The final temperature of the object is going to be determined by how well it can lose heat. As the object warms up, it will lose heat via three mechanisms: radiation, conduction, and convection. All three of these mechanisms are more efficient the hotter the object is. The final temperature will be reached when the rate of heat loss by these three mechanisms balances the amount of heat coming in to the object. So to get a hotter object, we need to make sure that it can’t lose heat very effectively.
To a certain extent, we can control the amount of heat that we lose to conduction and convection; we can have the object be held in place by highly insulating materials, or even rig up some kind of electromagnetic system where the object isn’t in contact with anything solid, and we could imagine putting the object in a clear lucite box that let through most of the sunlight’s energy but could be pumped down to a vacuum.
The one thing we can’t eliminate is radiation. So the best possible case would be an object in direct sunlight, in a complete vacuum, not touching any other objects, receiving the same amount of sunlight as the Earth does. As it happens, there’s a pretty well-known object satisfying these criteria: the Moon. The day side of the Moon is about 123 °C (253 °F), and this temperature is determined by the rate at which the Moon radiates away the incoming heat of the Sun.
I wouldn’t think that you could get significantly hotter than this. The Moon isn’t perfectly black (it reflects about 11% of the incoming light), so you might be able to get hotter for this reason. On the other hand, the Moon is above the Earth’s atmosphere, so it receives more solar energy; an object in a transparent vacuum chamber on Earth wouldn’t receive as much heat from the Sun. So you might not be able to get quite this hot after all. The actual answer isn’t clear to me without breaking out my thermodynamics textbooks, which, come on, it’s the weekend.
Actually jet fuel doesn’t have to melt them to make the building collapse. Steel loses a lot of strength when it is heated, and that is what makes blacksmithing possible. If they lose too much strength, they collapse.
Where you can gain is in noting that sunlight doesn’t have a uniform energy/wavelength distribution. If you have a material that adsorbs energy at those wavelengths that are strong in sunlight, but has low emissivity in the longer wavelengths that you would expect the peak of your object’s emission to be at the temperatures you hope to get to, you can do better than a black body.
This is the basis of various magic proprietary coatings used in solar hot water heaters. A quick Google turned up this site. And we see materials that when used in sunlight have a ratio of up to 11. So make your box out of stainless steel which has had been heated to oxidise the nickel content. It would appear that could get very toasty.
Also, if you put the plate in the top of a well insulated box with an open bottom, and reflect the sunlight upwards, you can get rid of buoyant advection as a transfer mechanism, and if your box is constructed like a Thermos dewar and it’s deep, you can get rid of all the conduction except for the very small conduction downward through a long column of air.
The OP rules out optical things that focus, but does not mention redirecting sunlight upwards without focusing, so I’m proposing that.
While the Moon as a whole is in a vacuum, the surface which is receiving radiation from the Sun is not. It’s touching the rocks below, and surface heat is conducted away to heat the inner portions that receive no direct sunlight. I’d think that the Moon as a whole is in thermal equilibrium and at some depth is at a pretty constant temperature, but the portion above goes through a month-long cycle of warming at the surface and conducting heat into the interior then cooling at the surface and having heat conducted from the interior back out. We know it must be doing this because the nighttime surface temperature is -173 °C.
So I’d think that the surface of the moon never gets as hot as a much smaller object in the same place gets – an object that would have a higher interior equilibrium temperature. How much hotter I don’t know.
To give a real life example.
We use devices that measure their own temperature.
They are small unpainted aluminum cased, laying on the ground. Generate a small amount of internal heat.
When the outside temperature is 30 C, the temperature inside can be over the measurement limit of 50 C, when in direct sunlight.
You can work it out from the Stefann-Boltzman law. at the orbit of the earth, equilibrium temperature works out to somewhere around 300K, which is about average Earth temperature. See here, for instance: