Rationale for passivating aluminium

Pure aluminium naturally forms a tough resistant oxide, alumina, almost immediately that protects it from further oxidation in most environments. Aluminium alloys, however, offer little protection against corrosion. Many alloys are "alclad" or coated with a thin layer of pure aluminium, although bare metal must be treated to prevent corrosion. Alodine, Iridite, and chemical film coatings can provide varying amounts of protection for aluminium alloy surfaces.

See also

• Layer (electronics)
• Pilling-Bedworth ratio

References

Notes

1.     ^ University of Bath & Western Oregon University

Bibiography

• Budinski, Kenneth G. (1988), written at Englewood Cliffs, New Jersey, Surface Engineering for Wear Resistance, Prentice Hall.
• Brimi, Marjorie A. (1965), written at New York, New York, Electrofinishing, American Elsevier Publishing Company, Inc.

Further reading

• ASTM A967 provides advice for the passivation of stainless steel parts
• Chromate conversion coating (chemical film) per MIL-DTL-5541F for aluminium and aluminium alloy parts
• A standard overview on black oxide coatings is provided in MIL-HDBK-205, Phosphate & Black Oxide Coating of Ferrous Metals. Many of the specifics of Black Oxide coatings may be found in MIL-DTL-13924 (formerly MIL-C-13924). This Mil-Spec document additionally identifies various classes of Black Oxide coatings, for use in a variety of purposes for protecting ferrous metals against rust

http://en.wikipedia.org/wiki/Passivation

 

 

Passivity (engineering)

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Passivity is a property of engineering systems, most commonly used in electronic engineering and control systems. A passive component, depending on field, may either refer to a component that consumes (but does not produce) energy, or to a component that is incapable of power gain. A component that is not passive is called an active component. An electronic circuit consisting entirely of passive components is called a passive circuit (and has the same properties as a passive component).

Thermodynamic passivity

In control systems and circuit network theory, a passive component or circuit is one that consumes energy and can not control the flow of electrons, but does not produce energy. Under this methodology, voltage and current sources are considered active, while resistors, tunnel diodes, glow tubes, capacitors, and other dissipative and energy-neutral components are considered passive. For memoryless two-terminal elements, this means that the current-voltage characteristics lie in the first and third quadrant. Circuit designers will sometimes refer to this class of components as dissipative, or thermodynamically passive.

While many books give definitions for passivity, many of these contain subtle errors in how initial conditions are treated (and, occasionally, the definitions do not generalize to all types of nonlinear time-varying systems with memory). We will state a correct, formal definition, taken from Wyatt^[1] (which also explains the problems with many other definitions):

Given an n-port R with a state representation S, and initial state x, define available energy E_A as:

Where the notation indicates that the supremum is taken over all T > 0 and all admissible pairs . A system is considered passive if E_A is finite for all initial states x. Otherwise, the system is considered active.

Incremental passivity

In circuit design, informally, passive components refer to ones that are not capable of power gain. Under this definition, passive components include capacitors, inductors, resistors, transformers, voltage sources, and current sources. They exclude devices like transistors, relays, glow tubes, tunnel diodes, and similar devices. Formally, for a memoryless two-terminal element, this means that the current-voltage characteristic is monotonically increasing. For this reason, control systems and circuit network theorists refer to these devices as locally passive, incrementally passive, increasing, monotone increasing, or monotonic. It is not clear how this definition would be formalized to multiport devices with memory -- as a practical matter, circuit designers use this term informally, so it may not be necessary to formalize it.^{[nb 1]}

Other definitions of passivity

In some very informal settings, passivity may refer to the simplicity of the device, although this definition is almost universally considered incorrect (indeed, maybe should not even be included here). Here, devices like diodes would be considered active,^[2] and only very simple devices like capacitors, inductors, and resistors are considered passive. In some cases, the term "linear element" may be a more appropriate term than "passive device." In other cases, "solid state device" may be a more appropriate term than "active device."

Stability

Passivity, in most cases, can be used to demonstrate that passive circuits will be stable under specific criteria. Note that this only works if only one of the above definitions of passivity is used -- if components from the two are mixed, the systems will, in general, not be stable under any criteria. In addition, passive circuits will not necessarily be stable under all stability criteria. For instance, a resonant series LC circuit will have unbounded voltage output for a bounded voltage input, but will be stable in the sense of Lyapunov, and given bounded energy input will have bounded energy output.

Passivity is frequently used in control systems to design stable control systems or to show stability in control systems. Passivity is also used in some areas of circuit design, especially filter design.

Passive filter

A passive filter is a kind of electronic filter that is made only from passive elements -- in contrast to an active filter, it does not require an external power source (beyond the signal). Since most filters are linear, in most cases, passive filters are composed of just the four basic linear elements -- resistors, capacitors, inductors, and transformers. More complex passive filters may involve nonlinear elements, or more complex linear elements, such as transmission lines.

Television signal splitter consisting of a passive hi-pass filter (left) and a passive low-pass filter (right). The antenna is connected to the screw terminals to the left of center.

A passive filter has several advantages over an active filter:

• Guaranteed stability
• Passive filters scale better to large signals (tens of amps, hundreds of volts), where active devices are often impractical
• No power consumption (aside from possibly taking some power out of the signal)
• Inexpensive (unless large coils are required)
• For linear filters, generally, more linear than filters including active (and therefore non-linear) elements

They are commonly used in speaker crossover design (due to the moderately large voltages and currents, and the lack of easy access to power), filters in power distribution networks (due to the large voltages and currents), power supply bypassing (due to low cost, and in some cases, power requirements), as well as a variety of discrete and home brew circuits (for low-cost and simplicity). Passive filters are less common in integrated circuit design, where active devices are comparatively inexpensive compared to resistors and capacitors, and inductors are prohibitively expensive.

Notes

1.     ^ This is probably formalized in one of the extensions to Duffin's Theorem. One of the extensions may state that if the small signal model is passive, under some conditions, the overall system will be stable. This needs to be verified.

References

1.     ^ IEEE Transactions on Circuits and Systems, CAS-28, No 1. January 1981, pp. 48–61. "Energy Concepts in the State-Space Theory of Nonlinear n-Ports: Part I — Passivity", by Wyatt, et al.

2.     ^ Young EC, passive, The Penguin Dictionary of Electronics, 2nd ed, ISBN 0140511873

Further reading

• Khalil, Hassan (2001). Nonlinear Systems (3rd Edition). Prentice Hall. ISBN 0130673897.  — Very readable introductory discussion on passivity in control systems.
• Chua, Leon; Desoer, Charles; Kuh, Ernest (1987). Linear and Nonlinear Circuits. McGraw-Hill Companies. ISBN 0070108986.  — Good collection of passive stability theorems, but restricted to memoryless one-ports. Readable and formal.
• Desoer, Charles; Kuh, Ernest (1969). Basic Circuit Theory. McGraw-Hill Education. ISBN 0070851832.  — Somewhat less readable than Chua, and more limited in scope and formality of theorems.
• Cruz, Jose; Van Valkenberg, M.E. (1974). Signals in Linear Circuits. Houghton Mifflin. ISBN 0395169712.  — Gives a definition of passivity for multiports (in contrast to the above), but the overall discussion of passivity is quite limited.
• Wyatt, J.L.; Chua, L.O.; Gannett, J.; Göknar, I.C.; Green, D. (1978). Foundations of Nonlinear Network Theory, Part I: Passivity. Memorandum UCB/ERL M78/76, Electronics Research Laboratory, University of California, Berkeley. 
  Wyatt, J.L.; Chua, L.O.; Gannett, J.; Göknar, I.C.; Green, D. (1980). Foundations of Nonlinear Network Theory, Part II: Losslessness. Memorandum UCB/ERL M80/3, Electronics Research Laboratory, University of California, Berkeley. 
  — A pair of memos that have good discussions of passivity.

http://en.wikipedia.org/wiki/Passive_component

 

 

 

English passive voice

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This article is about the passive voice in English. For grammatical voices generally, including the passive voice in other languages, see Grammatical voice.

In English, as in many other languages, the subject of a verb in the passive voice corresponds to the object of the same verb in the active voice. English's passive voice is periphrastic; that is, it does not have a one-word form. Rather, it is formed using a form of the auxiliary verb be together with a verb's past participle.

Passive voice can be used in any number of tenses.

Canonical passives

Passive constructions have a range of meanings and uses. The canonical use is to map a clause with a direct object to a corresponding clause where the direct object has become the subject. For example:

• John threw the ball.

Here threw is a transitive verb with John as its subject and the ball as its direct object. If we recast the verb in the passive voice (was thrown), then the ball becomes the subject (it is promoted to the subject position) and John disappears:

• The ball was thrown.

The original subject can typically be re-inserted using the preposition by:

• The ball was thrown by John.

Promotion of other objects

One non-canonical use of English's passive is to promote an object other than a direct object. It is usually possible in English to promote indirect objects as well. For example:

• John gave Mary a book. → Mary was given a book.
• John gave Mary a book. → Mary was given a book by John.

In the active form, gave is the verb; John is its subject, Mary its indirect object, and a book its direct object. In the passive forms, the indirect object has been promoted and the direct object has been left in place. (In "A book was given to Mary", the direct object is promoted and the indirect object left in place. In this respect, English resembles dechticaetiative languages.)

It is also possible, in some cases, to promote the object of a preposition:

• They talked about the problem. → The problem was talked about.

In the passive form here, the preposition is "stranded"; that is, it is not followed by an object. (See Preposition stranding.) Indeed, in some sense it doesn't have an object, since "the problem" is actually the subject of the sentence.

Promotion of content clauses

It is possible to promote a content clause that serves as a direct object. In this case, however, it typically does not change its position in the sentence, and an expletive it takes the normal subject position:

• They say that he left. → It is said that he left.

Stative passives

The passives described so far have all been eventive (or dynamic) passives. There exist also stative (or static, or resultative) passives; rather than describing an action, they describe the result of an action. English does not usually distinguish between the two. For example:

• The rule was broken.

This sentence has two different meanings, roughly the following:

• [Someone] broke (violated) the rule.
• The rule was in a broken (dysfunctional) state.

The former meaning represents the canonical, eventive passive; the latter, the stative passive. (The terms eventive and stative/resultative refer to the tendencies of these forms to describe events and resultant states, respectively. The terms can be misleading, however, as the canonical passive of a stative verb is not a stative passive, even though it describes a state.)

Some verbs do not form stative passives. In some cases, this is because distinct adjectives exist for this purpose, such as with the verb open:

• The door was opened. → [Someone] opened the door.
• The door was open. → The door was in the open state.

Adjectival passives

Adjectival passives are not true passives; they occur when a participial adjective (an adjective derived from a participle) is used predicatively (see Adjective). For example:

• She was relieved to find her car undamaged.

Here, relieved is an ordinary adjective, though it derives from the past participle of relieve,^[1] and that past participle may be used in canonical passives:

• He was relieved of duty.

In some cases, the line between an adjectival passive and a stative passive may be unclear.

Passives without active counterparts

In a few cases, passive constructions retain all the sense of the passive voice, but do not have immediate active counterparts. For example:

• He was rumored to be a war veteran. ← *[Someone] rumored him to be a war veteran.

(The asterisk here denotes an ungrammatical construction.) Similarly:

• It was rumored that he was a war veteran. ← *[Someone] rumored that he was a war veteran.

In both of these examples, the active counterpart was once possible, but has fallen out of use.

Double passives

It is possible for a verb in the passive voice — especially an object-raising verb — to take an infinitive complement that is also in the passive voice:

• The project is expected to be completed in the next year.

Commonly, either or both verbs may be moved into the active voice:

• [Someone] expects the project to be completed in the next year.
• [Someone] is expected to complete the project in the next year.
• [Someone] expects [someone] to complete the project in the next year.

In some cases, a similar construction may occur with a verb that is not object-raising in the active voice:

• ?The project will be attempted to be completed in the next year. ← *[Someone] will attempt the project to be completed in the next year. ← [Someone] will attempt to complete the project in the next year.

(The question mark here denotes a questionably-grammatical construction.) In this example, the object of the infinitive has been promoted to the subject of the main verb, and both the infinitive and the main verb have been moved to the passive voice. The American Heritage Book of English Usage declares this unacceptable,^[2] but it is nonetheless attested in a variety of contexts.^[3]

Other passive constructions

Past participle alone

A past participle alone usually carries passive force; the form of be can therefore be omitted in certain circumstances, such as signs, newspaper headlines, and reduced relative clauses:

• Couple found slain; Murder-suicide suspected. ^[1]
• The problem, unless dealt with, will only get worse.
• A person struck by lightning has a high chance of survival.

With get as the auxiliary

While the ordinary passive construction uses the auxiliary be, using get in its place can sometimes achieve the same effect:

• Jamie got hit with the ball.

This use of get is fairly restricted. First of all, it is fairly colloquial; be is used in news reports, formal writing, and so on. Second, it typically only forms eventive passives of eventive verbs.

Ergative verbs

Main article: Ergative verb

An ergative verb is a verb that may be either transitive or intransitive, and whose subject when it is intransitive plays the same semantic role as its direct object when it is transitive. For example, fly is an ergative verb, such that the following sentences are roughly synonymous:

• The airplane flew.
• The airplane was flown.
• [Someone] flew the airplane.

One major difference is that the intransitive construction does not permit an agent to be mentioned, and indeed can imply that no agent is present, that the subject is performing the action on itself. For this reason, the intransitive construction of an ergative verb is often said to be in a middle voice, between active and passive, or in a mediopassive voice, between active and passive but closer to passive.

Reflexive verbs

A reflexive verb is a transitive verb one of whose objects is a reflexive pronoun (myself, yourself, etc.) referring back to its subject. In some languages, reflexive verbs are a special class of verbs with special semantics and syntax, but in English, they typically represent ordinary uses of transitive verbs. For example, with the verb see:

• He sees her as a writer.
• She sees herself as a writer.

Nonetheless, sometimes English reflexive verbs have a passive sense, expressing an agentless action. Consider the verb solve, as in the following sentences:

• He solved the problem.
• The problem solved itself.

One could not say that the problem truly solved anything; rather, what is meant is that the problem was solved without anyone's solving it.

Similarly, certain transitive verbs can take a subject referring to a person and an object referring to the same person or to one of his body parts, again with a passive sense.^[4] Consider the verb break:

• Her leg was broken in a car accident.
• She broke her leg in a car accident.

The two sentences are almost synonymous, but the explicit passive construction is less idiomatic.

Gerunds and nominalization

Gerunds and nominalized verbs (nouns derived from verbs and referring to the actions or states expressed by them), unlike finite verbs, do not require explicit subjects. This allows an object to be expressed while omitting a subject. For example:

• The proof of the pudding is in the eating.
• Generating electricity typically requires a magnet and a solenoid.

The same applies to infinitive constructions:

• The easiest way to make more space would be to install more shelving.
• The first step is to read the manual.

Usage and style

Many English educators and usage guides, such as The Elements of Style, discourage the use or overuse of the passive voice, seeing it as unnecessarily verbose, or as potentially unclear.^{[5] [6]} However, the passive voice is frequently found in pedagogically valued writing, and its use in modern English continues as a meaningful mode of expression. In a sentence devoted to explaining why it should be avoided, The Elements of Style itself employs the passive voice^[7], concretely showing the utility of this grammar feature, despite prescriptivist ideals.

Notes

1.     ^ Language Log: How to defend yourself from bad advice about writing

2.     ^ The American Heritage Book of English Usage, ch. 1, sect. 24 "double passive." Boston: Houghton Mifflin, 1996. http://www.bartleby.com/64/C001/024.html. Accessed 13 November 2006.

3.     ^ Neal Whitman, "Double Your Passive, Double Your Fun", in Literal Minded. http://literalminded.wordpress.com/2005/05/16/double-your-passive-double-your-fun/. Accessed 13 November 2006.

4.     ^ Benjamin Zimmer, “Shia crushed his hand?”, in Language Log, 2008 August 5.

5.     ^ William Strunk, Jr. and E. B. White, The Elements of Style; available at http://www.crockford.com/wrrrld/style3.html#11.

6.     ^ Nature.com, “Writing for a Nature journal”.

7.     ^ Geoffrey K. Pullum, “Passive Aggression”, in Language Log, 2006 July 18

http://en.wikipedia.org/wiki/Passive_voice