Posts Tagged ‘Material’

Steps To SUCCESS In Buying The Most Appropriate Decking Material

The steps to successful buying of any decking material can easily be remembered by using the pneumonic – SUCCESS

S – Start with a clear view of what you are buying and why (consider benefits as well as features).

U – Understand what your own customers really want. It may be different to your opinion.

C – Comparative analysis of alternative materials.

C – Centre your buying activity on customer needs.

E – Educate yourself regarding alternatives.

S – Sincerity is the key to lasting customer relationships; do not try to sell your own prejudices.

S – Start from the beginning each time you buy, whilst remembering to build on previous relationships where value has been obtained.

During the mobilisation phase of any large project it is usual for the project procurement manager to issue procurement procedures specifically for the project well in advance of the first commitment being made. These procedures determine how the vendors will be selected and how awards will be made.

It is usual to state that the cheapest material that is technically acceptable will be procured. This stark requirement process is however in danger of ignoring any special requirements that your customers may have, and it is vital that these requirements should be fully established at an early stage so that SUCCESS can be achieved.

What are the minimum requirements that should be met when selcting decking material?

The following are minimum requirements that the decking material under consideration should meet, in no particular order of priority:

a)      It should be safe for pedestrian traffic

b)      It should be weather resistant and rot free

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c)      It should provide value for money and offer good whole life costs

d)     It should be structurally sound and durable

e)      It should be (environmentally friendly), sustainable in production, use and eventual disposal

There are of course a number of other desirable qualities that the decking should possess such as being splinter free, stain resistant and algae resistant (nonporous) but for the purpose of evaluation these other qualities can each be assigned to one of the main categories set out above

How do you ensure client requirements are met?

It is the first duty of the procurement manager to ensure that the client’s requirements are met, and within budget. A happy client will be likely to return again and again with repeat business and importantly is likely to recommend the contractors work to other potential clients – thus becoming a salesman for the organisation.

There are four main groups of decking material from which the modern procurement manager can make his selection: (1) hardwood from, ideally, a sustainable source; (2) recycled plastic decking; (3) glass reinforced plastic; and (4) decking manufactured using wood flour and polyester (composite decking). Each group has different benefits and disadvantages that can make a balanced evaluation difficult. There are however a number of aids that assist the procurement manager in his duty and these include, published data, technical papers, published test results and articles in trade papers.

Set out below is a comparison between recycled plastic decking and composite decking.

Recycled Plastic Decking

This is typically made from high quality recycled plastic residue mainly from the nutrition and packaging industry.  The various constituent plastics are ground, mixed and fused at high temperature and pressed into moulds.

Properties (relevant material characteristics):

Maintenance free – no algal growth
Potential life of 50/70 years
100% recycled input material
Polymeric material is recyclable after use
Does not exude toxic substances
Thermal expansion 0.1mm/m/oC
Fire resistance Class B2 DIN 4102
European manufacture

Composite Material Decking

Composite decking boards can be formed from a mixture of wood flour and ground recycled plastic mixed together with virgin polymer resins and curing agents before being extruded to the required section.  If polyester resins are used durability can be compromised.

Properties (relevant material characteristics)

Algal growth can feed on surface wood flour
Life 25 years (10 in extreme conditions)
Resin supplies have high oil demand
Polymer composites discarded after use
Resins & curing agents emit greenhouse gas
Some thermal expansion
Fire resistance Class 3 BS476 Part 7
Often Far East manufacture

When you compare the properties for recycled plastic and composite decking, it is clear that recycled plastic decking gives better value for money as it requires little or zero maintenance, lasts longer and is actually better for the environment.

When aiming for SUCCESS, more thought has to be given to alternatives to traditional hardwood or composite decking as the impact on maintenance schedules, safety and replacement timetables is huge. Recycled plastic decking is now readily available, and offers better value for money while maintaining a low negative impact on the environment compared to alternatives.

Techniques for Bulk Material Resistivity Measurements

Electrical resistivity is a basic property that defines how well a material conducts current. It’s determined by measuring the resistance of a material sample, and then applying geometry factors. The three basic types of bulk materials—metals, insulators, and semiconductors—have different ranges of resistivity. Metals are good conductors of current with typical resistivities of about 10-6 ohm-cm. Insulators are poor conductors with typical resistivities from about 109 to 1020 ohm-cm. Semiconductors conduct current better than insulators but not as well as metals; they may fall anywhere from about 10-3 to 107 ohm-cm.

Measuring the Resistivity of Good Conductors. Characterizing a metal’s resistivity requires the measurement of very low resistances (and therefore, low voltages) accurately. The techniques described here can also be used in applications involving other small voltage measurements, such as those needed in measuring the resistance of superconductors, nanowires, carbon nanotubes, graphene (a one-atom-thick form of carbon), and other nanomaterials in which applied power must be kept low to prevent material heating.

Imagine a conductive material sample in the shape of a thin block with thickness t, width w, and some arbitrary length. To find its volume resistivity, a current source is connected at both ends of the sample along its length. Voltmeter leads are placed a known distance apart, L, along the length of its surface. The resistivity can be found by sourcing a known current, I, measuring the voltage drop, V, then calculating the volume resistivity, r, from the measured voltage, the magnitude of the source current, the cross-sectional area, A, (=wt), and the distance between the voltmeter leads, L, using the equation:

r = (V / I) x (wt / L)

For metals and other good conductors, the voltage drop is usually just microvolts or nanovolts, so precise measurements are crucial. Potential error sources include test lead resistance, thermoelectric voltages, low frequency noise, external noise sources, Johnson noise, and the use of a voltmeter with insufficient sensitivity. Fortunately, special techniques can reduce the impact of these errors. For example, using the four-wire (Kelvin) measurement method, in which one set of leads are used to source the current and another set are used to measure the voltage drop across the sample, will eliminate the effects of lead resistance.

Thermoelectric voltages are a common source of error when making low voltage and low resistance measurements. These voltages are generated when different metals in the circuit are at different temperatures. To reduce thermoelectric voltages construct test circuits using the same materials for interconnects. Minimize temperature gradients within the test circuit and allow the test equipment to warm up and reach thermal equilibrium. Finally, use an offset compensation method to overcome these unwanted offsets, such as a current-reversal method or the delta mode offset compensation technique.

The delta mode technique for removing offsets and low frequency noise involves applying a current and measuring the voltage, then reversing the current and re-measuring the voltage. The difference between the two measurements divided by two is the voltage response of the sample to the applied current level. Repeating the process and using averaging reduces the noise bandwidth and therefore the noise. Although this was once a manual technique, which limited reversal speed to less than 1Hz, newer instruments, such as Keithley’s Model 2182A Nanovoltmeter and Model 6221 Current Source, now automate the technique. This increases the reversal speed, which sets the frequency that dominates the noise. Higher reversal speeds do a better job of removing low frequency noise and thermal drift, because these noise sources have lower power at higher frequencies.

External noise sources are interferences created by motors, computer screens, or other electrical equipment. They can be controlled by typical shielding and filtering techniques, or by simply eliminating the noise source. Because these noise sources are often at the power line frequency, avoid test frequencies that are exact multiples or fractions of 60Hz or 50Hz. When using DC instruments and reversal methods, the same result can be achieved by adjusting the instrument’s signal integration period for each measurement to an integer number of power line cycles.

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In any electrical resistance, thermal energy produces the motion of charged particles. This charge movement results in Johnson noise. Johnson noise may be reduced by reducing the bandwidth using analog or digital filtering, or by reducing the temperature of the device.

When considering instruments for resistivity measurements keep in mind that most digital multimeters (DMMs)can’t measure microvolt- or nanovolt-level voltage drops accurately. Generally, an instrument with around 1nV sensitivity is a better choice, which can be found in a good nanovoltmeter.

Measuring the Resistivity of Insulators. The techniques used to measure the resistivity of insulators such as paper, rubber, and plastics are very different from those used for conductors. The resistivity of an insulator is determined by applying a voltage to the sample for a specified period of time, measuring the resulting current with an electrometer or picoammeter, then calculating the resistivity based on Ohm’s law and geometric considerations.

Both volume and surface resistivity measurements can be made on insulators. Volume resistivity is a measure of the leakage current directly through the insulator. To set up this measurement, picture a sample of arbitrary shape, but with a known uniform thickness t. Two electrodes, each having the same area, A, are placed on the top and bottom of the sample. The lower electrode is connected to the high terminal of a voltage source, V. The low terminal of the voltage source is connected to the low terminal of an ammeter. The high terminal of the ammeter is connected to the top electrode. Although the magnitude of the applied voltage usually depends on the material under test, it is often 500V DC (per ASTM D257). After a specified electrification time, usually 60 seconds, the current, I, is measured using an ammeter capable of measuring nanoamps or lower. Volume resistivity, r, is calculated based on the area, A, of the electrodes and the thickness, t, of the sample using the equation:

r = (V / I) x (A / t)

Surface resistivity is the electrical resistance on the sample’s surface. This measurements is conducted on a sample of any thickness and length, but fabricated with a constant width, w. Two electrodes are placed across the width of the sample, separated by a constant distance, L. An ammeter’s high terminal is placed on one electrode; the voltage source’s high terminal is placed on the other. The low terminals of the ammeter and voltage source are connected together. A potential difference, V, is applied for a known period of time and the ammeter measures the resulting current, I. The surface resistivity, s, is calculated from:

s = (V / I) x (w / L)

Potential error sources when characterizing an insulator’s resistivity include choosing an ammeter without sufficient sensitivity, and using an inappropriate electrification time or test voltage. Other error sources include electrostatic interference, background currents due to charge stored in the material, and static or triboelectric charge, or piezoelectric effects.

Electrostatic interference occurs when an electrically charged object is brought near an uncharged object. High resistance materials do not allow the charge to decay quickly and unstable measurements may result. Electrostatic shielding will help to minimize these effects. Shield the material by using a conductive shielded enclosure and connecting the low terminal of the ammeter to the shield.

Background currents can equal or exceed the current stimulated by the applied voltage. To counter the effects of these spurious currents, an alternating polarity technique can be used. In this technique, a bias voltage of positive polarity is applied, then the current is measured after a specified delay. Next, the polarity is reversed and the current is re-measured using the same delay. This process can be repeated any number of times. The resistance is calculated based on a weighted average of the most recent current measurements. By calculating a weighted average current, the background current is cancelled out. Some instruments, such as Keithley’s Model 6517B Electrometer, have a built-in test sequence that automates this alternating polarity technique.

A high-quality electrode system that provides good contact to the test sample is essential. Conductive rubber on these electrodes enables good contacts to the sample, especially if the sample is a rigid material. Avoid electrodes that will add appreciable resistance to the measurement circuit or could contaminate the sample. Choose an electrode configuration that supports calculating the resistivity from the measured resistance. Several commercial systems now available provide this type of resistivity measurement.

Measuring Semiconductor Resistivity

The four-point collinear probe technique is the most common way of measuring the resistivity of semiconductor materials, particularly wafers being tested at a probe station. This technique involves the use of four equally spaced (collinear) probes in contact with the material sample. The outer probes (1 and 4) source current; the inner probes (2 and 3) measure the resulting voltage drop across the sample’s surface. The volume resistivity is calculated thus.

ρ = (π / ln2) x (V / I) x (tk),

where ρ = volume resistivity (ohm-cm), V = voltage between 2 and 3, I = source current from 1 to 4,
t = sample thickness (cm), and k = a correction factor based on the ratio of the probe to wafer diameter, and on the ratio of wafer thickness to probe separation. Using four probes in this manner eliminates measurement errors due to the probe and lead resistance, the spreading resistance under each probe, and contact resistance between each metal probe and the semiconductor material.

Another technique for measuring the resistivity of semiconductors is the van der Pauw method, which involves applying a current and measuring voltage using four small contacts on the circumference of a flat, arbitrarily shaped sample of uniform thickness. The current is forced between two adjacent terminals of the sample. The voltage is measured on the opposite pair of terminals. This method is particularly useful for measuring very small samples because the dimensions of the sample and the spacing of the contacts are unimportant. It requires making eight measurements around the periphery of the sample to compensate for offsets and geometric considerations. These measurements are combined mathematically to compute the resistivity.

Typical sources of error for van der Pauw measurements include voltage drops due to lead and contact resistances, voltage offsets, and incorrect instrumentation choices. In semiconductor material research, a parametric tester is often the instrumentation of choice when characterizing material resistivity. These systems often include a switch matrix to switch the current source and voltmeter to all sides of the sample, which facilitates measurement automation. Parametric test systems also include software to completely automate the measurements and perform resistivity calculations.

Special considerations must be taken into account when measuring semiconductor materials with resistances of hundreds of kilo-ohms or higher:

* To avoid leakage currents, use a 4-point collinear probe with excellent isolation between the probes to avoid leakage current errors.

* Choose a current source with high output impedance (≥1E14 ohms) to avoid loading errors, and with a built-in guard circuit to reduce the effects of shunt capacitance.

* Use voltmeters with high input impedance (≥1E14 ohms).

* Always shield the sample and all sensitive circuitry, and use shielded cabling to prevent errors due to electrostatic interference; connect the shield to the instrumentation’s low terminal.

* To avoid errors from common mode currents, use differential voltage measurement techniques. 

Summary

In such a brief article it is impossible to address all of complexities that help ensure the accuracy of bulk materials resistivity measurements. For more details, view Keithley’s free webinar on this topic at the following link: How to Make Electrical Resistivity Measurements of Bulk Materials.

Material Science in Chemical Engineering from HelpWithAssignment.com

 Material Science in Chemical Engineering

Life in the 21st century is every dependant on an unlimited variety of advanced materials. In our consumptive world, it is easy to take for granted the macro, micro and nanoscopic building blocks that comprise many any item ever produced. We are spoiled by the technology that adds convenience to our lives, such as microwave ovens, laptop computers, digital cell phones and improved modes of transportation. However, we rarely take time to think about and appreciate the materials that constitute these modern engineering feats.

The term material may be broadly defined as any solid-state component or device that may be used to address a current or future societal need. For instance, simple building materials such as nails, wood, coatings, etc address our need for shelter. Other more intangible materials such as nanodevices may not yet be widely proven for particular applications, but will be essential for the future needs of our civilization. Although the above definition includes solid nanostructural building blocks that assemble to form larger materials, it excludes complex liquid compounds such as crude oil, which may be more properly considered a precursor for materials.

There is a sharp distinction between the various classes of materials that we see today. For example: a thin film is defined as having a film of thickness less than 1μm; however, if the thickness drops below 100nm, the dimensions may be more accurately classified within the nanoscale regime. Likewise, liquid crystals are best described as having properties intermediate between amorphous and crystalline phases, and hybrid composite materials involve both inorganic and organic components.

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The broadly defined discipline of materials chemistry is focused on understanding the relationships between the arrangements of atoms, ions, or molecules comprising a material and its overall bulk structural/physical properties. By this designation, common disciplines such as polymer, solid-state and surface chemistry would all be placed within the scope of materials chemistry. This broad field consists of studying the structures/properties of existing materials, synthesizing and characterizing new materials and its overall bulk structural/physical properties. By this designation, common disciples such as polymer, solid-state and surface chemistry would all be placed within the scope of materials chemistry. This broad field consists of studying the structures/properties of existing materials, synthesizing and characterizing new materials and using advanced computational techniques to predict structures and properties of materials that have not yet been realized.

Although the study of materials chemistry is a relatively new entry in chemistry, it has always been an important part of chemistry. By most accounts, Neolithic man (10000 – 300) BC was the first to realize that certain materials such as limestone, wood, shells and clay were most easily shaped into materials used as utensils, tools and weaponry.

Applications for metallic materials date back to the Chalcolithic age (4000 – 1500) BC where copper was used for a variety of uses.

Metal alloys were first used in the Bronze age (1400 – 0) BC, where discovery that doping copper with other compounds drastically altered the physical properties of the material.

The Iron age (1000 – 1950) AD first brought about applications for iron based materials. Since the earth’s crust contains significantly more iron than copper, the latter was abandoned for material applications.

Although building and structural materials such as ceramics, glasses and asphalt have not dramatically changed since their invention, the world of electronics has undergone rapid changes. Many new architectures for advanced material design are surely yet undiscovered, as scientists are now attempting to mimic the profound structural order existing in living creatures and plant life, which is evident as one delves into their microscopic regimes.

Low friction nanocomposite material

Dr. Eric Loth’s research group. See more at nanotexture.tumblr.com. This one is slow motion water and oil droplets on a superhydrophobic and superoleophobic nanocomposite, taken with a high speed camera.

MoviTHERM – Composite Material Thermal Inspection

A demonstration of MoviTHERM’s Composite-Check solution for non-destructive testing of composite materials. For more information, visit www.movitherm.com