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Possible safer way to make ZnS:Cu phosphor?

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Conundrum

Wed Sept 22 2010, 07:10PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Hi all.
After reading "The Sun" article re. a fatal H2S poisoning case, I got nervous about having anything to do with this gas.

This got me thinking about alternatives.
One idea is to set up a polarised mini-Castner cell (?) with a copper electrode in a glass tube coated with ZnS and BaSO4 dried powder paint, and a layer of TiO2 paint followed with a hollow carbon "tube", crocodile clips and the whole thing is then buried in sand with the "tail" sticking upright.

this is then connected to a fairly high voltage (most likely 2.5KV DC from a collection of CCFL inverters in parallel as it is relatively tolerant of near S/C use) and left to cook at 800C or so with a temperature feedback to keep things safe.

The idea is that the ZnS partially dissociates at the Cu electrode hot spot and the copper electromigrates onto it, forming the desired ZnS:Cu, while the BaSO4 combines with the TiO2 to form BaTiO3.
The carbon absorbs the remaining free oxygen to form CO2 as long as the proportions are correct.

EDIT: I tried it using an old fluorescent CCFL fragment but it didnt work as well as hoped, seems that I missed an important step of pre-heating the mixture to drive out the moisture, then compressing it into a pellet.

EDIT:-

sound feasible or is this totally hopeless?

regards, -A

GluD

Wed Sept 22 2010, 09:48PM

Registered Member #1221 Joined: Wed Jan 09 2008, 06:17PM
Location: Odense, Denmark
Posts: 196

Why not just mix the Zn, Cu and S to a powder and light it up outside, could use a chicken-stick to light it with to get as much distance to the combustion as possible?

Even if your idea works it seems to be alot of work for something so simple.

Proud Mary

Wed Sept 22 2010, 10:58PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

GluD wrote ...

Why not just mix the Zn, Cu and S to a powder and light it up outside, could use a chicken-stick to light it with to get as much distance to the combustion as possible?

Even if your idea works it seems to be alot of work for something so simple.

I was going to suggest something very similar AndrÃ© - making an extremely dilute solution of your dopant i.e. CuSO4, and using this to make a slurry of Zn granules and S powder with a mortar and pestle. Put the slurry in a lidded crucible on a pipe clay triangle on top of a tripod, and heat it to red heat with propane or MAPP gas.

I have done this once or twice to make small quantities of doped sulphides with good effect.

Inevitably, some SO2 will be produced - though not that much with a lidded crucible - and this will form sulphurous acid with the moisture of eye and lung, so do take care to ensure adequate ventilation. SO2 is choking stuff, but has nothing like the toxicity of H2S, which had a brief but grisly career as a war gas in Flanders, so I'd suggest having nothing to do with it: see:

Bored Chemist

Thu Sept 23 2010, 08:00PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Proud Mary

Thu Sept 23 2010, 08:09PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A very vigorous reaction - perhaps not one of the sulphides to make in the back garden then!

I have a bottle of ZnS around somewhere, and could spare you 25 grams, rather than see you taking an avoidable risk
in trying to prepare the phosphor yourself.

Conundrum

Thu Sept 23 2010, 09:55PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

it appears to (somewhat) work.
I set up a "quick & dirty" setup with the ZnS,BaSO4 paint mixed with graphite powder on a brass doorknob, and a tiny bit of titanium dioxide paint on top.
Arced CCFL inverter through it and it glowed nicely while powered, as you can see here.
interestingly I got a short lived greenish glow in the middle of the arc zone when the power was interrupted so it looks promising.

also noticed a similar green fluorescence effect after arcing CCFL HV through a zinc gluconate tablet with a dab of paint, so the next step is to try each product on a piece of ITO with my EL inverter (when it turns up)..

EDIT:- didn't work, but I did find out that using solid phase materials saves a lot of hassle.

EDIT:- have ordered 3 vials of the EL phosphor and two of the dielectric, let me know if anyone wants some for a project etc.

Thanks for the safety tips guys..
regards, -A

Proud Mary

Tue Sept 28 2010, 10:45PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Aha, I have the whole thin film ZnS biz sorted with none of the hazards we looked at above:

CYCLIC VOLTAMMETRIC SYNTHESIS OF ZnS THIN FILMS USING TRIETHANOLAMINE AS A COMPLEXING AGENT

This requires no more than zinc chloride, sodium thiosulphate, TEA, and electricity. The amount of TEA in the electrolytic cell affects not only the rate of deposition of the ZnS film, but its bandgap too - a reaction you can do in a beaker with an ordinary bench PSU.

It looks likely that a thin ZnS film could be deposited on a piece of tin conductive glass - totally brill for a scintillation counter plate without clunky grains of ZnS in glue type coatings.

Conundrum

Wed Sept 29 2010, 07:13PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

neat..
could also be useful for making homemade EL sheet, assuming that flexible (i.e. touch screen cover plastic) can be used in the same way.

now to find the reagents... i already have some silver cleaner with thiourea, but i don't think this will work...
as for TEA,

seems to suggest that it can be found in some cosmetic removal creams...

regards, -A
"Bother" said Pooh, as his car blew its oxygen sensor...

Proud Mary

Wed Sept 29 2010, 08:24PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Conundrum wrote ...

neat..
could also be useful for making homemade EL sheet, assuming that flexible (i.e. touch screen cover plastic) can be used in the same way.

now to find the reagents... i already have some silver cleaner with thiourea, but i don't think this will work...
as for TEA,

seems to suggest that it can be found in some cosmetic removal creams...

The ingredients are inexpensive, uncontroversial, not especially hazardous or noxious, and easy to obtain from normal chemical dealers, so there is no need to try to extract them from expensive consumer products. In any case, a high degree of purity is required of all the raw materials if the thin films are to form reliably. I think I already have zinc chloride and sodium thiosulphate in stock, and have found a supplier of TEA, so will be ready to roll when I have some other work out of the way.

Dopants can be added to the electrolysis cell, so they will be co-deposited as defects in the thin film with the ZnS.

I lost your email address when I upgraded to a new computer, so if you'd like to send it to me in a PM, I'll send you the whole paper detailing the process, in a pdf attachment file.

Bored Chemist

Thu Sept 30 2010, 06:22PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"The ingredients are inexpensive, uncontroversial, not especially hazardous or noxious"
In the very real sense that thiourea is a suspect carcinogen.

Having said that the evidence is poor and it's not as if anyone is planning to drink the stuff.

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