfaces and coatings etc). Each of these three Figures should also be given a Figure caption.

Answer the following discussion questions. No other discussion is required.
From the three micrographs, what is the thickness (in microns) of:
The coating on the Nozzle (component 1) surface
The spring (component 3)
The surface-hardened layer in the smaller end-cap (component 11)
The EDX spectra for both the Nozzle (component 1) and the spring (component 3), showing Cr and/or Ni are present as well as Fe (steel). Explain why the ratio of Cr to Ni in the EDX spectra is different in these two components.
The microstructure of the Nozzle and spring steel show anisotropic (directional) features running parallel to the surface. Why are these anisotropic features present in these two samples, whereas in the end-cap (component 11) the microstructure appears fairly isotropic (uniform)?
The hardness of the smaller end cap (component 11) increases significantly as you get closer to the surface. How does this increase correlate with the change in its microstructure?
The EDX spectra produced by the SEM gives an indication of the near-surface elemental composition of each component. From how far below the surface are the X-rays in an EDX spectra generated, and what does this imply about its usefulness when analysing coated or surface treated components?
Include an Abstract (summary) of less than 250 words, covering the whole laboratory practical.

Technical Note Submission
Assessment Criteria

1. Results 30%
A table and graph of piezo properties
A table of the 3 component properties
The 3 micrographs 10%
10%
10%
2. Discussion 50%
Piezoelectric measurements and spark generation
5 questions 20%
30%
3. Abstract 20%
Write a sentence about what you did, why you did it, how you did it, the results and why it was important. (Roughly 250 words)

Submission Instructions

Download the Technical Note Template cover sheet in the RE: Reverse Engineering section of the Year 2 Labs VITAL site. If you are having difficulty please contact the LTAs by email for help.
Rename the document to include the date that you were in the lab and your name before submitting it – for example “RE Technical Note 21.11.14 John Smith.docx”.
The deadline is before midnight 7 calendar days from the date of the lab, including the day of the lab. So if you did the lab on Monday the 2nd, the deadline would be at 23:59 on Sunday the 8th.
The TurnItin submission link should be used only after you have completed a short, anonymous survey for feedback on the lab. The School of Engineering takes your opinions seriously and feedback from previous students informed significant changes to the lab which is updated each year.

Appendix

Table 1: Voltage generated by piezo-crystal during loading and unloading
Mass applied (kg) Force applied (N) Max. voltage readings on loading (V) Max. voltage readings on unloading (V) Average maximum voltage reading (V)

Figure 1: Graph of piezo voltage vs. Applied load

Table 2: Gas-lighter components: materials and process selection

COMPONENT HARDNESS FUNCTION MATERIALS MANUFACTURING PROCESS REASON FOR PROCESS- and MATERIALS-SELECTION
No 1
NOZZLE 125 HV Protective end-cover.
Acts as one of the spark electrodes.
Holds other plastic parts together.
No 2
ELECTRODE At centre: 540 HV
In the 25µm coating: 300HV Acts as one of the spark electrodes. Plain C steel wire
Zn-coated Work-hardened, drawn steel wire.
Galvanised Electrical conduction.
Zn coating stops corrosive layer forming, but cheaper than plating.
No 3
SPRING 130 HV Acts as the other spark electrode.
Helps keep other components in place. Flexes during operation.
No 4
SIDE ARM 100 HV Holds many parts in place. Plain low-C steel.
Zn coating. Rolled sheet
Stamped
Galvanised Zn coating provides corrosion resistance.
Steel is cheap and ductile (for rolling).
No 5
SIDE LEVER 200 HV Transmits and applies force to the piezoelectric crystals. Plain low-C steel
Zn coating Rolled sheet
Stamped
Galvanised Steel is cheap, but reasonably strong to transfer applied forces.
Zn coating provides corrosion resistance.
No 6
BOLT and PLATE Bolt Head: 400 HV

Bolt Thread: 365 HV

Plate: 210 HV Keeps piezoelectric crystals in a fixed position Both probably plain low-C steel Bolt is drawn rod, with head “cold-headed” (cold-worked/pressed), and thread probably cut/machined with a die tool.
Plate is stamped from rolled sheet. Bolt head slightly harder than thread due to cold-working. Head and thread both quite hard due to drawing.

Steel is cheap and ductile (can be rolled/drawn).
No 7
PIVOT At centre: 150 HV
Towards edge: 250 HV
At edge: 775 HV Part of electrical circuit.
Holds side-arm and side-lever in position.
Applies force to piezoelectric crystal Low/medium C-steel Stamped/drawn
Machined
Drilled
Surface hardened Cheap steel, with moderate electrical conductivity.
Easy to fabricate.
Surface is easily hardened, leaving tough interior
No 8
END PLATE At centre: 450 HV
At edge: 780 HV Holds almost all other component parts together Probably medium C steel Rolled sheet, subsequently stamped, machined and drilled.
Work hardened throughout, and some additional surface hardening Cheap steel.
Medium C steel has required hardness and can be surface hardened to improve wear resistance.

No 9
END CAP 458 HV Hold piezelectric crystals in place, indirectly transmitting force Medium C steel Drawn, machined and cut to size Cheap steel with required hardness. Easy to fabricate. Moderate electrical conductivity.
No 10
BRASS SPACER 155 HV Conductor Brass (Cu + Zn) Stamped out of rolled sheet Good electrical conductivity and corrosion resistance
No 11
SMALLER END CAP At centre: 185 HV
At edge: 590 HV Helps to transmit the applied force to the piezoelectric crystals
No 12
PIEZO-
CRYSTALS About 1000HV Generates voltage when pressure applied PZT crystals with copper coating to improve conductivity Powder compressed and sintered.
Copper plated. Produces voltage to generate spark

The Piezo-Electric Lighter: Further background information
EDX (energy-dispersive X-ray) spectra from an SEM: The presence of elements lighter than sodium is not normally revealed by this technique. This means that in steels there is no indication of the Carbon content, even though the Carbon content of steel is very important in determining its mechanical properties. It should also be noted that only the near-surface material is sampled by this technique; so the X-ray signal from a coating layer present on a component may be mixed in with that from the base metal, or for very thin coatings it may not be detected at all.
The spectrum obtained from plain carbon steels will be dominated by a pair of Iron peaks; there is very little else, but just to the left of the large peak a very small Manganese peak is usually visible. Low alloy steels will differ only in showing small additional peaks, usually corresponding to Chromium and/or Nickel. However large Ni and/or Cr peaks may also be from a Ni and/or Cr electroplated coating on a plain C-steel, or Cr and Ni peaks can be indicative of austenitic stainless steel (18% Cr – 8% Ni). You can sometimes tell which by looking at the relative heights of the Ni, Cr and Fe peaks.
A large zinc peak is usually indicative either of a zinc based die casing alloy (e.g. brass), or a Zn coating on plain C-steel (galvanised steel). Brass has a very distinctive large copper peak and a smaller zinc one.

Hardness Values (Vickers Hardness, HV): The condition of steel will be determined both by its intended usage and the method of fabrication. Mild steel (0.2 wt% C) is suitable for low strength applications and lends itself to fabrication as a thin sheet or strip (it is ductile) and shaping by stamping and bending. It may be used in its softest condition (HV100 if it has been annealed after cold working) or slightly harder (say 150HV) if in the cold worked state. Medium C steels (0.3 wt% C) are normally stronger than mild steel as fabricated (because of their higher %C) but are also capable of being made very hard (and strong) by quenching. Quenched steels develop a microstructure called martensite, with a hardness 500-1000HV, which has the distinctive metallographic appearance of needle-like “laths” (jagged/pointed platelets) when viewed in an optical microscope.

Optical Microscopy of steel microstructures: Plain carbon steels with carbon contents in excess of 0.2 wt% but which have not been hardened will show the presence of a finely layered constituent (pearlite) in their microstructure. The pearlite layers may be so fine that they can only be resolved at very high magnification. Its absence indicates mild steel (wt%C  0.2). More or less symmetrical grains show an annealed (softened) material; pancake shaped grains indicate material in the cold worked state. It is usually difficult to see a well-defined grain structure in a heavily cold worked material.
“Case hardening” of steel parts: (i.e. making the surface few microns much harder than the underlying bulk of a component, usually by adding extra C to the near-surface and quenching the component) is often visible as a dark rim on polished and etched x-sections just below the surface. Micro-hardness measurements are needed to establish the hardness of the hardened “case”. Under the microscope case-hardened material formed by carburising (diffusion of C into the surface) will usually show the needle-like, hard constituent (martensite) in the “case”, and normal pearlite in the core where the steel is not quenched as rapidly.
Galvanised (Zn) coatings: are detectable in metallographic cross-sections: the zinc layer is usually visible. Cr or Ni electroplated coatings may also just be visible metallographically, but the layers are rather thinner than in the case of zinc coatings.
Remember also that many stainless steels are non-magnetic!